WO2023094364A1

WO2023094364A1 - Process for preparing a gas stream comprising chlorine

Info

Publication number: WO2023094364A1
Application number: PCT/EP2022/082760
Authority: WO
Inventors: Gerhard Olbert; Hans SCHUYTEN; Holger Friedrich; Koenraad Vandewalle; Torsten Mattke
Original assignee: Basf Se
Priority date: 2021-11-23
Filing date: 2022-11-22
Publication date: 2023-06-01
Also published as: CN118284576A; EP4436916A1

Abstract

The present invention relates to a process for preparing a gas stream G(n) comprising Cl2 and a production unit for carrying out said process as well as a use of the production unit for continuously preparing chlorine.

Description

Process for preparing a gas stream comprising chlorine

The present invention relates to a process for preparing a gas stream G(n) comprising Ch and a production unit for carrying out said process as well as a use of the production unit for continu- ously preparing chlorine.

In the large-scale production of isocyanates by phosgenation of the corresponding amines, large amounts of HCI are obtained as a by-product. In addition to its use in other applications, the recovery of chlorine from the HCI and its use in phosgene synthesis is an attractive route (chlorine recycling).

Electrochemical processes are expensive both in terms of investment and operating costs. The oxidation of HCI to chlorine, the so-called Deacon process, is more economically attractive. The Ch produced can then be used to manufacture other commercially valuable products, and at the same time the emission of waste hydrochloric acid is curtailed. The Deacon process is based on the gas phase oxidation of hydrogen chloride. HCI is reacted with oxygen over a catalyst, for example copper chloride (CuCh), to form chlorine and water in the gas phase at temperatures of 200 to 500 °C. It is an equilibrium reaction with a slight exotherm. Cooled reactors are used to control the temperature development and avoid hot spots. Both tube-bundle reactors and fluidized beds are known.

To avoid corrosion damage, suitable materials are required that can withstand the aggressive substance system at high temperatures, including nickel and nickel-based alloys but also silicon carbide. These materials and their processing is comparatively expensive, which leads to corre- spondingly high costs for the reactor. In addition, a high-temperature cooling system is required, which causes additional costs. As a rule, a nitrate I nitrite molten salt is used as the cooling sys- tem. In the event of a leak, this can react with the reaction gas and damage the reactor.

W02007/134771 A discloses a method for producing chlorine by catalytic gas phase oxidation of hydrogen chloride using oxygen, wherein the reaction is carried on at least two catalyst beds under adiabatic conditions, wherein intermediate heat exchangers are required. However, there is still a need to provide new processes for preparing chlorine which permits to avoid the afore- mentioned issues while reducing production costs.

Therefore, there is a need to provide a new process for preparing chlorine which permits to avoid these problems while reducing production costs. Thus, the object of the present invention is to provide a new process for preparing chlorine which permits to improve the production of chlorine, avoid the problems of the prior art, such as deterioration of the production unit used for such processes, while reducing production costs.

Surprisingly, it was found that the process according to the present invention permits to obtain chlorine at good conversation rate starting from fresh starting materials or obtained from recycle streams and reduce the costs of production. Further, it has surprisingly been found that the pro- cess of the present invention permits to avoid the deterioration of the production unit and thus ensure high quality production of chlorine while reducing functioning costs.

Therefore, the present invention relates to a process for preparing a gas stream G(n) compris- ing Ch in an apparatus comprising n serially coupled reaction zones Z(i), i=1 ...n, n>2, wherein a reaction zone Z(i) contains a heterogeneous catalyst C(i) for the reaction of HCI with O2 to give CI2, wherein Z(1 ) is the most upstream reaction zone and Z(n) is the most downstream reaction zone, the process comprising

(a) providing a gas stream G(0) which comprises O2 and HCI;

(b) n successive process stages S(i), i=1 ...n, wherein in each S(i), when i=1 ...n-1 , a gas stream G(i-1 ) is fed into a reaction zone Z(i) and brought in contact with C(i) in Z(i), obtaining a gas stream Gp(i) which comprises CI2, O2, H2O and HCI; removing Gp(i) from Z(i); admixing Gp(i) with a liquid stream L(i) which comprises H2O and HCI, obtaining a gas stream G(i) which comprises CI2, O2, H2O and HCI; and wherein in S(n), a gas stream G(n-1 ) is fed into a reaction zone Z(n) and brought in contact with C(n) in Z(n), obtaining a gas stream G(n) which comprises CI2, O2, H2O and HCI; wherein CG(i)(Ch) > CGG-I/CL), CGO/CL) being the CI2 concentration in G(i) and CGG-I/CL) being the CI2 concentration in G(i-1 ).

Preferably n is in the range of from 2 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 7.

Preferably the process is a continuous process.

Preferably the reaction of HCI with O2 in at least one stage S(i), more preferably in all n stages S(i), is carried out under adiabatic conditions.

Preferably from 60 to 100 weight-%, more preferably from 90 to 100 weight-%, more preferably from 95 to 100 weight-%, of G(0) consist of O2 and HCI. In the context of the present invention, the gas stream G(0) can preferably be provided from the mixture of 02 gas stream and HCI gas stream, wherein said streams can preferably be recycled streams.

Preferably, the mole ratio of the amount of oxygen, in mol, to the amount of hydrogen chloride, in mol, in G(0) is in the range of from 0.1 :1 to 5:1 , more preferably in the range of from 0.2:1 to 2:1 , more preferably in the range of from 0.22:1 to 1 :1.

Preferably the gas stream G(0) has a temperature in the range of from 150 to 350 °C, more preferably in the range of from 200 to 300°C, more preferably in the range of from 250 to 280°C. Preferably the n heterogeneous catalysts C(i) are chemically and physically the same or differ- ent. More preferably the n heterogeneous catalysts C(i) are chemically and physically the same.

As to the catalyst C(i), it is preferred that it is selected from the group consisting of a Ru- containing catalyst, a Ce-containing catalyst, a Cu-containing catalyst and a mixture of two or more thereof, more preferably is selected from the group consisting of a Ru-containing catalyst, a Ce-containing catalyst and a Cu-containing catalyst. More preferably the catalyst C(i) is a Ru- containing catalyst. More preferably the catalyst C(i) comprises, more preferably consists of, particles having an average particle size in the range of from 1 to 10 mm, preferably in the range of from 1 to 4 mm, the average particle size being determined as described in the exam- ple section.

Preferably each of the n serially coupled reaction zones Z(i) comprises, more preferably is, a catalyst bed B(i), wherein V(B(i+1 )) > V(B(i)), more preferably V(B(i+1 )) > V(B(i)), V(B(i+1 )) be- ing the volume of the catalyst bed B(i+1 ) and V(B(i)) being the volume of the catalyst bed B(i).

Preferably the catalyst bed B(i) is an adiabatic catalyst bed.

Preferably the n serially coupled reaction zones Z(i) are located in a single reactor.

Alternatively, the n serially coupled reaction zones Z(i) are preferably each located in a reactor R(i), wherein the reactor R(i) is in fluid communication with the reactor R(i+1 ), more preferably via a pipe.

In the context of the present invention, it is preferred that from 75 to 100 weight-%, more prefer- ably from 80 to 100 weight-%, more preferably from 85 to 100 weight-%, of Gp(i) consist of Ch, O2 and one or more of H2O and HCI. Indeed, in the context of the present invention, it is noted that Gp(i) may also comprise inert components such as N2, Ar and/or CO2.

Preferably the gas stream G(i) has a temperature, T(G(i)), in the range of from 150 to 350 °C, more preferably in the range of from 200 to 300°C, more preferably in the range of from 250 to 280°C.

Preferably T(Gp(i)) > T(G(i)), T(Gp(i)) being the temperature of the gas stream Gp(i) and T(G(i)) being the temperature of G(i).

Preferably in the process of the present invention, when i=1 ...n-1 , neither Gp(i) exiting Z(i) or G(i) is cooled via a heat exchanger. Indeed, in the context of the present invention, the cooling is performed via the introduction and the mixing of the liquid stream L(i).

Preferably the gas stream Gp(i) has a temperature, T(Gp(i)), of at most 450 °C, more preferably of at most 430 °C. Preferably the gas stream G(n) has a temperature T(G(n)) of at most 400 °C, more preferably of at most 395 °C. Preferably the liquid stream L(i) has temperature T(L(i)) in the range of from 10 to 60 °C, more preferably in the range of from 15 to 30 °C. More preferably the liquid stream L(i) consists of HCl and water. Preferably from 10 to 60 weight-%, more preferably from 20 to 50 weight-%, more preferably from 20 to 40 weight-%, of the liquid stream L(i) consists of HCl. Preferably from 98 to 100 weight-%, more preferably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, of the liquid stream L(i) consists of water and HCl. In other words, it is preferred that the liquid stream L(i) consists essentially of, more preferably consists of, water and HCl. Preferably f(L(i)) < fL(i+1), f(L(i)) being the HCl + H2O mass flow in L(i) and f(L(i+1)) being the HCl + H2O mass flow in L(i+1). Preferably, in the n successive process stages S(i) according to (b), G_P(i) is admixed with the liquid stream L(i) and additionally a gas stream H(i), which comprises HCl. Preferably f_H(i)(HCl) < f_H(i+1)(HCl), f _H(i)(HCl) being the HCl mass flow in H(i) and f_H(i+1) being the HCl mass flow in H(i+1). Preferably from 98 to 100 weight-%, more preferably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, of the gas stream H(i) consists of HCl. In other words, it is preferred that the gas stream H(i) consists essentially of, more preferably consists of, HCl. Preferably the gas stream H(i) has a temperature, T(H(i)), in the range of from - 50 to 250°C, more preferably in the range of from 0 to 100 °C. Preferably the liquid stream L(i), and more preferably a gas stream H(i) defined in the foregoing, are introduced and admixed with G_P(i) downstream of the reaction zone Z(i) via a nozzle, more preferably a spray nozzle or a venturi nozzle, more preferably via a spray nozzle, more prefera- bly the liquid stream L(i), and the gas stream H(i) are introduced and mixed with G_P(i) via a dual- flow spray nozzle. More preferably the liquid stream L(i), and more preferably a gas stream H(i) defined in the foregoing, mixed with GP(i) are further mixed in one or more mixing devices, the mixing device being preferably one or more of a static mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more preferably the mixing device is a static mixer. More preferably the liquid stream L(i), and more preferably a gas stream H(i) defined in the foregoing, are introduced and admixed with GP(i) downstream of the reaction zone Z(i) via a spray nozzle and the mixed streams are subsequently mixed in one or more static mixers, more preferably two static mixers. Preferably, when i=1…n-1, neither GP(i) exiting Z(i) or G(i) is cooled via a heat exchanger. Preferably no heat exchanger is used between the reaction zones Z(i) and Z(i+1). Preferably from 20 to 70 weight-%, more preferably from 40 to 60 weight-%, the gas stream G(n) consists of chlorine. Preferably according to (a) the gas stream G(0) is provided continuously. Preferably n=5. Thus, the process preferably comprises (a) providing a gas stream G(0) which comprises O2 and HCl, more preferably continu- ously; (b) five successive process stages S(1), S(2), S(3), S(4) and S(5), wherein in S(1), - a gas stream G(0) is fed into a reaction zone Z(1) and brought in contact with C(1) in Z(1), obtaining a gas stream GP(1) which comprises Cl2, O2, H2O and HCl; - removing GP(1) from Z(1); - admixing G_P(1) with a liquid stream L(1), which comprises H₂O and HCl, and a gas stream H(1) comprising HCl, obtaining a gas stream G(1) which compris- es Cl2, O2, H2O and HCl; wherein c_G(1)(Cl₂) > c_G(0)(Cl₂), c_G(1)(Cl₂) being the Cl₂ concentration in G(1) and c_G(0)(Cl₂) being the Cl2 concentration in G(0); wherein in S(2), - the gas stream G(1) is fed into a reaction zone Z(2) and brought in contact with C(2) in Z(2), obtaining a gas stream GP(2) which comprises Cl2, O2, H2O and HCl; - removing G_P(2) from Z(2); - admixing GP(2) with a liquid stream L(2), which comprises H2O and HCl, and a gas stream H(2) comprising HCl, obtaining a gas stream G(2) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(2)(Cl₂) > c_G(1)(Cl₂), c_G(2)(Cl₂) being the Cl₂ concentration in G(2) and c_G(0)(Cl₂) being the Cl2 concentration in G(1); wherein in S(3), - the gas stream G(2) is fed into a reaction zone Z(3) and brought in contact with C(3) in Z(3), obtaining a gas stream GP(3) which comprises Cl2, O2, H2O and HCl; - removing G_P(3) from Z(3); - admixing G_P(3) with a liquid stream L(3), which comprises H₂O and HCl, and a gas stream H(3) comprising HCl, obtaining a gas stream G(3) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(3)(Cl₂) > c_G(2)(Cl₂), c_G(3)(Cl₂) being the Cl₂ concentration in G(3) and c_G(2)(Cl₂) being the Cl2 concentration in G(2); wherein in S(4), - the gas stream G(3) is fed into a reaction zone Z(4) and brought in contact with C(4) in Z(4), obtaining a gas stream GP(4) which comprises Cl2, O2, H2O and HCl; - removing G_P(4) from Z(4); - admixing GP(4) with a liquid stream L(4), which comprises H2O and HCl, and a gas stream H(4) comprising HCl, obtaining a gas stream G(4) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(4)(Cl₂) > c_G(3)(Cl₂), c_G(4)(Cl₂) being the Cl₂ concentration in G(4) and c_G(3)(Cl₂) being the Cl2 concentration in G(3); wherein in S(5), - the gas stream G(4) is fed into a reaction zone Z(5) and brought in contact with C(5) in Z(5), obtaining a gas stream G(5) which comprises Cl₂, O₂, H₂O and HCl; wherein c_G(5)(Cl₂) > c_G(4)(Cl₂), c_G(5)(Cl₂) being the Cl₂ concentration in G(5) and c_G(4)(Cl₂) being the Cl2 concentration in G(4). Preferably no heat exchanger is used between the reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5). Preferably the reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5) are located in a single reactor. Preferably the reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5) are located in reactors R(1), R(2), R(3), R(4) and R(5), respectively. Preferably from 20 to 70 weight-%, more preferably 40 to 60 weight-%, of the gas stream G(5) consist of chlorine. Alternatively, preferably n=6. Thus, the process preferably comprises (a) providing a gas stream G(0) which comprises O2 and HCl, more preferably continu- ously; (b) five successive process stages S(1), S(2), S(3), S(4), S(5) and S(6), wherein in S(1), - a gas stream G(0) is fed into a reaction zone Z(1) and brought in contact with C(1) in Z(1), obtaining a gas stream G_P(1) which comprises Cl₂, O₂, H₂O and HCl; - removing G_P(1) from Z(1); - admixing G_P(1) with a liquid stream L(1) which comprises H₂O and HCl, and a gas stream H(1) comprising HCl, obtaining a gas stream G(1) which compris- es Cl2, O2, H2O and HCl; wherein c_G(1)(Cl₂) > c_G(0)(Cl₂), c_G(1)(Cl₂) being the Cl₂ concentration in G(1) and c_G(0)(Cl₂) being the Cl₂ concentration in G(0); wherein in S(2), - the gas stream G(1) is fed into a reaction zone Z(2) and brought in contact with C(2) in Z(2), obtaining a gas stream GP(2) which comprises Cl2, O2, H2O and HCl; - removing G_P(2) from Z(2); - admixing G_P(2) with a liquid stream L(2) which comprises H₂O and HCl, and a gas stream H(2) comprising HCl, obtaining a gas stream G(2) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(2)(Cl₂) > c_G(1)(Cl₂), c_G(2)(Cl₂) being the Cl₂ concentration in G(2) and c_G(0)(Cl₂) being the Cl2 concentration in G(1); wherein in S(3), - the gas stream G(2) is fed into a reaction zone Z(3) and brought in contact with C(3) in Z(3), obtaining a gas stream GP(3) which comprises Cl2, O2, H2O and HCl; - removing G_P(3) from Z(3); - admixing GP(3) with a liquid stream L(3) which comprises H2O and HCl, and a gas stream H(3) comprising HCl, obtaining a gas stream G(3) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(3)(Cl₂) > c_G(2)(Cl₂), c_G(3)(Cl₂) being the Cl₂ concentration in G(3) and c_G(2)(Cl₂) being the Cl2 concentration in G(2); wherein in S(4), - the gas stream G(3) is fed into a reaction zone Z(4) and brought in contact with C(4) in Z(4), obtaining a gas stream G_P(4) which comprises Cl₂, O₂, H₂O and HCl; - removing GP(4) from Z(4); - admixing GP(4) with a liquid stream L(4) which comprises H2O and HCl, and a gas stream H(4) comprising HCl, obtaining a gas stream G(4) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(4)(Cl₂) > c_G(3)(Cl₂), c_G(4)(Cl₂) being the Cl₂ concentration in G(4) and c_G(3)(Cl₂) being the Cl2 concentration in G(3); wherein in S(5), - the gas stream G(4) is fed into a reaction zone Z(5) and brought in contact with C(5) in Z(5), obtaining a gas stream GP(5) which comprises Cl2, O2, H2O and HCl; - removing G_P(5) from Z(5); - admixing GP(5) with a liquid stream L(5) which comprises H2O and HCl, and a gas stream H(5) comprising HCl, obtaining a gas stream G(5) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(5)(Cl₂) > c_G(4)(Cl₂), c_G(5)(Cl₂) being the Cl₂ concentration in G(5) and c_G(4)(Cl₂) being the Cl2 concentration in G(4); wherein in S(6), - the gas stream G(5) is fed into a reaction zone Z(6) and brought in contact with C(6) in Z(6), obtaining a gas stream G(6) which comprises Cl2, O2, H2O and HCl; wherein c_G(6)(Cl₂) > c_G(5)(Cl₂), c_G(6)(Cl₂) being the Cl₂ concentration in G(6) and c_G(5)(Cl₂) being the Cl2 concentration in G(5). Preferably no heat exchanger is used between the reaction zones Z(1), Z(2), Z(3), Z(4), Z(5) and Z(6). Preferably the reaction zones Z(1), Z(2), Z(3), Z(4), Z(5) and Z(6) are located in a single reactor. Preferably the reaction zones Z(1), Z(2), Z(3), Z(4), Z(5) and Z(6) are located in reactors R(1), R(2), R(3), R(4), R(5) and R(6), respectively. Preferably from 20 to 70 weight-%, more preferably 40 to 60 weight-%, of the gas stream G(6) consist of chlorine. Preferably the process of the present invention consists of (a) and (b). The present invention further relates to a production unit for carrying out the process for prepar- ing a gas stream G(n) comprising Cl₂ according to the present invention, the apparatus compris- ing - n serially coupled reaction zones Z(i), i=1...n, n≥2, wherein each reaction zone Z(i) comprises -- a catalyst C(i); - an inlet means for passing the gas stream G(0) into the reaction zone Z(i); - an outlet means for removing the gas stream GP(i) from Z(i); - a means M(i) for introducing and admixing the liquid stream L(i) with the gas stream G_P(i); - a means for passing the gas stream G(i) into the reaction zone Z(i+1); - an outlet means for removing the gas stream G(n) from the reaction zone Z(n).

Preferably the reaction zone Z(i) comprises, more preferably is, a catalyst bed B(i) which com- prises the catalyst C(i).

Preferably the reaction zone Z(i) is an adiabatic reaction zone, more preferably the catalyst bed B(i) is an adiabatic catalyst bed. In other words, it is preferred that the reaction zone Z(i), more preferably the catalyst bed B(i), operates adiabatically.

Preferably the catalyst bed B(i) is isolated with ceramic walls.

In the context of the present invention, the total volume of the catalyst beds depend on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt this parameter. Preferably the total volume of the catalyst beds B(i) to B(n) is in the range of from 10 to 100 m³. For example, the total volume of the cata- lyst beds B(i) to B(n) can more preferably be in the range of from 20 to 40 m³, more preferably in the range of from 25 to 35 m³.

Preferably V(B(i)) < V(B(i+1 )), more preferably V(B(i)) < V(B(i+1 )), V(B(i) being the volume of the catalyst bed B(i) and V(B(i+1 )) being the volume of the catalyst bed B(i+1 ).

Preferably the catalyst C(i) is selected from the group consisting of a Ru-containing catalyst, a Ce- containing catalyst, a Cu- containing catalyst and a mixture of two or more thereof, more preferably selected from the group consisting of a Ru- containing catalyst, a Ce- containing cat- alyst and a Cu- containing catalyst. More preferably the catalyst C(i) is a Ru-containing catalyst.

Preferably the catalyst C(i) comprises, more preferably consists of, particles having an average particle size in the range of from 1 to 10 mm, preferably in the range of from 1 to 4 mm, the average particle size being determined as described in the example section.

Reactor Rs (single reactor)

Preferably the apparatus further comprises a reactor Rs, said reactor Rs comprising the n seri- ally coupled reaction zones Z(i).

Preferably the reactor Rs comprises an inlet end and an oulet end, wherein the inlet means for passing the gas stream G(0) into the reaction zone Z(i) is located at the inlet end of Rs and the outlet means for removing the gas stream G(n) from the reaction zone Z(n) is located at the outlet end of Rs.

Preferably each of the n serially coupled reaction zones Z(i) is separated from each other by a space T(i) in the reactor Rs, wherein the means M(i) for introducing and admixing the liquid stream L(i), and preferably the gas stream H(i), with the gas stream Gp(i) is located in T(i). Preferably the means M(i) is a nozzle, more preferably a spray nozzle or a venturi nozzle, more preferably a spray nozzle.

More preferably the means M(i) for introducing and admixing the liquid stream L(i), and the gas stream H(i), with the gas stream Gp(i) is a dual-flow spray nozzle.

Preferably the production unit further comprises, in the space T(i) and downstream of the means M(i), one or more mixing devices, the mixing device being more preferably one or more of a static mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more prefer- ably the mixing device being a static mixer.

In the context of the present invention, the inner diameter of the reactor Rs depends on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt such parameter. Preferably the reactor Rs has an inner di- ameter in the range of from 0.5 m to 8.0 m. For example, the reactor Rs may more preferably have an inner diameter which is in the range of from 1.0 m to 5.0 m, more preferably in the range of from 1.5 m to 2.5 m.

In the context of the present invention, the height of the reactor Rs depends on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled per- son knows how to adapt such parameter. Preferably the reactor Rs has a height in the range of from 7 to 25 m. For example, the reactor may more preferably have a height which is in the range of from 10 to 20 m.

Preferably the reactor Rs is made of corrosion-resistant material, more preferably of iron-based alloys, nickel-based alloys, nickel or nickel-cladded material, more preferably of nickel or nickel- cladded material.

Preferably the means M(i) for introducing and admixing the liquid stream L(i), and preferably a gas stream H(i) as defined in the foregoing, with the gas stream Gp(i) is made of silicon carbide.

Preferably the reaction zone Z(i) comprises, more preferably is, a catalyst bed B(i), wherein the catalyst bed B(i) more preferably is an adiabatic catalyst bed. In other words, it means that the catalyst bed (reaction zone) operates adiabatically. More preferably the catalyst bed B(i) has walls which are made of ceramic and metallic materials. In this context, it is noted that the at least 50 weight-%, more preferably at least 60 weight-%, of the catalyst bed B(i) is made of ce- ramic material. The ceramic material is used for isolation.

In the context of the present invention, the height of the catalyst bed B(1) depends on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt such parameter. Preferably h(B(1)) is in the range of from 0.05 to 0.5 m, h(B(1)) being the height of the most upstream catalyst bed B(1) of the production unit. For example, h(B(1)) can more preferably be in the range of from 0.1 to 0.4 m. Preferably when n=5, h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), more preferably h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), h(B(1)) being the height of the catalyst bed B(1), h(B(2)) being the height of the catalyst bed B(2), h(B(3)) being the height of the catalyst bed B(3), h(B(4)) being the height of the catalyst bed B(4) and h(B(5)) being the height of the catalyst bed B(5).

Reactors R(i)

As an alternative to reactor Rs, it is preferred that each of the n serially coupled reaction zones Z(i) is comprised in a respective reactor R(i), wherein the reactor R(i) is connected to the reactor R(i+1) via a pipe.

Preferably the most upstream reactor R(1) comprises an inlet end and an outlet end and the most downstream reactor R(n) comprises an inlet end and an outlet end, wherein the inlet means for passing the gas stream G(0) into the reaction zone Z(i) is located at the inlet end of the reactor R(1) and the outlet means for removing the gas stream Gp(n) from the reaction zone Z(n) is located at the outlet end of the reactor R(n).

In the context of the present invention, the inner diameter of the reactors R(i) depends on differ- ent factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt such parameter. Preferably the reactor R(i) has an inner di- ameter in the range of from 0.5 m to 8.0 m. For example, the reactor R(i) may more preferably have an inner diameter which is in the range of from 1.0 m to 5.0 m, more preferably in the range of from 1.25 m to 2.75 m.

In the context of the present invention, the height of the reactors R(i) depends on different fac- tors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt such parameter. Preferably the reactor R(i) has a height in the range of from 0.4 to 10 m. For example, the reactor R(i) may more preferably have a height which is in the range of from 0.5 to 8 m.

Preferably the reactor R(i) is made of corrosion-resistant material, more preferably of iron-based alloys, nickel-based alloys, nickel or nickel-cladded material, more preferably of nickel or nickel- cladded material.

Preferably the means M(i) for introducing and admixing the liquid stream L(i), and preferably the gas stream H(i), with the gas stream Gp(i) is located in the reactor R(i). Alternatively, it is pre- ferred that such means M(i) is located in the pipes between the reactors R(i).

Preferably the means M(i) is a nozzle, more preferably a spray nozzle or a venturi nozzle, more preferably a spray nozzle. More preferably the means M(i) for introducing and admixing the liquid stream L(i) and the gas stream H(i) with the gas stream Gp(i) is a dual-flow spray nozzle.

Preferably the production unit further comprises, downstream of the means M(i) and upstream of the reactor R(i+1 ), one or more mixing devices, the mixing device being more preferably one or more of a static mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more preferably the mixing device is a static mixer.

Preferably the means M(i) for introducing and admixing the liquid stream L(i), and optionally a gas stream H(i) as defined in the foregoing, with the gas stream Gp(i) is made of silicon carbide.

Preferably the reaction zone Z(i) comprises, more preferably is, a catalyst bed B(i), wherein the catalyst bed B(i) more preferably is an adiabatic catalyst bed. More preferably the catalyst bed B(i) has walls which are made of ceramic and metallic materials. In this context, it is noted that the at least 50 weight-%, more preferably at least 60 weight-%, of the catalyst bed B(i) is made of ceramic material.

In the context of the present invention, the height of the catalyst bed B(1 ) depends on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt such parameter. Preferably h(B(1)) is in the range of from 0.05 to 0.5 m, h(B(1 )) being the height of the most upstream catalyst bed B(1) of the production unit. For example, h(B(1 )) can more preferably be in the range of from 0.1 to 0.4 m.

Preferably when n=5, h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1 )), more preferably h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1 )), h(B(1)) being the height of the catalyst bed B(1 ), h(B(2)) being the height of the catalyst bed B(2), h(B(3)) being the height of the catalyst bed B(3), h(B(4)) being the height of the catalyst bed B(4) and h(B(5)) being the height of the catalyst bed B(5).

In the context of the present invention, the total volume of the catalyst beds depend on different factors, such as the type of catalyst, the particle size of the catalyst, the space velocity. The skilled person knows how to adapt these parameters. Preferably the total volume of the catalyst beds B(i) to B(n) is in the range of from 10 to 100 m³. For example, the total volume of the cata- lyst beds B(i) to B(n) can more preferably be in the range of from 20 to 40 m³, more preferably in the range of from 25 to 35 m³.

In the context of the present invention, it is preferred that n=5 or n=6.

Preferably the production unit is free of heat exchanger between the reaction zones. Indeed, it is preferred that no heat exchanger is present between reactors R(i) and R(i+1). Without wanted to be bound to any theory, it is believed that the liquid stream L(i) permits sufficient cooling of the gas streams comprising chlorine. The present invention further relates to a use of a production unit according to the present in- vention for the continuous production of chlorine. The present invention further relates to a process for preparing phosgene comprising preparing chlorine according to the process of the present invention; reacting the obtained chlorine with carbon monoxide in the presence of a catalyst, in gas phase, obtaining phosgene. The present invention is further illustrated by the following set of embodiments and combina- tions of embodiments resulting from the dependencies and back-references as indicated. In particular, it is noted that in each instance where a range of embodiments is mentioned, for ex- ample in the context of a term such as "The process of any one of embodiments 1 to 4", every embodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the word- ing of this term is to be understood by the skilled person as being synonymous to "The process of any one of embodiments 1, 2, 3, and 4". Further, it is explicitly noted that the following set of embodiments represents a suitably structured part of the general description directed to pre- ferred aspects of the present invention, and, thus, suitably supports, but does not represent the claims of the present invention. 1. A process for preparing a gas stream G(n) comprising Cl2 in an apparatus comprising n serially coupled reaction zones Z(i), i=1...n, n≥2, wherein a reaction zone Z(i) contains a heterogeneous catalyst C(i) for the reaction of HCl with O₂ to give Cl₂, wherein Z(1) is the most upstream reaction zone and Z(n) is the most downstream reaction zone, the process comprising (a) providing a gas stream G(0) which comprises O₂ and HCl; (b) n successive process stages S(i), i=1...n, wherein in each S(i), when i=1…n-1, - a gas stream G(i-1) is fed into a reaction zone Z(i) and brought in contact with C(i) in Z(i), obtaining a gas stream G_P(i) which comprises Cl₂, O₂, H₂O and HCl; - removing G_P(i) from Z(i); - admixing G_P(i) with a liquid stream L(i) which comprises H₂O and HCl, obtain- ing a gas stream G(i) which comprises Cl₂, O₂, H₂O and HCl; and wherein in S(n), - a gas stream G(n-1) is fed into a reaction zone Z(n) and brought in contact with C(n) in Z(n), obtaining a gas stream G(n) which comprises Cl₂, O₂, H₂O and HCl; wherein c_G(i)(Cl₂) > c_G(i-1)(Cl₂), c_G(i)(Cl₂) being the Cl₂ concentration in G(i) and c_G(i-1)(Cl₂) being the Cl₂ concentration in G(i-1). 2. The process of embodiment 1, wherein n is in the range of from 2 to 10, preferably in the range of from 3 to 8, more preferably in the range of from 4 to 7. 3. The process of embodiment 1 or 2, being a continuous process.

4. The process of any one of embodiments 1 to 3, wherein the reaction of HCI with O2 in at least one stage S(i), preferably in all n stages S(i), is carried out under adiabatic condi- tions.

5. The process of any one of embodiments 1 to 4, wherein from 60 to 100 weight-%, prefer- ably from 90 to 100 weight-%, more preferably from 95 to 100 weight-%, of G(0) consist of O₂ and HCI.

6. The process of any one of embodiments 1 to 5, wherein, the mole ratio of the amount of oxygen, in mol, to the amount of hydrogen chloride, in mol, in G(0) is in the range of from 0.1 :1 to 5:1 , preferably in the range of from 0.2:1 to 2:1 , more preferably in the range of from 0.22:1 to 1 :1.

7. The process of any one of embodiments 1 to 6, wherein the gas stream G(0) has a tem- perature in the range of from 150 to 350 °C, preferably in the range of from 200 to 300°C, more preferably in the range of from 250 to 280°C.

8. The process of any one of embodiments 1 to 7, wherein the n heterogeneous catalysts C(i) are chemically and physically the same or different, preferably the same.

9. The process of any one of embodiments 1 to 8, wherein the catalyst C(i) is selected from the group consisting of a Ru-containing catalyst, a Ce-containing catalyst, a Cu-containing catalyst and a mixture of two or more thereof, preferably is selected from the group con- sisting of a Ru-containing catalyst, a Ce-containing catalyst and a Cu-containing catalyst, more preferably is a Ru-containing catalyst; wherein the catalyst C(i) preferably comprises, more preferably consists of, particles hav- ing an average particle size in the range of from 1 to 10 mm, preferably in the range of from 1 to 4 mm.

10. The process of any one of embodiments 1 to 9, wherein each of the n serially coupled reaction zones Z(i) comprises, preferably is, a catalyst bed B(i), wherein V(B(i+1 )) > V(B(i)), preferably V(B(i+1)) > V(B(i)), V(B(i+1 )) being the volume of the catalyst bed B(i+1) and V(B(i)) being the volume of the catalyst bed B(i).

11 . The process of embodiment 10, wherein the catalyst bed B(i) is an adiabatic catalyst bed.

12. The process of any one of embodiments 1 to 11 , wherein the n serially coupled reaction zones Z(i) are located in a single reactor.

13. The process of any one of embodiments 1 to 11 , wherein the n serially coupled reaction zones Z(i) are each located in a reactor R(i), wherein the reactor R(i) is in fluid communi- cation with the reactor R(i+1), preferably via a pipe. 14. The process of any one of embodiments 1 to 13, wherein from 75 to 100 weight-%, pref- erably from 80 to 100 weight-%, more preferably from 85 to 100 weight-%, of Gp(i) consist of Ch, O2 and one or more of H2O and HCI.

15. The process of any one of embodiments 1 to 14, wherein the gas stream G(i) has a tem- perature, T(G(i)), in the range of from 150 to 350 °C, preferably in the range of from 200 to 300°C, more preferably in the range of from 250 to 280°C.

16. The process of any one of embodiments 1 to 15, wherein T(Gp(i)) > T(G(i)), T(Gp(i)) being the temperature of the gas stream GP(i) and T(G(i)) being the temperature of G(i).

17. The process of any one of embodiments 1 to 16, wherein the gas stream GP(i) has a temperature, T(GP(i)), of at most 450 °C, preferably of at most 430 °C.

18. The process of any one of embodiments 1 to 17, wherein the gas stream G(n) has a tem- perature T(G(n)) of at most 400 °C, preferably of at most 395 °C.

19. The process of any one of embodiments 1 to 18, wherein the liquid stream L(i) has tem- perature T(L(i)) in the range of from 10 to 60 °C, preferably in the range of from 15 to 30 °C, wherein the liquid stream L(i) preferably consists of HCI and water.

20. The process of any one of embodiments 1 to 19, wherein from 10 to 60 weight-%, prefer- ably from 20 to 50 weight-%, more preferably from 20 to 40 weight-%, of the liquid stream L(i) consists of HCI.

21 . The process of any one of embodiments 1 to 20, wherein from 98 to 100 weight-%, pref- erably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, of the liquid stream L(i) consists of water and HCI.

22. The process of any one of embodiments 1 to 21 , wherein f(L(i)) < fL(i+1 ), f(L(i)) being the HCI + H2O mass flow in L(i) and f(L(i+1 )) being the HCI + H2O mass flow in L(i+1).

23. The process of any one of embodiments 1 to 22, wherein, in the n successive process stages S(i) according to (b), Gp(i) is admixed with the liquid stream L(i) and additionally a gas stream H(i), which comprises HCI.

24. The process of embodiment 23, wherein fn /HCI) < f_H(i+i)(HCI), f _H(i)(HCI) being the HCI mass flow in H(i) and f_H(i+i) being the HCI mass flow in H(i+1 ).

25. The process of embodiment 23 or 24, wherein from 98 to 100 weight-%, preferably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, of the gas stream H(i) consists of HCI. 26. The process of any one of embodiments 23 to 25, wherein the gas stream H(i) has a tem- perature, T(H(i)), in the range of from - 50 to 250°C, preferably in the range of from 0 to 100 °C. 27. The process of any one of embodiments 1 to 26, wherein the liquid stream L(i), and pref- erably a gas stream H(i) defined in any one of embodiments 23 to 26, are introduced and admixed with G_P(i) downstream of the reaction zone Z(i) via a nozzle, preferably a spray nozzle or a venturi nozzle, more preferably a spray nozzle, wherein more preferably the liquid stream L(i), and the gas stream H(i) are introduced and mixed with GP(i) via a dual- flow spray nozzle; wherein preferably said streams are further mixed via one or more mixing devices down- stream of said nozzle, wherein the mixing device more preferably is one or more of a stat- ic mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more prefera- bly the mixing device is a static mixer. 28. The process of any one of embodiments 1 to 27, wherein, when i=1…n-1, neither GP(i) exiting Z(i) or G(i) is cooled via a heat exchanger. 29. The process of any one of embodiments 1 to 28, wherein from 20 to 70 weight-%, prefer- ably from 40 to 60 weight-%, the gas stream G(n) consists of chlorine. 30. The process of any one of embodiments 1 to 29, wherein according to (a) the gas stream G(0) is provided continuously. 31. The process of any one of embodiments 1 to 30, wherein n=5 and the process comprises (a) providing a gas stream G(0) which comprises O2 and HCl, preferably continuously; (b) five successive process stages S(1), S(2), S(3), S(4) and S(5), wherein in S(1), - a gas stream G(0) is fed into a reaction zone Z(1) and brought in contact with C(1) in Z(1), obtaining a gas stream G_P(1) which comprises Cl₂, O₂, H₂O and HCl; - removing GP(1) from Z(1); - admixing G_P(1) with a liquid stream L(1), which comprises H₂O and HCl, and a gas stream H(1) comprising HCl, obtaining a gas stream G(1) which compris- es Cl2, O2, H2O and HCl; wherein c_G(1)(Cl₂) > c_G(0)(Cl₂), c_G(1)(Cl₂) being the Cl₂ concentration in G(1) and c_G(0)(Cl₂) being the Cl₂ concentration in G(0); wherein in S(2), - the gas stream G(1) is fed into a reaction zone Z(2) and brought in contact with C(2) in Z(2), obtaining a gas stream GP(2) which comprises Cl2, O2, H2O and HCl; - removing G_P(2) from Z(2); - admixing G_P(2) with a liquid stream L(2), which comprises H₂O and HCl, and a gas stream H(2) comprising HCl, obtaining a gas stream G(2) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(2)(Cl₂) > c_G(1)(Cl₂), c_G(2)(Cl₂) being the Cl₂ concentration in G(2) and c_G(0)(Cl₂) being the Cl2 concentration in G(1); wherein in S(3), - the gas stream G(2) is fed into a reaction zone Z(3) and brought in contact with C(3) in Z(3), obtaining a gas stream GP(3) which comprises Cl2, O2, H2O and HCl; - removing G_P(3) from Z(3); - admixing GP(3) with a liquid stream L(3), which comprises H2O and HCl, and a gas stream H(3) comprising HCl, obtaining a gas stream G(3) which compris- es Cl₂, O₂, H₂O and HCl; wherein c_G(3)(Cl₂) > c_G(2)(Cl₂), c_G(3)(Cl₂) being the Cl₂ concentration in G(3) and c_G(2)(Cl₂) being the Cl2 concentration in G(2); wherein in S(4), - the gas stream G(3) is fed into a reaction zone Z(4) and brought in contact with C(4) in Z(4), obtaining a gas stream G_P(4) which comprises Cl₂, O₂, H₂O and HCl; - removing G_P(4) from Z(4); - admixing GP(4) with a liquid stream L(4), which comprises H2O and HCl, and a gas stream H(4) comprising HCl, obtaining a gas stream G(4) which compris- es Cl₂, O₂, H₂O and HCl; wherein cG(4)(Cl2) > cG(3)(Cl2), cG(4)(Cl2) being the Cl2 concentration in G(4) and cG(3)(Cl2) being the Cl₂ concentration in G(3); wherein in S(5), - the gas stream G(4) is fed into a reaction zone Z(5) and brought in contact with C(5) in Z(5), obtaining a gas stream G(5) which comprises Cl₂, O₂, H₂O and HCl; wherein cG(5)(Cl2) > cG(4)(Cl2), cG(5)(Cl2) being the Cl2 concentration in G(5) and cG(4)(Cl2) being the Cl₂ concentration in G(4). 32. The process of embodiment 31, wherein the reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5) are located in a single reactor. 33. The process of embodiment 31, wherein the reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5) are located in reactors R(1), R(2), R(3), R(4) and R(5), respectively. 34. The process of any one of embodiments 31 to 33, wherein from 20 to 70 weight-%, pref- erably 40 to 60 weight-%, of the gas stream G(5) consist of chlorine. 35. The process of any one of embodiments 1 to 30, wherein n=6 and the process comprises (a) providing a gas stream G(0) which comprises O₂ and HCl, preferably continuously; (b) five successive process stages S(1), S(2), S(3), S(4), S(5) and S(6), wherein in S(1), - a gas stream G(0) is fed into a reaction zone Z(1) and brought in contact with C(1) in Z(1), obtaining a gas stream GP(1) which comprises Cl2, O2, H2O and HCl; - removing GP(1) from Z(1); - admixing GP(1) with a liquid stream L(1) which comprises H2O and HCl, and a gas stream H(1) comprising HCl, obtaining a gas stream G(1) which compris- es Cl₂, O₂, H₂O and HCl; wherein cG(1)(Cl2) > cG(0)(Cl2), cG(1)(Cl2) being the Cl2 concentration in G(1) and cG(0)(Cl2) being the Cl2 concentration in G(0); wherein in S(2), - the gas stream G(1) is fed into a reaction zone Z(2) and brought in contact with C(2) in Z(2), obtaining a gas stream GP(2) which comprises Cl2, O2, H2O and HCl; - removing GP(2) from Z(2); - admixing GP(2) with a liquid stream L(2) which comprises H2O and HCl, and a gas stream H(2) comprising HCl, obtaining a gas stream G(2) which compris- es Cl2, O2, H2O and HCl; wherein cG(2)(Cl2) > cG(1)(Cl2), cG(2)(Cl2) being the Cl2 concentration in G(2) and cG(0)(Cl2) being the Cl₂ concentration in G(1); wherein in S(3), - the gas stream G(2) is fed into a reaction zone Z(3) and brought in contact with C(3) in Z(3), obtaining a gas stream G_P(3) which comprises Cl₂, O₂, H₂O and HCl; - removing GP(3) from Z(3); - admixing G_P(3) with a liquid stream L(3) which comprises H₂O and HCl, and a gas stream H(3) comprising HCl, obtaining a gas stream G(3) which compris- es Cl2, O2, H2O and HCl; wherein cG(3)(Cl2) > cG(2)(Cl2), cG(3)(Cl2) being the Cl2 concentration in G(3) and cG(2)(Cl2) being the Cl₂ concentration in G(2); wherein in S(4), - the gas stream G(3) is fed into a reaction zone Z(4) and brought in contact with C(4) in Z(4), obtaining a gas stream GP(4) which comprises Cl2, O2, H2O and HCl; - removing G_P(4) from Z(4); - admixing GP(4) with a liquid stream L(4) which comprises H2O and HCl, and a gas stream H(4) comprising HCl, obtaining a gas stream G(4) which compris- es Cl₂, O₂, H₂O and HCl; wherein cG(4)(Cl2) > cG(3)(Cl2), cG(4)(Cl2) being the Cl2 concentration in G(4) and cG(3)(Cl2) being the Cl2 concentration in G(3); wherein in S(5), - the gas stream G(4) is fed into a reaction zone Z(5) and brought in contact with C(5) in Z(5), obtaining a gas stream G_P(5) which comprises Cl₂, O₂, H₂O and HCl; - removing GP(5) from Z(5); - admixing GP(5) with a liquid stream L(5) which comprises H2O and HCl, and a gas stream H(5) comprising HCl, obtaining a gas stream G(5) which compris- es Cl₂, O₂, H₂O and HCl; wherein cG(5)(Cl2) > cG(4)(Cl2), cG(5)(Cl2) being the Cl2 concentration in G(5) and cG(4)(Cl2) being the Cl₂ concentration in G(4); wherein in S(6), - the gas stream G(5) is fed into a reaction zone Z(6) and brought in contact with C(6) in Z(6), obtaining a gas stream G(6) which comprises Cl₂, O₂, H₂O and HCl; wherein cG(6)(Cl2) > cG(5)(Cl2), cG(6)(Cl2) being the Cl2 concentration in G(6) and cG(5)(Cl2) being the Cl₂ concentration in G(5). 36. The process of embodiment 35, wherein the reaction zones Z(1), Z(2), Z(3), Z(4), Z(5) and Z(6) are located in a single reactor. 37. The process of embodiment 35, wherein the reaction zones Z(1), Z(2), Z(3), Z(4), Z(5) and Z(6) are located in reactors R(1), R(2), R(3), R(4), R(5) and R(6), respectively. 38. The process of any one of embodiments 35 to 37, wherein from 20 to 70 weight-%, pref- erably 40 to 60 weight-%, of the gas stream G(6) consist of chlorine. 39. A production unit for carrying out the process for preparing a gas stream G(n) comprising Cl₂ according to any one of embodiments 1 to 38, the apparatus comprising - n serially coupled reaction zones Z(i), i=1...n, n≥2, wherein each reaction zone Z(i) com- prises -- a catalyst C(i); - an inlet means for passing the gas stream G(0) into the reaction zone Z(i); - an outlet means for removing the gas stream Gp(i) from Z(i);

- a means M(i) for introducing and admixing the liquid stream L(i) with the gas stream Gp(i);

- a means for passing the gas stream G(i) into the reaction zone Z(i+1);

- an outlet means for removing the gas stream G(n) from the reaction zone Z(n).

40. The production unit of embodiment 39, wherein the reaction zone Z(i) comprises, prefera- bly is, a catalyst bed B(i) which comprises the catalyst C(i).

41 . The production unit of embodiment 40, wherein the catalyst bed B(i) is an adiabatic cata- lyst bed.

42. The production unit of embodiment 40 or 41 , wherein the catalyst bed B(i) is isolated with ceramic walls.

43. The production unit of any one of embodiments 40 to 42, wherein the total volume of the catalyst beds B(i) to B(n) is in the range of from 10 to 100 m³.

44. The production unit of any one of embodiments 40 to 43, wherein V(B(i)) < V(B(i+1 )), preferably V(B(i)) < V(B(i+1 )), V(B(i) being the volume of the catalyst bed B(i) and V(B(i+1 )) being the volume of the catalyst bed B(i+1 ).

45. The production unit of any one of embodiments 39 to 44, wherein the catalyst C(i) is se- lected from the group consisting of a Ru-containing catalyst, a Ce- containing catalyst, a Cu- containing catalyst and a mixture of two or more thereof, preferably is selected from the group consisting of a Ru- containing catalyst, a Ce- containing catalyst and a Cu- con- taining catalyst, more preferably is a Ru-containing catalyst.

46. The production unit of embodiment 45, wherein the catalyst C(i) comprises, preferably consists of, particles having an average particle size in the range of from 1 to 10 mm, preferably in the range of from 1 to 4 mm.

47. The production unit of any one of embodiments 39 to 46, wherein the apparatus further comprises a reactor Rs, said reactor Rs comprising the n serially coupled reaction zones Z(i).

48. The production unit of embodiment 47, wherein the reactor Rs comprises an inlet end and an oulet end, wherein the inlet means for passing the gas stream G(0) into the reaction zone Z(i) is located at the inlet end of Rs and the outlet means for removing the gas stream G(n) from the reaction zone Z(n) is located at the outlet end of Rs.

49. The production unit of embodiment 47 or 48, wherein each of the n serially coupled reac- tion zones Z(i) is separated from each other by a space T(i) in the reactor Rs, wherein the means M(i) for introducing and admixing the liquid stream L(i), and preferably a gas stream H(i) as defined in any one of embodiments 23 to 26, with the gas stream Gp(i) is located in T(i).

50. The production unit of embodiment 49, wherein the means M(i) is a nozzle, more prefera- bly a spray nozzle or a venturi nozzle, more preferably a spray nozzle; wherein more preferably the means M(i) for introducing and admixing the liquid stream L(i) and the gas stream H(i) with the gas stream Gp(i) is a dual-flow spray nozzle.

51 . The production unit of embodiment 49 or 50, further comprising in the space T(i), one or more mixing devices, the mixing device being preferably one or more of a static mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more preferably the mix- ing device is a static mixer.

52. The production unit of any one of embodiments 47 to 51 , wherein the reactor Rs has an inner diameter in the range of from 0.5 m to 8.0 m.

53. The production unit of any one of embodiments 47 to 52, wherein the reactor Rs has a height in the range of from 7 to 25 m.

54. The production unit of any one of embodiments 47 to 53, wherein the reactor Rs is made of corrosion-resistant material, more preferably of iron-based alloys, nickel-based alloys, nickel or nickel-cladded material, more preferably of nickel or nickel-cladded material.

55. The production unit of any one of embodiments 47 to 54, wherein the means M(i) for in- troducing and admixing the liquid stream L(i), and preferably a gas stream H(i) as defined in any one of embodiments 23 to 26, with the gas stream Gp(i) is made of silicon carbide.

56. The production unit of any one of embodiments 39 to 55, wherein the reaction zone Z(i) comprises, preferably is, a catalyst bed B(i), wherein the catalyst bed B(i) preferably is an adiabatic catalyst bed, wherein the catalyst bed B(i) more preferably has walls which are made of ceramic and metallic materials.

57. The production unit of embodiment 56, wherein h(B(1 )) is in the range of from 0.05 to 0.5 m, h(B(1)) being the height of the most upstream catalyst bed B(1) of the production unit.

58. The production unit of 57 or 58, wherein n=5 and h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), preferably h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), h(B(1)) being the height of the catalyst bed B(1), h(B(2)) being the height of the catalyst bed B(2), h(B(3)) being the height of the catalyst bed B(3), h(B(4)) being the height of the catalyst bed B(4) and h(B(5)) being the height of the catalyst bed B(5). 59. The production unit of any one of embodiments 39 to 44, wherein each of the n serially coupled reaction zones Z(i) is comprised in a respective reactor R(i), wherein the reactor R(i) is connected to the reactor R(i+1) via a pipe.

60. The production unit of embodiment 59, wherein the most upstream reactor R(1 ) comprises an inlet end and an outlet end and the most downstream reactor R(n) comprises an inlet end and an outlet end, wherein the inlet means for passing the gas stream G(0) into the reaction zone Z(i) is located at the inlet end of the reactor R(1 ) and the outlet means for removing the gas stream Gp(n) from the reaction zone Z(n) is located at the outlet end of the reactor R(n).

61 . The production unit of embodiment 59 or 60, wherein the reactor R(i) has an inner diame- ter in the range of from 0.5 m to 8.0 m.

62. The production unit of any one of embodiments 59 to 61 , wherein the reactor R(i) has a height in the range of from 0.4 to 10 m.

63. The apparatus of any one of embodiments 59 to 62, wherein the reactor R(i) is made of corrosion-resistant material, more preferably of iron-based alloys, nickel-based alloys, nickel or nickel-cladded material, more preferably of nickel or nickel-cladded material.

64. The production unit of any one of embodiments 59 to 63, wherein the means M(i) for in- troducing and admixing the liquid stream L(i), and preferably the gas stream H(i) as de- fined in any one of embodiments 23 to 26, with the gas stream Gp(i) is located in the reac- tor R(i) or in the pipes between the reactors.

65. The production unit of embodiment 64, wherein the means M(i) and admixing the liquid stream L(i), and preferably the gas stream H(i) as defined in any one of embodiments 23 to 26, with the gas stream Gp(i) is a nozzle, preferably a spray nozzle or a venturi nozzle, more preferably a spray nozzle; wherein more preferably the means M(i) and admixing the liquid stream L(i) and the gas stream H(i) with the gas stream Gp(i) is a dual-flow nozzle.

66. The production unit of any one of embodiments 59 to 65, wherein the means M(i) for in- troducing and admixing the liquid stream L(i), and preferably a gas stream H(i) as defined in any one of embodiments 23 to 26, with the gas stream Gp(i) is made of silicon carbide.

67. The production unit of embodiment 66, further comprising, downstream of the means M(i) and upstream of the reactor R(i+1 ), one or more mixing devices, the mixing device prefer- ably being one or more of a static mixer, a dynamic mixer, an ejector, a venturi nozzle and a spray nozzle, more preferably the mixing device being a static mixer. 68. The production unit of any one of embodiments 59 to 67, wherein the catalyst beds B(i) in the reactor R(i) have walls which are made of ceramic and metallic materials.

69. The production unit of embodiment 68, wherein h(B(1 )) is in the range of from 0.05 to 0.5 m, h(B(1)) being the height of the most upstream catalyst bed B(1) of the production unit.

70. The production unit of embodiment 68 or 69, wherein n=5 and h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), preferably h(B(5)) > h(B(4)) > h(B(3)) > h(B(2)) > h(B(1)), h(B(1)) be- ing the height of the catalyst bed B(1), h(B(2)) being the height of the catalyst bed B(2), h(B(3)) being the height of the catalyst bed B(3), h(B(4)) being the height of the catalyst bed B(4) and h(B(5)) being the height of the catalyst bed B(5).

71. The production unit of any one of embodiments 68 to 70, wherein the total volume of the catalyst beds B(i) to B(n) is in the range of from 10 to 100 m³.

72. The production unit of any one of embodiments 39 to 71 , wherein n=5 or n=6.

73. The production unit of any one of embodiments 39 to 72, being free of heat exchanger between the reaction zones.

74. Use of a production unit according to any one of embodiments 39 to 73 for the continuous production of chlorine.

75. A process for preparing phosgene comprising preparing chlorine according to the process of any one of embodiments 1 to 38; reacting the obtained chlorine with carbon monoxide in the presence of a catalyst, in gas phase, obtaining phosgene.

In the context of the present invention, a term “X is one or more of A, B and C”, wherein X is a given feature and each of A, B and C stands for specific realization of said feature, is to be un- derstood as disclosing that X is either A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. In this regard, it is noted that the skilled person is capable of transfer to above abstract term to a concrete example, e.g. where X is a chemical element and A, B and C are concrete elements such as Li, Na, and K, or X is a temperature and A, B and C are concrete temperatures such as 10 °C, 20 °C, and 30 °C. In this regard, it is further noted that the skilled person is capable of extending the above term to less specific realizations of said feature, e.g. “X is one or more of A and B” disclosing that X is either A, or B, or A and B, or to more specific realizations of said feature, e.g. “X is one or more of A, B, C and D”, disclosing that X is either A, or B, or C, or D, or A and B, or A and C, or A and D, or B and C, or B and D, or C and D, or A and B and C, or A and B and D, or B and C and D, or A and B and C and D.

The present invention is further illustrated by the following Examples section and Figures. Examples

Method for measuring the average particle size

The particle size can be defined as the diameter of a sphere with equal volume. Particles are normally uniform and average dimensions can be derived from microscopic pictures of the par- ticles.

Reference Example 1 : Process for preparing chlorine not according to the present inven- tion

A set-up of 6 reactors in series with intermediate cooling (heat exchangers) and distributed HCI feed as described in example 5 of W02007/134771 A was taken as reference. Figure 1 shows a system according to Reference Example 1. The system was described using thermodynamic simulations coupled with the known equilibrium of Deacon reaction. Thereby it was assumed, that in each separate adiabatic catalyst bed the system runs into equilibrium.

The pressure, not given in the above mentioned example, was chosen at 1 bara. The feeds (amount) were the same like in example as well as the inlet temperatures of the single reactors. The results are shown in table below.

Table 1

Due to assumption of thermodynamic equilibrium in each stage and estimated pressure the out- let temperatures differ slightly from values given in the example. The overall HCI conversion was 83.7% compared to 82.4% mentioned in example 5 of W02007/134771 A.

Example 1 : Process for preparing chlorine according to the present invention

The same set-up as described in Reference Example 1 was used, but the intermediate cooling was skipped - no heat exchanger is used between the reactors - and the intermediate feeds of a gaseous, pre-heated HCI were replaced by mixed streams of gaseous HCI (25°C) and liquid, aqueous HCI (30 wt% HCI, 25°C). With mixing of the reaction mass of the upstream reactor with the mixed HCI stream the temperature is decreased due to evaporation of liquid HCI/water and heat-up of both HCI feed streams. In the downstream reactor the conversion of added HCI leads to adiabatic temperature rise as known. The composition of the mixed HCI streams were cho- sen to fit the same outlet temperature like in Reference Example 1. The results are summarized in Table 2 below:

Table 2

The dosing of mixed HCI streams lead to slightly higher inlet temperatures. The overall HCI conversion is 81% and slightly lower than the set-up with intermediate cooling devices. Howev- er, there is no need for installation of intermediate heat-exchangers anymore and the costs for the reactor set-up can be decreased.

Example 2: Process for preparing chlorine according to the present invention

A tray reactor contains 5 catalyst beds in series in a single reactor. Mixed HCI feeds as de- scribed in Table 3 are fed in between the catalyst beds. The system was described using ther- modynamic simulations coupled with the known equilibrium of Deacon reaction. Thereby it was assumed, that in each separate adiabatic catalyst bed the system runs into equilibrium. Mixed streams of gaseous HCI (25°C) and liquid, aqueous HCI (30 wt% HCI, 25°C) were introduced after catalyst beds 1 to 4. With mixing of the reaction mass of the upstream reactor with the mixed HCI stream the temperature is decreased due to evaporation of liquid HCI/water and heat-up of both HCI feed streams. In the downstream reactor the conversion of added HCI leads to adiabatic temperature rise as known. The reactor is illustrated in Figure 4 and a catalyst bed is illustrated in Figure 5.

Table 3

Description of the figures Figure 1 shows a set-up of 6 reactors in series according to the prior art. The production unit comprises reactors 1-6 arranged in series with intermediate heat exchangers a-f, positioned after each of the reactors 1-6 respectively. HCl (feed 1) and O₂ (feed 2) are introduced continuously at the inlet end of the reactor 1. Figure 2 shows a set-up of 5 reactors with no intermediate heat exchangers (Figure 2(a)). This production unit is according to a preferred embodiment of the present invention. The production unit comprises five reactors R(1), R(2), R(3), R(4) and R(5), wherein the reactor R(1) is positioned upstream of the reactor R(2), the reactor R(2) is posi- tioned upstream of the reactor R(3), the reactor R(3) is positioned upstream of the reactor R(4) and the reactor R(4) is positioned upstream of the reactor R(5). The re- actor R(1) comprises an inlet end and an outlet end, wherein at the inlet end, the gas stream G(0), comprising HCl and O₂, is introduced in the reaction zone Z(1) comprising, preferably consisting of a catalyst bed B(1). The catalyst bed B(1) com- prises the catalyst C(1). The gas stream G(0) reacts with the catalyst C(1) in the catalyst bed B(1) to obtain a gas stream GP(1) comprising Cl2, O2 and one or more of H₂O and HCl, preferably Cl₂, O₂, H₂O and HCl. Said gas stream is removed from the reaction zone Z(1) in a downstream portion of the reactor R(1) where it is mixed with a liquid stream L(1), comprising HCl and H2O, and a gas stream H(1) compris- ing HCl, the gas stream G(1) is obtained and removed from the reactor R(1) through a pipe. L(1) and H(1) are introduced into the downstream portion of the reactor R(1) via a means M(1), preferably a multi-component nozzle like the one described in Figure 2b (type of spray nozzle). The gas stream G(1) is optionally passed through one or two mixing devices, preferably static mixers, prior to entering the reactor R(2) at its inlet end. The gas stream G(1), comprising Cl₂, O₂, HCl and H₂O, is introduced in the reaction zone Z(2) comprising, preferably consisting of a catalyst bed B(2). The catalyst bed B(2) comprises the catalyst C(2). The gas stream G(1) reacts with the catalyst C(2) in the catalyst bed B(2) to obtain a gas stream GP(2) comprising Cl2, O2 and one or more of H2O and HCl, preferably Cl2, O2, H2O and HCl. Said gas stream is removed from the reaction zone Z(2) in a downstream portion of the reac- tor R(2) where it is mixed with a liquid stream L(2), comprising HCl and H₂O, and a gas stream H(2) comprising HCl, the gas stream G(2) is obtained and removed from the reactor R(2) through a pipe. L(2) and H(2) are introduced into the downstream portion of the reactor R(2) via a means M(2), preferably a multi-component nozzle like the one described in Figure 2b. The gas stream G(2) is optionally passed through one or two mixing devices, preferably static mixers, prior to entering the re- actor R(3) at its inlet end. The gas stream G(2), comprising Cl₂, O₂, HCl and H₂O, is introduced in the reaction zone Z(3) comprising, preferably consisting of a catalyst bed B(3). The catalyst bed B(3) comprises the catalyst C(3). The gas stream G(2) reacts with the catalyst C(3) in the catalyst bed B(3) to obtain a gas stream GP(3) comprising Cl₂, O₂ and one or more of H₂O and HCl, preferably Cl₂, O₂, H₂O and HCl. Said gas stream is removed from the reaction zone Z(3) in a downstream por- tion of the reactor R(3) where it is mixed with a liquid stream L(3), comprising HCl and H2O, and a gas stream H(3) comprising HCl, the gas stream G(3) is obtained and removed from the reactor R(3) through a pipe. L(3) and H(3) are introduced into the downstream portion of the reactor R(3) via a means M(3), preferably a multi- component nozzle like the one described in Figure 2b. The gas stream G(3) is op- tionally passed through one or two mixing devices, preferably static mixers, prior to entering the reactor R(4) at its inlet end. The gas stream G(3), comprising Cl₂, O₂, HCl and H₂O, is introduced in the reaction zone Z(4) comprising, preferably consist- ing of a catalyst bed B(4). The catalyst bed B(4) comprises the catalyst C(4). The gas stream G(3) reacts with the catalyst C(4) in the catalyst bed B(4) to obtain a gas stream GP(4) comprising Cl₂, O₂ and one or more of H₂O and HCl, preferably Cl₂, O₂, H₂O and HCl. Said gas stream is removed from the reaction zone Z(4) in a downstream portion of the reactor R(4) where it is mixed with a liquid stream L(4), comprising HCl and H₂O, and a gas stream H(4) comprising HCl, the gas stream G(4) is obtained and removed from the reactor R(4) through a pipe. L(4) and H(4) are introduced into the downstream portion of the reactor R(4) via a means M(4), preferably a multi-component nozzle like the one described in Figure 2b. The gas stream G(4) is optionally passed through one or two mixing devices, preferably stat- ic mixers, prior to entering the reactor R(5) at its inlet end. The gas stream G(4), comprising Cl2, O2, HCl and H2O, is introduced in the reaction zone Z(5) comprising, preferably consisting of a catalyst bed B(5). The catalyst bed B(5) comprises the catalyst C(5). The gas stream G(4) reacts with the catalyst C(5) in the catalyst bed B(5) to obtain a final gas stream G(5) comprising Cl2, O2 and one or more of H2O and HCl, preferably Cl₂, O₂, H₂O and HCl, which is removed from the reactor R(5) at its outlet end through a pipe, wherein c_G(5)(Cl₂) > c_G(4)(Cl₂) > c_G(3)(Cl₂) > c_G(2)(Cl₂) > c_G(1)(Cl₂), c_G(5)(Cl₂) being the Cl₂ concentration in G(5), c_G(4)(Cl₂) being the Cl₂ con- centration in G(4), cG(3)(Cl2) being the Cl2 concentration in G(3), cG(2)(Cl2) being the Cl₂ concentration in G(2) and c_G(1)(Cl₂) being the Cl₂ concentration in G(1). No inter- mediate heat exchanger are present between the reactors R(1) to R(5). In Figure 2, a1-a6 represent the pipes and b1-b4 represent the mixing devices, preferably static mixers. Figure 2(b) shows one of the means M(i), in particular M(4), wherein L(i) and H(i), in particular wherein L(4) and H(4), are introduced tangentially via a multicom- ponent nozzle to the reactor downstream of the catalyst bed to be mixed with GP(i), in particular GP(4), the means M(4) preferably being a multicomponent nozzle. Figure 3 shows one of the reactors of the set-up illustrated in Figure 2(a). Figure 3 shows a reactor R(2) comprising an inlet end and an outlet end, wherein at the inlet end of the reactor R(2) and upstream of the reaction zone Z(2) comprising the catalyst bed B(2), the reactor R(2) comprises a gas distributor D(2) for distributing the gas stream G(1) entering the reactor into the reaction zone Z(2). The reactor R(2) has walls sur- rounding (see d2 in Figure 3) the catalyst bed B(2) which can be made of an insulat- ing material, preferably ceramic material. At the bottom of the catalyst bed B(2) a removable support grid c2 can preferably be placed. The gas stream G(1) then re- acts with the catalyst C(2) (not shown) to form the gas stream GP(2) which is re- moved from the catalyst bed to be mixed with the liquid stream L(2) and the gas stream H(2) which were introduced in the reactor R(2) via a means M(2), preferably one or more tubes with openings at the end directed tangentially related to sym- metry axis of R(2). The three streams are then mixed in the downstream part of the reactor R(2) obtaining the gas stream G(2). The gas stream G(2) is passed through a pipe and optionally two mixing devices, such as static mixers. Figure 4 shows a set-up of 5 catalyst beds in a single reactor. This production unit is accord- ing to a preferred embodiment of the present invention. The production unit com- prises five reaction zones Z(1), Z(2), Z(3), Z(4) and Z(5) disposed serially in a single reactor Rs, wherein the reaction zone Z(1) is positioned upstream of the reaction zone Z(2), the reaction zone Z(2) is positioned upstream of the reaction zone Z(3), the reaction zone Z(3) is positioned upstream of the reaction zone Z(4) and the re- action zone Z(4) is positioned upstream of the reaction zone Z(5). The reactor Rs comprises an inlet end and an outlet end, wherein at the inlet end, the gas stream G(0), comprising HCl and O₂, is introduced in the reaction zone Z(1) comprising, preferably consisting of a catalyst bed B(1). The catalyst bed B(1) comprises the catalyst C(1). The gas stream G(0) reacts with the catalyst C(1) in the catalyst bed B(1) to obtain a gas stream GP(1) comprising Cl₂, O₂ and one or more of H₂O and HCl. Said gas stream is removed from the reaction zone Z(1) in a downstream por- tion of the reactor Rs, named T(1), where it is mixed with a liquid stream L(1), com- prising HCl and H₂O, and a gas stream H(1) comprising HCl, the gas stream G(1) is obtained and removed from T(1) to enter the reaction zone Z(2) at its inlet end. The gas stream G(1), comprising Cl2, O2, HCl and H2O, is introduced in the reaction zone Z(2) comprising, preferably consisting of a catalyst bed B(2). The catalyst bed B(2) comprises the catalyst C(2). The gas stream G(1) reacts with the catalyst C(2) in the catalyst bed B(2) to obtain a gas stream GP(2) comprising Cl₂, O₂ and one or more of H2O and HCl. Said gas stream is removed from the reaction zone Z(2) in a downstream portion of the reactor R(2), named T(2), where it is mixed with a liquid stream L(2), comprising HCl and H₂O, and a gas stream H(2) comprising HCl, the gas stream G(2) is obtained and removed from T(2) to enter the reaction zone Z(3) at its inlet end. The gas stream G(2), comprising Cl₂, O₂, HCl and H₂O, is introduced in the reaction zone Z(3) comprising, preferably consisting of a catalyst bed B(3). The catalyst bed B(3) comprises the catalyst C(3). The gas stream G(2) reacts with the catalyst C(3) in the catalyst bed B(3) to obtain a gas stream GP(3) comprising Cl₂, O₂ and one or more of H₂O and HCl. Said gas stream is removed from the reac- tion zone Z(3) in a downstream portion of the reactor R(3), named T(3), where it is mixed with a liquid stream L(3), comprising HCl and H2O, and a gas stream H(3) comprising HCl, the gas stream G(3) is obtained and removed from T(3) to enter the reaction zone Z(4) at its inlet end. The gas stream G(3), comprising Cl₂, O₂, HCl and H2O, is introduced in the reaction zone Z(4) comprising, preferably consisting of a catalyst bed B(4). The catalyst bed B(4) comprises the catalyst C(4). The gas stream G(3) reacts with the catalyst C(4) in the catalyst bed B(4) to obtain a gas stream GP(4) comprising Cl₂, O₂ and one or more of H₂O and HCl. Said gas stream is removed from the reaction zone Z(4) in a downstream portion of the reactor R(4), named T(4), where it is mixed with a liquid stream L(4), comprising HCl and H2O, and a gas stream H(4) comprising HCl, the gas stream G(4) is obtained and re- moved from T(4) to enter the reaction zone Z(5) at its inlet end. The gas stream G(4), comprising Cl₂, O₂, HCl and H₂O, is introduced in the reaction zone Z(5) com- prising, preferably consisting of a catalyst bed B(5). The catalyst bed B(5) comprises the catalyst C(5). The gas stream G(4) reacts with the catalyst C(5) in the catalyst bed B(5) to obtain a final gas stream G(5) comprising Cl₂, O₂ and one or more of H2O and HCl which is removed from the reactor Rs at its outlet end through a pipe, wherein cG(5)(Cl2) > cG(4)(Cl2) > cG(3)(Cl2) > cG(2)(Cl2) > cG(1)(Cl2), cG(5)(Cl2) being the Cl2 concentration in G(5), c_G(4)(Cl₂) being the Cl₂ concentration in G(4), c_G(3)(Cl₂) being the Cl₂ concentration in G(3), c_G(2)(Cl₂) being the Cl₂ concentration in G(2) and cG(1)(Cl2) being the Cl2 concentration in G(1). No intermediate heat exchangers are present in the reactor Rs. The gas streams GP(1), G(1), GP(2), G(2), GP(3), G(3), GP(4) an G(4) are not shown in said figure. Figure 5 shows a possible design of T(2) of the single reactor Rs shown in Figure 4. Ax- isymmetric guiding plates are introduced to first drive GP(2) to the middle of the re- actor followed by another plate to direct the flow radially outwards. In this section, the streams L(2) and H(2) are introduced into GP(2) via several dual-flow nozzles. This mixture is redirected radially inwards by another plate and mixing elements in this section ensure complete mixing of GP(2), L(2) and H(2) to get G(2). G(2) is dis- tributed finally via distribution plates (Orifice plates) equally across the reactor cross area before entering Z(3) . Orifice plates f2 can be added for the distributing GP(2) equally into the following catalyst bed B(3) not shown here. Cited literature

Claims

Claims 1. A process for preparing a gas stream G(n) comprising Cl2 in an apparatus comprising n serially coupled reaction zones Z(i), i=1...n, n≥2, wherein a reaction zone Z(i) contains a heterogeneous catalyst C(i) for the reaction of HCl with O₂ to give Cl₂, wherein Z(1) is the most upstream reaction zone and Z(n) is the most downstream reaction zone, the process comprising (a) providing a gas stream G(0) which comprises O₂ and HCl; (b) n successive process stages S(i), i=1...n, wherein in each S(i), when i=1…n-1, - a gas stream G(i-1) is fed into a reaction zone Z(i) and brought in contact with C(i) in Z(i), obtaining a gas stream G_P(i) which comprises Cl₂, O₂, H₂O and HCl; - removing G_P(i) from Z(i); - admixing G_P(i) with a liquid stream L(i) which comprises H₂O and HCl, obtain- ing a gas stream G(i) which comprises Cl2, O2, H2O and HCl; and wherein in S(n), - a gas stream G(n-1) is fed into a reaction zone Z(n) and brought in contact with C(n) in Z(n), obtaining a gas stream G(n) which comprises Cl₂, O₂, H₂O and HCl; wherein c_G(i)(Cl₂) > c_G(i-1)(Cl₂), c_G(i)(Cl₂) being the Cl₂ concentration in G(i) and c_G(i-1)(Cl₂) being the Cl₂ concentration in G(i-1). 2. The process of claim 1, wherein n is in the range of from 2 to 10, preferably in the range of from 3 to 8, more preferably in the range of from 4 to 7. 3. The process of claim 1 or 2, wherein the reaction of HCl with O2 in at least one stage S(i), preferably in all n stages S(i), is carried out under adiabatic conditions. 4. The process of any one of claims 1 to 3, wherein, the mole ratio of the amount of oxygen, in mol, to the amount of hydrogen chloride, in mol, in G(0) is in the range of from 0.1:1 to 5:1, preferably in the range of from 0.2:1 to 2:1, more preferably in the range of from 0.22:1 to 1:1. 5. The process of any one of claims 1 to 4, wherein the gas stream G(0) has a temperature in the range of from 150 to 350 °C, preferably in the range of from 200 to 300°C, more preferably in the range of from 250 to 280°C. 6. The process of any one of claims 1 to 5, wherein the n heterogeneous catalysts C(i) are chemically and physically the same or different, preferably the same; wherein the catalyst C(i) is selected from the group consisting of a Ru-containing catalyst, a Ce-containing cat- alyst, a Cu-containing catalyst and a mixture of two or more thereof, preferably is selected from the group consisting of a Ru-containing catalyst, a Ce-containing catalyst and a Cu- containing catalyst, more preferably is a Ru-containing catalyst. 7. The process of any one of claims 1 to 6, wherein the n serially coupled reaction zones Z(i) are located in a single reactor. 8. The process of any one of claims 1 to 6, wherein the n serially coupled reaction zones Z(i) are each located in a reactor R(i), wherein the reactor R(i) is in fluid communication with the reactor R(i+1), preferably via a pipe. 9. The process of any one of claims 1 to 8, wherein T(G_P(i)) > T(G(i)), T(G_P(i)) being the temperature of the gas stream GP(i) and T(G(i)) being the temperature of G(i). 10. The process of any one of claims 1 to 9, wherein the liquid stream L(i) has temperature T(L(i)) in the range of from 10 to 60 °C, preferably in the range of from 15 to 30 °C, where- in the liquid stream L(i) preferably consists of HCl and water. 11. The process of any one of claims 1 to 10, wherein, in the n successive process stages S(i) according to (b), G_P(i) is admixed with the liquid stream L(i) and additionally a gas stream H(i), which comprises HCl. 12. The process of any one of claims 1 to 11, wherein the liquid stream L(i), and preferably a gas stream H(i) as defined in claim 11, are introduced and admixed with GP(i) downstream of the reaction zone Z(i) via a nozzle, preferably a spray nozzle or a venturi nozzle, more preferably a spray nozzle. 13. The process of any one of claims 1 to 12, wherein, when i=1…n-1, neither GP(i) exiting Z(i) or G(i) is cooled via a heat exchanger. 14. A production unit for carrying out the process for preparing a gas stream G(n) comprising Cl₂ according to any one of claims 1 to 13, the apparatus comprising - n serially coupled reaction zones Z(i), i=1...n, n≥2, wherein each reaction zone Z(i) com- prises -- a catalyst C(i); - an inlet means for passing the gas stream G(0) into the reaction zone Z(i); - an outlet means for removing the gas stream G_P(i) from Z(i); - a means M(i) for introducing and admixing the liquid stream L(i) with the gas stream G_P(i); - a means for passing the gas stream G(i) into the reaction zone Z(i+1); - an outlet means for removing the gas stream G(n) from the reaction zone Z(n). 15. Use of a production unit according to claim 14 for the continuous production of chlorine.