FR2675399A1 - Pressurised heat exchanger-reactor comprising means for controlled combustion - Google Patents

Abstract

Heat exchanger-reactor under pressure and at high temperature. At one of its ends the heat exchanger reactor (14) comprises an entry for pressurised heating gas containing oxygen, connected to a calandria (14) of the exchanger-reactor in which the said gases circulate, and means (13) for feeding a gas mixture, connected to a plurality of reaction tubes (12), the said calandria comprising means of turbulence (51) adapted to produce a chicane circulation of gas circulating therein, the said exchanger-reactor comprising, at the other end, an exit (17) for the said gases and means (24) for discharging a hot gaseous effluent, connected to the said reaction tubes, the said exchanger-reactor additionally comprising means (15, 16) for controlled in-situ combustion inside the calandria (14), adapted to carry out, using appropriate injections of gaseous fuel and optionally of inert gas in contact with the said gases, a suitable combustion to maintain the temperature of the gases substantially constant all along the tubes, the said means of combustion being connected to means (5b) for introducing fuel into the calandria and optionally to means (5c) for introducing inert gas. Application to the steam-cracking of a hydrocarbon feedstock.

Description

The invention relates to a pressurized and high temperature heat exchanger-reactor for carrying out endothermic chemical reactions.

The invention applies to the implementation by an example of a steam cracking reaction of a hydrocarbon feed such as naphtha.

The reactions involved in these applications involve quantities of heat which must be supplied for a very short time and therefore it is advantageous to be able to have a reactor capable of delivering a high thermal flux, according to a square type temperature profile, or rectangular.

Heating at very high temperature (for example 1000 ° C. and above) of reaction systems such as steam cracking of hydro-carbides involves, according to the prior art, burners supplied with fuel in an oven of the radiative type.

The tubes in which the reactants flow, are subjected to a heat exchange of mainly radiative type, the contribution of convection heating being minimal.

The radiative heat exchange implies that there is no screen between the heat source and the tubes to be heated. Under these conditions, the number of tubes inside a burner chamber, even with multiple walls, can only be limited. In addition, the pressure drop and residence time constraints mean that the length of the individual tubes cannot be adjusted at will in order to compensate for this limited number of tubes. This generally results in a compromise in the heat transfer system between a large burner chamber and a rather small total surface area for heat exchange. These are in fact the radiative heat transfer mechanism and exposure to a very high temperature. (close to that of the flame) which are the compensating factors on which it is possible to intervene to reach the required level of heat exchange.

On an industrial scale, construction constraints as well as economic constraints require the parallel installation of a good number of identical ovens to reach the maximum heating capacity. For example, it may be necessary to use up to thirteen identical pyrolysis furnaces in a unit in the case of steam pyrolysis of naphtha to produce 400,000 tonnes of ethylene per year.

A disadvantage of the technology described above concerns the size requirement of the installations where a need for a large quantity of structural steel and a large surface area on the ground.

Another drawback of this type of technology is linked to exposure to a very high temperature affecting the resistance to stress of the materials. Indeed, a tube subjected to a very high temperature tends to expand both axially and transversely. This transverse expansion can be exalted by the pressure prevailing inside the tubes all the more that the pressure outside the tubes is substantially close to atmospheric pressure. It follows that the thickness of the tubes must be increased, to prevent the risk of ruptures, which ultimately costs more.

Another drawback linked to exposure to a very high temperature concerns an increase in coking inside the reaction tubes, which requires more frequent shutdowns of the installations.

 There are also known shell and tube heat exchangers in which a charge circulates. The heating gases circulating in the calender convection heat the load but there is a temperature gradient between the entry and the exit of the gases of the calender which is detrimental to the realization of an endothermic reaction for which a temperature profile of substantially square or rectangular type is preferable. Admittedly it can be recommended to increase by a factor of about 2 to 3 the gas flow rates at high temperature and under pressure at the inlet of the radiator grille but it then appears at the outlet of the exchanger an excess of power that one cannot always value and that is then lost.

The invention overcomes these drawbacks and provides excellent results.

Specifically, the invention relates to a heat exchanger reactor (11) under pressure of elongated shape, preferably vertical, with tubes (12) and with calender (14) comprising at one of its ends a gas inlet for heating (10) under pressure and containing oxygen connected to the shell (14) of the exchanger reactor in which said gases circulate, and means (13) for supplying a suitable gas mixture comprising at least one hydrocarbon connected to a plurality of reaction tubes (12), said tubes preferably being substantially parallel to each other and preferably substantially parallel to the longitudinal axis of the reactor-exchanger, said shell comprising means for turbulence of the gases flowing therein, heating said tubes by indirect heating , said reactor-exchanger comprising at the other end, an outlet (17) of said gases having circulated through the calender and means of evacuation (24/26) of an e hot gas stream connected to said reaction tubes, said exchanger-reactor further comprising means (15, 16) of controlled combustion in situ inside the calender (14) adapted to be produced by appropriate injections of gaseous fuel and possibly inert gas in contact with said gases circulating in the shell, combustion suitable for keeping the temperature of the gases substantially constant along the tubes, said combustion means being connected to means (5b) for introducing fuel into the shell and possibly to means (5c) for introducing inert gas.

According to a preferred configuration, the reactor-heat exchanger is vertical and it is suitable for co-current circulation of the gas mixture in the reaction tubes and of the heating gases in the shell from one end to the other and preferably from the bottom to the top.

According to a characteristic of the device, it is generally wise to keep the temperature of the reaction tubes substantially constant. To do this, the in-situ combustion means inside the reactor-heat exchanger may comprise at least one injection tube preferably (15) substantially parallel to the axis of the exchanger and substantially parallel to the tubes. reaction vessels, advantageously arranged in the center of a set of reaction tubes, for example, arranged in a circle, connected by one of its ends, to the means (5b) for introducing the fuel and possibly to the means (5c) of introduction of inert gas, the other end being closable, said injection tube being pierced at its periphery with a plurality of orifices (16) advantageously arranged in several circles at an appropriate distance, said orifices having a surface of increasingly smaller opening in the direction of flow of said heating gases.

By inert gas is meant gases such as CO2, N2, water vapor or their mixture.

Advantageously, the number of injection tubes judiciously distributed throughout the exchanger can represent 2 to 20% of the number of reaction tubes and preferably 5 to 10%.

Preferably water vapor can be mixed with the gaseous fuel to control its flow and to avoid hot spots at the injection ports. The ratio of water vapor to gaseous fuel can be adjusted so as to avoid cracking of the hydrocarbons present in the fuel.

The process implemented by the above device makes it possible, for example, to carry out the steam cracking of the hydrocarbon molecules at very high temperature, maintained substantially constant along the reaction tubes. Furthermore, the residence time of the product in the reactor-exchanger is very short. The reaction tubes are generally of small diameter, for example 10 to 40 mm. The difference in pressure of the heating gases between the outside of the tubes and the inside is such that it prevents, at least in part, the radial expansion of the tubes, so that the risks of rupture of the walls of the tubes are minimized. It follows that the steel thickness of these walls can be reduced. Certainly, the axial expansion remains, but a thermal expansion joint at the outlet end of the tubes can generally dampen the variations in elongation of these last.

In addition, the quench is immediate at the exit of the heating zone. There is in fact not the equivalent of connecting the tubes of the different chambers as in the units of the prior art. According to the invention, the volume of piping necessary for transfer to the quench exchanger downstream of the reactor-exchanger is minimal and the temperature profile of the reaction is substantially more of the square type, than it is according to the invention. prior art.

The heating gases which are introduced into the shell of the reactor-heat exchanger can come from a gas generator, for example a jet engine in which an amount of air and an amount of fuel have been passed and which is suitable for supplying heating gases at a pressure of 4 to 20 bar (1 bar = 0.1 MPa) and a temperature of 600 to 1400 "C and also generally containing 10 to 21% of oxygen by volume.

The best results are obtained when the temperature is 1000 to 1200 "C and the pressure is 6 to 10 bar.

Furthermore, with an oxygen content advantageously between 15 and 19% by volume, combustion in situ is obtained, making it possible to reach the desired temperature profile under excellent conditions.

Finally, the mechanical energy which can be recovered downstream by a power recovery turbine is substantially the same as that which is available at the inlet of the reactor-heat exchanger, which contributes to making the process of using this very economical reactor
The means of turbulence of the heating gases in the shell are generally those listed in the literature. Interns were advantageously used, alternating between a solid central disc and a pierced disc (disc and donut; GA Skrotzki, Power, June 1954).

The reactor-heat exchanger can also contain a plurality of reaction tubes, the number of which is for example between 500 and 1500. They are generally made of Incoloy type steel, with high nickel content and their diameter is generally from 10 to 40 mm. for a length usually between 8 and 20 meters.

The invention will be better understood from the attached figure illustrating an embodiment applicable to the production of ethylene and propylene by steam pyrolysis of naphtha combining the recovery of mechanical energy by a power recovery turbine. . A gas generator, not shown in the figure which can be a jet engine, comprises an axial air compressor which compresses it to approximately 10 to 20 bar. The air temperature increases by about 300 to 450 "C by compression. The compressed air passes through a combustion chamber which is an integral part of the jet engine and which is supplied with fuel. This fuel can be methane to fuel oil. An exothermic combustion is initiated and takes place at constant pressure to increase the air temperature to around 1000 to 120,000. The air and very hot combustion products are then expanded to a turbine high pressure power recovery which drives the air compressor through a drive shaft.

The air and the products of combustion leave the gas generator at a pressure of approximately 5 to 8 bar and at a temperature of approximately 700 to 80,000 and are sent to the outlet of the turbine, to a post-combustion chamber. in which fuel is introduced through a line. The temperature of the combustion fumes and the air (heating gas) which may still contain 15 to 19% by volume of oxygen can reach approximately 1100 to 1400. These gases are used as a heat source in a heat exchanger reactor. below and their temperature can be controlled by appropriate means slaving a fuel supply valve connected to the post-combustion.

They are sent by a line (10) to the lower end of a reactor-heat exchanger (11) which is vertical and of elongated shape. This tube and shell type reactor-exchanger comprises an enclosure (50) coated with a refractory material containing a shell (14) of normal steel without alloy, for example, adapted to withstand a pressure of approximately 20 bar. This grille includes internals (51) adapted to achieve a baffled circulation of the gases and promoting turbulence and the mixing of these gases, for example internals comprising alternately a solid central disc and a pierced disc (disc and donut; GA Skrotzki , Power, June 1954).

The reactor-exchanger also contains a plurality of reaction tubes (12), about one thousand of 20 mm in diameter, made of Incoloy type steel, with high nickel content, substantially parallel to each other, substantially parallel to the axis of the reactor- heat exchanger and maintained by a floating transverse wall (40) and a transverse wall (41) welded to the enclosure (50). These tubes are adapted to receive, thanks to a parallel supply, a mixture preheated to 580-6500C at 1.5 to 3 bar, water vapor and hydrocarbons, a naphtha cut for example, by a line (13) opening at the lower end of the reactor (11), so that the gaseous hydrocarbon mixture circulates from below above in the reactor-exchanger under conditions such that its residence time is limited to approximately 100 to 300 ms.

The tubes (12) are heated essentially by convection substantially at the temperature of the heating gases which circulate in the calender (14) connected to the line (10) by means of, for example, a nozzle not shown in the figure in the base of the grille. Thus, the oxygen-rich heating gases (10 to 19%) preferably introduced tangentially circulate in the same direction of flow as that of the gas mixture in the tubes, which promotes a greater heat supply at the very beginning of the reaction.

The reactor-exchanger comprises at one of its ends, on the discharge side of the gaseous effluent for example, a chamber (29) of mixture of gaseous fuel and dilution water vapor, substantially cylindrical, impervious to heating gases and to the gas mixture constituting the charge. It is delimited, on the end side of the reactor-exchanger, by the floating circular wall (40) which supports the reaction tubes passing through this chamber and on the shell side by a circular and floating wall (42) supporting the injection tubes which open into said chamber . This is connected to means for introducing (5b, 5c) gaseous fuel (methane for example) and water vapor. It is suitable for preheating the fuel and water vapor and cooling the walls of the chamber. It also contributes to the mixture of fuel and water vapor which is all the better achieved as the arrivals of fuel and water vapor in the cylindrical chamber are tangential and diametrically opposite. In the figure, the mixing chamber is located at the end on the discharge side of the gaseous effluent. It allows, according to this particularly advantageous embodiment, to make a pre-tempering of the effluent from 5 to 50 "C for example, in the reaction tubes cooled by the mixture, over a length representing from 1/50 to 1/10 of their length and advantageously from 1/25 to 1/15 of their length.

In order to keep the temperature substantially constant around 1000 "C throughout the reaction, a plurality of fuel injection tubes (15) are installed in incoloy (about 5% of the number of reaction tubes), the end of which is upper opens into the mixing chamber (29) and the other lower end is closed.

These tubes are substantially parallel to the reaction tubes and are arranged so as to be substantially in the center of a circle formed by the neighboring reaction tubes.

These gaseous fuel injection tubes are pierced at their periphery with calibrated orifices (16) advantageously arranged in circles judiciously distributed along these tubes, for example every meter, so that the local temperature increase is generally around 20 to 100 "C and advantageously 40 to 600 C. These orifices are usually located between the discs containing the internals. Given the pressure drop existing in the calender, the preferably circular orifices have surfaces of increasingly smaller openings, in the direction of gas flow in the grille to be able to introduce substantially the same flow rate at each injection level.

To control the thermal level of the reaction, to possibly avoid hot spots and to minimize the risk of cracking of the fuel, in particular hydrocarbons other than methane, water vapor can be added to the fuel supply. makes it possible to dilute the mixture, generally in a vapor to fuel ratio of between 0.2 and 2.0 and advantageously between 0.8 and 1.2 by weight. Finally, the very high temperature difference and therefore the possibility of carrying out a very large rapid thermal transfer, combined with these local additions of thermal energy, contribute to inducing a substantially square temperature profile throughout the reaction, which favors better yield of ethylene.

The heating gases still containing 10 to 12% by volume of oxygen are collected and evacuated at the top of the reactor-heat exchanger at about 900-1050 "C and under about 5 to 7 bar by a line (17) and directed to a conventional gas-gas heat exchanger and then to a power recovery turbine (not shown in the figure) .The conventional exchanger is suitable for preheating counter-current by convection in appropriate tubes the gaseous reaction mixture of hydrocarbons and steam at 580-650 C. The mixture, once preheated, is sent to the lower end of the reactor-heat exchanger by line (13).

The gaseous effluent at the outlet of the reaction tubes (12) of the reactor-exchanger (11) is collected at a temperature of 800 to 900 "C by a suitable internal collector 24 and provided with an expansion joint (25). A very short transfer line (26) is connected to the upper end of the reactor-collector and leads the effluent from the collector to a conventional type member, suitable for carrying out rapid cooling (quench) and which generates steam at very high pressure, around 100 bar.

The reactor-exchanger finally comprises means for controlling its operation, not shown in the figure. They include an automatic controller connected by a transmission line to a temperature probe preferably arranged on the evacuation line of the steam cracking effluent (26). The controller controls by a transmission line a valve for partially opening the line (5b) of the gaseous fuel and at the same time by another transmission line, in a mixing chamber (29) a valve for partially opening the line (5c) bringing the water vapor, so that the value of the ratio of water vapor to hydrocarbons remains substantially that initially chosen, for example equal to 1.0.

The following example illustrates the process and the device according to the invention and in particular the quantity of energy involved. For a production of 400,000 tonnes / year of ethylene from the steam pyrolysis of naphtha , the following operating conditions are adopted:
Water to naphtha ratio 0.5
Quantity injected with naphtha 135 T / h
Mechanical energy required downstream 60 MW
Gas generator (GE LM 5000) 2 X 30 MW
Air quantity 2 x 450 T / hx 2
Quantity of heating gas produced 2 x 460 T / h at 1150 "C and 7 bar
Fuel injected in situ 4 x 3.5 T / h
Preheating temperature of
load 620 "C at 2.5 bar
Reactor-heat exchanger:
tube and grille type
Number 4
Number of tubes 1200
Outside diameter 20 mm
Length 12m
Heat transfer area 1150 m2 per reactor-exchanger
To provide all the heat of reaction to the charge and the water vapor (164
Gcal / hour) by the sensible heat of the hot gas without combustion in situ in the reactor-exchanger, the weight flow rate of hot gas should be increased by a factor of 2, so as not to exceed 100 "C in temperature loss of hot gas , which would result in a final recovery of power in excess of 50 MW.

To avoid this excess power, fuel and water vapor are injected in a weight ratio via a tube having ten injection points calibrated along the height of the tube to reach the level of energy required and a substantially isothermal temperature profile. This combustion in the reactor-exchanger provides 123 GcaVh.

The gases leave the reactor-exchanger at around 950 "C at 6 bar and pass through an exchanger where they are cooled to around 840" C, supplying 28.4 Gcal / h of preheating heat to the load. These cooled gases, at a pressure of 5 bar, are expanded in two power recovery turbines delivering 30 MW each for utilities, ie 50 GcaVh. The exhaust gases, at around 5000C, from these turbines are then sent to a common heat recovery chamber where 80 Gcal / h of heat can be recovered for a first preheating of the load and the production of steam while the gases cooled at the end to 2000C are discharged through a chimney into the atmosphere. The heat balance shows a thermal efficiency of 81.0% calculated as follows:
Amount of energy consumed by combustion Gcal / h Gas generator 275.0. In-situ 123.0
TOTAL 398s0
Amount of heat absorbed Reaction 164.0 Preheating 28.4
Recovery 80.0
SUB-TOTAL 272.4
Quantity of power recovered 60 MW or 50.0
TOTAL 322.4

Claims (8)

 optionally to means (5c) for introducing inert gas.  connected to means (5b) for introducing fuel into the shell and  substantially constant along the tubes, said combustion means being  grille, adequate combustion to maintain gas temperature  gaseous and possibly inert gas in contact with said gases flowing in the  calender, (14) adapted to be produced by appropriate fuel injections  further means (15, 16) of controlled combustion in situ inside the  hot connected to said reaction tubes, said reactor-exchanger comprising  circulated through the radiator grille and means for discharging (24) a gaseous effluent  reactor-exchanger comprising at the other end, an outlet (17) of said gases having  gases circulating therein adapted to heat said tubes by indirect heating, said  reaction tubes (12), said calender comprising turbulence means (51)  suitable gas comprising at least one hydrocarbon connected to a plurality of  which said gases circulate, and means (13) for supplying a mixture  and containing oxygen connected to the shell (14) of the reactor-exchanger in  having at one of its ends an inlet for pressurized heating gas  pressure of elongated shape, preferably vertical with tubes (12) and calender (14)  CLAIMS 1. Heat exchanger reactor (11) for carrying out an endothermic reaction under 2. Reactor-exchanger according to claim 1 wherein said means for  combustion inside the reactor-exchanger include at least one tube  injection (15), advantageously arranged in the center of a set of tubes  reaction, connected by one of its ends, to the means 5b for introducing the  fuel and possibly the means for introducing inert gas (5c), said  injection tube being pierced at its periphery with a plurality of orifices (16)  advantageously arranged in several circles at an appropriate distance,  said orifices having an increasingly smaller opening surface in the direction of  the flow of said heating gases. 3. Reactor-exchanger according to one of claims 1 to 2 wherein the number of  injection tubes represents 2 to 20% of the number of reaction tubes and  advantageously 5 to 10%. 4. Reactor-exchanger according to one of claims 1 to 3 wherein the reactor  exchanger comprises at one of its ends a mixing chamber (29) of the  fuel and inert gas substantially cylindrical, impervious to said gases  heating and said gas mixture, delimited on the end by a wall (40)  floating circular which supports the reaction tubes (12) passing through said chamber  and by a floating circular wall (42), grille side, supporting the tubes (15)  injection which open into said chamber, said chamber being connected to the  means for introducing (5b, 5c) the fuel and inert gas, said chamber being  suitable for preheating the fuel and inert gas and the  cooling of said walls of the chamber. 5. Reactor-exchanger according to claim 4, in which the mixing chamber is  located at the end of the reactor-exchanger, discharge side of the gaseous effluent and  is suitable for pre-tempering the effluent in the reaction tubes on  a length representing from 1/50 to 1/10 of their length and advantageously from  1/25 to 1/15 of their length. 6. Reactor-exchanger according to one of claims 1 to 5 characterized in that it  includes control and servo means comprising a controller  automatic connected to a temperature sensor placed on the evacuation means  effluent and adapted to control at least one fuel flow valve  supplying the reactor-exchanger by the in situ combustion means (15, 16). 7. Exchanger reactor according to one of claims 1 to 6, in which the number of  conventional tubes is between 500 and 1500. 8. Use of the reactor-heat exchanger according to one of claims 1 to 7  for steam cracking of a hydrocarbon feed.