SE502038C2 - Procedure for washing and cooling gases in the black liquor gasification - Google Patents

Abstract

Process for recovering chemicals and energy from black liquor, where the black liquor is gasified with CO, CO2, CH4, H2, and H2S, in gaseous form, and Na2CO3, NaOH and Na2S, in the form of drops of smelt, being principally formed. The mixture of gas and smelt is cooled, in a first stage, by direct contact with a cooling liquid, whereupon a part of the cooling liquid is volatilized, and the smelt drops are separated off and dissolved in the remaining part of the cooling liquid with the formation of a liquid bath (6) of green liquor. In a second stage, the gas is washed and saturated with moisture by direct contact with a washing liquid bath (13). After the gas has been washed in the second stage, energy in the form of thermal energy and condensation heat is recovered from the gas in an indirect condenser (16). The contact between the gas and the liquid bath (6), consisting of green liquor, in the first stage is substantially less than the contact between the gas and the washing liquid bath (13), in the second stage. This is achieved, by the gas in the first stage not being permitted to bubble through the liquid bath (6), consisting of green liquor, in contrast to the second stage, in which the gas is caused to buble through the washing liquid bath (13).

Description

502 038 2 However, this process, although considerably simpler and smoother than the Tomlinson process, also shows potential for improvement. A disadvantage is, for example, that undesired bicarbonate is formed in the green liquor in connection with the carbon dioxide in the pyrolysis gas coming into contact with the same when gas and melting droplets are cooled in the first step. In addition, extremely small, almost hydrophobic particles present in the gas from the gas scrubber remain in the gas, which the scrubber according to SE 448 173 is not able to effectively separate. Another disadvantage is that the energy recovery from the physical calorific value of the gas cannot be driven completely optimally for the production of high-quality process steam, but only steam of relatively moderate pressure can be produced.

SOLUTION The present invention is a further development of the concept according to SE 448 173 and effectively eliminates the disadvantages of this prior art.

The idea of the invented method is to provide an opportunity to produce green liquor without undesired hydrogen carbonate being formed in the same and to provide an opportunity to make optimal use of the gas content of thermal energy and vapor barrier while the gas content of small particles, so-called fumes or micro- solids, effectively separated.

The principle is that the outgoing gas / melt mixture from the reactor is cooled in a first step by means of direct contact with a coolant which mainly consists of water in the form of condensate. Intense contact between the gas and green liquor which is formed when molten droplets and hydrogen sulfide are dissolved in the coolant is avoided as much as possible. This avoids that carbon dioxide in the gas reacts with sodium carbonate in the green liquor to form sodium bicarbonate and that the carbon dioxide reacts with sodium hydroxide and image. It is good if the sodium hydroxide formed is not converted to sodium carbonate, since sodium hydroxide is the desired end product after causticizing the green liquor. In the causticization, sodium carbonate is converted to sodium hydroxide by reaction with slaked lime.

The gas scrubber in the second stage, on the other hand, is designed so that maximum intensive contact occurs between the gas and the washing liquid, which mainly consists of water in the form of condensate. In a preferred embodiment, this is solved by quenching in two steps, where the gas in the first quench step is not allowed to bubble through the liquid bath of green liquor collected in the bottom of the vessel. In the second quench step, the gas is washed by bubbling it through a second liquid bath consisting mainly of water in the form of condensate. In this way an efficient purification and moisture saturation of the hot gas is achieved. From the hot moisture-saturated gas, energy is then preferably extracted in the form of a condensing fan and thermal energy by using a countercurrent indirect cooler. With the help of, for example, a countercurrent drop film condenser, you can efficiently create high-quality steam in a single unit, preheat feed water and produce hot water. The gas content of small particles, micro-solids mainly consisting of sodium ricarbonate, acts as condensation nuclei in the cooler and is therefore effectively separated from the gas to collect and dissolve in the condensate.

Admittedly, it is known per se from U.S. Patent 4,328,008 (Texaco) to cool and separate reaction products from a gasification reactor in two consecutive quench steps. However, this patent differs from the present invention partly in that it relates to a poorer range of use, combustion of solid fuel, without the possibility of green liquor production which is the main object of the present invention but also in that the preferred concept of optimal energy recovery from hot saturated gas using countercurrent condenser is missing. In addition, the two quench stages of US 4,328,008 are not designed to achieve minimal contact between gas and liquid phase product in the first stage and maximum contact in the second stage which is the case of the invention described herein.

The invention will be described in more detail below with reference to a preferred embodiment with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a preferred embodiment of the concept according to the invention.

Figure 2 shows a preferred embodiment for energy recovery from combustion gas by indirect countercurrent cooling. 502 038 i 4 DETAILED DESCRIPTION OF THE DRAWINGS Part number 1 in Figure 1 indicates a pressure vessel comprising a ceramic-lined pre-gasification reactor 2. The reactor is provided with an inlet 3 for black liquor and an inlet 4 for oxygen or oxygen-containing gas and a burner (not shown).

The reactor chamber opens downwards in the form of a downpipe 5 which opens just above the liquid surface into a green liquor core core 6 below. A number of nozzles 7 for coolant open into the downpipe. Produced green liquor is transported from the chamber 6 through a line 8 via a pump 9 and a heat exchanger 10 to subsequent process steps for white liquor generation or another process step where green liquor is used. The fuel gas from the first vessel is passed through a line 11 to a second pressure vessel 12 for gas treatment and energy recovery. This line 11 opens into the pressure vessel 12 below the liquid surface in a washing liquid chamber 13 at the bottom of the vessel. Above the chamber 13 there is an indirect cooler of the countercurrent drop type condenser 16. At the top of the second pressure vessel 12 there is an outlet 17 for cooled combustion gas.

The liquid in the second vessel washing liquid chimney can be fed via a line 14 via a pump 15 to the first vessel to serve as diluent or as coolant via the spray nozzles 7. Feed water for steam generation is supplied to condenser 16 via a line 18 and produced steam has its outlet via a line 19. (Energy recovery is shown in more detail in Figure 2.) Cold water is supplied to the upper part of the condenser via a line 20 and produced hot water has its outlet via line 21. Supplementary water for maintaining fluid balance is supplied to the system via a line 22.

Step 1 - Gasification and cooling.

The gasification method itself is not to be described in more detail here, but is clear from patent SE 448 173. The product after carried out fl ash pyrolysis with understochiometric oxygen addition is in the preferred example mainly a mixture of gaseous hydrogen, carbon monoxide, carbon dioxide, water vapor and hydrogen sulfide and molten sodium carbonate droplets , sodium hydroxide and sodium sulphide with a temperature of about 950 ° C and an absolute pressure of 26 bar. The gas velocity is high and helps to transfer the molten droplets, which partly form an mlm on the reactor walls, to the green liquor chamber 6, which is arranged below the gasification reactor 2. The outlet from the reactor consists of a downcomer 5 in which coolant is sprayed through a number of nozzles 7 for 502 038 maximum contact with the melt / gas mixture. The coolant consists mainly of water, which water will partially evaporate on contact with hot gas and melt at reactor temperature. The melt droplets and the molten film along the reactor walls dissolve in the remaining part of the coolant and thereby form green liquor which falls into the liquid vessel crowner 6. Alternatively, the melt droplets fall directly into the liquid chamber 6 and only then dissolve in the green liquor already there. The cooling of the molten droplets then takes place by evaporating water in the green liquor bath.

The downcomer 5 murmurs just above the liquid level in the liquid core 6. This is very important to avoid intense contact between the gas and the formed green liquor. If the pipe had opened below the liquid surface, the gas would have been forced to bubble through the green liquor with the consequence that bicarbonate had formed by reaction between the carbon dioxide present in the gas and the sodium hydroxide and sodium carbonate present in the green liquor. The temperature after cooling is controlled by the selected operating pressure, and is related to the temperature of the saturated steam at this pressure. Thus, at an operating accident of 26 bar, an iron weight temperature of 20 DEG C. of green liquor and gas can be expected in the cooling stage if the vapor partial pressure is 60%.

The green liquor learns the first pressure vessel 1 through a line 8 and is pumped by means of a pump 9 through a heat exchanger 10 where heat energy is extracted from the green liquor by cooling the same.

A small part of the green liquor is used for wetting the inside of the downpipe 5 by returning it there and allowing a thin film to form on the inside of the downpipe.

Step 2 - Gas washing.

The cooled, partially moisture-saturated gas leaves the first vessel 1 via a conduit 11 which opens into the second vessel 12. The conduit 11 opens in the form of a liquid lock, consisting of a downcomer and a riser, below the liquid surface in the washing liquid chamber 13 located at the bottom in the vessel. Because the line opens below the liquid surface in this way, the gas is forced to bubble through the liquid, which mainly consists of water, via the liquid lock, in order to be able to rise upwards. This results in a total moisture saturation of the gas while sanitizing it as it is washed on its content of residual chemicals. Due to the intensive contact between gas and washing liquid which is made possible, some of the very difficult-to-separate small particles present in the gas, so-called micro-solids or fumes, will also be largely washed out. The temperature of the washing liquid bath 13 and the gas leaving the bath is substantially the same 502 os 6 as the temperature of the green liquor liquid chamber 6 in the first vessel 1. This enables a high vapor partial pressure in the shielding gas.

The washing liquid in the liquid cannon 13 is returned to the first vessel 1 through a line 14 to constitute diluent for the green liquor in the liquid chamber 6 or to constitute the coolant which is sprayed into the downcomer 5 via the nozzles 7.

After energy recovery in condenser 16, the gas leaves the system in a 17 fate 17.

Any remaining sulfur-containing compounds are then washed out of the gas with alkaline washing liquid, for example sodium carbonate.

Step 3 - Energy recovery.

Figure 2 shows the concept of energy recovery according to the invention in a preferred embodiment at the system pressure 26 bar. The principle is based on the use of a countercurrent drop liquid condenser 16 which is installed above the washing liquid bath 13 in the second pressure vessel 12. The moisture-saturated gas of the temperature 200 ° C which teaches the washing liquid bath 13 is caused to condense by indirect heat transfer to preheated feed water about 1800 ° C. The feed water is thereby evaporated and the resulting steam of about 10 bar excess and about 184 ° C can via a steam 24 leave the system in a desert 25 to be used elsewhere in the industry. The condensate leaving the gas falls into the washing liquid chamber 13 and will there in part constitute washing liquid for incoming gas II. This gas will thus be washed in "own" hot condensate. The liquid in the washing liquid chamber 13 consists mainly of water, but also washed-out chemicals which have been in the gas phase and dissolved in the condensate are of course found in this liquid.

After partial condensation, the gas maintains a temperature of about 180 DEG C. and can now be used for preheating said feed water. This feed water enters the condenser via a line 18 and maintains a temperature of about 70 ° C. An indirect transfer of heat from the gas reaches the feed water approx. 180 ° C and can, after passing the steam dome 24, be used for steam generation as described above.

After preheating the feed water, the gas maintains a temperature of about 80 ° C and the remaining calorific value down to about 30 ° C can be used in the same condenser unit to generate hot water. In this case, cold water enters the condenser through a line 20 at about 15 ° C and leaves the condenser through a line 21 at about 70 ° C.

Water addition to balance liquid loss from the system, among other things in the form of extracted green liquor 8, is supplied in an amount of about 2.1 m3 per tonne of pulp to the upper part of the condenser in a 22. fate 22. By the countercurrent process, this water is heated to 200 ° C before it combines with the condensate in the washing liquid chamber 13.

The gas leaves the condenser at about 30 ° C in a desert 17 and its chemical energy is utilized by burning it, whereby steam and / or electricity is produced, for example according to the concept in SE 448 173.

ADVANTAGES Through the countercurrent principle of energy recovery, the heat of vaporization from the cooling stage in the first vessel is fully utilized as a condensing fan in the second vessel. A high thermal level, which enables the generation of high-quality steam, is based on the principle that the gas in the second stage is washed in "own" hot condensate. By refluxing this condensate to the cooling stage, it is also possible that the cooling demand for melt / gas from the reactor to 90 - 100% consists of evaporative heat.

Moisture-saturated fuel gas contains about four times more water vapor than gas compared to a specific volume. This means by the countercurrent process that the steam / gas mixture at the inlet to the condenser has, for example, a speed of about 10 rn / s, after which the gas velocity decelerates when the moisture condenses so that the speed at the outlet becomes about 4-5 m / s. This simplifies droplet separation of droplets entrained with the gas.

The residual content of the gas of extremely difficult-to-separate small particles, so-called micro-solids or fumes, of the size 0.01 - 1.0 μm is used as condensation cores and separation of these particles is thereby made possible.

Due to the low gas velocity required in the countercurrent principle, a longer residence time is given for wetting these almost hydrophobic particles than is usually the case with cooling according to the cocurrent principle.

Another advantage of the countercurrent process is that the sensitive, clutter-prone lower cooling surfaces of the condenser are kept free of coating by flushing with the entire condensate at high temperature. 502 038 s In terms of equipment technology and control, a compact fl functional gas treatment unit with a single place for condensate collection constitutes a significant simplification compared with a conventional concept.

By separate quenching of melt in the first stage and gas in the second stage, production of bicarbonate in the green liquor is avoided.

Experience shows that the liquid in the quenching step of the first vessel has a tendency to foam during temporary pressure drops in the system. The present method enables such foam from the first stage to be absorbed by the washing liquid in the second quench stage.

ALTERNATIVE EMBODIMENTS The embodiment as described above is a preferred one.

However, the invention is not limited to this description but can be varied within the scope of the claims. The different sub-steps in the countercurrent condenser, when the condensation heat and thermal energy of the hot moisture-saturated gas are extracted, can of course be fewer or more and the given temperatures of, for example, feed water and gas in the different sub-steps can be at other values.

The gasification temperature in the reactor can be 500 - 1600 ° C, preferably 700 - 130 ° C and more preferably 800 - 100 ° C and the system pressure can be up to 150 bar absolute pressure, preferably 21 - 50 bar, but also atmospheric pressure is conceivable even if it is not possible. to generate high-quality steam.

The temperature level of condensate and green liquor should be as high as possible, but limited by the saturation temperature at any given system pressure. The temperature can, for example, be about 170 - 260 ° C if the system pressure is 21 - 50 bar and the vapor partial pressure in the cooling stage in the first vessel about 35 - 90%. At, for example, 83% vapor partial pressure and 26 bar absolute pressure, the temperature becomes 216 ° C.

In addition to the upper part of the condenser, water for maintaining the fluid balance can be supplied directly to the condensate, for example to the line 14.

The principle of condenser 16 can also be used in connection with other system solutions with similar needs. The condenser 16 does not have to be housed in the same vessel as the washing liquid bath 13, although this is advantageous, but cooling and condensing of the moisture-saturated gas can be carried out in a separate cooler with return of the condensate from this cooler to the vessel containing the washing liquid bath 13. Optionally, this separate cooler can consist of a conventional cooler, not countercurrent to the fall protection condenser, whereby of course the advantages of such a condenser are not achieved.

The design of the two quenchers / shower cooling baths with shower pipes and liquid baths can be designed in different ways. In the first vessel 1, for example, dry separation can be carried out in such a way that the melt droplets are allowed to fall into the cooling bath 6 without coolant being sprayed into the stream of melt droplets. The gas is then diverted from the stream of melt droplets and the coolant is instead sprayed directly into this gas stream, whereby any entrained droplets of melt dissolve and fall into the cooling bath. In the second vessel 12, the quench can be designed for unlimited mammoth pump power, i.e. the possibility for gas to lift liquid, whereby good circulation and washing power is obtained. Alternatively, a so-called venturi quench can be used in one or both quench stages, possibly with a diverter sensor.

The two liquid baths 6 and 13 can also be accommodated in one and the same vessel where they are kept separated, for example by a partition wall.

The recovery concept is of course also applicable to chemical recovery in processes with completely different types of liquors and recycled chemicals, such as bleach liquors, liquids from the production of semi-chemical pulp (eg CTMP) or liquids from a pulp process based on potassium instead of sodium.

Claims (10)

5 0 2 0 3 8 10 PATENT REQUIREMENTS A process for the extraction of chemicals and energy from black liquor obtained by pulp production by chemical digestion of raw material, the black liquor being gasified to form mainly CO, CO 2, CH 4, H 2 and H 2 S in gaseous form and Na 2 CO 3, NaOH and Na 2 S, in the form of droplets of melt. The resulting mixture of gas and melt is cooled, in a first step, by direct contact with a coolant consisting mainly of water, whereby a part of the coolant is evaporated and the melt droplets are separated and dissolved in the remaining part of the coolant to form a liquid bath (6). of green liquor. In a second step, said gas is washed and saturated by direct contact with a washing liquid bath (13) consisting mainly of water. After the gas washing in the second stage, energy is extracted in the form of thermal energy and condensation heat from the hot moisture-saturated gas in an indirect cooler (16). The method is characterized in that the contact between the gas and the liquid bath (6) consisting of green liquor in the first step is substantially less than the contact between the gas and the washing liquid bath (13) mainly consisting of water in the second step. Method according to claim 1, characterized in that the energy content of the hot moisture-saturated gas is recovered by means of an indirect cooler (16), the resulting hot condensate being collected from the cooler (16) to form part, preferably constitute a main part, of the washing liquid bath ( 13) in the second step. A method according to claim 1 or 2, characterized in that the indirect cooling is effected by a countercurrent method. The heat of condensation in the gas and the most high-quality part of the gas's thermal energy are thereby used to generate high-quality steam. Residual thermal energy is used for the use of feed water for said steam generation and production of hot water. A method according to claim 2, characterized in that liquid balance is maintained in the liquid baths (6, 13) by liquid addition (22), preferably consisting of water, to the upper part of the cooler (16). 11 502 038 5. A method according to claim 2, characterized in that hot liquid is returned from the washing liquid bath (13) in a desert (14) to the first step to constitute coolant and solution liquid there to the green liquor liquid bath. A method according to claim 1, characterized in that in the first step the gas is not allowed to bubble through the liquid bath (6) consisting of green liquor. A method according to claim 1, characterized in that the gas in the second step is caused to bubble through the washing liquid bath (13). A method according to claim 2, characterized in that the indirect cooler (16) consists of a countercurrent drop capacitor. 9. A method according to claim 1, characterized in that the pressure in the system is up to 150 bar, preferably 21-50 bar. A method according to claim 1, characterized in that the temperature in the washing liquid bath (13) is substantially the same as the temperature of gas entering the washing liquid bath and the temperature in the green liquor bath (6).