EP2736625A1

EP2736625A1 - Heat recovery in absorption and desorption processes

Info

Publication number: EP2736625A1
Application number: EP12733595.8A
Authority: EP
Inventors: Johannes Menzel
Original assignee: ThyssenKrupp Industrial Solutions AG
Current assignee: ThyssenKrupp Industrial Solutions AG
Priority date: 2011-07-28
Filing date: 2012-06-27
Publication date: 2014-06-04
Also published as: CA2842981A1; AU2012289276A1; DE102011108749A1; TW201315529A; WO2013013749A1; US20150078973A1; US9573093B2; AR087306A1

Abstract

The invention relates to a method for removing components to be separated from industrial gases by means of absorption and desorption processes that use liquid absorbents, wherein at least one absorption device (20) and one desorption device (22) are provided, at least a part of the laden solution leaving the absorption device (20) is diverted before being heated and is delivered to the head of the heat transfer section (22a), and said laden partial stream is heated by the steam rising from the lower part of the desorption device (22b) through heat exchange in the heat transfer section (22a). The remaining stream of cold, laden solution (5a) leaving the absorption device (20) is expanded by means of the relief valve (25) and via the heat exchanger (21) into a pressure relief vessel (26), such that the stream leaving the heat exchanger (21) separates into a liquid and a gaseous state, wherein the pressure in the pressure relief vessel (26) is lowered in such a way that the total energy demand in absorption and desorption processes is reduced.

Description

Heat recovery in absorption and desorption processes

The present invention relates to an economical process for removing components to be separated from industrial gases by means of absorption and desorption processes.

The technical gases are usually natural gas or synthesis gas, wherein the synthesis gas is obtained from fossil fuels such as petroleum or coal and from biological raw materials. Natural gas and synthesis gas contain in addition to the useful valuable gases and interfering components such as sulfur compounds, in particular sulfur dioxide, carbon dioxide and other components to be separated, as well as hydrogen cyanide and water vapor. In addition to natural gas and synthesis gas, flue gases from combustion of fossil fuels belong to the group of technical gases, from which also disturbing components, such as e.g. Carbon dioxide are removed. The components to be separated can also be useful gases which are to be separated for a specific purpose.

Both physical and chemical absorbents can be used for absorption. Chemically acting absorbents are z. As aqueous amine solutions, alkali salt solutions, etc. Selexol, propylene carbonate, N-methyl-pyrrolidone, Mophysorb, methanol, etc. belong to the physical absorbents.

From the prior art, it is known to remove components to be separated from the technical gases by means of absorption and desorption in a circuit. In the absorption device, the components to be separated are absorbed by the liquid absorbent. While the solvent-insoluble gas leaves the absorption device at the head, the components to be separated remain dissolved in the liquid absorbent and leave the absorption device at the bottom. Before the loaded solution is fed to the head of the desorption desorption apparatus, the loaded solution is usually preheated by heat exchange with the hot, regenerated solution, recovering some of the energy needed for desorption in the desorption apparatus.

A reboiler located at the bottom of the desorption is generated by means of a heating medium vapor by partial evaporation of the solvent at the bottom within the desorption. The steam thus produced acts as a stripping medium in order to expel the component to be separated from the loaded solution. In countercurrent, the loaded solution is removed with stripping medium from the recorded components to be separated. The expelled components to be separated leave the desorption apparatus overhead, wherein the vapor portion of the stripping medium is condensed in a top condenser and fed back to the desorption apparatus. The regenerated solution freed from the components to be separated leaves the desorption device at the sump, wo in the solution after the heat exchange is usually cooled returned to the head of the absorption device. This completes the cycle of absorption and desorption processes.

In the absorption, which has taken place in most cases at an operating pressure of 1 to 100 bar, an absorption temperature of 20 ° C up to 70 ° C proved to be favorable to remove the components to be separated from the technical gas.

The laden with components to be separated solution can be regenerated by relaxing to a lower pressure and / or stripping, the components to be separated are released again and / or stripped off by steam. After the regeneration process, the absorbent can be cooled and reused accordingly.

The temperature required for desorption in a desorption device is higher than the temperature for absorption by the absorbent in an absorption device. The desorption is operated in most cases at a temperature between 80 ° C to 140 ° C, and an absolute pressure of 0.2 to 3 bar.

In absorption and desorption heat recovery can be achieved by the heat exchange between aufzuwärmender and cooling absorbent solution by means of a heat exchanger. By means of this heat exchange, on the one hand, the medium to be heated is desirably preheated, on the other hand, the medium to be cooled is likewise cooled down in a desirable manner, as a result of which the regeneration energy requirement to be supplied from outside is significantly reduced.

Even with an ideal heat exchange in which the temperature approach between the hot regenerated solution and the warmed loaded solution is close to zero, absorption and desorption processes still require a great deal of external energy to effect the regeneration of the solvent. For economic reasons, the heat exchanger is usually designed for a minimum temperature approximation of about 10 K between the hot regenerated solution and the warmed loaded solution. This leads to an increase in the externally supplied regeneration energy demand.

EP 1 606 041 B1 discloses a method for the selective removal of acid gas components from natural gas or synthesis gas, wherein the sour gas component is selectively removed within two absorption stages, in which the loaded solution is depressurized in two stages in a flash vessel to a selected pressure and subsequently is introduced into the desorption for desorption.

Also disclosed is a method of removing sour gas from natural gas in DE 10 2005 030 028 A1, in which the pressure of the loaded solution between the absorption column ne and the stripping column is gradually regulated accordingly that as little additional energy is needed.

WO 2010/086039 A1 teaches a method and apparatus for separating carbon dioxide from an exhaust gas of a fossil-fired power plant. In the power plant connected absorption and desorption process, the driving modes "split-feed" and "Lean Solvent Flash" combined, only the combination of both steps leads to a more favorable overall system efficiency of the power plant process. The implementation of the method according to WO 2010/086039 A1 requires compared to the prior art, a significantly higher equipment complexity, since in this case both a vacuum compression stage, as well as a further compression stage would be needed.

EP 1 736 231 A1 discloses a method and a device for removing carbon dioxide, in which case different variants have been presented in order to improve energy efficiency. Due to the thermal shading presented there, however, only a part of the energy which is supplied to the regeneration device can be recovered, since most of the energy still present in the vapors originating from the flash container described is not returned to the regeneration device but is removed by means of an external cooler and thus lost to the system. In order to be able to transfer the heat sufficiently from and to the enriched solution of the flash tank, a significantly higher level of additional equipment is required, due to additional heat exchangers, coolers, etc. For example, a required intermediate task in the absorber increases the required total height of the absorber and thus also the costs.

Due to the growing demand for resources, an economic procedure in all areas has long been an important basis for further development. Therefore, an efficient and cost-saving method is sought, which reduces the energy requirement for the entire system.

The invention is therefore based on the problem to provide an economically improved process for the removal of component to be separated from industrial gases by means of absorption and desorption with heat recovery available, in particular to further reduce the energy consumption required externally.

The object is achieved by a method for removing components to be separated from technical gases, in which the method by means of absorption and desorption processes, which use liquid absorbent, is realized, wherein at least one absorption device (20) is provided, the at least one mass transfer - Section includes, in which the components to be separated are absorbed by the liquid absorbent, and at least one desorption device (22) is provided, wherein the desorption device (22) at least one heat transfer section (22a), a Stripping section (22b) and a reboiler (23) at the sump, wherein the heat transfer section (22a) is located above the stripping section (22b) and the temperature in the desorption device (22) is higher than the temperature in the absorption device (20 ).

The absorption device (20) leaving, loaded with components to be separated solution is warmed by a heat exchanger before this solution of the desorption device (22) is supplied. The additional energy needed for regeneration is supplied by the reboiler (23) in the sump of the desorption device (22). The components to be separated by the stripping medium leave the head of the stripping section (22b) as vapors, which are further introduced into the heat transfer section (22a), cooled accordingly, and leave the desorption device (22) over the head. The freed after desorption of the components to be separated solution leaves the desorption device (22) at the bottom, exchanges the heat in the heat exchanger (21) with the enriched solution, is then cooled and is returned to the absorption device (20).

At least a portion of the absorbent device (20) leaving laden solution is diverted before warming and abandoned on the head of the heat transfer section (22a). This loaded partial stream is warmed up by the heat rising from the lower part of the desorption device (22b) by heat exchange in the heat transfer section (22a). The residual flow of the cold, laden solution (5a) leaving the absorption device (20) is released by means of the expansion valve (25) and via the heat exchanger (21) into a pressure relief tank (26), so that the flow leaving the heat exchanger (21) flows into separates a liquid and gaseous state, wherein the pressure in the pressure expansion tank (26) is lowered so that the total energy requirement is reduced in absorption and desorption processes.

Heat is transferred from the regenerated solution to the enriched solution in the heat exchanger (21). For economic reasons, the temperature difference between the hot, regenerated solution and the warmed, laden solution, as well as between the cooled, regenerated solution and the cold, warm-up, loaded solution should normally be not less than 10K. In the event that only a partial flow of the cold, laden solution for the cooling of the larger flow of the regenerated solution is available, inevitably results in a temperature difference of greater than 10 K, since the flow rate of the enriched solution is smaller than the flow rate of the regenerated solution , In order nevertheless to use the heat present in the regenerated solution as completely as possible, and to reduce the temperature difference between the cooled, regenerated solution and the cold solution to be heated, laden solution again to about 10 K, the pressure by means of the expansion valve (25 ) In the heat exchanger (21) on the side of the enriched solution stream so lowered that by the consequent Partial evaporation of the loaded solution more heat is transferred from the hot, regenerated solution to the cold, laden solution. As a result, the heat present in the circuit and in the desorptive device is used efficiently, whereby the additional amount of external energy required in the reboiler (23) is reduced. The energy gain results from the fact that according to the procedure of the invention, the heat exchanger, despite smaller flow rate, the same amount of heat transfers, as in the prior art, the entire flow of the enriched solution is passed through the heat exchanger and additionally the energy, which is recovered from the stripping vapors in the heat transfer section (22a) to the substream of the enriched solution. This reduces the total energy demand during absorption and desorption processes.

Generally, the reboiler at the bottom of the desorption device (22) continuously supplies the necessary heat in which the stripping medium is heated to the stripping steam by the reboiler. The stripping steam expels the components to be separated from the liquid solvents. The released by the pressure reduction in the pressure relief tank (26) steam is withdrawn from the head of the pressure relief tank (26) and abandoned below the heat transfer section (22a), where he gives his heat to the solution to be heated and cools down as desired. The cooled, separated components leave the desorption apparatus overhead and are ready for further processing, with no condenser or only a significantly smaller condenser needed to cool down the separated components.

It is known that some pressure must be present to promote the solution through a heat exchanger and then to the top of the desorption device. Thus, according to the prior art, a pressure of about 5 to 6 bar behind the heat exchanger (21) is required. This pre-pressure is required to overcome the geodetic height of the desorption, to compensate for the line resistance and to have sufficient control reserves in the expansion control valve to the desorption available. Further pre-pressure is required to reach the normal operating pressure of the desorption device. Due to the high pre-pressure, therefore, the vapor content in the laden solution after warming in the heat exchanger (21) is correspondingly low. The pressure can now be lowered in the heat exchanger to a pressure, so that a much higher partial evaporation allowed in the heat exchanger. According to the invention, the pressure in the pressure relief tank (26) to a pressure which is at most 1, 5 bar greater than the pressure at the top of the desorption (22), relaxed. Thus, the expanded gas phase can be easily abandoned on the desorption device (22).

Depending on the solvent, the pressure can be lowered to 1 or even 0.1 bar greater than the pressure at the top of the desorption device (22). At a pressure reduction to 0.1 bar greater than the pressure at the top of the desorption (22) increases the vapor content. The pressure, if convenient, can also be lowered below the pressure at the head of the desorption device (22), the gas phase then having to be conveyed to the top of the desorption device using a gas compressor.

The pressure release can be carried out in several series-connected pressure relief tank. This is advantageous if the expansion pressure should be lowered below the pressure in the desorption, since then only this proportion of the vapor must be compressed in order then to promote this in the desorption.

The released by the pressure reduction in the pressure expansion tank (26) steam is withdrawn from the head of the pressure relief tank (26) and above the stripping section (22b) of the desorption device (22) abandoned.

The liberated by the pressure reduction in the pressure relief tank (26) liquid part is withdrawn from the bottom of the pressure relief tank (26) and the stripping section (22b) of the desorption (22) supplied to expel the remaining components to be separated from the solvent.

The heating by the heat transfer section (22a) may be a direct or indirect heat transfer. The vapor rising from the stripping section (22b) releases its heat to the laden solution to be heated. In a direct heat transfer, the heat transfer section (22a) has a mass transfer section equipped with mass transfer elements in which direct heat transfer is carried out using as mass transfer elements all the in-column internals used for heat and mass transfer, e.g. Packings, structured packing, trays (bells, valves, sieve trays) etc. can be used. The trickled down solution absorbs the heat from the rising vapor, the vapor is cooled accordingly. In indirect heat transfer, the heat transfer section (22a) may be implemented as a heat exchanger in which indirect heat transfer is performed. By this apparatus, on the one hand, the rising vapor cooled as required, on the other hand, the laden warm-up solution is warmed up as desired.

The expansion valve (25), heat exchanger (21) and the pressure expansion tank (26) are generally arranged on the floor. An advantageous arrangement of the apparatus may, for example, be such that the expansion valve (25), heat exchanger (21) and the pressure relief tank (26) are applied above the height level of the stripping section (22b). As a result, no further pump for conveying the solution from the pressure relief tank (26) and required on the head of the desorption. However, the devices can be arranged arbitrarily, that the inventive method can be performed. The partial flow heated up via the heat transfer section (22a) is applied to the stripping section (22b).

In this method, a physically or chemically acting absorbent can be used. In particular, the process can be used to remove acid gas components from industrial gases.

The method according to the invention is explained below by means of an example by means of the drawings and tables.

Fig. 1 illustrates a prior art.

Fig. 2 illustrates the procedure according to the invention.

Fig. 3 illustrates an alternative prior art.

From a crude gas with about 13 vol% C0 ₂ is about 90% of the existing in the crude gas C0 ₂ 's are removed, the raw gas amount is 150000 NnfVh. The components C0 ₂ to be separated off are to be removed with the aid of an aqueous MDEA solution as absorbent with a solvent circulation of about 1100 t / h. The following results are obtained by means of a process simulation program:

Table"! :

According to the process of the invention, the laden laden solution leaving the absorption device is divided into two streams, wherein the proportion of the residual stream (5a, 5b) is approximately (1169-248) / 1 169 = 79% of the total solvent recycle stream. Although only about 79% of the total solvent flow is available, nearly the same amount of energy can be transferred by means of the heat exchanger to the desorption apparatus as in the prior art.

In Fig. 3, the loaded solution is passed through a heat exchanger without pressure release. One recognizes here that obviously less heat (79.1 MW) at the desorption device is transferred, so that in the end even about (34.5 - 31) / 31 = 1 1% more external energy is needed for the desired C0 ₂ removal. That is, the pressure reduction set appropriately for the solvent is thus essential for the procedure according to the invention.

With the procedure according to the invention, however, it is possible to make do with significantly less external energy in the desorption. Thus, for this example, up to (31-23) / 31 = 26% of the externally required energy in the reboiler can be saved for the regeneration of the solution.

From the table it can also be seen that the vapor content in the inventive procedure with 6% is significantly higher than that compared to the prior art.

LIST OF REFERENCE NUMBERS

1 feed gas

2 product gas

3 Loaded solution stream

4 Loaded partial flow

5a Loaded residual flow before the expansion valve

5b Loaded residual flow after the expansion valve

6, 6a Preheated electricity

6b, 6c vapor phase of the relaxed solution

7a, 7b Liquid fraction of the relaxed solution

8, 9 Regenerated solvent stream

10 Regenerated solvent stream

11 Solvent flow after heat exchange

12 Cooled regenerated solution

13 Separated component

14 Cooled separated component

15 reflux pump

16, 27 pump

17 heat exchangers

18 heat exchangers

19 reflux tank

20 absorption device

21 heat exchangers

22 desorption device

22a heat transfer section

22b stripping section

23 heat exchangers

24 diversion

25 relaxation valve

26 pressure relief tank

28 compressors

Claims

claims

A process for the removal of components to be separated from industrial gases by means of absorption and desorption processes which use liquid absorbents, wherein

• at least one absorption device (20) is provided which includes at least one mass transfer section in which the components to be separated are taken up by the liquid absorbent, and

• at least one desorption device (22) is provided, wherein the desorption device (22) comprises at least one heat transfer section (22a), a stripping section (22b) and a reboiler (23) at the sump, and wherein the heat transfer section (22a) above the Stripping section (22b) is arranged, and

The temperature in the desorption device (22) is higher than the temperature in the absorption device (20), and

The solution laden with components to be separated is heated by a heat exchanger before this solution is fed to the desorption device (22) and the remainder of the energy required for desorption is supplied through the reboiler (23) at the bottom of the desorption device (22);

• the components to be separated driven by the stripping medium leave the head of the stripping section (22b) as vapors, and

The vapors are further introduced into the heat transfer section (22a) and cooled accordingly, leaving the desorption device (22) overhead, and

The solution freed from the components to be separated after desorption leaves the desorption device (22) at the sump, is cooled and returned to the absorption device (20),

characterized in that

• at least a portion of the laden laden solution (4) leaving the absorption device (20) is branched off prior to its heating and is applied to the top of the heat transfer section (22a), and

This charged partial flow is heated by the steam rising from the lower part of the desorption device (22b) by heat exchange in the heat transfer section (22a), and

• the residual stream of the cold, laden solution (5a) leaving the absorption device (20) is expanded by means of the expansion valve (25) and via the heat exchanger (21) into a pressure relief tank (26), so that the Heat exchanger (21) separating stream leaving in a liquid and gaseous state,

• wherein the pressure in the pressure expansion tank (26) is lowered so that the total energy requirement is reduced in absorption and desorption processes.

2. The method according to claim 1, characterized in that the pressure in the pressure expansion tank (26) is expanded to a pressure which is at most 1, 5 bar greater than the pressure at the top of the desorption device (22).

3. The method according to claim 1, characterized in that the pressure is released in several successive pressure relief tanks.

4. The method according to claim 2 or 3, characterized in that by the

Pressure reduction in Druckentspannungsbehälter (26) released steam is withdrawn from the head of the pressure relief tank (26) and above the stripping section (22b) of the desorption device (22) is abandoned.

5. The method according to claim 2 or 3, characterized in that by the

Pressure reduction in Druckentspannungsbehälter (26) liberated liquid part withdrawn from the bottom of the pressure relief tank (26) and the stripping section (22b) of the desorption device (22) is supplied.

6. The method according to claims 1 to 5, characterized in that the heat transfer section (22a) has a mass transfer section, which is equipped with mass transfer elements, in which a direct heat transfer is made.

7. The method according to claims 1 to 5, characterized in that the heat transfer section (22a) is designed as a heat exchanger in which an indirect heat transfer is performed.

8. The method according to any one of the preceding claims 1 to 7, characterized in that via the heat transfer section (22a) heated partial stream () on the stripping section (22b) of the desorption device (22) is abandoned.

9. Use of the method according to claims 1 to 8, wherein a physically acting absorbent is used.

10. Use of the method according to claims 1 to 8 in which a chemically acting absorbent is used.

11. Use of the method according to claims 1 to 10 for the removal of sour gas components from industrial gases.