RU2656661C1 - Method for removing carbon dioxide from gas mixtures - Google Patents

Abstract

FIELD: technology of water and air purification.

SUBSTANCE: invention relates to the field of purification from carbon dioxide of various gas mixtures such as natural gas, hydrocarbon conversion gases, flue gases, etc. by absorption. Method for removing carbon dioxide from gas mixtures comprises absorbing carbon dioxide with an aqueous solution of an absorbent with a monoethanolamine (MEA) content of 10–14 % by weight at a saturation degree of the absorption solution not less than 0.65 M_CO2/M_IEA, desorption of carbon dioxide at a temperature of 70–90 °C to produce carbon dioxide and a carbon dioxide-depleted solution of the absorbent that is returned to the absorption step completely without further purification.

EFFECT: minimization of losses of MEA associated with high-temperature degradation, reduction of equipment corrosion, heat consumption and increase in the degree of removal of carbon dioxide.

1 cl, 1 tbl, 1 ex, 2 dwg

Description

The invention relates to the field of carbon dioxide purification of various gas mixtures, such as natural gas, hydrocarbon conversion gases, flue gases, etc. by absorption method. More narrowly, the invention relates to a technology for the absorption of carbon dioxide in aqueous solutions of ethanolamines, and in particular monoethanolamine (MEA). The development of these technologies is determined by the need to exclude the release of carbon dioxide into the environment, as in the case of flue gases, or by the need to purify various process gases from it with its subsequent disposal.

To date, a typical widely used carbon dioxide removal technology is as follows. At the first stage, carbon dioxide is absorbed in the nozzles of a packed or plate type, while the gas to be cleaned is supplied from below, and the absorber is irrigated with a countercurrent flow to the gas to be cleaned. The temperature in the absorber is 35-40 ° C, the pressure is 0.9 ÷ 4 MPa. As a rule, the absorbent is 30% wt. a solution of monoethanolamine (MEA) in water. The degree of saturation of the absorbent is 0.4-0.6 M _CO2 / _MEA , preferably 0.5 M _CO2 / _MEA . The saturated solution then passes through a system of recuperative heat exchangers, where it is heated by a stream of desorbed solution, and enters the desorber, where CO ₂ is desorbed at a temperature of 100-140 ° C, as a rule, to its content not lower than 0.2 M _CO2 / _MEA . The purified absorbent solution (regenerated absorbent) is returned to the absorber through the same system of regenerative heat exchangers. The temperature in the stripper is maintained by heating its bottom part with either a built-in or remote boiler, heated by steam. The disadvantage of this technology is the fact that in the bottom part of the stripper, as a rule, the temperature is maintained at 115-140 ° C. Under these conditions, thermal or thermo-oxidative degradation of the MEA occurs with the formation of a number of corrosive substances that accumulate in the absorbent stream - these are the so-called thermostable salts (TSS). The formation of TSS has two negative consequences - the loss of absorbent efficiency by reducing its concentration and the development of the corrosion process of equipment and pipelines. A special balance study of this problem [Peter Mosera, Sandra Schmidta, Knut Stahla. // Investigation of trace elements in the inlet and outlet streams of a MEA-based post-combustion capture process Results from the test program at the Niederaussem pilot plant. Energy Procedia, 4 (2011), 473-479] showed that for a year of operation, amine losses can reach ≈150% of the initial load. This means that during the operation of such an MEA regeneration system, constant compensation of its losses in the amount of 50% of the initial load is necessary throughout the year. Given that the actual degree of saturation of CO ₂ in the absorber is 0.4-0.5 M _CO2 / _MEA , and desorption is carried out to a residual CO ₂ content _of 0.2 M _CO2 / _MEA , the efficiency of using MEA is not more than 40% (almost the maximum saturation of MEA CO ₂ may be 0.74).

Typically, the stripper operates at a temperature of 115-140 ° C.

In FIG. 1 you can see the dependence of the growth of MEA losses over time on the composition of the absorbent and temperature.

In FIG. 1, top to bottom:

1 - absorbent contains 40% wt. MEA, CO ₂ saturation — 0.4 M _CO2 / M _MEA , desorber temperature — 135 ° C;

2 - the absorbent contains 30% wt. MEA, CO ₂ saturation — 0.4 M _CO2 / M _MEA , desorber temperature — 135 ° C;

3 - absorbent contains 30% wt. MEA, CO ₂ saturation 0.2 _MCO2 / _MEA , desorber temperature 135 ° C;

4 - the absorbent contains 30% wt. MEA, CO ₂ saturation - 0.4 M _CO2 / M _MEA , desorber temperature - 120 ° C;

5 - absorbent contains 30% wt. MEA, CO ₂ saturation is 0.4 M _CO2 / M _MEA , the temperature in the stripper is 100 ° C.

FIG. 1 indicates that in the case of an absorbent containing 30% wt. MEA and at a saturation of 0.4 M _CO2 / M _MEA , even at 100 ° C after 3 weeks of operation, TSS appear and then their number increases. This is the reason for the increase in MEA losses even under relatively mild desorption conditions [Chi, S., Rochelle, GT, 2001. Oxidative degradation of monoethanolamine. In: First National Conference on Carbon Sequestration, Washington, DC, May 14-17, 2001].

In FIG. Figure 2 presents a graph for determining the degree of saturation of MEA solutions by their specific electrical conductivity (UEOP).

There are many proposals for reducing IEA losses. One group of methods involves ion exchange and electrodialysis.

Thus, US Pat. No. 5,268,155 A, 1993, describes a process for removing TCC from an aqueous MEA solution, including pre-filtering this solution with subsequent purification by ion exchange, the purified amine solution first passing through a cation exchange column and then through an anion exchange.

The method is unacceptable for large-scale production: a large consumption of ion-exchange resins, regeneration reagents, large volumes of wastewater make it economically inefficient.

A typical method for removing TCC from an MEA absorbent using electrodialysis is given in US Pat. No. 6,517,700 B2, February 11, 2003. According to the patent, part of the CO ₂ depleted stream is selected from the cold portion of the loop of the MEA regeneration system. Before being fed to the electrodialyzer, the selected stream is treated with a sodium hydroxide solution to convert the residual carbon dioxide to the carbonate form and destroy the TCC also with the formation of the corresponding sodium salts. The formed sodium salts are removed in the electrodialyzer and the purified absorbent stream is recycled before the absorber. The method has not found large-scale application due to the need for reagents and a large volume of wastewater.

For example, there is a known method for regenerating an MEA absorbent using chemical reagents, described in US application No. 20090155889 A1, 06/18/2009. To remove TCC with a small amount of CO _{2, it was} proposed to treat the absorbent solution with, for example, basic zeolites, transition metal compounds, enzymes of the type “carbonic anhydrase”, which are catalysts for the reaction of CO ₂ + H ₂ O = H ₂ CO ₃ . Disadvantages - large enterprises have high reagent costs and the formation of large amounts of waste as a result of CO ₂ absorption.

In US application 20120235087 A1, 09/20/2012, a method for removing TCC from an amine (MEA) absorbent into a carbon dioxide absorption process is proposed. The method involves the selection of a portion of the amine absorbent depleted in the CO ₂ depleted stream from the stripper cube in a volume of preferably 0.01-10% and feeding this part to an additional unit, where at a temperature of 120 ° C or higher, residual CO _{2 is} removed from the extracted absorbent stream while the release of TCC, followed by the return of this additionally purified absorbent with a reduced concentration of TCC in the absorption-desorption recycling of CO ₂ absorption. The removal of CO ₂ from the specified stream occurs due to the throttling of pressure, while the gas stream of CO ₂ is removed from the system, the stream freed from CO ₂ enters the electrodialyzer through the refrigerator. The concentrate from the electrolyzer (amine solution with TCC) is sent to the sludge collector, and the diluate (purified absorbent) is sent to the regenerated absorbent line, which is returned to the absorption stage. The amount of this purified stream is 75-80% of the low CO ₂ withdrawn from the line, or 8% of the total stream. The technology for minimizing IEA losses described in this patent only partially achieves its goals. Firstly, carbon dioxide desorption, both in the main circulation line and in the additional one before the solution is fed to the electrodialyzer, is carried out at high (from 120 ° C and above) temperatures, which is the reason for the formation of TSS.

Most methods for reducing MEA losses belong to another group and boil down to removing part of the absorbent loaded with carbon dioxide from the distillation zone of the stripper, deeply treating the diverted stream from the TSS in a remote boiler, and returning the purified stream to the stripper.

In US patent 8927450 B2, 01/06/2015, to intensify the absorption process, the gas stream to be cleaned before the absorber is cooled in a refrigerator irrigated with chilled water, and then fed to the absorber. The bottom part of the stripper is also connected with a remote boiler, which heats the saturated CO ₂ absorbent passing through it to 150 ° C. Degraded MEA and TCC are removed from the boiler as sludge, and the pairs of absorbent and water of the purified part of the absorbent are returned to the stripper.

The closest analogue of the proposed method is the method described in PCT application No. WO2010142716 A1, December 16, 2010. According to the method for removing carbon dioxide from gas mixtures, it is absorbed by an aqueous absorbent solution, then carbon dioxide is desorbed from a saturated absorbent solution when heated to 120 ° C . Among the listed absorbents, MEA monoethanolamine is mentioned. The stream withdrawn from the bottom zone of the stripper with a temperature of 120 ° C is fed to a remote boiler, where the temperature is maintained (130-140 ° C), which ensures evaporation of the absorbent. The vapors are then returned to the stripper, and the liquid phase containing TCC is withdrawn. This technique allows you to increase the depth of extraction of CO ₂ . At the same time, at the outlet from the top of the stripper, a vacuum is created using a vacuum pump or an ejector. Carbon dioxide and water vapor are discharged from the upper part of the stripper and pass through a refrigerator-condenser. Water condensate is returned to the stripper, and purified carbon dioxide is sent for processing.

A common technique in these processes — taking part of the saturated absorbent stream from the bottom of the stripper and deeply removing carbon dioxide from it at a higher temperature than in the stripper, with the simultaneous removal of MEA degradation products from this selected portion of the stream — does not solve the problem of amine degradation, since this post-treatment of part of the absorbent is carried out in harsh temperature conditions, which initiates further degradation of the MEA.

Based on the above examples, it can be concluded that the formation of TCC can be eliminated or minimized only by lowering the desorption temperature of the MEA-saturated carbon dioxide absorption solution.

The aim of the invention is to reduce the consumption of adsorbent and to eliminate or minimize the formation of amine degradation products, such as thermostable salts (TCC), as well as corrosion under the influence of equipment and pipelines, while increasing the degree of removal of carbon dioxide from various gas mixtures by absorption.

This goal is achieved in that in a method for removing carbon dioxide from gas mixtures by absorbing carbon dioxide with an aqueous absorbent solution — MEA monoethanolamine — to obtain an absorbent solution of carbon dioxide saturated with carbon dioxide, to desorb carbon dioxide from this saturated absorbent solution by heating to produce carbon dioxide and depleted in dioxide carbon solution of the absorbent and returning it to the absorption stage, use an absorbent solution with a monoethanolamine content of 10-14% wt., the degree of saturation the absorption solution is at least 0.65 M _CO2 / M _MEA , desorption is carried out at a temperature of 70-90 ° C, and said depleted solution is returned to the absorption step completely without further purification.

Most preferably, an absorbent solution with a monoethanolamine content of 12% by weight is used.

As gas mixtures, natural gas, hydrocarbon conversion gases, flue gases, etc. can be used.

When used as absorbents in aqueous solutions, for example, monoethanolamine with a concentration of 10-14% wt. degree of saturation in CO ₂ to 0.74 M _CO2 / M _MEA . Moreover, according to the proposed method, the desorption of a stream saturated with carbon dioxide is carried out at a temperature of 70-90 ° C. When the desorption process is carried out under the indicated conditions, thermal degradation of the MEA is not observed, which eliminates or minimizes the loss of absorbent during operation.

Carbon dioxide desorption example

The efficiency of desorption of CO ₂ from absorbent solutions containing MEA (% wt. And M _MEA / l): 7.5 / 1.25; 10 / 1.64; 12 / 1.97; 14 / 2.29; 30 / 4.92), determined experimentally. A vessel equipped with a reflux condenser and a stirrer is filled with a saturated MEA solution. The degree of saturation of the MEA solutions was determined by the values of electrical conductivity (SEC) using calibration, an example is presented in FIG. 2. At a zero concentration of CO ₂ the value of the SEC for the initial MEA solution is 1.17 mS / cm.

The desorption of a solution saturated with carbon dioxide is carried out at 90 ° C. The carbon dioxide released together with water vapor passes through a condenser-refrigerator, where water vapor condenses, and CO ₂ is collected through a vacuum compressor in a storage tank for further disposal. The temperature of the cooling water in the condenser refrigerator is 20 ° C. The content of CO ₂ in solutions is determined by the measured values of electrical conductivity for solutions of the corresponding concentrations of MEA. The results of several experiments on the desorption of carbon dioxide are presented in the table.

The results obtained indicate that the desorption of the absorbent with an MEA concentration of 10-14% wt. It successfully occurs at 90 ° C with an intensity comparable to the parameters generally accepted today and reaches values of the degree of extraction exceeding 70%. After regeneration, the solution can be returned to the absorption stage without additional purification steps. Optimal is a solution containing 12% MEA.

Resistance to 90 ° C was tested by heating a saturated 12% MEA solution (0.74 M _CO2 / M _MEA ) for 4 weeks. Under the indicated conditions and exposure time, the formation of TSS was not detected, since the EEC of the solution remained unchanged.

At a lower desorption temperature, TSS will also not form, but its decrease to a value below 70 ° C will lead to a decrease in the efficiency of CO ₂ removal.

The process of regeneration of MEA absorption solutions according to the invention minimizes MEA losses and corrosion of equipment, reduces heat consumption: the desorbed solution can be heated with steam condensate instead of steam at 2-5 atm. As can be seen from the table, a decrease in the desorption temperature and the MEA content in the absorbent with an increase in the degree of saturation of the initial CO ₂ solution also leads to a cleaner regenerated absorbent at the same time (saturation of the purified CO ₂ solution is not higher than 0.2 M _CO2 / M _IEA ) and increased CO ₂ removal.

Claims (1)

The method of removing carbon dioxide from gas mixtures by absorbing carbon dioxide in an aqueous solution of an MEA monoethanolamine absorbent is to obtain a carbon dioxide-saturated absorbent solution, to desorb carbon dioxide from this saturated absorbent solution by heating to obtain carbon dioxide and a carbon dioxide-depleted absorbent solution and returning it to the stage absorption, characterized in that use an absorbent solution with a content of monoethanolamine 10-14 wt.%, the degree of saturation of the absorption solution the ora is not less than 0.65 M _CO2 / M _MEA , desorption is carried out at a temperature of 70-90, and said depleted solution is returned to the absorption stage completely without further purification.

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