Patent Application
20190291042
The present invention relates to a method for separating CO2 based on regenerative chemical absorption, which uses an absorber where the CO2 remains retained in an absorbent liquid, and a regenerator where the CO2 is released, obtaining a regenerated absorbent that is re-used in the absorber. The invention proposes a configuration of the entire capturing process which allows a more efficient operation and, therefore, significantly reduces the energy requirements mainly associated with the regeneration of the absorbent, as well as a lesser thermal degradation of same.
The regenerative chemical absorption of acid gases has been used since the 1930s in a number of industrial processes. The basic process was patented by R. R. Bottoms (U.S. Pat. No. 1,783,901), and after it, a number of configurations and/or chemical absorbents have been developed over the years for the purpose of optimizing the entire process for separating CO2 and, in particular, significantly reducing the energy consumption mainly associated with the regeneration of the absorbent.
Different variants of the conventional arrangement of the absorber have also been proposed, described in specialized literature (U.S. Pat. No. 8,192,530). On the other hand, most of the patents registered in this field concern modifications applied in the regenerator: harnessing energy from the sensible heat of the outlet stream of the regenerator, preheating the condensates at the inlet of the regenerator, partially evaporating the stripped at the outlet of the regenerator such that the total energy input into drum of the regeneration unit is minimized, pressurizing the upper section of the regenerator to lower the water/CO2 ratio in the stripping stream of the regeneration unit, etc. All these modifications have led to important energy reductions in the regeneration of the absorbent compared with the conventional arrangement.
In recent years, processes for separating CO2 based on chemical absorption have sparked enormous interest on an industrial level as a result of the possibility of being used in the field of CO2 capture and storage technologies, mainly in processes for producing electric energy, in the cement industry and in the production of steel. The earliest references to patents for applying chemical absorption for mitigating CO2 emissions into the atmosphere in specialized literature date back to the mid-1990s (WO 1995/021683). Despite being considered a mature technology in the industrial field, the application of chemical absorption for separating CO2 coming from combustion and/or process gases entails certain difficulties which must be solved in order to develop it on a commercial scale, which still have not been solved in an effective manner.
These difficulties are mainly summarized in the high energy consumption associated with the regeneration of the absorbent and in the degradation said absorbent experiences due to oxidative and thermal mechanisms occurring during the capture process. These aspects mean that the use of a chemical absorption unit in an electrical production facility based on the combustion of fossil fuels can involve up to a 10 point-loss of net yield in the power cycle, rendering its implementation on an industrial scale economically inviable today (Pulverized coal oxycombustion power plants. Volume 1: Bituminous coal to electricity, Final report 1291. DOE-NETL, 2007, pp. 5-6).
With respect to the degradation of the absorbent, this parameter is extremely relevant in the definition of the operating conditions in the regeneration unit. According to G. T. Rochelle (Rochelle, G. T.; Current Opinion in Chemical Engineering, 2012, 1(2): 183-90), the process for separating CO2 by means of chemical absorption based on compounds with rapid kinetics, as occurs with the use of primary and secondary amines, is favored by operating at high temperature and high pressure in terms of energy consumption per ton of CO2. Therefore, the operating temperature in the desorption unit is defined on the basis that it produces the maximum degradation allowable in the absorbent, that is, that the observed degradation ratios are offset by a significant reduction in the specific consumption per ton of CO2 captured during the regeneration of the absorbent (Oexmann, J.; Ather, A., International Journal of Greenhouse Gas Control, 2010, 4(1), 36-43).
Today, there are a number of companies holding licenses for chemical absorption processes on a commercial scale applied specifically to the capture of CO2 in industrial processes, which shows the interest that these developments spark in the field of environmental technology and chemical and industrial processes where the separation of CO2 acid gases from a gas stream is required.
The present invention proposes an alternative configuration with respect to the conventional system of separating CO2 from a gas stream by means of chemical absorption, based on the optimization of the cyclic operating capacity of the absorbent used by means of a particular arrangement of the streams involved in the CO2 absorption-desorption process and a very thorough control of the operating conditions of the inlet streams into the regenerator, mainly in terms of temperature and distribution of feed flow rates to the equipment.
In this sense, the energy consumption associated with the regeneration process is significantly reduced and favors lower thermal degradation of the absorbent. This invention has been developed to be applied in technologies for capturing CO2 from stationary sources, but they may be applicable for any process which requires separating acid gases from a gas stream.
The invention consists of a process and a system for regenerative chemical absorption applied to the capture of CO2 from stationary sources, which allows adjusting the degree of regeneration required by the absorbent by significantly reducing the energy consumption of the process. In general, the objective of the proposed configuration is to optimize the cyclic capacity during operation of the absorbent so as to minimize the energy requirements in the drum of the absorber system.
To achieve this objective, the present invention provides a method for regenerative chemical absorption applied to the capture of CO2 from stationary sources, which allows adjusting the degree of regeneration of the absorbent by means of using the described system, and in which the treatment of the different gas streams generated takes place.
Therefore, a first aspect of the present invention relates to a method for separating CO2 coming from a gas stream, comprising the following steps:
a) absorbing the CO2 coming from a gas stream to be treated at a temperature preferably less than 60° C., preferably between 40° C. and 60° C., and a pressure comprised in a range of between 1 and 1.5 bar, by means of putting said stream in contact in an absorber with an absorbent solution in which the CO2 will be retained;
b) recirculating up to 75% of the stream comprising the CO2-rich absorbent solution coming from step a) to the lower bed of the absorption system. The operation under these conditions allows adjusting the cyclic working capacity range of the absorbent during operation in an optimized manner;
c) desorbing CO2 in a regenerator from the stream comprising the CO2-rich absorbent solution coming from step a) not recirculated to step b) at a temperature of between 80° C. and 120° C., a pressure of between 1.5 and 5 bar and a steam stripping flow rate of between 10 and 90% by volume with respect to the desorbed CO2 flow rate, where said stream is split into at least two streams by means of a set of heat exchangers, prior to the inlet of the regenerator;
d) recovering the absorbent solution resulting from step c) from the absorber of step a).
In a preferred embodiment, the CO2 is absorbed from the stream to be treated in step a) of the method of the invention in the absorber unit from the gas phase to the liquid phase, where it is dissolved and chemically bonds with the absorbent or absorbent solution. It is also possible to use absorbents which only operating with physical mechanisms, and not chemical mechanisms, of absorption.
In a preferred embodiment, the absorbent solution contained in the absorption unit comprises any one aqueous solution of CO2 absorbents, and more preferably an aqueous solution of a compound having an amine base, which can be selected, though without being limited to one amine from the list comprising monoethanolamine (MEA), triethanolamine (TEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA) and diglycolamine (DGA), piperidine (PP), piperazine (PZ), 2-amino-2-methyl-1-propanol (AMP), monomethylethanolamine (MMEA), etc., or any of their combinations.
Another object of the invention is the absorption system used in this method which is based on the fundamental incorporation of an absorber which receives the gas to be treated with CO2, which is absorbed by means of a absorbent solution, a set of heat exchangers conditioning the temperature of the CO2-rich absorbent solution exiting the absorber and a regenerator, in which the absorbent solution is regenerated, releasing it from the CO2, for re-using it and incorporating it back into the absorber.
Based on this basic configuration, the system of this invention proposes first the incorporation of a recirculation line directed to the absorber constituting a bypass of the outlet of the CO2-rich absorbent solution, which is partially conducted back to the absorber for the purpose of optimizing the CO2 absorption capacity of the absorbent used. Secondly, the system incorporates a particular set of heat exchangers which, besides thermally conditioning the CO2-rich solution, divides it into at least two streams which are introduced in the regenerator in areas located at different heights, stratifying the feed into the regenerator, which causes a decrease in the temperature profile of the regenerator, achieving a reduction in energy consumption associated with the regeneration of the absorbent.
In this manner, the system allows significantly reducing the specific consumption associated with the regeneration of the absorbent compared with a conventional configuration of the absorption system. It has been demonstrated that the level of reduction of consumption is higher the more concentrated the acid gas is in the gas stream to be treated.
The invention assures, therefore, an operation of the regenerator at a thermal level that is lower than the level proposed in conventional operating modes. As a result, it is possible to work with a higher load or concentration of CO2 in the regenerated absorbent and, in this manner, to shift the cyclic operating capacity thereof to areas where the energy consumption associated with desorbing CO2 is lower. The decrease obtained in the temperature profile of the regenerator reduces the degradation rate of the absorbent associated with thermal mechanisms.
To complement the description that is being made and for the purpose of helping to better understand the features of the invention, according to a preferred practical embodiment thereof, a set of drawings is attached as an integral part of said description, wherein the following has been depicted with an illustrative and non-limiting character:
A preferred embodiment of the system object of this invention is described below.
Specifically, the absorption-desorption system of CO2 including the elements described below has been depicted in
As can be observed in
The inlet stream of regenerated absorbent solution (23) coming from the regenerator (15) is at a temperature which has been adjusted to values close to that of the gas stream to be treated (1) by means of using a second heat exchanger (8B).
On the other hand, the absorber (2) incorporates an inlet for recirculated CO2-rich absorbent solution recirculation line (7), which is conducted back to the lower bed of the absorber (2) for the purpose of increasing the load thereof by means of a first heat exchanger (8A) which lowers its temperature.
In a preferred embodiment, the design of the absorber (2) requires an increase in section in the lower bed with respect to the rest of the column, as shown in
It can also be seen in
The set of heat exchangers (9) receives the mentioned CO2-rich absorbent solution (6), where the temperature of this stream is adjusted in an optimized manner before being split and directed to the regenerator (15), and it receives a stripped regenerated absorbent solution (21) coming from the regenerator (15), and the inlet stream of regenerated absorbent solution (23) exits the set of heat exchangers (9), directed to the absorber (2), and a primary stream (10) and a secondary stream (13) also exit as a consequence of the mentioned splitting of the CO2-rich absorbent solution (6).
The set of heat exchangers (9) comprising the following elements can be seen in
The set of exchangers (9) depicted in
The distribution of the CO2-rich absorbent solution (6) between the primary stream (10) and the secondary stream (11) is preferably established in the range of between 0.25 and 0.75. The primary stream (10) is then preheated in an indirect contact second exchanger (11) indirect contact using the outlet stream from the regenerator (16), at a temperature greater than 100° C., giving rise to a main inlet stream into the regenerator (12).
The regenerator (15) receives the stream from the absorber (2) at different heights and temperatures, such that the degree of regeneration of the absorbent is adjusted in an optimal manner.
The main inlet stream into the regenerator (12) is introduced in the upper part of the regenerator (15). On the other hand, the secondary stream (13) is introduced at a temperature less than the temperature set for the primary stream (10) in an intermediate bed of the regenerator (15), Achieving a temperature profile which optimizes the energy requirements of the entire capturing process. The secondary stream (13) can in turn be split into another additional stream (14) in order to be fed in at different heights of the regenerator (15).
This configuration allows obtaining a partial regeneration of the absorbent, shifting the cyclic capacity thereof into areas with a lower energy requirement of CO2 desorption. The energy necessary for the regeneration of the absorbent to occur is provided to the regenerator (15) by means of a drum (20) preferably using vapor as the working fluid.
On the other hand, the outlet stream (16) at the upper part of the regenerator, which stream is primarily made up of CO2 and water vapor, is introduced in a separator (17), where the stream having a high concentration of CO2 saturated in water (18) and a condensate stream (19) are obtained, which is subsequently recirculated to the regenerator (15).
Lastly, the stripped regenerated absorbent solution (21) is removed from the lower part of the regenerator (15) and impelled by means of a second pump (22) to the set of exchangers (9) prior to being reincorporated into the absorption system (23).
The regenerator (15) preferably works in a pressure range comprised between 1.5 and 5 bar, and at a maximum temperature less than 120° C., more preferably, in a temperature range comprised between 100° C. y 120° C., such that lesser degradation of the absorbent is assured.
The invention is illustrated below by means of tests performed by the inventors, which clearly shows the specificity and effectiveness of the method of the invention for capturing CO2.
Particularly, a process for separating CO2 from a synthetic gas stream has been performed in a laboratory-scale unit based on two operative configurations which correspond on one hand to a conventional configuration and on the other to a configuration according to the system of the invention.
In this sense, A synthetic gas flow rate of 7 L/min, with a composition of 60% v/v CO2, saturated with water vapor and completed with N2 has been used. Monoethanolamine in aqueous solution at 30% w/w has been used as absorbent, as it is a reference absorbent. The amount total of absorbent used in the system is 2 L. The absorption of CO2 is performed at a pressure of 1 atm and at a temperature of 50° C. in a column having 3 cm in diameter and 2 m in height, using as an absorption bed 6 mm ceramic Raschig rings. The regeneration of the absorbent is performed at a pressure of 2 bar in a column having 3 cm in diameter and 1 m in height using 6 mm stainless steel 316L Raschig rings.
The conventional configuration consisted of having a recirculation rate in the absorber of 0 (7), a single internal heat exchanger (24) makes up the set of exchangers (9) and the infeeding of the regenerator (15) is performed by means of using a single primary stream (10) introduced at the upper part of the regenerator (15). The absorbent flow rate was set at 7.01 kg/h, which corresponds with an L/G ratio equal to 12, with the inlet temperature into the absorber being 49° C.
The configuration of the invention uses a partial recirculation of the stream of recirculated CO2-rich absorbent solution (7), a set of exchangers (9) made up of internal heat exchangers (24, 25), and the inlet stream has been distributed to the regenerator in two streams: a primary stream (10) in the upper part of the regenerator (15) and a secondary stream (13) in the intermediate area of the regenerator (15). This secondary stream (13) was removed at the outlet of the first internal heat exchanger (24) of the set of exchangers (9). The absorbent flow rate was set at 8.18 kg/h, which corresponds with an L/G ratio equal to 14, with the inlet temperature of the gas into the absorber being 47° C.
The most relevant operating conditions and the results obtained are summarized in Table 1. The operation by means of the method of the invention allowed increasing the cyclic capacity of the absorbent and the CO2 separation yield during the separation operation as a result of a higher load of the rich absorbent in the absorption step. This increase in load is primarily due to the recirculation of part of the recirculated CO2-rich absorbent solution (7). Stratifying the feed into the regenerator (15) caused a decrease in the temperature profile in the regenerator (15) and, therefore, a stripped solution with a higher load of CO2. This shift in the cyclic operating capacity of the absorbent allowed the use of the new configuration to achieve an 11% reduction of the specific consumption of energy associated with the regeneration of the absorbent, producing a net benefit with respect to the conventional configuration of processes of this type. Furthermore, the lower thermal level obtained in the regenerator favors a reduction of the degradation of the absorbent associated with thermal mechanisms.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201600519 | Jun 2016 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2017/000073 | 6/19/2017 | WO | 00 |
