CA3061855A1 - Method and process for efficient regeneration of co2 rich solvents - Google Patents

Abstract

ABSTRACT The invention relates to a method and a device for reducing energy demand at regeneration of CO2 rich chemical solvent. Application of chemical absorbents such as amines to remove CO2 is widely established so far. However, high-energy demand to regenerate the absorbent solution is a risk factor in application of such technology in particular for post combustion system. Herein, the invented system is able to regenerate the CO2 rich solvent in a faster rate and less energy demand, assisted by the acceleration of release of trapped CO2 gas in the solvent by a device working on hydrodynamic cavitation concept. 1 CA 3061855 2019-11-17

Description

DESCRIPTION

SOLVENTS
FIELD OF THE INVENTION
The present invention provides an economically viable small to medium scale method for the regeneration of CO2 rich solutions in chemical absorption systems with highlights of lowering energy demand for carbon valorization purpose. The novel regeneration system relies on hydrodynamic cavitation concept induced by an external power accelerating the release of CO2 gas trapped in the liquid solution.
BACKGROUND OF THE INVENTION
Global warming due to rising levels of greenhouse gases, such as atmospheric CO2 is the major challenge that threatens the earth future. In the last years, in parallel with the development of Carbon Capture and Storage (CCS) technologies, a new vision about the CO2 is rising, focused on the development of technologies able to reuse CO2 instead of storing or emitting it to the atmosphere.
CO2 is the sole source of carbon and its conversion to a stable form of carbon will be a promising approach that addresses the concerns with respect to storage and transportation of CO2 by-products.
Then, the concept of Carbon Capture and Valorization (CCV) has been proposed as an attractive alternative to CO2 sequestration that is based on the use of captured carbon as a reagent for producing useful valuable chemicals. The CO2 is first captured and separated from gas stream produced from the source, such as power plants, biomethane producers, and chemical manufacturing, then the captured CO2 is used as a reagent for producing useful chemicals either through biological, chemical or electrochemical methods. The success of this technology platform depends on finding the economic solution for capturing and regeneration of CO2.
In the past few years, some different technologies have been developed for capturing the CO2 from either post-combustion or biogas production sources, including chemical and physical absorption, solid adsorption, and membrane separation._Of all the CO2 capture technologies for downstream streams containing CO2, the chemical absorption of CO2 by a solvent is considered so far the most suitable one and most possible to be implemented in the near future. The most commonly used chemical solvents for CO2 absorption of post-combustion systems are alkanolamine solutions, single or mixed primary, secondary, and tertiary amines. The biggest resistance to the implementation of conventional absorption system is still the operational costs mainly with respect to the regeneration of CO2 saturated solution. Traditionally the CO2 saturated solution is regenerated by adding heat to the solution, directly or indirectly. The source of heat is typically the saturated or superheated steam.
The large quantity of steam needed for this regeneration boots the plant cost including capital and operation expenses that is the main challenge for the implementation at the low pressure CO2 capture systems such as CCV, specifically at small to medium scales. For example, MEA
(monoethanolamine) is the most advanced amine for such applications, however, due its high activation energy, the MEA-based solvent generally needs a high-energy level to regenerate and is prone to the thermal degradation at such conditions. Therefore, there is a great demand on the innovative technologies to reduce the cost of regenerating the CO2 saturated absorption liquid with special attention to the small to medium scale applications.
The concept of cavitation, either in acoustic or hydrodynamic shapes, has been widely used in nanomaterial synthesis, chemical reactions, material homogenization, the food and pharmaceutical industries. Examples of such applications are: WO 2012/161603A2, WO
2013/003496A1, US
7,771,582 82, US 2008/0319375 Al, WO 2011/003556 A2. Cavitation is known by two sorts of hydrodynamic or acoustic, and can be produced by different means, either by hydrodynamic devices including venturi nozzles, high-velocity rotation, rotor-stator siren-like devices, or acoustic generators such as ultrasonic transducers. In all these systems, input energy is transformed into the liquid to create low and high pressure cycles. During the low-pressure cycle, the nucleus bubbles are formed and merged together to make larger vacuum bubbles.. Continued operation and entering to the high pressure cycle concludes the collapse of these bubbles and emitting some energy to the bulk of liquid that is dissipated into the solution. The fraction of the input energy that is transformed into cavitation depends on several operational factors and the cavitation generating equipment. The cavity generation by means of an ultrasound-induced device follows a different mechanism compared to hydrodynamic cavitation generation.

2 One promising use of cavitation devices is for accelerating the degassing of liquids. The enhanced fermentation of sake, beer and wine using the ultrasound device, as the CO2 release rate is promoted, is the example of this application as described at prior arts JPH 0795873A, CN
106434837A, US
9,221,734 B2. With respect to CO2 absorption-regeneration processes, there are few examples of using the ultrasound device in the regeneration of CO2 absorption solvent, where a promoted regeneration rate was addressed (CN 103977683A, US 9.238,192 B2, WO
2009/127440 Al).
Although a sort of ultrasound based systems have been applied to promote the CO2 release from CO2 loaded chemical absorbents, these systems do not provide a convenient and economic method of degassing the CO2 loaded chemical solvent mainly due to low energy efficiency of ultrasound device and their problematic issues in scaling up. Accordingly, what is needed is an efficient and convenient cavitation working system for degassing of CO2 loaded chemical absorbents. The present invention provides such a procedure.
SUMMARY OF THE INVENTION
The present invention relates to propose a process to facilitate the release of captured CO2 at the chemical absorption plant, applicable to CO2 capture systems at various sources such as chemical industry, natural gas production, power plants, biogas producing plants, etc.
The CO2 in process streams is first absorbed by a chemical solvent including but not limited to alkanolamines and amino acid salts (single or mixture) thereof. Then, the CO2-rich solution is subjected to treat by a flow-through controlled hydrodynamic cavitation unit that accelerates the CO2 escape from the solution. In one embodiment, the said cavitation unit is used individually at downstream of CO2 absorption unit.
In the other embodiments, the cavitation device is placed at the upstream of the conventional stripper column and serves as auxiliary CO2 escape accelerator. Processing of CO2-rich solution by the invented cavitation technology needs a lower energy demand than traditional approach and reduces the time for regeneration that adds value to the economy of CO2 capture system specifically for post-combustion applications.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a schematic diagram of an embodiment for treating the CO2-rich solution coming from absorption unit by an individual hydrodynamic cavitation device.
Fig. 2 is a schematic view of diverse hydrodynamic cavitation orifice device.

3 Fig. 3 is a schematic diagram of another embodiment for treating the CO2-rich solution from absorption unit by an integrated hydrodynamic cavitation device into upstream of traditional stripper column.
Fig. 4 is a schematic diagram of the other embodiment where the CO2-rich solution splits into two streams of heated and unheated, and the heated one is treated by an integrated hydrodynamic cavitation device into upstream of traditional stripper column.
DETAILS DESCRIPTION OF EMBODIMENTS
Commonly used absorbents for CO2 capture are aqueous solutions of alkanolamines, which are weak bases, and can react with CO2 to form complexes having weak chemical bonds.
These chemical bonds are easily broken upon a proper heating, leading to absorbent regeneration.
The CO2 regeneration section is typically energy-intensive, which significantly rises the overall cost related to CO2 capture at post-combustion or biogas production systems. The aqueous primary, secondary and tertiary amine solutions, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) have been widely used to remove CO2 from effluent streams. The primary amines have a more CO2 absorption rates, but they need a higher heat demand for the regeneration.
To reduce the cost corresponding to CO2 removal at aforementioned systems, a process is proposed here to accelerate the regeneration of CO2-rich solvent at a lower energy intensive environment, including but not limited to diverse types of alkanolamines (MEA, MDEA, DEA, etc), amino acid salts (potassium or sodium derivatives of glycine, L-alanine, taurine, sarcosine, etc) single or their mixture or their mixture with other amine such as piperazine, sterically hindered amine thereof. The regeneration system relies on the CO2 release acceleration from the heated rich solvent by means of a hydrodynamic cavitation device.
Hydrodynamic cavitation science has matured over the years such that scale-up is now more straightforward than it would be particularly for ultrasonic cavitation.
Hydrodynamic cavitation exhibits effects similar to those of ultrasound cavitation, but is more energy efficient and enables a better energy transfer to the fluid using a simple treatment unit. The large electricity consumption by an ultrasound unit to create the cavitation bubbles is a matter of concern at the system scale up.
Generally speaking only 5 to 10% of energy transferred to the liquid in the form of cavitation and the remaining is wasted in the form of heat to increase the solution temperature.
The ultrasound generator unit cost and its complexity are also the other challenges with acoustic cavitation driven systems.

4 Therefore, the invented process here applies hydrodynamic-driven concept to assist wipe of CO2 from the saturated absorbent solution at moderate temperatures and atmospheric to vacuum pressures.
In a hydrodynamic cavitation system, microbubbles are generated due to a sudden increase in liquid velocity through a nozzle or high-velocity rotor and an accompanying reduction in the liquid pressure, such that the pressure drops below the vapor pressure of the liquid. The microbubbles expand quickly and then, due to pressure recovery after the cavitation zone, implode or collapse. The implosion or collapse of the microbubbles releases energy in the form of shock waves, with vigorous shearing forces and localized hot spots at the interface of the microbubbles. The energy released from the cavitation is dispersed in the bulk and the elevated temperature in the hot spots returns to near the original value almost instantaneously.
Cavitation number (Cv) is a dimensionless number which is used to relate the conditions of flow stream with the cavitation intensity:
P2 ¨Pro C = (1) v where P2 is the recovered downstream pressure, Pv is the vapor pressure of the liquid and vi is the velocity of the liquid at the constriction area. The inception of cavitation occurs at the number called the cavitation inception number Cy,. Generally, the cavitation inception is established at Cvi equals 1.0, and below this inception number, the cavitational effects are prone to be significant. The cavitation can be also achieved at higher number of Cvi, which might be mostly attributed to the dissolved or trapped gas in the liquid solution. The cavitation bubble collapse at recovery zone steadily occurs at Cv, range of cavitation number at 0.1 to 1Ø Operating the cavitational reactor at lower numbers leads to lock with supercavitation regime with no bubble collapse happening. The cavitation number is an indication of the extent of the region that is filled with vapour bubbles inside the nozzle or downstream side. The upstream and downstream pressures, the liquid temperature and the volumetric flow of liquid are determining factors for intensity of cavitation and the region in which cavitation occurs, partially or fully developed cavitation regime.
The principle behind the cavitation-based solvent regeneration at this invention is to form stable microbubbles by working at supercavitation conditions. The CO2 loaded solution is subjected to rapid changes of pressure that cause the formation of cavities in the liquid where the pressure is relatively low. Bubbles produced at the low-pressure zone have a high surface area and behave like a sink to draw dissolved CO2 gas out of the liquid and into the gas bubble. The bubbles can grow in size and, and their density .value and intensity of generated cavities will determine the hydrodynamic regime of cavities after cavitation unit. The critical time of bubble implosion is important in cavitation unit where the bubbles will collapse and released CO2 will be dissolved back into the liquid. The bubble must be ideally released upward prior to collapse in the liquid phase, that is, the solution to be preferentially degassed at cavitation zone. So, the layout of cavitation unit is important to meet this goal. Therefore, the invented cavitation-based device is preferentially integrated with the adjacent degassing vessel, is more preferentially attached to the vessel body to minimize the travel time of supercavitated fluid.
Fig. 1 represents the process diagram of an embodiment in current invention including CO2 absorption by a chemical solvent at absorption tower (101) and CO2-release accelerator units as regeneration unit (105). The CO2 in flue gas or product gas (100) is removed through contact with lean solvent (112) at the absorption column (101), and leaves the column from top (113). The liquid absorbent rich in CO2 (111) then exits from the absorption unit, pressurized by a pump (114), and passes through a heat exchanger (104), preferentially shell & tube or plate & frame heat exchangers in order to exchange heat between cold rich absorbent stream from the absorption column (114) and hot lean absorbent stream (117) from the CO2 degassing vessel (106). In this particular embodiment, the semi-heated rich absorbent liquid (116) passes then through a heat exchanger working with hot oils or steam to meet the required temperature at entrance of following cavitation-based regeneration device (105), which is fully loaded at start-up stage. This emerging internal energy accelerator unit relies on cavitation concept, preferentially on hydrodynamic cavitation concept, prior to enter to the downstream degassing vessel. The device can be any of flow orifice, or venturi nozzles, or high-velocity rotation, rotor-stator siren-like device. Fig. 2 indicated a schematic of cavitation-based device, flow orifice layout (101). The device can have a single hole (100) or multi holes (102) depending on the desired intensity of cavitation for the processing. Each single hole is typically ranged from 0.1 to 5 mm, preferentially 1.0 to 3.0 mm. A symmetrical hole size is preferred at multi hole layout than unsymmetrical one. In Fig. 1, the pressure, temperature and flowrate of inlet stream to cavitation device 105 are adjusted such that the solvent becomes degassed at supercavitation zone, preferentially below Cv, of 0.1. The temperature of liquid is maintained close to the boiling point of solvent, for example, at case MEA, preferentially close to 100 to 110 C. The pressure at inlet stream to unit 105 is set at a proper number to gain an efficient cavitation intensity, can be even elevated to 600 psig. The cavitation-based unit can be single or several parallel devices depending on capacity of fluid has to be treated. The cavitation device provides a medium for the creation of tiny low-pressure bubbles and their growth. The enlarged voids provide a unique surface contact permitting the trapped or released CO2 bubbles in the solution become dissolved into the aforementioned vacuum voids.
Hence, the diffused CO2 bubbles into these low-pressure voids can be easily escaped upon expansion at a following vessel. The outlet stream (118) enters to the degassing vessel (106) where the trapped CO2 bubbles are released. The degassing vessel (106) is preferentially at atmospheric condition, more preferentially working at vacuum or partial vacuum condition. It is expected that this system reduces the energy consumption needed for regeneration compared to the conventional stripper columns by elimination of need of reboiler system. The degassed lean solvent (119) is pressurized by a pump (107) and returning back to the loop through heat exchanger 104. The escaped gas stream from degassing vessel (120) is flowed to a stacked or elevated individual condenser (108) to wipe the water from the product gas. The water is collected at accumulation vessel (109) and return back to the degassing vessel by gravity (122). The product CO2 gas is sucked by a blower or vacuum pump (110) and leaves the unit (124).
Fig. 3 represents the process diagram of another embodiment in current invention including conventional absorption tower and integrated cavitation-based regeneration device into regular stripper column. This invented layout will decrease the amount of steam consumption at stripper column, then improves the energy efficiency of the process. The heated pressurized CO2-rich solvent (119) enters to the cavitation device (104) at a pressure can be elevated up to 600 psig. The treated stream (120) by the unit 104 is directed to the degassing vessel (105) where the hot CO2 bubbles created by the cavitation device are released (121) and enter to the top section of stripper column (107), inlet side of stacked or elevated condenser (110). The water vapours are condensed, collected, and returned back to the stripper column from the top (130). The non-condensable gases are sucked by a vacuum pump or blower (113) and delivered as product stream (132). The hot and partially regenerated solvent from degassing vessel (105) is pumped to the stripper column (123). The degassing vessel (105) and the stripper column (107) are preferentially working near atmosphere, more preferentially at vacuum or partially vacuum conditions.
Fig. 4 demonstrates the process diagram of the other embodiment in current invention including the split of CO2-rich solvents to two streams sending to top part of stripper column and cavitation device that integrated to the conventional stripper column. In this embodiment, the pressurized rich solvent (117) from the bottom of absorption column (101) splits into two branches, one goes directly to the top part of stripper column (135), and the other one is heated by two series heat exchangers (102 and 104) and enters to cavitation regeneration device (120). The second heater behaves as a supplementary heat source to adjust the desired temperature at 120, and may or may not be use. The temperature of liquid at inlet to cavitation device is preferentially close to the solvent boiling point that accelerates the creation of numerous stable bubbles. The pressure of system is also determined as desired to meet a proper cavitation intensity. The branched cold stream (135) routed to the top section of stripper column helps to wipe the vapours leaving the top section of stripper column from the water and itself becomes heated. Then part of waste energy at top condenser (112) is conserved inside the stripper unit. The treated stream by cavitation device (121) is then degassed at degassing vessel (106) and the released gas (125) then enters to top section of stripper column, when the excess heat is counter-current exchanged by cold solvent at this section. The stripper (108) and degassing vessel (106) are preferentially working near atmosphere, more preferentially at vacuum or partial vacuum conditions.
The hot and partially regenerated solvent is then flowed to stripper column to release the remaining CO2 in the solution and meets the loading factor spec. The major energy consumption belongs to the reboiler section, however, by implementation of the proposed layout, the energy load on reboiler is remarkably decreased.

Claims (16)

We claim:

1. A hydrodynamic cavitation based system for the regeneration of CO2 rich solvent comprising: a process liquid pump to elevate the CO2 rich solvent pressure to desired value, a preheater to heat up the said fluid to desired temperature, a hydrodynamic cavitation device to create supercavitation flow regime at outlet of cavitation device.

2. The process of claim 1, wherein said hydrodynamic cavitation device is any of flow orifice, or venturi nozzles, or high-velocity rotation, rotor-stator siren-like device.

3. The process of claim 1, wherein the said CO2 gas in feed stream is absorbed at the absorption unit by a chemical solvents, including but not limited to alkanolarnines (MEA, MDEA, DEA, etc), amino acid salts (potassium or sodium derivatives of glycine, L-alanine, taurine, sarcosine, etc) single or their mixture or their mixture with other amine such as piperazine, sterically hindered amine thereof.

4. The process of claim 1, wherein the said CO2 gas trapped in the supercavitational bubbles are released from the liquid bulk at degassing vessel working at a lower pressure than upstream of cavitation device, preferentially vacuum to atmospheric pressures.

5. The process of claims 1 to 4, wherein the water carryover with the released CO2 is condensed at the overhead condenser and the CO2 is sent out by a blower or vacuum pump for using at downstream unit.

6. The process in claims 1 to 5, wherein the combination of preheater, cavitation device and degassing vessel is enough to meet the desired spec of CO2 loading at regenerated solvent mixture.

7. The process of claim 1, wherein hydrodynamically-driven cavitation solvent regeneration system regenerates the CO2 rich solvent at a faster rate and a less energy intensive environment at the at the expense of a reasonable additional liquid pumping power.

8. The process of claim 1, wherein the hydrodynamic based cavitation system consumes a less energy for the regeneration of rich in CO2 solvent in comparison to the ultrasound driven apparatus.

9. The process of claims 1 to 5, wherein the cavitation device is the supplementary to the downstream stripper column, and the degassed semi-regenerated CO2 absorbent is pumped to the stripper column for the completion of CO2 regeneration process.

10. The process of claim 9, wherein the released CO2 and water carryover from degassing vessel is sent to the stripper column overhead.

11. The process of claim 9, wherein the stripper column is preferentially working at vacuum to atmospheric pressures.

12. The process of claim 9, wherein the reboiler duty of stripper column and the dimension of stripper itself is reduced since a considerable part of the CO2 is released after treating the rich solvent by the cavitation device.

13. The process of claim 9, wherein the solvent degradation rate decreases due to a lower solvent exposure time to superheated steam from reboiler; a reduction in regeneration temperature of CO2 rich MEA solvent by 17 C results in the MEA degradation four times less (Davis, J. and Rochelle, G. Energy Proc. 1:327(2009)).

14. The process of claims 9 to 11, wherein the CO2 rich solvent splits to two branches at downstream of feed pump and prior to enter to preheater followed by the cavitation device.

15. The process of claim 14, wherein the one branch of the CO2 rich solvent is heated and directed to the cavitation device, and another branch of solvent (cold fluid) is routed to the top of the stripper column.

16. The process of claim 15, wherein the heat exchanged between the hot gases in the stripper column and nonregenerated cold solvent stream at top section of tower helps to recover a large portion of waste heat of leaving gases from the top of stripper and adds more energy efficiency to the process by lowering the reboiler duty.