Heat recovery in absorption and desorption processes

Abstract

A method for removing components to be separated from industrial gases using an absorption and desorption processes having liquid absorbents. At least one absorption device and one desorption device are provided, at least a part of the laden solution leaving the absorption device is diverted before being heated and delivered to the head of the heat transfer section. The laden partial stream is heated by the steam rising from the lower part of the desorption device through heat exchange in the heat transfer section. The remaining stream of cold, laden solution leaving the absorption device is expanded by so the relief valve and the heat exchanger into a pressure relief vessel, such that the stream leaving the heat exchanger separates into a liquid and a gaseous state. The pressure in the pressure relief vessel pressure is lowered so that the total energy demand in absorption and desorption processes is reduced.

Claims

1. A process for the removal of at least one of sour-gas components, sulfur compounds, carbon dioxide, hydrogen cyanide, water, or combinations thereof as components to be separated, from natural gas, synthesis gas, flue gas from incineration of fossil fuels or combinations thereof as a technical gas, by an absorption and desorption process using a liquid absorbent, with reduced energy demand, the process comprising: absorbing the components to be separated into the liquid absorbent in an absorption device containing at least one mass transfer section, forming a laden absorbent solution; desorbing the components to be separated from the laden absorbent solution in a desorption device comprising at least one heat transfer section, at least one stripping section below the heat transfer section, and a bottom reboiler, the desorption device operated at a higher temperature than the absorption device; splitting the laden absorbent solution into a first part stream and a second part stream; heating the second part stream in a heat exchanger and supplying heat to the heat exchanger from a bottoms stream of absorbent from the desorption device, the bottoms stream free of components to be separated; wherein between the heat exchanger and the desorption device is at least one relief valve and a subsequent flash vessel, and flashing the second part stream of the laden absorbent solution by means of the relief valve, separating the laden absorbent solution into a liquid phase and a gas phase, thus lowering the pressure in the subsequent flash vessel, and removing the components to be separated as an exhaust stream from the stripping section, and introducing the exhaust stream into the heat transfer section of the absorption device and cooling the exhaust stream by routing the first part stream of laden absorbent solution to the top of the absorbent solution to the top of the heat transfer section of the desorption device, and removing a cooled exhaust stream of components to be separated; and routing a cooled stream of absorbent free of component to be separated from the heat exchanger to the absorption device.

2. The process of claim 1, wherein the heat transfer section for the desorption device is provided with a mass transfer section comprising mass transfer elements for direct heat transfer.

3. The process of claim 1, wherein the heat transfer section of the desorption device comprises a heat exchanger for indirect heat transfer.

4. The process of claim 1, wherein the adsorbent is a physically acting absorbent.

5. The process of claim 1, wherein the adsorbent is a chemically acting absorbent.

6. The process of claim 1, wherein the components to be separated comprise sulfur dioxide, carbon dioxide, hydrogen cyanide, or a combination thereof.

7. The process of claim 1, wherein the adsorbent comprises at least one of an aqueous amine solution or alkali salt solution.

8. The process of claim 1, wherein the liquid absorbent comprises at least one selexol, propylene carbonate, N-methyl-pyrrolidone, morphysorb, or methanol.

9. The process of claim 1, wherein the liquid absorbent comprises at least one of propylene carbonate, N-methyl-pyrrolidone, or methanol.

10. The process of claim 1, wherein the pressure of the flash vessel is maximally 1.5 bar greater than the pressure in the desorption device between the heat transfer section and the stripping section.

11. The process of claim 10, wherein the gas phase from the flash vessel is fed to the desorption device above the stripping section.

12. The process of claim 10, wherein the liquid phase from the flash vessel is fed to the stripping section of the desorption device.

13. The process of claim 1, wherein a plurality of series connected flash vessels are used.

14. The process of claim 13, wherein the gas phase from the flash vessel is fed to the desorption device above the stripping section.

15. The process of claim 13, wherein the liquid phase from the flash vessel is fed to the stripping section of the desorption device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents a prior art process for removal of technical gases.

(2) FIG. 2 represents one embodiment of the invention.

(3) FIG. 3 represents a prior art process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) The solution leaving the absorption device (20) and laden with the components to be separated is heated by a heat exchanger before this solution is fed to the desorption device (22). The additional energy required by the desorption is supplied by the reboiler (23) at the bottom of the desorption device (22). The components to be separated, which have been stripped off by the stripping agent, leave the top of the stripping section (22b) as exhaust steam, which is then introduced into the heat transfer section (22a), cooled as required and leaves the desorption device (22) at the top. The solution which, after desorption, is free of the components to be separated leaves the desorption device (22) at the bottom, undergoes heat exchange with the enriched solution in the heat exchanger (21), is then cooled and returned back to the absorption device (20).

(5) At least part of the laden solution leaving the absorption device (20) is branched off before being heated and fed to the top of the heat transfer section (22a). This laden part-stream is heated by the steam rising from the bottom part of the desorption device (22b) via heat exchange in the heat transfer section (22a). The residual stream of cold, laden solution (5a) leaving the absorption device (20) is flashed by means of the relief valve (25) and via the heat exchanger (21) into a flash vessel (26), so that the stream leaving the heat exchanger (21) is separated into a liquid and a gas phase, the pressure in the flash vessel (26) being reduced to such an extent that the whole energy demand in the absorption and desorption processes is reduced.

(6) In the heat exchanger (21) heat is transferred from the regenerated solution to the enriched solution. For economical reasons, the temperature difference between the hot, regenerated solution and the heated, laden solution as well as between the cooled, regenerated solution and the cold, laden solution to be heated should normally not be smaller than 10 K. In the event that only a part-stream of the cold, laden solution is available for cooling the large mass flow of the regenerated solution, the resulting temperature difference will inevitably be greater than 10 K, as the mass flow of the enriched solution is smaller than the mass flow of the regenerated solution. To utilise the heat of the regenerated solution as fully as possible in spite this and to reduce the temperature difference between the cooled, regenerated solution and the cold, laden solution to be heated to approx. 10 K again, the pressure on the side of the enriched solution stream is lowered in accordance with the invention by means of the relief valve (25) in the heat exchanger (21) such that by the resulting partial evaporation of the laden solution more heat is transferred from the hot, regenerated solution to the cold, laden solution. In this way, the heat present in the circuit and in the desorption device is efficiently utilised causing a reduction of the amount of external energy required in the boiler (23). The gain in energy results from the fact that according to the process embodying the invention the heat exchanger, despite the smaller mass flow, transfers the same amount of heat as according to the state of the art, with the whole mass flow of the enriched solution being passed through the heat exchanger and, in addition, the energy which is transferred from the stripping steam in the heat transfer section (22a) to the part-stream of the enriched solution being recovered. This reduces the overall energy demand in absorption and desorption processes.

(7) The reboiler at the bottom of the desorption device (22) generally ensures continuous supply of the required heat, in which the the stripping agent is heated to stripping steam by the reboiler. The stripping steam strips the components to be separated from the liquid solvents. The steam released by the pressure reduction in the flash vessel (26) is withdrawn from the top of the flash vessel (26) and fed below the heat transfer section (22a), with the steam transferring its heat to the solution to be heated and cooling down as desired. The cooled, separated components leave the desorption device at the top and are ready for downstream processing, with no condenser or only a significantly smaller condenser being required to cool down the separated components.

(8) It is known that a certain pressure is required to convey the solution through the heat exchanger and subsequently to the top of the desorption device. Thus, according to the state of the art, a pressure of approx. 5 to 6 bar is required downstream the heat exchanger (21). This input pressure is required to overcome the geodetic height of the desorption device, to compensate the line resistance and to dispose of sufficient control reserves in the relief control valve of the desorption device. Further input pressure is required to reach the normal working pressure of the desorption device. On account of the high input pressure, the steam fraction in the laden solution after being heated in the heat exchanger (21) is correspondingly low. The pressure in the heat exchanger can now be reduced to a pressure allowing significantly higher partial evaporation in the heat exchanger. According to the invention the pressure in the flash vessel (26) is reduced to a pressure which is maximally 1.5 bar larger than the pressure at the top of the desorption device (22). As a result, the flashed gas phase can be fed to the desorption device (22) without further ado.

(9) Depending on the solvent, the pressure may be reduced to down to 1 or even 0.1 bar greater than the pressure at the top of the desorption device (22). If the pressure is reduced to down to 0.1 bar greater than the pressure at the top of the desorption device (22), the steam fraction will increase. The pressure mayif advantageousbe reduced to below the pressure at the top of the desorption device (22); in such case a gas compressor is required for conveying the gas phase to the top of the desorption device.

(10) Pressure reduction can also be implemented in several flash vessels connected in series. This is of advantage in cases where it is intended to reduce the flash pressure to below the pressure in the desorption device, since in that case it is only this fraction of the steam that needs to be compressed to subsequently feed it to the desorption device.

(11) The steam released by pressure reduction in the flash vessel (26) is withdrawn from the top of the flash vessel (26) and fed above the stripping section (22b) of the desorption device (22).

(12) The liquid fraction released by the pressure reduction in the flash vessel (26) is withdrawn from the bottom of the flash vessel (26) and fed to the stripping section (22b) of the desorption device (22) to strip the remaining components to be separated from the solvent.

(13) Heating in the heat transfer section (22a) may be implemented by direct or indirect heat transfer. The exhaust steam rising from the stripping section (22b) transfers its heat to the laden solution to be heated. In the case of direct heat transfer, the heat transfer section (22a) is provided with a mass transfer section, which is equipped with mass transfer elements where direct heat transfer is implemented, in which all internals of a column used for heat and mass exchange, such as packing material, structured packings, trays (bubble, valve, sieve trays), etc. can be used as mass transfer elements. The laden solution which trickles downwards absorbs the heat from the rising exhaust steam while the exhaust steam is being cooled accordingly. In the case of indirect heat transfer, the heat transfer section (22a) can be provided in the form of a heat exchanger where indirect heat transfer is implemented. This process vessel on the one hand cools the rising exhaust steam as required and on the other hand heats the laden solution to be heated as desired.

(14) The relief valve (25), heat exchanger (21) and flash vessel (26) are generally arranged on the floor. An advantageous arrangement of the process vessels can, for example, provide for the relief valve (25), heat exchanger (21) and flash vessel (26) being located above the level of the stripping section (22b). In this way, no additional pump is required for conveying the solution from the flash vessel (26) and to the top of the desorption device. The devices can, however, be arranged in any desired form to run the process embodying the invention.

(15) The part-stream heated in the heat transfer section (22a) is fed to the stripping section (22b).

(16) This process can be run with a physically or a chemically acting absorbent. The process can be used in particular for the removal of sour-gas components from technical gases.

(17) The process embodying the invention is explained below by means of drawings and tables referring to an example.

(18) From a crude gas of approx. 13% by vol. CO.sub.2, approx. 90% of the CO.sub.2 contained in the crude gas shall be removed, with the amount of crude gas being 150,000 Nm.sup.3/h. The CO.sub.2 components to be separated shall be removed by means of an aqueous MDEA solution as absorbent at a solvent recycle rate of approx. 1100 t/h. The use of a process simulation program yields the following results:

(19) TABLE-US-00001 TABLE 1 Process State of embodying State of the art the invention the art (FIG. 1) (FIG. 2) (FIG. 3) CO.sub.2, crude gas kmol/h 870 870 870 CO.sub.2, purified gas kmol/h 95.6 96.3 95.2 CO.sub.2, exhaust gas kmol/h 774 773.4 774.8 Recycling solvent, m.sup.3/h 1166 1169 1185 reg. (9, 10) Solvent stream (4) m.sup.3/h 0 248 250 Laden solution, HE, C. 41.1 41.1 40.9 in (5b) Laden solution, HE, C. 113 115 115 out (6a) Reg. solution, HE, C. 125.4 125.2 124.8 in (10) Reg. solution, HE, C. 48.4 52.1 62.9 out (11) Pressure, laden bar 4.5 2.2 4.5 solution, HE, out Steam fraction, % 2 6 2 laden solution (6a) Heat transferred MW 96.2 91.7 79.1 Logarithmic C./K 9.6 10.6 15.1 temp. diff. Heat exchange m.sup.2 12700 10700 6500 surface (21) Reboiler output MW 31 23 34.5 (23)

(20) According to the process embodying the invention the laden solution leaving the absorption device is divided into two streams, with the share of the residual stream (5a, 5b) amounting to approx. (1169248)/1169=79% in the total recycling solvent stream. Even though only approx. 79% of the total solvent stream can be used, nearly the same energy amount as according to the state of the art can be transferred to the desorption device by means of the heat exchanger.

(21) In FIG. 3 the laden solution is passed through a heat exchanger without reducing the pressure. One recognises here that evidently less heat (79.1 MW) is transferred to the desorption device so that in the end even approx. (34.531)/31=11% more external energy are required for the desired removal of CO.sub.2. This means that the pressure reduction adapted to the requirements of the solvent is essential for the mode of operation according to the invention.

(22) The mode of operation according to the invention, however, allows operating with significantly less external energy in the desorption device. In this example, up to (3123)/31=26% of the externally required energy in the reboiler can be saved for the regeneration of the solution.

(23) The table also shows that the steam fraction of 6% in the mode of operation according to the invention is significantly higher than that achieved according to the state of the art.

LIST OF REFERENCE NUMBERS AND DESIGNATIONS

(24) 1 Feed gas 2 Product gas 3 Laden solution stream 4 Laden part-stream 5a Laden residual stream upstream of the relief valve 5b Laden residual stream downstream of the relief valve 6, 6a Pre-heated stream 6b, 6c Steam phase of the flashed solution 7a, 7b Liquid fraction of the flashed solution 8, 9 Regenerated solvent stream 10 Regenerated solvent stream 11 Solvent stream after heat exchange 12 Cooled regenerated solution 13 Separated component 14 Cooled separated component 15 Reflux pump 16, 27 Pump 17 Heat exchanger 18 Heat exchanger 19 Reflux drum 20 Absorption device 21 Heat exchanger 22 Desorption device 22a Heat transfer section 22b Stripping section 23 Heat exchanger 24 Branch 25 Relief valve 26 Flash vessel 28 Compressor