CA1068638A

CA1068638A - Regeneration of regenerable aqueous scrubbing solutions used for removing acidic gases from gas mixture

Info

Publication number: CA1068638A
Application number: CA242,505A
Authority: CA
Inventors: Francis Van Hecke
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-12-24
Filing date: 1975-12-22
Publication date: 1979-12-25
Also published as: FR2295781A1; AT369421B; ATA979075A; ES443792A1; ZA757939B; IE41981L; NZ179638A; BE836929A; PH12514A; DE2557531C2; BR7508536A; IT1051858B; AU503541B2; DE2557531A1; DK588975A; IE41981B1; IN144785B; GB1484050A; FR2295781B1; AU8781975A

Abstract

ABSTRACT OF THE DISCLOSURE

The disclosure relates to a process for the regenera-tion of regenerable aqueous scrubbing solutions which are used for the bulk removal of acidic gases, by absorption from gas mixtures containing these acidic gases. The process is a cyclic one in which the aqueous scrubbing solutions are regenerated in a regeneration system by steam stripping before being recycled to the absorption stage, a part of the scrubbing solutions being regenerated in a main regeneration section of the regene-ration system, whereas the other part of the scrubbing solu-tions is regenerated in an auxiliary regeneration section of the regeneration system. The process comprises the steps of select-ing the pressure in the auxiliary regeneration section indepen-dently from the pressure in the main regeneration section and of obtaining at least a part of the stripping steam required in the auxiliary regeneration section through flashing of the scrubbing solution in a reduced pressure zone connected to the suction side of a steam-jet thermocompressor, the mixture of thermocompressor motive steam and recompressed flashed vapour being directly discharged into said auxiliary regeneration section.

Description

863~
This invention relates to improvemen-ts to the regeneration of regenerable a~lueous scrubbin~ solutions, which -are used for the bulk removal of acidic gases~ such as C02, H2S, HCN, COS, S02 mercap-tans, etc., by absorption from gas ml~tures con-ta:ining these acidic gases, in a cyclic process :i.n wllich sai~ a~ueous scrubbing solu-tions are regenerated by steam stripping before bein~ recycled to the absorption stage.
The invention rela-tes particularly to a pro-cess for the regeneration of regenerable aqueous scrubbing solutions which are used for the bulk removal of acidic gases, by absorption from gas mixtures containing these acidic gases, in a cyclic process in which said aqueous scrubbing solutions are regenerated in a regeneration system by steam stripping ` before being recycled to the absorption stage, a part of the scrubbing solutions being regenerated in a main regene-.. . .
~ ~ ration section of the regeneration system, whereas the other .
par-t of the scrubbing solutions is regenerated in an ~;` auxiliary regeneration section of the regeneration system.

The gas mixture which must be purified may be , either a natuxal ~as stream or an industrial process gas.

The most commonly used aqueous scrubbing solu-: tions used in a cyclic process are solu-tions of allcaline car- ;
: ,.,, : .
bonates, particularly potassium carbonate which may con-tain ~ -~ any of the well kno~ activation additives such as arsenites, ~ -`~` borates, aminoacids, alkanolamines or other additives which increase the rates of absorption and desorption of the acidic gas in the scrubbing solution. In addition to the alkaline `; carbonate solution, other regenerable solutions such as aqueous solutions of ethanolamines and other alkanolamines, of alkali metal salts of aminoacids, of alkaline phosphates, -~
alkaline phenates, alkaline borates and other similar compo- -" ~ .

6~8 nents are used either separately or in admixture, For -the absorp-tion of S02 it is sometimes pre~erred to use a solution of alka]i me-tal sulfites and hisulfi-tes. It is common practice -to incorporate in these solutions a corrosion inhibitor~
The regeneration of the spent scrubbing solution by steam strippin~ and boiling takes place at a regeneration pressuIe ~hich is usually close to -the a-tmospheric pressure, correspondin~ to boiling temperatures which are commonly in -the range of 70C to 150C.
.. . . .
; 10 The strippin~ steam may be produced by boilin~
thc aqueous scrubbing solution in an indirectly heated re-boiler, or be obtained from an outside stcam source supply-~ ing live steam. The spent scrubbing solution tJhich leaves i -the absorber ~Jhere the absorp-tion of -the ac:idic gas has nor-~ .. . .
mally been carried out under a superatmospheric pressure, is first submitted to a pressure reduction and flashing before being introduced in the steam stripping zone of the regenera-tion section.
; The regeneration equipmen-t which is commonly llsed , comprises a regeneration column equipped wi-th packing mate-`; , rials, b~lbble cap plates, sieve trays or other suitable means for ensurin~ intimate contact between the solution and the ~ stripping steam. Similar type equipment is used ~or the .- absorption stage.
.. .. .
~ According to the temperature at which the rege-``~ nerated scrubbing solution is recycled -to the absorber and at which the spent scrubbing solution leaves said absorber, the cyclic process may be defined as "isothermal", "optimal"
~ or "classic". In the classic cycle the absorber operates a-t ;~- 30 a moderate temperature which may be close to ambient tempe- -; ~ .
~`~ rature. As the re~eneration by steam stripping takes place ~ -at a higher tempera-ture, several heat exchangers and/or :
~` - 2 .~, , .

, ~6863~

solu-tion heaters and coolers will be required in addition to the usual absorption and regeneration equipment. Such a classic cycle is for ins-tance preferred for the removal of C02, when the scrubbin~ solution is an aqueous monoethanol-amine so]ution.
en the gaseous mixture which is to be purified is available at such a pressure -that the partial pressure of -the acidic gas consti-tuent which MUS't be removed is relative-'' ly high, i-t becomes possible to increase the temperature of the absorption stage. The preferred cyclic process will then be either of the isothermal or the optimal type.
The difference between the so-called isothermal cycle and the optimal cycle can most easily be sho~n in the case of a t~o stage absorption and regeneration process.
' In the usual -two stage absorp-tion and regeneration cycle, the total flow of spent scrubbing solution is sent to the ' top of the regenerator, from where a major part of partially ~` regenerated solution is withdrawn at an intermediate level, ,` whereas the remainin~ minor part of solution is more fully re~enerated.'Only this minor par-t of "lean'l solution is '' , sent. to the top of the absorber, while the major stream of ' "semi-lean" solution is recycled to the absorber at an in- ' termediate level. ~;' ~'`' ' In the so-called "isothermal" cycle only the -~
minor stream of lean solution is cooled before being intro- '' ' duced at -the top of the absorber, whereas the major stream ' -'' of semi-lean solu-tion is recycled to the absorber without - ' any in-termediate cooling whatsoever. In the 'loptimal'l cycle, ~ i~
; the minor stream of lean solution is coolecl before recycling "~'' ~30 to the top of -the absorber, in order to ad~iust the vapour ,~ pressure of the solution to a value corresponding -to the ~' required de~ree of purification, whereas the temperature .. . ' ' .. ' ' ',' ~:':
:" ',' ' ` ~6~3B

o~ the semi-lean solu-tion which is recycled to an intermediate level is adjusted by coolin~ to such a value th~t -the optimal -tempera-ture profile for an efficient absorp-tion process can ; be achieved.
This optimal thermal profile is some-times important in order to control or limit some unwanted side reactions.
~ hen the optimal cycle is to be supplied to a sincle s-tage spllt-flow regeneration system, both the minor flow o~ regenera-ted solution which is recycled to the top of the absorber and the major flow of solution which is recycled to an intermediate level of the absorber are cooled although .
-to different de~rees. By comparison, in the so-called isother-mal single stage split-flow system only the minor flow of re-generated solu-tion is cooled, whereas the major flow of rege-~ . .
nerated solu-tlon is recycled to an intermediate level of the ~-absorber without cooling.
;` It is already known that the overall heat require- ~ -ment of these cyclic processes can be reduced if a dual-pres-sure regeneration system is used, in which the major par-t of -~ 20 the spent scrubbing solu-tion is regenera-ted in a main regene-~ ; ration column operating at a higher pressure level, while `~ the minor part of the solution is regenerated in a second `~ auxiliary regeneration column operating at a lower pressure level, by a flow of stripping steam which has been produced "~ exclusively by flashing, following pressure let-do~n, of the major part of regenerated solution from the higher pressure level to the lower pressure level.
~` ~ccording to this system essentially all the heat required in order to produce the stripping steam used for regeneration both in the higher pressure and in the lower pressure regeneration columns is introduced in the system at the higher pressure level, where the temperature '' '~ ' .

.', ,,. ~.
, - , . , . ~ ,- , , '": '' ' .' '' ' ', increasc o~ the solution serves as a hea-t storage from which upon pressure let-down to -the lower pressure le~Tel, the strip- ~
ping steam for the lower pressure regeneration column will be obtained.
; The thermal efficiency o~ this clual-pressure re~eneration system increases ~lhen the pressllre di~ference between the -two pressure levels increases. ~s in general the acidicgases whichare desorbed during the regeneration process have to be collec-ted for further trea-tment, it is usually pre~erable to opera-te the lower pressure re~eneration column ~;

at a pressure which is slightly superatmospheric, so as to decrease -the ener~y requirement for moving t;he desorbed gas to the next processing stage.
This means that in practice the relative heat `~
savinC will be dependent on the increase of pressure, and therefore of temperature, at the columnoperating at the higher pressure level.
This increase in tempera-ture constitutes sometimes an important drawback in view o~ some well known undesirable side-reactions such as -the thermal de~radation of various ~ scrubbing a~ents, notably the alkanolamines or aminoacids, i~
-` or -the formation o~ formates, as well as a more important risk o~ corrosion.
According to a modification of the above mentioned -dual-pressure regeneration process, part of the strippin~
steam required in the column operatin~ at the lower regenera-tion pressure can be produced in a solution reboiler operat-ing at said lower regeneration pressure leve:L. I~ therefore milder operating conditions are desired, it is no longer required that the major part o~ the solution should be re~e-nera-ted at the higher pressure level nor is it necessary to increase -the pressure in the hi~her pressure level -to the same extent.

.` . .
' . ' .

However such decreases of the severity of operation ~ can only be obtained at the expense of an increase of the over-all heat requirement. This increased heat requirement is intro-. duced in the system through two solution reboilers, one at the ~ higher pressure level and one at the lower pressure level~ In this two-reboiler version of the dual-pressure regeneration . system, the specific problems associated with solu-tion reboil-.. ers, i.e. risk o~ solution overheating and reboiler corrosion are increased in relation to the increased reboiler duty. :
10 According to the present invention, it was found -that, ; contrdr~ to all expectations, the severity of operation and the overall heat requirement can be decreased simultaneously to-gether with a reduction of the solution reboiler duty.
. . .
The process according to the present invention, in which a part of the aqueous spent scrubbing solution is regene~
. :
;~ rated in a main regeneration section or column, whereas the `.
other part of said spent scrubbing solution is regenerated in an auxiliary regeneration section or column of the regeneration . system, is essentially characterizèd by the fact that the pres- ~-20 sure in the auxiliary regeneration section is selected indepen- :
dently from the pressure in the main regeneration section and ~
:.. -~ .
; at least a part of the stripping steam required in ~he auxiliary regeneration section is obtained through flashing of the scrub-bing solution in a reduced pressure æone connected to the suc-tion side of a steam-jet thermocompressor, the mixture of ther- .. :~
mocompressor motive steam and recompressed flashed vapour being .
- , directly discharged into said auxiliary regeneration section.
~:.`. According to a broad aspect of the invention, there .~; is provided a process for the regeneration of regenerable aqueous scrubbing solutions which are used for the bulk removal of ~;

acidic gases, by absorption from gas mixtures containing thes~
acidic gases, in a cyclic process in which sai.d aqueous scrubbing -6- :`
... ..

, iB~3~3 solutions are regenerated in a regeneration system by steam strippi.ng before being recycled to the a~sorption stage, a part of the scrubbing solutions being regenerated in a main regene-ration sec-tion of the regeneration system, whereas the other par-t of the scrubbing solutions is regenerated in an auxiliary regeneration section of the regeneration system, the process ~:
comprising the steps of selecting the pressure in the auxiliary regeneration section independently from the pressure in the main regeneration section and of obtaining at least a part of the stripping steam required in the auxiliary regeneration section throuyi~ flashing of the scrubbing solution of which at least a part consists of the solution stream withdrawn from said ~.

auxiliary regeneration section in a reduced pressure zone :
: .
connected to the suction side of a steam-jet thermocompressor, the mixture of thermocompressor motive steam and recompressed ~. .
flashed vapour being directly discharged into said auxiliary ~.
regeneration section.
In a particular embodiment of the process according to this invention, the pressure in the auxiliary regeneration section is lower than the pressure in the main regeneration section and a part of the stripping steam required in said auxiliary regeneration section is obtained by direct flashing of the scrubbing solution upon pressure reduction from the , .:
.

..

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i ' ' ' ,~;:: :-. . . .
~, ' : .
i:; . .

, L ~ i -6a-~```, .. :

.. . .

068~3~3 higher pressure of the main regeneration sec-tion to the lower pressure of the auxiliary regeneration section.
Contrary to the kno~ln practice, it was surprisingly found that the pressure in the auxiliary regeneration section may also, according to this invention, be equal to or higher than the pressure in the main regeneration section.
The process according to this invention enables a substantial .improvement of the thermal efficiency of the :
regeneration procedure, due to the fact that only a part of the stripping steam required at the lower regeneration pres-sure level is ob-tained through flashing resulting from a pres-. ~ . . .
~- sure let-down of the scrubbing solution from the higher pres-sure regeneration section to the lower pressure regeneration . section, whereas the remainder of said required stripping steam is obtained through additional flashing of the scrub-. bing solution to a still lower pressure level and recompres-` sion of the flashed vapour mixture by way of a steam-jet ther-mocompressor. This technique avoids all risk of solution over-~- heating in an additional solution reboiler, and recovers under form of useful stripping vapour a cer-tain amount of heat stored in the solution which would otherwise be lost in the coolers which are used to trim the temperature of the solution before ; recycle to the absorber. .
. :
According to an additional feature of the inven-tion, the flashing to the reduced pressure induced by the operation of the steam-jet ejector is performed on a partial-. .
ly or totally regenerated scrubbing solution obtained from ~` either the main regeneration section or the auxiliary regene-ration section.
According to still another feature of the inven-`. tion, a part of the mixture of thermocompressor motive st~am ` and recompressed flashed vapour is discharged in the main regeneration section.

_ 7 _ ' . .

i3~3 Other details and features of -the invention will appear from the following detailed description, wherein -reference is made to ~igures 1 to 4 of the attached drawings which represent flow diagrams illustrating, by way of non-limitative examples, four embodiments of the process accord-ing to this invention.
In the following description, reference is made, by way of example, to a specific operation, i.e. the bulk removal of CO2 ~rom gas mixtures, using, as scrubbing solu-tion, an aqueous concentrated solu-tion of potassium carbonate : ... . .
containing known appropriate additives.
The flow diagram shown in figure 1 refers to an optimal cycle of regeneration of the spent scrubbing solu-tion, which due to its possibility of selective temperature adjustment of both the lean and the semi-lean scrubbing :, .
solution streams allows enough flexibility to achieve the desired temperature profile in the absorber.
It is thus possible to maintain constant the spent solution outlet conditions in the absorption stage even when modifications in the regeneration section tend to ~odify the temperature of the semi-lean and l~an scrub-bing solution streams which are to be recycled.
;~ The process gas which has to be puri~ied is in-troduced by line 30 at the base of the absorber ~ and leaves ; the top of said absorber by line 31 after acidic gas removal.
~- The hot spent scrubbing solution leaving the bot-tom of absorber 1 through line 21 is sent partly to a main regeneration colùmn 2 through a pressure let-down valve 52 and line 22 and partly to an auxiliary regeneration column 3 through a pressure let-down valve 53 and a line 23.
In the flash zone 7 o~ the main regeneration column 2 which operates at the higher pressure level, a cer .
,~ .
.' ~ ~ ...

. . . , ' ! , ~ ; ~ ; ; " " ~ "

LO~ ;3~3 ,, tain amount of steam and acidic gas is given off, while the , , remaining solution, which has been cooled by flashing to its boiling temperature under the prevailing pressure, flows downward towards the stripping zone 15.
In this stripping zone the solution flows counter-currently to the strlpping steam and is progressively heated by said stripping steam, thereby maintaining the solution at its boiling temperature as it progressively regenerates.
Part of the solution is withdrawn at an interme~
diate level from draw-off pan 40 after having been partially regenerated, whilst the remainder of the solution flows down through the stripping section 16 to the base of the main column 2 where it is heated in the solution reboiler 90.
The fully regenerated "lean" solution leaving the base of the regeneration column 2 by line 25 flows through ~
the pressure reducing valve 57 to the expansion vessel 4 ;
which is maintained at the same pressure as the base of the .~ ~
auxiliary regeneration column 3. The flashed gas mixture liberated in 4 flows to the lower part of column 3 where it is used as stripping steam9 whereas the lean solution, which has been cooled by flashing, leaves vessel 4 by line 26 and flows to exchanger 99 where it preheats the regeneration con densate before being pumped by pump 81 to the top of the absorber 1 after temperature adjustment in cooler 91.
;` The partially regenerated "semi~lean" solution which is withdrawn from draw-off pan 40 flows through the ` pressure reducing valve 56 and line 24 to the flash zone 9 at the base of the auxiliary column 3.
The gas mixture which is generated by flashing, ~` mixes wi-th the vapour coming from expansion vessel 4 and -~ flows upwards in column 3 where it is used as stripping steam.

_ g _ :. . . ;." ,. -~6~363~ ~

At the top of said auxiliary column ~, the solu-. .
tion which has been fed through a pressure let-down valve 53 flows through a flash zone 8 towards a regeneration section 17 where i-t is regenerated by the flow of stripping steam :~
which rises from the base of the column. , ~ .
After regeneration, the solution which collects . ' in a draw-off pan 41 is withdrawn through a line 271 and mixes : :~
with solution from the flash zone 9 which flows through a line 27. These combined streams flow through a pressure reducing ' 10 valve 58 to an expansion vessel 5 which is connected to the ~-suction side of a steam-jet thermocompressor or ejector 80.
The steam-jet ejector 80 is operated by the motive steam,sup- ,:
plied by a line 34, in order to maintain in the expansion ~
vessel 5 a suction pressure which is lower than the pressure "' prevailing in the auxiliary regeneration column 3, while the discharge pressure of the ejector 80 is adjusted so that the mixture of flashed vapour exhausted from expansion vessel 5 .' is recompressed to the pressure prevailing in the auxiliary ~': ' regeneration column 3. '' ' ~'' ~ , 20 The recompre,ssed mixture o~ flashed vapour and : ~ ' motive steam is delivered through line 35 to the auxiliary regeneration column 3 where it is used as stripping steam. ~'.:'--''~
The total amount of stripping steam available in the auxiliary stripping column 3 will thus be made up in ,, part of the steam liberated by flashing of the solution' supplied from the main stripping column 2, through lines ,:
24 and 33 and in part by the mixture o~ flashed vapour and motive steam provided by the steam~jet thermocompressor 80 through line 35. '~, , .
Depending on the relative pressure differences ., .................................................................... : . .
which exist between the columns 2 and 3 on the one hand, ,, , ' and the column 3 and expansion vessel 5 on the other hand, ' ~- . . .
. .. .

' . '' , :, '' '' ' ', . ,.', ` ' ~ ' .~ ` .'.! ' , ' . ' , . . . .

: ~0~63~3 ;
the relative amounts of these steam supplies will vary in importance.
After flashing in expansion vessel 5, the semi-lean solution flows through a line 28 and is pumped by a pump 82 to an intermediate level o~ the absorber 1 through a line 281 and a cooler 92 where its temperature can be adjusted to the desired value. The mixture of desorbed acidic gas and resi-dual stripping steam at the top of the auxiliary column 3 flows through a line 32 where it mixes with the desorbed acidic gas and steam mixture released by the main column 2 through a - pressure reducing valve 54 - The combined flows are cooled in a cooler 93 where ; the major part of the steam condenses. The regeneration con-densa-te which is collected in a reflux drum 6, from where the acidic gas leaves the unit, through a line 37, flows through a line 29 to a pump 83. After preheating in an exchanger 99, itis deliveredeither to a boiling ket-tle 95 where it is vapour-.
ized to produce motive steam to be used in the steam ejector, or returned through a valve 62 to the main regeneration co-~-` 20 lumn 2 as process reflux, or if desired evacuated out of the ,;, ~
` system through a valve 63. The preheating of the condensate ! ' which in this case occurs in an exchanger 99, can be realized in various ways.
Sometimes it is preferred to recover for this - purpose the heat contained in the acidic gas-steam mixture of line 32 before it is cooled in exchanger 93, or it may be chosen to use the hot semi-lean solution o~ line 28 as i- the heating medium. These various modifications have in common that the preheating of the condensate is achieved with heat which is available within the regeneration system, in order to reduce the overall external heat requirement.
~ If a separate cooler and condensate drum are provided for ~ 11 :,:
'`', ~ ' , ~

. . .

l~o ~:

the overhead vapour from column 2, it is possible to avoid any pressure reduction of said acidic gas stream, which can thus be delivered at same the higher pressure level a-t which it has been desorbed.
When the process feed gas which has to be purified in absorber 1 is available at a sufficiently high temperature as is the case for some industrial process gases, it can con-veniently be used as the heating medium in ket-tle 95 where the motive steam can be produced, As the motive steam is usually required at a pressure of 5 to 6 kg/cm2, the tempera-ture of the process gas leaving the kettle 95 through line 36 will still be high enough to be used as heating medium in the ~ ~`
solution reboiler 90.
....
After having passed through the heating coils 36 of the reboiler 90, the process gas has to be cooled further-down and this can for instance occur in a boiler feed water -heater.
; After such a series of cooling operations, the . process gas can then be introduced in the absorption column 1 ` -~
through line 30. :
; ~ . When the process gas which must be puri~ied is `~ available at a lower temperature, as is the case with some :~ natural gas streams, it is usually more convenient to use medium or low pressure steam as a heating medium in the solu-- tion reboiler 90 and the steam kettle 95. ;~ -;- . ... ~
~ The flow-sheet of fig. 1 shows clearly the dif~e-~, ,.
rence between the process according to this invention and the previously known dual-pressure regeneration methods, accord-ing to which the solution from the auxiliary column 3, which -.~ 3 operates at the lower pressure level, flows directly through ,, ~
: the lines 27 and 271 to the line 229 and pump ~2 ~rom where ` . it is returned to the absorber 1 after temperature adjustment ~;
:'` ' ' .;'': ~
- 12 - ~ ~

~: .~, :

,............ . . , ., .. ; .....

` ~L061!3638 in cooler 92.
One obvious difference between the two methods of operation is that in the new method of this invention part of the cooling of the solution is realized in expansion :
vessel 5 and that consequently an important part of the heat which mus-t otherwise be wasted in cooler 92 is recovered in a useful manner by way of the steam-jet thermocompressor 80, An advantage of the process according to this in-vention is that this heat saving can be used either to de-- crease the overall heat requirement of the system or to de-. .
crease the severity of the operating conditions without however increasing the heat consumption.
Further research has shown that the same overall thermal efficiency can be achieved even if the pressure dif-ference between the main regeneration column 2 and the auxi-liary regeneration column 3 is reduced, provided that a suf-,; .
ficient pressure difference is applied by the steam-jet ..
;.~ thermocompressor between the expansion vessel in which the . 20 solu-tion flashes and the regeneration column in which the ; recompressed flashed mixture is discharged.
- It is thus possible, as sh~wn in fig~ 2, to adjust the flow-sheet of the regeneration process in such a way that the direct flashing of the solution from the pressure level of the main regeneration column 2 to the pressure level of the auxiIiary regeneration column 3 is avoi.ded, so that - all the flashing occurs upon pressure let-down between the ` pressure levels of anyone of the two regeneration columns and the pressure level of an expansion vessel connected to the suction side of a steam-jet ejector.
` : :
; As the transfer of heat and of stripping steam :~ from the main regeneration column 2, which receives the .
, - ' ' . . : .',, '; :
.;

6~638 ... .
reboiler heat, to the auxiliary column 3 which has no reboiler is now controlled by the operation of steam-jet thermocompres--sors 80 and 88, it becomes possible to var-~ the relative pressure levels of the main and of the auxiliary columns in such a way that9 if desired, the main regeneration column 2 with solution reboiler 90 may operate at a lower pressure level than the auxiliary regeneration colurnn 3.
It has also been found that it is possible to adjust the operating conditions so that the reboiler heat duty of the solution reboiler 90 of the main regeneration column 2 is decreased to such an extent that less than half of the total heat requirement for the production of stripping steam in the regeneration system is supplied through the so-lution reboiler. Although it is always possible to eliminate ~ completely the solution reboiler by using live steam, such i~ a practice requires that an equivalent quantity of process condensate should be removed from the system. The reduction of reboiler duty to which reference is made corresponds to ` a water balance of the regeneration system which is maintained `
in equilibriùm, so that there is no need to evacuate process ;~, :. ...
; condensate out of the system.
As shown in fig. 1 and 2, the spent scrubbing solu- -tion from the absorber 1 is split in two fractions which are regenerated separately, in two regeneration columns 2 and 3.
. :., ~s the pressure may be adjusted independently on each column, it also follows that the two fractions of desorbed acidic gas . .; .
` may be obtained at two different pressure levels. ~
- .:
; This implies that, for the fraction which is ~" obtained at the higher pressure level, the cost of recom-; 3 pression for further processing is decreased.
As shown in fig. 2, the two streams of desorbed `
acidic gas from columns 2 and 3 are cooled separately in " -' , ' .`; ., " ' ', . :', . ' . . ' : . - ~
-: . . .. . : . ~ , ;.. . . ..

~68638 coolers 93 and 96 and evacuated through separate lines 37 and 39. The process condensates which have been collected separately in two different reflux drums 6 and 10 may however conveniently be mi~ed in line 29 for further reuse.
If it is desired to obtain the -totality of the desorbed acidic gas at the same pressure, it may be more advantageous to deliver all the spent scrubbing solution coming from the absorber 1 to a single flashing zone after ; which one fraction of the flashed solution will be regene-rated in the main regeneration column 2 and a second frac-tion will be regenerated in the auxiliary column 3, in which, according to the present invention, the stripping steam is supplied either exclusively or partially by ~lashing of the solution which has been induced by a pressure reduction produced by a steam-jet ejector which also recompresses the flashed vapour mixture to the required pressure level, the other part of the steam supply being obtained by f1ashing of the solution upon pressure let-down from the higher rege-~ neration pressure level to the lower regeneration pressure -~ 20 level.
Further research has shown tha-t the process of this invention also applies if, after the initial flashing of the single stream of spent solution coming from the absorber 1, some regeneration has been performed in the . . .
main regeneration column and the separation of the solution - in two fractions occurs under form of the wi-thdrawal of a side-stream of already partially regenerated solution ~ which is then further regenerated separately in an auxiliary "~ column with stripping steam which is supplied according to ;~
` 30 the method o~ this invention.
For a better understanding o~ this embodiment ,.
~ of the invention, reference is made to fig. 3 of the drawings.
.. , ' ~' , ``'; - ~
., ' ' ' ' ':~ . ' ~

6)6863~3 The spent scrubbing solution leaving the base of absorber 1 by line 21 flows through the pressure reducing valve 52 to the main regeneration column 2.
After flashing in the flash zone 7, the solution flows downwards to the regeneration zone 18. A fi.rst part of ,. . ...
the partially regenerated solution is withdrawn from draw-of~ plate 42, whilst the remainder of the solution flows -to the regeneration zone 15. A second fraction of the solution may be withdrawn from the column at the draw~off plate 40, and the remainder of the solution flows to the regeneration section 16, and from there to the solution reboiler 90, and the bottom of the regeneration column 2.
According to the embodiment of the invention, as represented in fig. 3, the auxiliary regeneration column 3 is situated at a lower elevation level than the draw-off pan 4~. The first fraction of solution, withdrawn from draw-off pan 42, flows through line 23 to -the valve 53 and auxiliary column 3. The difference in level between draw-off pan 42 ~-and the au~iliary regeneration column 3 can easily be adjust-ed so that the hydrostatic head created in line 23 is suffi-cient to overcome the higher operating pressure which is maintained in the auxiliary column 3.
As the solution coming ~rom draw-off plate 42 enters in column 3, there is no flashing in flash zone 89 ;.
because of the hi~her operating pressure. In the auxiliary column 3, the solution flows to the regeneration zone 17, ;
where it is heated to its boiling temperature l~der thè
, prevailing higher pressure, and regenerated by the counter- ~
: current stream of stripping steam which rises ~rom the base --` 30 of the column 3.
`~ The stripping steam and the desorbed acidic gas ` flow through line 321 and valve 55 to the main regeneration i ' ' ' -', ' ., . , . ' ., . ~ . ,.:.

column 2, The regenerated solution which collects at the base of column 3 flows through line 27 and pressure reduc-tion valve 58 to the expansion vessel 5 which is connected to the suction side of the steam-jet ejector 80, which main--tains in said vessel -the desired lower pressure.
The vapour mixture which is given off by flashing is recompressed in the steam-jet thermocompressor 80 and discharged -together with the motive steam through line 35 at the base of the auxiliary column 3.
, 10 The flashed solution collecting at the base of expansion vessel 5 flows through line 28 and is pumped by pump 82 to an in-termediate level of absorber 1 after tem perature adjustment in cooler 92. A second fràction of partially regenerated solution which may be withdrawn from the main regenerator 2 at draw-off pan 40 flows through pressure reducing valve 56 and line 24 to the expansion vessel 5 where the ~lashed vapour mixture is evacuated and recompressed by the steam-jet ejector 80 which discharges through line 35 in column 3. The lean solution which collects at the base of the main regeneration column 2 flows -through line 25 and pressure reducing valve 57 to the expansion vessel 4 which is connected to the suction side of steam-jet ejector 88. The flashed vapour mixture is evacuated and re-compressed by the steam-jet ejector 88 to the pressure pre-vailing in column 3 and the mixture of motive steam and . .
recompressed flashed vapour is discharged through line 33 ~:
and valve 60 to the auxiliary column 3. :.
: The supply of stripping steam in the auxiliary regeneration column 3 is thus made up of the discharge flow of the steam-jet thermocompressors 80 and 88.
` As shown in fig. 3, it is possible, if the to-tal stripping steam supply exceeds the requirements of column 3, .
: .:
; . - 17 -~C~G~638 ;~ .
to discharge part or all of the output of the steam-jet thermocompressor 88 to column 2 through valve 61~ ~
The lean flashed solution in expansion vessel 4 flows through line 26 to pump 81 and is pumped in split-~low partly through line 261 to the top of the absorber 1 after ;.: ., cooling in cooler 91, and partly through line 262 to an intermediate level of the absorber 1 after temperature adjustment in cooler 94. At the top of the main regeneration column 2, the mixture of desorbed acidic gas and overhead steam is cooled in the overhead cooler-condenser 93. The acidic gas is evacuated from the condensate reflux drum 6 throuc~h line 37. The regeneration condensa-te flows through line 29 and pump 83 to the steam kettle 95.
The application of the invention is not limited to the regeneration of one single scrubbing solution. It is also suited for improving the regeneration of an acidic gas scrubbing system where two different scrubbing solutions ~-; are used, in order to ac~ieve a more complete removal of the acidic gas.
Reference is made to fig. 4 of the drawings which ~ shows the flow-sheet of a C02 removal unit, in which a first ; scrubbing solution is used for the removal of the major fraction of the acidic gas, and where a second aqueous solu- ;
tion is used to remove more completely the remaining frac- ;
.... .
` tion of acidic gas.
The first scrubbing solu-tion is for instance a potassium carbonate solution containing c~ly of the usual ., , activating additives, such as diethanolamine, while the second solution may be another aqueous solu-tion capable of reducing more effectively the residual partial pressure of the acidic gas in the process gas which must be purified.
It is most convenient to use as second scrubbing solution ~,; .
- "~
~ - 18 -.
, . .
" : :
, . . . . . .

` " 10~8638 an aqueous solution of diethanolamine or an aqueous solution of -the potassium salt of an aminoacid such as diMethylglycine.
i The process gas stream containing the acidic gas is introduced through line 30 at the base of the absorber 1, where it flows upwards successively throu~h a first absorption zone 14 which is usually maintained at a temperature close to the atmospheric boiling temperature of the solution and a se-concd absorption zone 12 which is main-tained at a significantly lower temperature, and leaves the top of the absorber 1, after purification through line 31.
The spent hot scrubbing solution from the first absorption zone 14 collects at the base of the absorber 1 -and flows through line 21, in part to the main regeneration column 2 and in part to the auxiliary column 3. The part of the solution which flows to column 2 through -the pressure .~ reducing valve 52 and line 22 flashes in flash zone 7 and flows -towards the regeneration zone 15 where it is regenerated by the stripping steam rising from the lower part of the co-lumn, and collects in draw-off pan 40. This regenerated solu-tion leaves column 2 through line 2L~ and flows through the ; pressure reducing valve 56 to the expansion vessel 5 which is maintained at the lower pressure by -the suction side of the `~. steam-je-t ejector 80.
The second part of the scrubbing solu-tion from line : 21 flows through the pressure reducing valve 53 and line 23 to ; the auxiliary column 3, where after passing through the flash ` zone 8 it flows downwards to the regeneration zone 17, coun-ter-.- currently to the rising flow of stripping ste~am. The regene-: rated solution collects a-t the base of column 3 and flows through line 27 and the pressure reclucing valve 58 to the expansion vessel 5, where the steam-je-t ejector 80 main-tains a lower pressure. ~-,.~ . - 19 - ~:
.~, ,.,.:, '' ,;:~ .

., . , : ~

i8E;38 .
The total flow of -the first scrubbing solution is -thus collected in expansion vessel 5 and flows through line -.~
28 to pump 82 from where it is recycled to the first absor.p- .
: . tion zone 14 of absorber 1, after temperature adjustment in ; cooler 92.
The second scrubbing solution f:rom the colder second absorption zone 12 leaves the absorber 1 at draw-off : ~.
pan 43 by line 20, and flows to the heat exchanger 100 where it is reheated by the recycled regenerated solution, and from there through the pressure reducing valve 64 to the -: -. lower regeneration section of the main regeneration column . :
.. 2. After flashing in flash zone 9, it flows to the regenera~
tion zone 16 and to solution reboiler 90 where the stripping ~:
~: s-team is produced. The regeneration solution leaves the base of column 2 by line 25 and flows through pressure reducing :
;, ~, .
. valve 57 to expansion vessel 4 where a lower pressure is : maintained by -the suction side of the steam-jet ejector 88~
. The flashed vapour is evacuated and recompressed ~. by the steam-jet ejector 88 which discharges the mixture of .. 20 . motive steam and recompressed flashed vapour through line 33. . ..
Depending on the relative steam requirements of the two regeneration columns 2 and 3, it is possible to .
; supply the stripping steam mixture produced by thermocom-~ pressor 88 to either column, or to apportion part of it to ~-~ each~
., :'. ..
After flashing, the regenerated solution leaves `.~ expansion vessel 4 through line 26 and is pumped by pump 81 to heat exchanger 100 where it preheats the spent solution ~` of line 20 which flows towards the regeneration section. . ~
0.` 30 After final temperature adjustment in cooler 91 it is return- ~ -.~ ed to the second absorption zone 12 of the absorber 1. :-~
~he acidic gas which has been desorbed in the auxiliary column 3 is cooled in cooler 96 and is evacuated ' ` , - ~068638 from the condensate reflux drum 10 by line 39, whereas the ac-idic gas desorbed in the main coiumn ~ flows through line 32 to heat exchanger 9~ where it preheats the process conden-sate and then to cooler-condenser 93. It leaves -the conden-sate drum 6 -through line 37. The regeneration condensate ~rom the two condensate drums 10 and 6 flows to pump 83 and after prehea-ting in heat exchanger 99, is sent to vapour-izing kettle 95 or may also be returned to the process as reflux through valve 62. The steam produced in kettle 95 is 10 delivered as motive steam in steam collector 34 for the steam-jet ejectors 80 and 88.
.
It will be noted that in column 2 the stripping ~ ~
steam required in the second regeneration zone ~6 is obtain- ;
ed mainly b~ reboiling the second stripping solution in reboiler 90.
After passing through this lower regeneration zone 16, the mixture of stripping steam and acidic gas which has been desorbed in said zone flows upwards to the first regeneration zone 15 where it is used to regenerate the ~.
first scrubbing solution. As in the absorber 1 only a minor fraction of the acidic gas has been absorbed in the second ~
scrubbing solution, the quantity of acidic gas in the strip- :
ping steam emerging from the second regeneration zone 16 remains small, and said stripping steam mixture can be used efficiently in the first regeneration zone 15. ;: .
The stripping steam required in the auxiliary , column 3 is supplied by the discharge of the thermocompres- ~.
sors 80 and 88.
As it was indicated previously, the invention makes it possible to select for each regeneration column the most appropriate pressure level, as the determining .
factor is the pressure difference which is applied between ~:.

- 21 - .:~

''~ ~ ' ' 06863~3 ~ each of -the regeneration columns and the expansion vessels.
.. ..
As already pointed out, it has been chosen to -describe the various single solution flow sheet modifications with reference to an absorber operating always under the same terminal conditions (temperature of top and bottom of the absorber), accordin~ to an "optimal" cycle which incorporates the possibility of temperature adjustment of all the recycled solution streams. This particular feature was chosen only for .,:. . .
the purpose of a meaningful comparison of the various modifi-ca-tions~
It is however clear to anyone skilled in the art that the application of the invention is not limited to a ~ so-called "optimal" absorption cycle. The benefits of the .~ invention, which refers specifically to the regeneration of ~: the scrubbingsolution, are equally applicable to any other .
, operating conditions of the absorber, whether a temperature . adjustmen-t or cooling of the recycled solution streams is . applied or not. The invention refers in fact to the method .
according to which the spent solution stream or streams is ;`; 20 or are regenerated before recycling. `~
, The following examples which illustrate the inven-- tion refer to the removal of C02 from an industrial process .~ gas resulting from a catalytic steam reforming of natural gas, . and which a~ter C02 removal will be used for the synthesis of ~i ammonia.
; It is well kno~Jn that C02 is an acidic gas for which the kinetics of absorption and desorption are relative-.
:` ly slow, as a result of which the regeneration of the spent scrubbing solution is somewhat more difficult than in the case -:
of other acidic gases such as H2S. It is thus appropriate to illus-trate the invention with reference to a C02 removal ` process.
. ' , .
- - 22 - ;

.

EX~IPL~ 1 .. . .
P~e~erence is again Made to fig. 1 of -the drawings which shows a cyclic acidic gas removal procèss, in which an aqueous potassium carbonate scrubbing solution is usedO The process gas stream fror.1 which C02 must be removed is available 1 ~ at a -temperature of 250C, a pressure of 2~rl;g/cm2, and a ; flow ra-te of 45,700 Nm3jhr (dry basis). It contains ~ Nm3 of water vapour per Nm3 of dry gas. ~s an inlet -temperature to the absorber of 80C is desired, it is customary to recover ; 10 the heat contained in the process feed gas by use of one or more hea-t exchangers, ~hich may if desired be used to supply part or all of the heat requirements of the acidic gas removal ~ system. For instance the hot process gas may be used first to ; produce low pressure steam at a pressure of 6 kg/cm2 in the ; steam kettle 95 in which it enters through ]ine 47. The pro-cess gas leaving kettle 95 through line 36 may then be sent to the solution reboiler, for the production of stripping steam in the main regeneration column 2, after which it may `~` be further cooled to 80C in a boiler-feed water heater (not shown).
The relative quantities of steam produced in kettle 95 and solution reboiler 9~ will depend on the inter- ~ -` mediate temperature of the process gas in line 36, For a tem-` ` perature of 167C, the steam production in kettle 95 will amount to 5,900 kg/hr.
Before entering the absorber 1 ~/hich operates at a pressure of_~r kg/cm , the condensate which has been formed ~
upon cooling of the feed gas is removed in a separator drum ~ ;
` (not sho~n).
The process gas ~1hich en-ters the absorber 1 through line 30 contains 17.6 c~ by volume of C02. The C02 which is absorbed by the scrubbing solution reacts with the potassium ,~

,:` "
.: `'`: ' `
, .

~C 16~6319 :.
carbonate to form potassium bicarbonate according to the reaction :
2C3 + C2 + H20----~2 KHCo The scrubbing solution -thus contains a mixture of unconverted ;~
potassium carbonate and of potassium bicarbonate. The relative proportions of I(2C03 and ~IC03 are indicated by the percent conversion.
The "potassium bicarbonate fraction" or "percent conversion" as used herein means the proportion of original K2C03 expressed in percent which has been converted to potassium bicarbonate by reaction with C02. For example, a solution having a percent conversion of 20 ~ is obtained by the conversion of 20 mols percent of the potassium carbo-nate content of the original solution to potassium bicarbo-nate, such that the ratio of the potassium ions present as K2C03 to the potassium ions present as ~HC03 is 80/20. Since ~ two mols of K~IC03 are produced for each mol of K2C03, the : mol ratio of K2C03/I~HC03 at 20 % conversion is 80/40~ As used herein the concentration of the solution, indicated as weight percent potassium carbonate, is referred to the original so ~ lution free from bicarbonate, i.e. with zero percent conver-~ ,.
sion.
; The spent scrubbing solution which collects at ~`~- the base of absorber 1 has a concentration of 29 % by weight of K2C03 and contains in addition to the usual corrosion ` inhibitors 3 So by weight of diethanolamine. As -the process ;
feed gas flows upwards in the absorber 1 through the absorp-` tion zones 14 and 12, it is scrubbed by the hot semi-lean ~-, . . .
` solution which is introduced at an intermediate level and ` 30 by the cooled lean solution which is introduced at the top of the absorber at a temperature of ~0C. The residual C02 content of -the cleaned process gas which leaves the absorber ",'.~

- 2~
"" . .

., -- ' . ' ~ ', -06863~
::
at a temperature of ~0C throug1l line 31, is no-t more than 0.1 ~ by volume.
The lean solu-tion which has a conversion of 22 ,o , is introduced in the absorber at a flow rate equivalent to 1~ ~o of -total flow rate of the spent scrubbing solution.
The semi-lean solution ~hich is introduced in the absorber `~
at a temperature of 101C and a flow ra-te corresponding to ~2 ~'J of the -total flow rate of the spent scrubbing solution has a conversion of 42.8 ,~.
The spent scrubbing solution which leaves the ' base of the absorber 1 through line 21 a-t a flow rate of - 37~,000 l~g/hr has a conversion of 8l~ ' and a tempera-ture of` 10~C. The load factor of the solution corresponds -to 0.45 ;. mole of C02 per mole of K2C03, which is equivalent to an ; overall percent conversion change of L~5 %. S
` The spen-t scrubbin~r solution flow is divided in two parts. The major fraction of 60 ,0 flows -throu~h pres-` sure reducing valve 52 and line 22 to the main regeneration column 2 and flash zone 7, where the operating pressure is ,~, 20 set at 1.9 lcg/cm2. The minor fraction of 40 /~ flows through~ pressurè reducing valve 53 and line 23 to thé auxiliary rege--` neration column 3 and flash zone ~ where the pressure is set at 1.3 l~/cm2. In the main regenera-tion column 2, -the solu-tion flows downwards, after flashin~, towards the regeneration zone 15 where it is scrubbed by the countercurrent flow of ; stripping steam which desorbs the acidic ga,s.
At the draw-off pan 40, a fraction of 70 ~ of the partially regenerated solution f]ow is withdrawn at a ;
temperature of 120C and conversion of 46.3 ', while the remaining frac-tion of 30 ~' flows downwards to regeneration zone 16 and the solution reboiler 90. The heat which is ~ -supplled in the reboiler 90 by the ho-t feed gas supplied by ."' , ; .
- 25 - ~

:` ~'~,.

- ~6~3638 : line 36, upon cooling from 167C to 12~C corresponds to a steam production of 10,300 kg/hr which satisfies the strip- -ping steam requirement in the main regenera-tion column 2.
The C02 desorption rate in column 2 corresponds to 60 ,S of . . .
the total acidic gas removal rate of ~,000 Nm3/hr of C02.
The stripping steam requirement in column 2 thus corresponds to approximately 2.14 kg of steam per Nm3 of C02 desorbed in the column. Part of the steam which is supplied at the base of the column is condensed in the regeneration zone in order to satisfy -the endothermic desorption heat requirement and ;~
also in order -to maintain the solution at i-ts boiling tempe-rature. The remainder emerges at the top of the column as ;: residual stripping stcam.
In view of the fact that the heat content of the steam supply may vary according to the temperature and pres-. .
- sure of the system, it is preferable to refer to the regene-; ration heat requirement in terms of kilocalories per Nm3 of r~ desorbed C02, which in this case corresponds to 1,165 kc/Nm3 ''~. C02, , , The lean solution which collects at the base of column 2 a-t a temperature of 125C has a conversion of 22 %.
The minor fraction of 40 ,6 of the spen-t scrubbing solution in the auxiliary column 3 must be regenerated to ~ ~ .
~;` a conversion of 39 S6 in order to achieve the specified load factor or overall percent conversion change of 45 ,S.
The heat requirement for -this regeneration du-ty corresponds to 1,090 kc/Nm3 of C02 desorbed in the auxiliary column 3, or approximately 6,400 ~g of s-tearn for the desorp- -tion duty of 3,200 Nm3 C02, i.e. ~0 ,~ of the total acidic gas , ............................. . .
~" 30 removal duty, whichis-tobe performedin said auxiliary column.
The semi-lean solu-tion from draw-off pan 40 flows a-t the rate of 155,000 kg/hr, i.e. 42 ,0 of the total solu-tion .
` ' -.
:. , ~ ....... .

- ~068~;38 .
-~ flow, -through the pressure red~cing valve 56 and line 24 to :- .
the flash zone 9 at -the base of the auxiliary column 3 where the operating pressure of 1~4 kg/cm2 is slightly higher -than ; at -the top of the column. ~y flashing the solution produces 2,000 kg/hr of s-team thereby cooling to 110C. In addition ; to the steam, a small amount of C02 is releasedO Due to the slow kine-tics of the desorption process the quantity of C02 which is given off by the flashing process is sufficiently ; small so that i-t does not interfere in a significan-t ~lay with the regeneration process of the auxiliary column, and there is therefore no need, for the understanding of this example, to -take it in account. ;
; The lean solu-tion which leaves the base of column 2 through line 25 and pressure reducing valve 57, flows to expansion vessel 4 where essentially the same pressure of 1.4 kg/cm2 prevails as in flash zone 9 at the base of column 3.
Steam is genera-ted by flashing at the rate of l,000 kg/hr ` thereby cooling the solution to 113C.
; The ratio of C02 to steam in the vapour mixture generated by flashing is in this case even smaller than in .. ...
the case of the flashing of the semi-lean solution, and the small amount of C02 thus liberated can be neglected for the :, purpose of the present example.
The total amount of flashed s-team produced both in flash zone 9 and in expansion vessel 4 corresponds to

3,000 kg/hr or approximately 0.94 kg of steam per Nm3 of C02 desorbed in column 3. As the heat thus supplied only amounts to 506 kc/Nm3 C02, an additional heat input of 5~1 kc/Nm3 C02 will be needed in order to satisfy the regenera-tion heat requi-~; 30 rement.
According to the present inven-tion, this additional heat supply is obtained through use of thermocompressor ~0 " : .
.. ...
~ - 27 -.''.: . - ' ' ' ' ' . , . . . ... , , , .. . :.. ., ., . ,. .. .. , ~ . ~ " , . .

~;863~3 which is operated with motive steam from line 3~ produced at - a pressure of 6 kg/cm2 in kettle 95 by vapourization of pro- -cess condensate. i The minor fraction of 40 O of scrubbing solution , . ~
which is regenerated in regeneration zone 1'7 of the auxiliary column 3 is collected on draw-off pan ~1 at a temperature of 111C and a percent conversion of 39 ~'o.
It flows through line 271 and pressure reducing ; valve 58 to -the expansion vessel 5, together with the semi-lean solution leaving the bottom of column 3 through line 27.
This combined flow of 305,000 kg/hr produces upon flashing to the reduced pressure of 1.2 kg/cm2 which is maintained by the suction side of the steam-jet ejector 80 in vessel 5, 1,400 kg/hr of s-team thereby cooling the combined solution flow which has now a conversion of 42.8 ~' to 107C.
The motive steam at a pressure of 6 i~g/cm2 used by the steam-jet ejector 80, in order to recompress the flashed steam to the pressure of 1.4 kg/cm2 which exists at .~ ~ . . .
the base of column 3, amounts to 2,000 kg/hr.
The combined flow of 3,400 kg/hr of recompressed flashed steam and motive steam is discharged through line 35 at the basé of column 3. As explained earlier, the small ~
amount of C02 liberated by flashing may also be neglected ~ ;
for the purpose of the presen-t explanation; it may be noted that i-ts relative importance is further decreased by the diluting effect of the motive steam.
. ~ .
By use of this steam~jet thermocompressor 80, ; which uses 1 kg of motive steam at a pressure of 6 kg/cm2 ` to recompress 0~7 kg of flashed steam, thus producing 1.7 kg ; .
~" 30 of strippin~ steam, the external heat supply which is required ` in order to satisfy the regeneration heat demand of column 3 amounts to 59 % only of the addi-tional heal demand of 581 ~c/Nm3 C02.
`.,': , .;

:-~. . ,--- -863~3 The external heat input thus amounts to 341 kc/Nm3 --C2 of motive steam, the balance being obtained from the ~ :
. . , . . -solution itself by flashing and recompression. ::
The overall regeneration heat requirement of . .
the cyclic process is defined by combining the external heat .
supply to the main regeneration column 2 through the solution r~boiler 90, and the external heat supply -to the auxiliary ; column 3 under form of motive steam.
..
To the external regenera-tion heat supply in the . 10 solution reboiler o~ 1,165 kc/Nm3 C02 for the desorption of .. : ~

4,800 Nm3/hr of C02 in the main column 2, must be added the . .... . :~:
~:. supply of 341 kc/Nm3 C02 for the 3,200 Nm3/hr of C02 desorbed ..
in the auxiliary colwmn 3. This corresponds -to an overall : hea-t requirement of 83~ kc/Nm3 of C02 calculated on the basis of -the total acidic gas removal duty of ~,000 Nm3/hr of C02.
:: The semi-lean solution which leaves expansion ~:
vessel 5 through line 28 is pumped by pump ~2 through line 281 ~
to the intermedia-te level of absorber 1, a.ter having been ~ .
: cooled to 101C in cooler 92. ~.
. 20 The lean solution from expansion vessel 4 flows , to heat exchanger 99, where it preheats the process conden-sate to 100C. It is pumped by pump 81 through line 261 to the cooler 91 where it is cooled to ~0C ancL from there to the top of absorber 1.
: The desorbed C02 and the residual stripping steam .~ .; .
. from -the auxiliary column 3 flow through line 32 and cooler '~. condenser 93 to -the condensate separator drwn 6 which are all .:
maintained at essentially the same pressure level of 1.3 kg/cm2.
;: The desorbed C02 from the main column 2, which represènts ~0 S~
;.- 30 of the total C02 or 4,~00 Nm3/hr flows together with the ;! residual s-tripping steam through the pressure reducing valve 54 in-to line 32 where it mixes with the flow coming from the .: -.
.~ auxiliary regenera-tion column 3. The combined stream of ,. . ~-~,~, , ``` 29 ~ 06~38 8,000 Nm~/hr of desorbed C02 is evacuated at a temperature of 60C from the unit through line 37, while the overhead process condensate leaves vessel 6 through line 29 at a rate of 7,L~oo kg/hr. The process condensate is pumped by pump 83 to the exchanger 99, ~/here it is heated from 60C to 100C, after which part of it is sent to the kettle 95 to produce the requirèd amount of motive s-team for the steam-jet ejector -80 and the remainder is re-turned as process reflux to the main re6eneration column, through valve 62.
It is qui-te obvious that, if desired, the major flow of 4,~00 Nm3/hr of C02 which has been desorbed at a pressure of 1.9 kg/cm2 can easily be discharged from the acid gas removal unit at this higher pressure by way of a suitable piping set up such as is illustrated in fig. 2.
This example can easily be compared with the ;
prior ar-t practice of regeneration.
It was indicated that the stripping steam demand of the auxiliary column 3 amo~mted to approximately 2. kg of steam per Nm3 of C02 desorbed in the column, or a total of 6,40C kg/hr, whereas the steam produced upon flashing in flash zone 9 and expansion vessel 4 amounted only to 3,000 kg/hr. The additional re~uirement of 3,~00 kg/hr could of course be satisfied by way of a solution reboiler Instead of sending the semi-lean solution streams from column 3 to the expansion vessel 5, they could be cir-culated throu~h a solution reboiler, where an additional heat input of 5~1 kc/Nm3 C02 would produce the required amount of addi-tional s-tripping steam.
As compared with the method of the invention, the use of the reboiler requires a greater heat input, to-gether with the increased solution temperature which prevails in a reboiler, as against a solution cooling in expansion vessel 5.

', ` ~06~;3!3 ~ ~

Accordin~ to anoth~r me-thod of -the prior art, . .
it is possible to operate the cyclic process of fig. 1 ~ .~
without an additional reboiler or without use of -the present :
invention, if the combin.ed ser~ lean solution flow from co-lumn 3 is sent directly through line 229 -to pump ~2. The ~.
opera-ting conditions must then be modified i.n order to balance .:~
the s-team demand and supply in column 3. If the same regenera- ~: :
tion pressure levels of 1.9 kg/cm2 and 1.3 kg/cm2 are maintain- ` .~ .
ed, the major fraction of solution regenerated in column 2 . ~ :
must be increased to 76.2 ,Ç and the solution flow to the ~ ~ .
auxiliary column decreased to 23.8 %.
The overall heat lnput to the system, which is ~:
now limited to the single solution reboiler 90, has increased .
in the ratio of solution flow to the main column and corres- -ponds to ~8 kc/Mm3 C02. It is ho~ever possible to increase :~ ~;
the th~rmal efficiency of this me-thod of operation by increas-ing the pressure on the main regeneration column in order to ~.:
decrease as much as possible -the heat loss resulting from the ~
ilashing in flash zone 7. If the pressure is increased to a .;: -value which is equivalent to the vapour pressure of the spent . ... :
scrubbing solution, essentially no flashing would occur in flash zone 7. In the case of this example, a pressure of 3 kg/cm2 would be required. There is a corresponding increase .. . .
of the heat requirement in column 2, ~hich no~ amounts to 1,435 kc/Nm3 o~ desorbed C02, but the overall heat requirement .- :
is decreased to ~65 kc/Nm3 as only 60.3 S' of -the spent scrub-bing solution must be regenerated in column ~. The temperature .
of -the solution at -the base of column 2 has however increased ?,;. .
to 139C.

The data concerning example 1 and the comparison .

with prior art practice are summarized in Table 1. ..

Experiment n 1 refers to the prior art prac-tice as sho~in in ' - . ~ ' ;3~

fig. 1 of the drawings, where the combined semi-lean solution streams leaving column 3 through lines 27 and 271 flow through line 229 directly to pump 82 and are recycled to the absorber 1 without further treatment.
Experiment No. 2 re~ers to the prior art practice where an additional reboiler is used for the auxiliary column 3.
Experiment No. 3 refers to example 1 of the present invention.

Experiment ~o. 1 2 3 1 Pressure at top 10of column 2/3 (kg~cm2)3/1.3 1.9/1.3 1.9/1.3 2 Temperature at base of column 2/3 C 139/111 125/111 125/111 3 Total external heat input kc/Nm3 C02 865 931 836 EX~MPLE 2 Reference is made to fig. 2 of the drawings.
The same process feed gas and scrubbing solution are used as in example 1. The same amount of C02 is removed from the feed gas in absorber 1 where the operating conditions are the same as in example 1, except for the following modification. The lean solution flow rate has been decreased to 15% of the equivalent total flow rate of spent scrubbing solution. Its temperature and conversion are maintained at 80C and 22%.
The semi-lean solution flow rate is now equivalent to 85% of the total flow rate of spent scrubbing solution. Its conver-sion is now 42% and the temperature 100C. The spent scrubb- -ing solution collects at the base of the absorber at the same temperature and conversion of 108C and 84% and its flow rate is unchanged a~t 378,000 kg/hr. The load factor of 0.45 mole C2 per mole K2C03 is unchanged.
The spent scrubbing solution flowing rhrough line 21 is split in two equal fractions of 50% which are ~(~6~3638 regenerated separately in the n1ain regeneration column 2 and in the auxiliary column 3~ The operating pressure of the main -regeneration column is set a-t 1.3 kg/cm2 wh:ile the operating pressure of the auxiliary column is adjusted -to the slightly hi~rher pressure of 1.5 kg/cm . The solution flowing through valve 5~ and line 22 to the main regeneration column 2 flashes ~
in flash zone 7 and flows to the regeneration zone 15. A frac- ~ `
tion of 70 ~ of the partially regenerated semi-lean solution with percent conversion of 46.3 S~ is withdra~n from draw-off plate 40, at a temperature of 109C and the remaining fraction of 30 % equivalent to 15 ~ of the total solution flow flows to regeneration zone 1~ of the main regeneration column 2 and :
the solution reboiler 90. -After reboilingj the lean solution with a con-version of 22 ,o and a temperature of 115C collects at the bottom of the column 2. :~
The second fraction of 50 ~ of the spent scrub- ~;
. . .. . ..
bing solution which flows through pressure reducing valve 53 and line 23 to the auxiliary regeneration column 3 flashes -in flash zone 8 and flows towards the rege~eration zone 17.
. The semi-lean solution from column 3 must be regenerated to a conversion of 39 % in order to achieve the specified overall percent conversion change of 45 5~. This requires a regeneration heat supply of 1,115 kc/Nm3 of C02 desorbed in the column 3, which is equivalent to approxima-tely ~,200 kg/hr o~ stripping steam. This stripping steam `~
j will be supplied by the s-team-jet thermocompressor 80 which ;~
discharges throu~h line 35 into column 3.
The suction side of the steam-jet ejector 80 is ~
connected to the expansion vessel 5 where a pressure of ;-1.1 kg/cm2 is maintained. The semi-lean solution flow of 130,000 kg/hr ~hich is withdrawn from draw--off pan 40 flows :, . , : ~ . .

'. ''''''. ~:

., . ~ : .

through line 24 and pressure reducing valve 56 to expansion vessel 5 where 900 kg/hr of steam are ~enerated by ~lashing, thereby cooling the solution to 103C. The semi-lean solution which collects at base of auxiliary column 3 at a temperature :
o~ 115C and conversion of 39% flows at the rate o~ 185,000 .
kg/hr through line 27 and pressure reducing valve 58 to .-expansion vessel 5 where 2,200 kg/hr of steam are produced by flashing thereby cooling the solution to 105C. The motive steam o~ 6 kg/cm pressure produced in kettle 95 is supplied to the steam-jet ejector 80 by line 34 at a flow rate of 5,100 kg/hr and the recompressed mixture of ~lashed vapour and motive steam is discharged by the therrnocompressor 80 through line 35 at the pressure of 1.64 kg/cm2 whicll prevails at the base of column 3, at a flow rate o~ 8,200 k~/hr.
The regeneration heat demand of column 3 has thus :~
been satisfied t:o an extent of 62% by an external supply of motive steam and to an extent of 38% by heat extracted from the solution itself by the thermocompression action of a steam-jet ejector.
The external heat supply to auxiliary column 3 therefore amounts to 62% of the regeneration heat requirement o~ 1,11~ kc/Nm3 C02, i.e. 692 kc per Nm3 o~ C02 desorbed in ~.
column 3. ~-As the heat demand of auxiliary colurnn 3 is ~ully satisfied by the steam supplied by the steam-jet thermo-compressor 80, it is possible to use the additional steam supply discharged by thermocompressor 88 as stripping steam in the main regeneration column 2.
The lean solution at the base of column 2 ~lows ~:
through line 25 and pressure reducing valve 57 to the expansion vessel 4, which is connected to the suction side of the stearn-jet ejector 88, which maintains in said vessel .:
: _ 34 _ - . .

~. . -, ~ , ,~

~6~631~ ;
a reduced pressure o~ l kg/cmZ. Steam is generated by flash-ing a-t a ra-te of 600 kg/hr thereby cooling -the solution to 106C.
Motive steam produced at a pressure of 6 kg/cm2 in ~settle 95 i5 supplied -through line 34 at the rate of 1,000 kg/hr to the Steam-jet ejector 8~ and the recompressed mixture of flashed vapour and motive steam is discharged -through valve ~1 at the pressure of 1.5 kg/cm2 which prevails a-t the base of column 2.

When it is thus chosen to return the total output o~ 1,600 kg/hr of steam ~rom thermocompressor ~ to column 2, valve 60 of line 33 is closed. The regeneration heat require-ment in main column 2 amounts to 1,044 kc per Nm3 of C02 desor bed in the column, which is equivalent to a stripping steam supply of 7,700 kg/hr. ~s the steam-jet thermocompressor ~
already supplies approximately 20 ~o of the total demand of column 2, the heat supply by solution reboiler 90 will be . ~. .
reduced in the same proportion and will amount -to ~3~ kc per Nm3 of C02 desorbed in column 2. The total external regenera-tion heat supply to column 2 will thus be made up of 83~ kc/Nm3 C2 from reboiler 90 and 62 kc/Nm3 C02 under form of motive steam, or a total of 900 kc/Nm3 C02 desorbed in column 2.
As the -total regeneration duty is divided in equal parts between the main column 2 and the auxiliary column 3, where the respective external heat requirements are 900 kc/Nm3 C02 and 692 kc/Nm3 C02, the combined overall external heat supply ;~ -will amo~mt to 796 kc per Nm3 of C02 removed from the spent scrubbing solution. ~ - -~ fter flashing in vessel 4, the lean solution flows through exchanger 99 where the re~reneration condensate is preheated to 100C and is pumped by pump ~1 through line 261 to the cooler 91 ~here it is cooled to ~0C before entering ,-~ , . , ,, . . : ~::
, . .

~' ~o~63~ , the top of absorber 1. The combined semi-lean solution ~low from expansion vessel 5 with a percent conversion of 42 ~
flows through line 28 to pump 82 from where it is recycled to absorber 1 after cooling in cooler 92 -to a temperature of 100C. The overhead mi~ture of steam and C02 from the main column 2 ~lows through line 32 to cooler 93 and reflux drum 6 ~here -the condensate is separated wllile the cooled C2 is dischar$cd a-t a pressure of approximately 1.3 kg/cm2 and a temperature of 60C through line 37, a-t a flow rate o~
4,000 Nm3/hr. The other fraction of acidic gas which is de-sorbed in auxiliary column 3 is evacuated at the higher pres-sure of 1.5 1~g/cm2 through line 39 after having been cooled to 60OC in cooler 96. The condensate from reflux drum 10 flows through pressure reducing valve 50 into line 29 through which it flows jointly with the condensate from reflux ~rum 6 at Q combined flow rate o~ 8,700 kg/hr and -temperature of 60C to pump ~3. From there it is pumped to heat exchanger 99 where it is rcheated to 100C' after which it flows at a rate of 6,100 lg/hr to vapourizing kettle '35 where the correspondin~ amount of motive steam is produced at a pres-sure of 6 k~/cm2 by the hot feed gas which is supplied by line 47 and leaves the kettle 95 by line 36 a-t a temperature of 164C. The remainder of the regenera-tion condensate is returned as reflux to the main column 2 through valve 62.
It will be noted that, accordingr to this mode of operation, the solution reboiler is operated at the lower regeneration pressure, and that the reboiler heat duty has been reduced to 419 1-c/Nm3 C02.
EX~IPL~ 3 Reference is made to fig. 3 of tlle drawings.
The scrubbing solution composition and load factor is the -~
same as in example 2, as well as the percent conversion, - 36 ~

' . - ' ' ,:' ': - ''.. ' .; ~ ' , , : . . . . .. .
~- ': ' , ;' , ...

' ~6~63l3 temperature and flow ra-te of -the streams of lean and semi-lean solu-tions entering the absorber and of spent scrubbing --solution leaving -the absorber. The recycl~ solution line 262 and cooler 94 are not used. The ~ame feed ga~ is used as in example 20 The spent scrubbing solution, wi-th a conversion ;
of ~ Q~ and temperature of 108C, flows through line 21 and .... ... ...
pressure reducing valve 52 to the flash zone 7 of the main regeneration column 2 where the pressure is set at 1.5 kg/cm2.
~t draw-off pan 42, below regeneration zone 18, where the pressure is approximately 1.6 kg/cm2, a fraction of 50 ~ of the partially regenerated solu-tion is withdrawn at a temperature of 110C an~ conversion of 55 ~. The remain-ing 50 ~ fraction flows down to regeneration zone 15 and draw-off pan 40 where a ~raction of 70 % of the remaining semi-lean solution, i.e. 35 % of the total flow entering -the top of the column is withdrawn at a temperature of 113~C
and conversion of 46.3 %. The remaining fraction of 15 ~ -~
flows to regeneratiQn zone 16~ and after passing through 20 solution reboiler 90, collects at the bo-ttom of -the column , at a temperature of 118C and conversion of 22 ~
... . . .
The partiaily regenerated solution withdrawn from ::-. . .
draw-off pan 42 flows through line 23 to the auxiliary co- -~
lumn 3, which is situated a-t a lower elevation~ which is such that the hydrostatic pressure of the solution in line 23 `
is sufficient to overcome the difference in pressure between columns 2 and 3. In this example, -the difference in height between draw-off pan ~2 and pressure reducing valve 53 is - 2 m., which corresponds to a hydrostatic head of approxima-tely 0.25 kg/cm2, and the pressure in auxiliary column 3 is 1.7 k~/cm2, i.e. 0.1 kg/cm2 more than at draw-off pan 42.

, . .

., ' ' , . ~ ,. . ..... .

Because of the increased pressure in flash zone 8, there is no flashing of the solution, which after regenera- -tion in regeneration zone 17 collects at the base of the column.
In order to satisfy the constant load factor of the solution of ~5 ~'~ overall percent conversion change, the solution stream in column 3 must be regenerated to a conver-sion of 39 ~6. For this regeneration duty, an increased amount of stripping steam is however required, so that the partial pressure of acidic gas in the vapour mixture which emerges at the top of regeneration zone 17 and is subsequently return-ed to the main column 2, by line 321, is maintained at a value which is sufficiently low to ensure that said vapour mixture will still have an adequate stripping efficiency when trans-ferred in -the main column 2.
As a conversion change of only 16 % is performed on a fraction of 50 Y of the total solution, this will cor-respond to a desorption of 1,420 Nm3/hr of C02. The strip~
ping steam which must be supplied to column 3 corresponds to 5.7 lsg steam per Mm3 of C02 or approximately 8,100 kg/hr.
, According to the inven-tion, this stripping steam ~`
~ , - demand is satisfied by the discharge flow of the steam-jet ejector 80. The semi-lean solution regenerated in column 3 to a conversion of 39 % and a temperature of 116C flows `
through line 27 and pressure reducing valve 58 to the expan-sion vessel 5, in which the suction side of the steam-jet ~
ejector 80 maintains a reduced pressure of~1.2 kg/cm2. The ^
flashing induced by this decrease in pressure generates 2,100 kg/hr of steam thereby cooling the solution to 105C.
Thè semi-lean solution which is wi-thdrawn from the main column 2 at draw-off pan 40, flows through line 24 and pres-sure reducing valve 56 to expansion vessel 5, where the ~;'` .
- 38 ~
- ''~ . ' `
.: . .

flashing generates I,200 kg/hr of steam, thereby cooling the solution to 105C. ` - ;
The motive steam7 which is supp]ied from kettle 95 through line 3l~, at a pressure of 6 kg/cm2 and a flo~r ra-te of ~ 00 kg/hr recompresses`-the combined flashed vapours to a pressure of 1.3 kg/cm2 which corresponds to the pressure at the base of column 3, where it discharges -the mix-ture of mo~
tlve steam and recompressed flashed vapour through line 35.
The regeneration heat requirement of the auxiliary column 3 has thus been satisfied by an external heat supply under form of live motive steam, equivalent to 1,~51 kc per Nm3 of C02 ~ -desorbed in said column.
The regeneration heat demand in the main column 2 is now satisfied in part by the stripping steam supplied by the auxiliary column 3 through line 321, and the additional re~uirement corresponds to a stripping steam supply of 7,800 kg/hr at -the base of the column, of which part will be supplied by the thermocompressor ~ through line 33 and valve 61, and part through the solution reboiler 90.
The lean solution at the bottom of the main column 2 flows through line 25 and pressure reducing valve 57 to the expansion vessel 4 where the suction side of the steam-jet ejector ~ maintains a reduced pressure of 1.15 kg/cm2.
The induced flashing generates 700 kg/hr of steam thereby cooling the solution to 108C. The mo-tive s-team which is sup-plied at a pressure of 6 kgjcm2 by kettle 95 through line 34, at a flow rate of 1,000 kg/hr recompresses the flashed vapour to the pressure of 1.7 kg/cm2 which prevails at the base of . . .
column 2, where it discharges the mixture of motive steam and recompressed flashed vapour through line 33 and valve 61 .
a-t a flo~J rate of 1,700 kg/hr. During this operation valve 60 is closed so thàt no discharge steam from thermocompressor . : .. .
- 39 - ~ `
':

. . .

: . , ; ' . ' ' .; ' ' ' , ' ' . . , ' , . ' :, ' ' `. ', . . . , ' , ,, ,1 ` ! ' . ` , .
,' `, ,' .' . .,:. ' ., . .,' . . .' .. `'. ' .," '`': " ', ' ,:' ": . ` .' ', '. ` ' ' '`: '. ', ' ", .` , ' : , ' , ' : ' ' .

-~ ~06863~3 ........
~8 is allowed to enter the auxillary column 3.
The balance of 6,100 kg/hr of s-tripping steam supply is obtained from reboiier 90 which is heated by the hot synthesis feed gas. -~ -If the heat supply to the reboiler is rela-ted to the overall acidic ~as removal duty of ~,000 Nm3/hr of C02 i-t corresponds to lr17 ~sc/~m3 C02 desorbed (overall). The motive steam used in the steam-jet thermocompressors ~0 and ~ -a-t a combined flow rate of 5,~00 kg/hr corresponds to a heat supply, ~lhich is introduced in the regeneration system through vapourizing kettle 95, of 397 kc/Nm3 C02 desorbed overall.
The combined regeneration heat demand of this re~eneration method thus amounts to 81~Isc/Nm3 C02 removed.
All the C02 which has been desorbed in the regeneration sys-tem is collected at the top of the main regeneration column 2 at a pressure of 1.5 kg/cm2, from where it flows through line 32 to cooler 93 and dischar~e line 37 where it leaves the unit at a temperature of ~0C.
.~ . . .
The overhead regeneration condensate which col-lects in reflux drum 6 at a rate o~ ~,000 l.&r/hr flows through ~; line 29 and is pumped by pump ~3 to heat exchanger 99 where it is preheated to 100C, after which part of it is supplied to kettle 95 for the production of r.1otive steam and the remainder is returned to the main column 2 as process reflux.
According to fig. 3, it is l~nown -that this reflux is returned through valve 62 at the top of column 2. It is however well kno~m that it is equally satisfactory to rcturn said reflux either to an lntermediate level, or at -the base of the regeneration column, or even direc-tly into the side stream flowing into the solution reboiler ~0. `~
According to another well lsno~l practice, it is ;
also possible -to return the cold process condensate into ' ~'.; . ' .
~'.. : . :
: ~', ' ' .

68~3~ ~
-the system, without preheatin~ in exchanger 99, directly into ,~, line 26.
These various modifications are equally applicable within the scope oI -the presen-t inven-tion.
After passing through hea-t exchanger 99, the lean solution is recycle~ by pump 81 and line 261 to the -top of the absorber 1 after cooling to 80C in coo]er 91.
The semi-lean solution from vessel 5 with conver~
sion of 42 ~ is recycled to the absorber 1 by pump 82 through :
line 281 and cooler 92 where its tempera-ture is adJusted to `
100C.
EX~LE 4 `
Reference is made to fig, 4 of the drawin6s, which shows a dual solution scrubblng system in which an :~ -aqueous potassium carbonate solution is used for the bulk ;~
removal of C02 in the first absorption zone 14 of absorber 1, and a 20 So by weight aqueous diethanolamine (DE~) solution is ;c ; .. - ...
used in the second absorption zone 12 of ab,~,orber 1 for final cleaning of the gas. The same process feed gas is used as in example 1. After passing through the kettle 95, where upon cooling to 163C it vapourizes 7,200 lcg/hr of motive steam at a pressure of 6 kg/cm2, solution reboiler 90 and another ~ ~ -exchanger (not sho~n), the process feed gas enters absorber 1 which operates at a pressure of ~ kg/cm2 through line 30 at ~r"
a flow rate of 45,700 Nm3/hr (dry basis), After passing `~
through the first absorption zone 14, where the C02 conten-t ;~
is reduced to 1.6 s~ by volu~e, the gas is further cleaned in ~, the second absorption zone after which it leaves the absorber through line 31 at a temperature of 45C and with a residual C02 content of not more than 0.01 % by volume. The regenerated ~-~
po-tassium carbonate solu-tion is recycled to the absorber 1 through line 2~1 at a temperature of 101C and a conversion ','' ' ~

9~06~63~ ~
of 41.5 So. The spent potassium carbonate so:Lu-tion l~hich col-lects at the base of absorber 1 has a concentration of 29 50 by weight of potassium carbonate7 and contains, in addition to a small amount of corrosion inhlbitor, 3 5~ by weight of diethanolamine. It leaves -through line 21, a-t a temperature o~ 110C and a conversion of ~4 50 at a flow rate of 371,000 kg/hr. The lean DEA (diethanolamine) solution is recycled to the top of the absorber 1 through line 261 at a temperature of 45QC and an acidic gas load of 0.0~ mole C02 per mole of DEA The spent DEA solution is wi-thdrawn from absorber 1 at draw-off pan 43, at a temperature of 77C and an acidic gas load of 0.38 mole per mole of DEA. It fiows through line 20, at a fIow rate of 47,000 kg/hr to heat exchanger 100, where it is preheated to 95C, and after pressure reduction in --valve 64, enters the lower regeneration section of the main regeneration column 2. After flashing in flash zone 9, where the operating pressure is 1.45 kg/cm2, the solution flows to regeneration zone 16 and to the solution reboiler 90. The heat supplied by the hot process feed gas frQm line 36, gene-rates 4,800 kg/hr of steam. Part of this stripping steam is ;. ~
used in regeneration zone 16 to supply the heat for the endo-thermic desorption process and to heat the solution to its boiling temperature, while the remainder, together with the desorbed C02 flows upwards to the upper regeneration section where the potassium carbonate solution is regenerated.
The spent potassium carbonate solution leaving ~
absorber 1 is divided in two parts. A fraction of 30 S' is ~ -sent through pressure reducing valve 52 and line 22 to the ~: ~
, ~
main regeneration column and flash zone 7 where the pressure is set at 1.3 l~g/cm2.
After flashing the solution flows to regeneration zone 15 and to draw-off pan 40 from where it is withdra~ at ~-~.. . . . . .
. . : .

~ 42 -: ~ . ~''" ' ,. ~ ' :'~', '.:: ..

i[)68~i3~3 :: .
a temperature of 109C and a conversion of I~G ~. The regene-ration steam demand in zone 15 amoun-ts to 4,600 kg/hr of - - -which only 2,550 kg/hr are rising from the lower regeneration section. In order to satisfy the additional requirement, the lean DEA solution which leaves the bottom of the main column 2 through line 25 a-t a temperature of 112C, is led to expansion vessel 4, where the suction side of the steam ejector ~& main-tains a reduced pressure of 1.05 kg/cm2. The flashin~ of this solu-tion upon pressure reduction -through valve 57, generates ~00 kg/hr of steam, ~hicll are recompressed by the thermo-compressor 8~, which is operated with 1,250 kg/hr of motive steam at a pressure of 6 kg/cm2, supplied b~ kettle 95 through ~ -the steam collector 34. The thermocompressor ~ discharjges the mixture of recompressed flashed vapour and mo-tive steam through line 33 and valve 611 to the flash zone 9 à-t a flow ra-te of 2,050 kg/hr. During this operation, the valves 61 and 60 are maintained closed. '~ ~
The second fraction of 70 ~ of the spen-t potassium r,: , carbonate solution flows through the pressure reducing valve 53 and line 23 to the auxiliary regeneration column 3 and !'' flash zone ~ where the pressure is set at 1.4 kg/cm2.
~ In order to satisfy the specified overall conver-sion change of 42.5 %, the regeneration in the auxiliary column must be carried through to a conversion of 39 ~.
The corresponding regeneration steam demand of 10,100 ~g/hr is supplied by the steam-jet ejector &0. The regenerated solution from draw~off pan 40 flows through line 24 and pres~
-sure reducing valve 56 to the expansion vessel 5 where the suction side of the steam-jet ejector ~0 maintains a reduced pressure of 1 kg/cm2. Upon flashing s-team is generated at the rate of 1,000 1~g/hr thereby cooling the solution -to 101C.
The solution which has been regenerated in the auxiliary co- ;

,`,; ' , . .
' '; . ' ' 10~;~363~
lumn 3, flows by line 27 at a -temperature of 112C -through the pressure reducing valve 5~, to the expansion vessel 5, where a steam flow of 3,100 l~g/hr is grenerated, thereby cool-ing the solution to 101C.
A supply of 6,ooo ~g/hr of motive steam is sup-plied by line ~4 to the s-team-jet thermocompressor ~0, which recompresses the flashed vapour to the pressure of 1.54 kg/cm2 which prevails a-t base of column 3, and discharges the mix-ture of mo-tive steam and recompressed vapour at a flow ra-te `
of 10,100 kg/hr through line 35.
The combined flow of regenerated potassium car- ~ -bonate solution, with a conversion of 41.5 %, flows at a tem- A, ~'~'' ;
pera-ture of 101C through line 28 -to pump ~2 and is recycled -~ -:, .
to absorber 1 through line 281.
No cooling is reqùired in cooler 92. The lean DEA
solution which leaves expansion vessel 4 at a temperature of 102C flows through line 26 to pump 81, and is pumped to heat exchanger 100 where it preheats the spent scrubbin~ solution, ~ -From there it flows to cooler 91 where its temperature is ~ adjusted to 45C. me flow of dèsorbed C02 from the auxiliary column 3 is cooled in cooler 96 and leaves -the regeneration ;
unit at a pressure of 1.5 kg/cm2 by line 39. The C02 which has been desorbed in the main column flows through line 32 `-to the heat exchanger 99 where it preheats the process con-densate and then to cooler 93 before leaving the uni-t by line ~37 at a pressure of 1.3 kg/cm2. r '' ' The regeneration condensate which collects in the condensate drums 10 anci 6 flows through line 29 at a com- ~
bined flow rate of 9,~00 kg/hr to pump 83, from where it is ~ -pumped to heat exchan~er 99. This heat exchi~n~er could equal-ly well be heated by the overhead C02 and vapour s-tream from ~-column 3. ~fter preheating to 94C, the conclensate flows -to .''.' ': -~ .
_ 44 , ~ ~,o~63 )so ettle 95 at a rate of ~72C0 l~/hr and the remainder is re-turned -through valve 62 to the lower section of the main regeneration column as process reflux. `~
,~. .
The total e~ternal hea-t requirement of the re~eneratiorl syster.1 is satisfied by the solution reboiler s-tea~ procluction of 4,~00 k~/hr and the motive steam supply of 7,250 l~g/hr to the -thermocompressors 80 and ~8. A total quantity of 5,040 Nm3/hr of C02 has been rernoved from ~he process gas, and -thc specific heat consumption amoun-ts to ~25 kc/Nm3 of C02 removed.
In this exar,~ple the two separate absorption zones 12 and 14 were combined in the same absorpt;ion column. It is ~ -obvious that the regeneration method according to the present invention applies equally well if the separa-te absorption zones ^~
are arranged in dif~erent columns.
I-t is to be noted that the flow diagral1s of .
figures I to 4 do not include all the necessary devices need-ed for the operation of an industrial plant, Mowever, the additional devices needed for that purpose will be obvious for -those sliilled in the art.
. It is also to be no-ted that the invention is not limited to the four above described embodiments and that the flow dia~rams may be modified wi-thin the scope of -this inven-tion.
.;. , - -- .

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,~
~,~.. .... .

.. . ..

"''' "'

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the regeneration of regenerable aqueous scrubbing solutions which are used for the bulk removal of acidic gases, by absorption from gas mixtures containing these acidic gases, in a cyclic process in which said aqueous scrubbing solutions are regenerated in a regeneration system by steam stripping before being recycled to the absorption stage, a part of the scrubbing solutions being regenerated in a main regene-ration section of the regeneration system, whereas the other of the scrubbing solutions is regenerated in an auxiliary regene-ration section of the regeneration system, the process compris-ing the steps of selecting the pressure in the auxiliary rege-neration section independently from the pressure in the main regeneration section and of obtaining at least a part of the stripping steam required in the auxiliary regeneration section through flashing of the scrubbing solution of which at least a part consists of the solution stream withdrawn from said auxiliary regeneration section in a reduced pressure zone connected to the suction side of a steam-jet thermocompressor,the mixture of ther-mocompressor motive steam and recompressed flashed vapour being directly discharged into said auxiliary regeneration section.

2. A process according to claim 1, in which all the stripping steam required in the auxiliary regeneration section is obtained by flashing of the scrubbing solution in at least one reduced pressure zone connected to the suction side of a steam-jet thermocompressor, the mixture of thermocompressor motive steam and recompressed flashed vapour being directly introduced into said auxiliary regeneration section.

3. A process according to claim 1, in which the pressure in the auxiliary regeneration section is lower than the pressure in the main regeneration section and a part of the stripping steam required in said auxiliary regeneration section is obtained by direct flashing of the scrubbing solution upon pressure reduction from the higher pressure of the main regeneration section to the lower pressure of the auxiliary regeneration section.

4. A process according to claim 1, in which the pressure in the auxiliary regeneration section is at least equal to the pressure in the main regeneration section.

5. A process according to claim 3, in which the flashing to a reduced pressure, induced by the operation of a steam-jet ejector, is performed on a solution comprising at least one further stream of at least a partially regenerated scrubbing solution withdrawn from at least one further point of any one of the regeneration sections.

6. A process according to claim 4, in which the flashing to a reduced pressure, induced by the operation of the steam-jet ejector is performed on a solution comprising at least one further stream of at least a partially regenerated scrubbing solution withdrawn from at least one further point of any one of the regeneration sections.

7. A process according to claim 1, in which a part of the mixture of thermocompressor motive steam and recompressed flashed vapour is discharged in the main regeneration section.

8. A process according to claim 1, in which the spent scrubbing solution is separated in two fractions which are introduced respectively into the main regeneration section and into the auxiliary regeneration section.

9. A process according to claim 1, in which all the spent scrubbing solution is introduced into the main regeneration section and a side stream of partially regenerated scrubbing solution is withdrawn from the main regeneration section and introduced into the auxiliary regeneration section, operating in parallel with said main regeneration sections.

10. A process according to claim 1, in which two different scrubbing solutions which have been used in separate gas absorption sections are regenerated in a single regeneration system comprising one main regeneration section and one auxiliary regeneration section, the stripping steam which has been used for the regeneration of one of the spent scrubbing solution being reused as stripping steam for the regeneration of at least part of the other spent scrubbing solution.

11. A process according to claim 10, in which at least a part of the stripping steam used in the auxiliary section is obtained by flashing at least one stream of any one of the at least partially regenerated different scrubbing solutions, whereby each of said streams is withdrawn from one point of any one of the regeneration sections and flashed in one correspond-ing reduced pressure zone connected to the suction side of a corresponding steam-jet thermocompressor, and whereby each obtained mixture of thermocompressor motive steam and recom-pressed flashed vapour is discharged directly into said auxiliary section.

12. A process according to claim 10, in which at least a part of the stripping steam used in the auxiliary regeneration section is obtained by flashing at least one stream of any one of the least partially regenerated different scrubbing solutions, whereby each one of said streams is withdrawn from one point of any one of the regeneration sections and flashed in one corres-ponding reduced pressure zone connected to the suction side of a corresponding steam-jet thermocompressor, and whereby at least one obtained mixture of thermocompressor motive steam and recompressed flashed vapor is discharged, at least, partially, into said auxiliary regeneration section, and any remaining mixture of thermocompressor motive steam and recompressed flashed vapour is discharged into the main regeneration section.

13. A process according to claim 1, wherein said acidic gas is selected from the group consisting of C02, H2S, HCN, COS, SO2 or a mercaptan.

14. A process according to claim 1, wherein said acidic gas is CO2.

15. A process according to claim 1, wherein said regener-able aqueous scrubbing solution is a solution of a material selected from the group consisting of an alkaline carbonate, an alkanolamine, an alkali metal salt of an amino acid, an alkaline phosphate, an alkaline phenate, an alkaline borate, an alkali metal sulfate or an alkali metal bisulfate.

16. A process according to claim 15, wherein said material is potassium carbonate.

17. A process according to claim 15, wherein said material is diethanolamine.