CA1105676A - Process of regenerating alkali carbonate solutions formed by the desulfurization of hot gases - Google Patents
Process of regenerating alkali carbonate solutions formed by the desulfurization of hot gasesInfo
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- CA1105676A CA1105676A CA306,850A CA306850A CA1105676A CA 1105676 A CA1105676 A CA 1105676A CA 306850 A CA306850 A CA 306850A CA 1105676 A CA1105676 A CA 1105676A
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Abstract
ABSTRACT OF THE DISCLOSURE:
In a process for selective removal of H2S from hot gases containing H2S and C02, wherein the gases are contacted at superatmospheric pressure with an alkali metal carbonate solution and the resultant alkali metal carbonate solution which contains hydro-sulfides and bicarbonates is regenerated by pressure-relieving the same, the so pressure-relieved solution is thereafter stripped and re-used for selective removal of H2S from hot gases containing the same and carbon dioxide, the improvement which comprises adjusting the alkali metal bicarbonate content of the pressure-relieved solution to be stripped so that it is at least 55% of the total alkali content of the solution and re-using said solution, following stripping, without cooling the same.
In a process for selective removal of H2S from hot gases containing H2S and C02, wherein the gases are contacted at superatmospheric pressure with an alkali metal carbonate solution and the resultant alkali metal carbonate solution which contains hydro-sulfides and bicarbonates is regenerated by pressure-relieving the same, the so pressure-relieved solution is thereafter stripped and re-used for selective removal of H2S from hot gases containing the same and carbon dioxide, the improvement which comprises adjusting the alkali metal bicarbonate content of the pressure-relieved solution to be stripped so that it is at least 55% of the total alkali content of the solution and re-using said solution, following stripping, without cooling the same.
Description
~etallgesellschaf-t AG 1~5~ii 29. Juni 1978 Reuterweg 14 WGN/CPA
6000 Frankfurt/Main Prov. No. 8017 LO
_ _ Process of Regenerating Alkali Carbonate Solutions Formed by the De-sulfurization of Hot Gases _ This invention relates to a process of regenerating alkali carbo-nate solutions which contain hydrosulfides and bicarbonates and have been formed by the desulfurization of CO2-containing hot gases by ab-sorption under superatmospheric pressure and comprises pressure-relieving and stripping the solutions.
It is known that gases formed, e, g" by the reaction of liquid or solid carbonaceous Euels with gases which contain free oxygen and with water vapor can be desulfurized by being scrubbed under superat-mospheric pressure with a concentrated aqueous solution of one or more alkali salts of weak inorganic acids at a temperature which is near the atmospheric boiling point of the solution and the laden solution can be regenerated by being pressure-relieved, heated, and stripped with water vapor (Printed German Application 2,127,768).
In the known process steam is consumed at a high rate to regene-rate the solution if only a small sulfur content is permissible in the re-generated solution in order to enable a substantial desulfurization of the gas, The gas cannot be desulfurized satisfactorily if steam is avail-able`only at a restricted rate for the regeneration. Besides, the H2S
content of the exhaust gas formed by the regeneration will necessarily depend on the rate at which CO2 is scrubbed off too as the gas is de-sulfurized. Particularly when the gas to be desulfurized has a low H2S
content, the processing of the resulting exhaust gas to form elemen-tary sulfur with a high conversion, e. g., in Claus process plants, will involve a high e~penditure if a substantial emission of polluting sulfur compounds is to be avoided.
.,. : , . ...,: .:, ;: .: , :
' ' `, ' ~567~ s . :
It is an object o the invention to avoid these and other disadvantages of the prior art, to reduce the rate at which steam is consumed in regenerating the laden absor-bent to improve the desulfurization of the yas~ and to enable the recovery of an exhaust gas haviny a higher H2S concentration.
The present invention is concerned with a process Eor selective removal of H2S Erom hot gases containing H2S
and CO2, wherein the gases are contacted at superatmospheric pressure with an alkali metal carbonate solution and the . resultant alkali metal carbonate solution which contains : hydro-sulfides and bicarbonates is regenerated by pressure-re.lieving the same, the so pressure-relieved solution is . thereafter stripped and re-used for selective removal of H2S from hot gases containing the same and carbon dioxide, the improvement which comprises adjusting the alkali metal bicarbonate content of the pressure-relieved solution to be stripped so that it is at least 55% of the total alkali content of the.solution and re-using said solution, followiny stripping, without cooling the same.
According to a preEerred Eurther Eeature of the invention, a decrease of the alkali bicarbonate content of the laden solution before the stripping thereof, is avoided in that the solution is cooled below its boiling temperature . ~ .
: at the pressure relief end before being pressure-relieved pressure, preferably to 80-95C.
According to a further preferred feature of the ~ invention, the cooled solution is treated under normal or : superatmospheric pressure at 50 to 100C with a yas which consists predominantly of water vapor.
In another embodiment of the invention, the stripped solution is cooled once more and is stripped at 50 to 100C with a gas which consists p~edominant~ly of C2 and i5 subsequently stripped with a gas which consists predominantly of water vapor. Within the scope of the inven-tion that mul~istage treatment can be repeated several times.
The stripping gas consisting predominantly of water vapor is desirably obtained by an indirect heating ~ :
of the stripped solution so tha-t the solution is stripped with its - 2a -':
own vapor, sui-tably in a countercurrent.
Within the scope of the invention, a suitable high-C2 gas for saturating the cooled solution can be obtained in that the exhaust gas, ~
.
/
___ . .. .... _ . . . _.. ., .. . . .. . . , . .. . ....... ..... ,.. _ __.. _
6000 Frankfurt/Main Prov. No. 8017 LO
_ _ Process of Regenerating Alkali Carbonate Solutions Formed by the De-sulfurization of Hot Gases _ This invention relates to a process of regenerating alkali carbo-nate solutions which contain hydrosulfides and bicarbonates and have been formed by the desulfurization of CO2-containing hot gases by ab-sorption under superatmospheric pressure and comprises pressure-relieving and stripping the solutions.
It is known that gases formed, e, g" by the reaction of liquid or solid carbonaceous Euels with gases which contain free oxygen and with water vapor can be desulfurized by being scrubbed under superat-mospheric pressure with a concentrated aqueous solution of one or more alkali salts of weak inorganic acids at a temperature which is near the atmospheric boiling point of the solution and the laden solution can be regenerated by being pressure-relieved, heated, and stripped with water vapor (Printed German Application 2,127,768).
In the known process steam is consumed at a high rate to regene-rate the solution if only a small sulfur content is permissible in the re-generated solution in order to enable a substantial desulfurization of the gas, The gas cannot be desulfurized satisfactorily if steam is avail-able`only at a restricted rate for the regeneration. Besides, the H2S
content of the exhaust gas formed by the regeneration will necessarily depend on the rate at which CO2 is scrubbed off too as the gas is de-sulfurized. Particularly when the gas to be desulfurized has a low H2S
content, the processing of the resulting exhaust gas to form elemen-tary sulfur with a high conversion, e. g., in Claus process plants, will involve a high e~penditure if a substantial emission of polluting sulfur compounds is to be avoided.
.,. : , . ...,: .:, ;: .: , :
' ' `, ' ~567~ s . :
It is an object o the invention to avoid these and other disadvantages of the prior art, to reduce the rate at which steam is consumed in regenerating the laden absor-bent to improve the desulfurization of the yas~ and to enable the recovery of an exhaust gas haviny a higher H2S concentration.
The present invention is concerned with a process Eor selective removal of H2S Erom hot gases containing H2S
and CO2, wherein the gases are contacted at superatmospheric pressure with an alkali metal carbonate solution and the . resultant alkali metal carbonate solution which contains : hydro-sulfides and bicarbonates is regenerated by pressure-re.lieving the same, the so pressure-relieved solution is . thereafter stripped and re-used for selective removal of H2S from hot gases containing the same and carbon dioxide, the improvement which comprises adjusting the alkali metal bicarbonate content of the pressure-relieved solution to be stripped so that it is at least 55% of the total alkali content of the.solution and re-using said solution, followiny stripping, without cooling the same.
According to a preEerred Eurther Eeature of the invention, a decrease of the alkali bicarbonate content of the laden solution before the stripping thereof, is avoided in that the solution is cooled below its boiling temperature . ~ .
: at the pressure relief end before being pressure-relieved pressure, preferably to 80-95C.
According to a further preferred feature of the ~ invention, the cooled solution is treated under normal or : superatmospheric pressure at 50 to 100C with a yas which consists predominantly of water vapor.
In another embodiment of the invention, the stripped solution is cooled once more and is stripped at 50 to 100C with a gas which consists p~edominant~ly of C2 and i5 subsequently stripped with a gas which consists predominantly of water vapor. Within the scope of the inven-tion that mul~istage treatment can be repeated several times.
The stripping gas consisting predominantly of water vapor is desirably obtained by an indirect heating ~ :
of the stripped solution so tha-t the solution is stripped with its - 2a -':
own vapor, sui-tably in a countercurrent.
Within the scope of the invention, a suitable high-C2 gas for saturating the cooled solution can be obtained in that the exhaust gas, ~
.
/
___ . .. .... _ . . . _.. ., .. . . .. . . , . .. . ....... ..... ,.. _ __.. _
- 2~ - ~
:
:
-3 which is formed by the stripping step and consists mainly of water vapor and CO2 and contains also H2S is cooled, the aqueous condensate is se-parated and the cooled gas is scrubbed with a partial stream of the alkali carbonate solution at a temperature below 60 C, preferably 25 to 35 C.
In this way most of the hydrogen suifide contained in the cool_ I
gas is selectively scrubbed off so that all or part of the scrubbed low-~I2S gas which consists predominantly of C02 can be used to saturate the cooled solution that is to be regenerated.
Within the scope of the invention, the saturating step, in which part of the CO2 is transferred from the gas into the solution, may be succeeded by the use of the non-absorbed gas for stripping the partial stream of the alkali carbonate solution which has been used to scrub the cooled exhaust gas formed by the stripping step and the non-absorbed gas may then be discharged whereas the stripped partial stream is re- turned to the main Gycle of the alkali carbonate solution ~ 9 a result, virtual}y all hydrogen sulfide which has been stripped from the sblution to be regenerated is contained in the exhaust gas which is discharged The concentration of hydrogen sulfide in that exhaust gas will be increased if only part of the scrubbed gas, which consists predominantly of ~2~ ~is used first to saturate the cooled solution to be regenerated, and subsequently to strip the high-H2S
partial stream of the solution, and the remainig portion is discharged as a low-H2S exhaust gas.
In accordance with the invention the high-H2S partial stream of the alkali carbonate solution which has been used to scrub the cooled exhaust gas formed by the stripping step can be regenerated by stripping said partial stream at its boiling temperature with a gas consisting pre-dominantly of water vapor so as to remove substantially ~1~ hydrogen - sulfide, whereafter the resulting gas, which contains hydrogen sulfide, is disoharged and the stripped-partial stream is returned to the main cycle o~ the alkali carbonate solutlon.
In this way most of the hydrogen suifide contained in the cool_ I
gas is selectively scrubbed off so that all or part of the scrubbed low-~I2S gas which consists predominantly of C02 can be used to saturate the cooled solution that is to be regenerated.
Within the scope of the invention, the saturating step, in which part of the CO2 is transferred from the gas into the solution, may be succeeded by the use of the non-absorbed gas for stripping the partial stream of the alkali carbonate solution which has been used to scrub the cooled exhaust gas formed by the stripping step and the non-absorbed gas may then be discharged whereas the stripped partial stream is re- turned to the main Gycle of the alkali carbonate solution ~ 9 a result, virtual}y all hydrogen sulfide which has been stripped from the sblution to be regenerated is contained in the exhaust gas which is discharged The concentration of hydrogen sulfide in that exhaust gas will be increased if only part of the scrubbed gas, which consists predominantly of ~2~ ~is used first to saturate the cooled solution to be regenerated, and subsequently to strip the high-H2S
partial stream of the solution, and the remainig portion is discharged as a low-H2S exhaust gas.
In accordance with the invention the high-H2S partial stream of the alkali carbonate solution which has been used to scrub the cooled exhaust gas formed by the stripping step can be regenerated by stripping said partial stream at its boiling temperature with a gas consisting pre-dominantly of water vapor so as to remove substantially ~1~ hydrogen - sulfide, whereafter the resulting gas, which contains hydrogen sulfide, is disoharged and the stripped-partial stream is returned to the main cycle o~ the alkali carbonate solutlon.
- 4 -: ~ .
s~
According to a preferred feature of the invention the gas con-sisting predominantly of CO2 and used to saturate the cooled solution is formed by the regeneration of a second cycle of alkali carbonate solution used to scrub the desulfurized, CO2-containing hot gas under super-atmospheric pressure In this embodiment, the first des,.lfurizing cycle is succeeded in the gas flow path by a second cycle of all~ali carbonate solution~ which supplies the CU2 required to regenerate the laden solution in the first cycle, BesidesJ the second scrubbing of the gas un- ;
der superatmospheric pressure with absorbent flowing in a second cycle causes additional hydrogen sulfide to be removed from the gas so that its desulfurization can also be improved. The alkali carbonate ~;
solutions in the two c~cles may differ in cornposition, For instance, a concentrated solution may be used in the first cycle and a more dilute solution in the second or vice versa.
Within the scope of the invention it is preferable to use an alkali carbonate solution to which sodium tetraborate has been added, preferab-ly in an amount of 1 to 20 mole percent of the total alkali content of the solution, i e., 1 to 20 ~0 of the total alkali which can be lltrated with acid should be added as sodium tetraborate, Na2B4O7.
The advantages afforded by the invention reside in that an alkali : .
carbonate solution which contains hydrosulfide and bicarbonate and has been obtained by the desulfurization of CO2-containing hot gases under superatmospheric pressure can be selectively regenerated by the pro-cess described in that almost all of the combined hydrogen sulfide con-tained in the solution as hydrosulfide but only part of the combined car- ~;bon dioxide present as bicarbonate are released and stripped off. This will be particularly significant if a selective removal of hydrogen sulfide from such gas is desired, i. e., when it is desired to scrub as much hydrogen sulfide as possible and as little CO2 as possible from the gas, as is required in various applications, e. g., in the desulfurization of fuel gas for a gas turbine process in which the mass of the humid gas should be preserved as far as possible before it is burnt. Besides, ., ' ~
' ' ' ` `'' ` ' . .'. ': ' , ,, ,. '.".' ` .' ! '; ' `
' ' ' ~ ` ` ' ' , , ,, '. ' . ' ~ .. '.' ., ' ,'';; ': ' ;, " ' ` ~ , ' ' ' . ',', "'~ i .'; i' _ 5 --the process renders the driven-off hydro~en sulfide available in a higher concentration Moreover, the further processing is facilitated and the investment and supplies required for the further processing are reduced.
Finally, the quantity of polluting gases emitted into the environrnent may be decreased.
The reactions taking place in the alkali carbonate solution are known and may be summarized as follows:
Absorption: 2C 3 2 2 = 2 K~IC03 (lj K2C03 + ~I2S = K~IC03 + KHS (2) The reactions take place in the opposite direction during the re-generation:
2 I~IC03 = I~-~C03 + C02 + H20 (3) KlIS ~ K~IC03 = K2C03 -~ H2S (~) From equation (4) it is apparent th~ bicarbonate is required for the decomposition of the hydrosulfide which has been absorbed. Such alkali carbonate solutions formed by the desulfurization of C02-containing gases contain in more cases more bicarbonate than hydrosulfide. But when the solutions are regenerated in the conventional manner by being pressure-relieved3 heated and stripped with steam, equal percentages of bicarbonate and hydrosulfide are always decomposed and driven off as C2 und H2S In that case, when 90 % of the h~rdrosulfide are to be re-moved by the regeneration, about 90 % of -the bicarbonate must also be decomposed and driven off. The regenerated solution will then contain only 10 %-of the hydrosulfide content but will also contain only 10 % of the bicarbonate content of the unregenerated solution. Such regenerated solution contains almost only alkali carbonate and for this reason can form rnuch bicarbonate when it is used to desulfurize the C02-containing gas under superatmospheric pressure As a result, C02 is scrubbed . ,, . :" :
:: - . , ~. : , . . .
,, ~ . :
:
. . : ~ ,. . , :
, ~ ' , ~: ' ~567~ -off at a high rate so that the desulfurization of the gas is less selective, with all disadvantages listed hereinbefore.
Whereas much bicarbonate is required for a substantial regene-ration of alkali carbonate solutions which contain hydrosulfideJ part of the C02 required for this pur~.ose may be recirculated and re-used in the regenerating stage in the process according to the invention if this is required. For this reason the regenerated solution may have a relatively high bicarbonate content and yet be freed almost completely from hydrosulfide, i.e., subjected to a selective desulfurization.
A preferred feature of the invention is based on the recognition that the purified gas which has been obtained by the desul~urization can be used to special advantage in a combined power plant process. For that purpose the hot desulfurized hot gas which is under superatmos-pheric pressure is used for an ecologically satisfactory production of electric power in a combined power, plant process~ in which energy is recovered from the gas by a pressure relief and/or combustion and is utilized to drive gas turbines and/or to produce steam, -That practice affords the advantage that the C02 left in the de-sulfurized gas greatly increases the efficiency of the combined power plant process because the energy contained in the C02 under super-atmospheric pressure can be utilized.
Another advantage of the invention resides in that the H2S-con-taining exhaust gas formed by the regeneration of the absorbent has a higher H2S content than the exhaust gas formed in a regenerating pro-cess known in the art, As a result, the plants for processing such ex-haust gas may be smaller and less expensive and can operate more economically, The invention will be described more fully hereinafter. The in-vention is shown diagrammatically and by way of example in Fig. 1 to 3.
The drawings show only the most important elements of the process, whereas pumps and valvesJ e, g,, have been omitted for the sake of clearne s s .
... . - ;, ,, .. , . -., :
~: : . ': ~. ;
Example 1 The process according to the inven-tion is shown in its simplest form in Fig. 1. Tha-t embodiment may be used if the alkali carbonate solution which has been formed by the desulfurization of the gas and is to be regenerated contains at least S5% of i-ts al~ali as bicarbonate. This solution is fed through conduit 1 at a temperature of 115 C under a pressure of 20 bars and consists of an aqueous solution of 251 grams KHC03/
liter, 5.0 grams KHS/liter, 62.5 grams K2C03/li-ter and 60 grams Na2B~07.10 H20/liter. The KHC03 content thus amoun-ts to 66% of -the total content of titratable alkali.
The solution is cooled to 90 C in the cooler 2 and is pressure-relieved in the pressure-relief valve and -then enters the regenera-ting column L~, which is under a pressure of 1.5 bars, Almost no gas and water vapor are released by -the pressure rellef, which does not change -the bicarbo-na-te and hydrosulfide conten-ts.
In the regenerator 4, -the solu-tion is heated and decompoesd by -the gases which rise from the lower por-tion and consi~ predominantly of water vapor. As a result, the solution releases C02 and H2S~ In the lower portion, the solution is circulated in conduits 5 and 6 and the indirectly heated heater 7, whereby mainly wa-ter vapor is produced, which rises in the regenerator L~, The regenerated solution runs off through conduit 8 and now contains only 72.3 grams K~C03/liter and 1.1 grams KHS/liter. The borax conten-t has not been changed and the K2C03 conten-t has increased -to 189.9 grams per li-ter. The bicarbona-te content has thus been decreased to 28.8~' of its initial value and the hydrosulfide content ~ 22.0% of its initial value.
It is apparent -that the hydrosulfide content has been decreas-ed more than the bicarbonate content, which means that the regeneration is selective with respect to hydrosulfide or H2S. The solution may now be re-used for a ~5~67~
selective desulfurization of gas. Gases and vapors from the top of regenerator 4 flow through conduit 9, are cooled and partially condensed in the indirect cooler 10. Gases and condensate are separated in the separator 11. The remaining gases are removed through the conduit 12. The condensate flows back to the top of the regenerator 4 through conduit 13.
_ontrol Example When the cooler 2 is not operated, in accordance with the state of the art, the incoming solution at 115C is pressure-relieved in the valve 3 to a KHC03 content of 49% of the total alkali content, and the heater 7 is supplied with steam at the same rate as in the preceding Example, the resulting regénerated solution contains 77.8 grams KHCO3/liter, 1.5 grams KHS/liter, 60 grams borax/liter and 185.7 grams K2CO3/liter.
In this operation, which is in accordance with the prior art, the KHCO3 content has been decreased by 69% and the KHS
content by 70%, which means that the portions driven off are about the same. The regeneration is not selective. When the regenerated solution is used to desulfurize gas, this unselective regeneration results in a higher residual H2S
content of the scrubbed`gas, in the removal of more CO2 by scrubbing, and in a lower concentration of H2S in the H2S-containing exhaust gas.
Example 2 This Example is illustrated in Fig. 2, which represents a preferred embodiment of the invention. The solution to be regenerated is supplied through conduit 1 under a pressure of 20 bars and a temperature of 140C and per liter contains 205.4 grams potassium hydrogen carbonate,
s~
According to a preferred feature of the invention the gas con-sisting predominantly of CO2 and used to saturate the cooled solution is formed by the regeneration of a second cycle of alkali carbonate solution used to scrub the desulfurized, CO2-containing hot gas under super-atmospheric pressure In this embodiment, the first des,.lfurizing cycle is succeeded in the gas flow path by a second cycle of all~ali carbonate solution~ which supplies the CU2 required to regenerate the laden solution in the first cycle, BesidesJ the second scrubbing of the gas un- ;
der superatmospheric pressure with absorbent flowing in a second cycle causes additional hydrogen sulfide to be removed from the gas so that its desulfurization can also be improved. The alkali carbonate ~;
solutions in the two c~cles may differ in cornposition, For instance, a concentrated solution may be used in the first cycle and a more dilute solution in the second or vice versa.
Within the scope of the invention it is preferable to use an alkali carbonate solution to which sodium tetraborate has been added, preferab-ly in an amount of 1 to 20 mole percent of the total alkali content of the solution, i e., 1 to 20 ~0 of the total alkali which can be lltrated with acid should be added as sodium tetraborate, Na2B4O7.
The advantages afforded by the invention reside in that an alkali : .
carbonate solution which contains hydrosulfide and bicarbonate and has been obtained by the desulfurization of CO2-containing hot gases under superatmospheric pressure can be selectively regenerated by the pro-cess described in that almost all of the combined hydrogen sulfide con-tained in the solution as hydrosulfide but only part of the combined car- ~;bon dioxide present as bicarbonate are released and stripped off. This will be particularly significant if a selective removal of hydrogen sulfide from such gas is desired, i. e., when it is desired to scrub as much hydrogen sulfide as possible and as little CO2 as possible from the gas, as is required in various applications, e. g., in the desulfurization of fuel gas for a gas turbine process in which the mass of the humid gas should be preserved as far as possible before it is burnt. Besides, ., ' ~
' ' ' ` `'' ` ' . .'. ': ' , ,, ,. '.".' ` .' ! '; ' `
' ' ' ~ ` ` ' ' , , ,, '. ' . ' ~ .. '.' ., ' ,'';; ': ' ;, " ' ` ~ , ' ' ' . ',', "'~ i .'; i' _ 5 --the process renders the driven-off hydro~en sulfide available in a higher concentration Moreover, the further processing is facilitated and the investment and supplies required for the further processing are reduced.
Finally, the quantity of polluting gases emitted into the environrnent may be decreased.
The reactions taking place in the alkali carbonate solution are known and may be summarized as follows:
Absorption: 2C 3 2 2 = 2 K~IC03 (lj K2C03 + ~I2S = K~IC03 + KHS (2) The reactions take place in the opposite direction during the re-generation:
2 I~IC03 = I~-~C03 + C02 + H20 (3) KlIS ~ K~IC03 = K2C03 -~ H2S (~) From equation (4) it is apparent th~ bicarbonate is required for the decomposition of the hydrosulfide which has been absorbed. Such alkali carbonate solutions formed by the desulfurization of C02-containing gases contain in more cases more bicarbonate than hydrosulfide. But when the solutions are regenerated in the conventional manner by being pressure-relieved3 heated and stripped with steam, equal percentages of bicarbonate and hydrosulfide are always decomposed and driven off as C2 und H2S In that case, when 90 % of the h~rdrosulfide are to be re-moved by the regeneration, about 90 % of -the bicarbonate must also be decomposed and driven off. The regenerated solution will then contain only 10 %-of the hydrosulfide content but will also contain only 10 % of the bicarbonate content of the unregenerated solution. Such regenerated solution contains almost only alkali carbonate and for this reason can form rnuch bicarbonate when it is used to desulfurize the C02-containing gas under superatmospheric pressure As a result, C02 is scrubbed . ,, . :" :
:: - . , ~. : , . . .
,, ~ . :
:
. . : ~ ,. . , :
, ~ ' , ~: ' ~567~ -off at a high rate so that the desulfurization of the gas is less selective, with all disadvantages listed hereinbefore.
Whereas much bicarbonate is required for a substantial regene-ration of alkali carbonate solutions which contain hydrosulfideJ part of the C02 required for this pur~.ose may be recirculated and re-used in the regenerating stage in the process according to the invention if this is required. For this reason the regenerated solution may have a relatively high bicarbonate content and yet be freed almost completely from hydrosulfide, i.e., subjected to a selective desulfurization.
A preferred feature of the invention is based on the recognition that the purified gas which has been obtained by the desul~urization can be used to special advantage in a combined power plant process. For that purpose the hot desulfurized hot gas which is under superatmos-pheric pressure is used for an ecologically satisfactory production of electric power in a combined power, plant process~ in which energy is recovered from the gas by a pressure relief and/or combustion and is utilized to drive gas turbines and/or to produce steam, -That practice affords the advantage that the C02 left in the de-sulfurized gas greatly increases the efficiency of the combined power plant process because the energy contained in the C02 under super-atmospheric pressure can be utilized.
Another advantage of the invention resides in that the H2S-con-taining exhaust gas formed by the regeneration of the absorbent has a higher H2S content than the exhaust gas formed in a regenerating pro-cess known in the art, As a result, the plants for processing such ex-haust gas may be smaller and less expensive and can operate more economically, The invention will be described more fully hereinafter. The in-vention is shown diagrammatically and by way of example in Fig. 1 to 3.
The drawings show only the most important elements of the process, whereas pumps and valvesJ e, g,, have been omitted for the sake of clearne s s .
... . - ;, ,, .. , . -., :
~: : . ': ~. ;
Example 1 The process according to the inven-tion is shown in its simplest form in Fig. 1. Tha-t embodiment may be used if the alkali carbonate solution which has been formed by the desulfurization of the gas and is to be regenerated contains at least S5% of i-ts al~ali as bicarbonate. This solution is fed through conduit 1 at a temperature of 115 C under a pressure of 20 bars and consists of an aqueous solution of 251 grams KHC03/
liter, 5.0 grams KHS/liter, 62.5 grams K2C03/li-ter and 60 grams Na2B~07.10 H20/liter. The KHC03 content thus amoun-ts to 66% of -the total content of titratable alkali.
The solution is cooled to 90 C in the cooler 2 and is pressure-relieved in the pressure-relief valve and -then enters the regenera-ting column L~, which is under a pressure of 1.5 bars, Almost no gas and water vapor are released by -the pressure rellef, which does not change -the bicarbo-na-te and hydrosulfide conten-ts.
In the regenerator 4, -the solu-tion is heated and decompoesd by -the gases which rise from the lower por-tion and consi~ predominantly of water vapor. As a result, the solution releases C02 and H2S~ In the lower portion, the solution is circulated in conduits 5 and 6 and the indirectly heated heater 7, whereby mainly wa-ter vapor is produced, which rises in the regenerator L~, The regenerated solution runs off through conduit 8 and now contains only 72.3 grams K~C03/liter and 1.1 grams KHS/liter. The borax conten-t has not been changed and the K2C03 conten-t has increased -to 189.9 grams per li-ter. The bicarbona-te content has thus been decreased to 28.8~' of its initial value and the hydrosulfide content ~ 22.0% of its initial value.
It is apparent -that the hydrosulfide content has been decreas-ed more than the bicarbonate content, which means that the regeneration is selective with respect to hydrosulfide or H2S. The solution may now be re-used for a ~5~67~
selective desulfurization of gas. Gases and vapors from the top of regenerator 4 flow through conduit 9, are cooled and partially condensed in the indirect cooler 10. Gases and condensate are separated in the separator 11. The remaining gases are removed through the conduit 12. The condensate flows back to the top of the regenerator 4 through conduit 13.
_ontrol Example When the cooler 2 is not operated, in accordance with the state of the art, the incoming solution at 115C is pressure-relieved in the valve 3 to a KHC03 content of 49% of the total alkali content, and the heater 7 is supplied with steam at the same rate as in the preceding Example, the resulting regénerated solution contains 77.8 grams KHCO3/liter, 1.5 grams KHS/liter, 60 grams borax/liter and 185.7 grams K2CO3/liter.
In this operation, which is in accordance with the prior art, the KHCO3 content has been decreased by 69% and the KHS
content by 70%, which means that the portions driven off are about the same. The regeneration is not selective. When the regenerated solution is used to desulfurize gas, this unselective regeneration results in a higher residual H2S
content of the scrubbed`gas, in the removal of more CO2 by scrubbing, and in a lower concentration of H2S in the H2S-containing exhaust gas.
Example 2 This Example is illustrated in Fig. 2, which represents a preferred embodiment of the invention. The solution to be regenerated is supplied through conduit 1 under a pressure of 20 bars and a temperature of 140C and per liter contains 205.4 grams potassium hydrogen carbonate,
5.7 grams potassium hydrogen sulfide, 60.0 grams borax and 93.6 grams potassium carbonate. The solution is cooled to 92C in the indirect cooler 2 and is pressure-relieved in the valve 3. Virtually no CO2 and no H2S are released by the .,~
~ ,...... . .
5~7~
-pressure relief. After the pressure relief -the solution enters the C02 satura-tor 4, which is under a pressure of 1.2 bars and in which the solu-tion at 85C absorbs C02 from the gas which rises through the gas transfer pipe 6, and consists - 8a -predominantly of C02. As a result, the KHC03 content increases to 235.8 grams/liter and the K2C03 content is decreased. 62% of the total alkali are now present as KHC03. The remaining gas is conducted through a conduit 13 and a compressor 14 into the upper portion of a regenerator 12, which is under a pressure of 1.5 bars-. The solution to be regenerated, which is saturat-ed with C02, flows through conduit 11 into the lower por-tion of the regenerator 12 and is stripped there at its boiling temperature by the gas which rises from the bottom and consists mainly of water vapor. That gas is produced in that the regenerated solution which runs off through conduit 16 is circulated in the conduit 17 and heated by an indirectly heated reheater 15.
Under the inlet 11 for -the solution, part of the gas which rises in the lower portion of the regenera-tor and consists predominantly of water vapor is withdrawn through conduit 18, which incorporates a con-trol valve 19, and the wi-thdrawn gas is cooled in an indlrect cooler 20.
The resulting aqueous condensate is separeted in a separator 21 and is recycled to the regenerator 12 through a conduit 22. The gas leaving the separator 21 consists mainly of C2 and also contains H2S and is conducted through conduit 7 to the H2S scru~ber 5~ A partial stream of the regenerated solution is fed through conduit 8 and the indirect cooler 9 to the scrubber 5, in which a major part of the H2S con-tent of the gas is scrubbed off at 30 C and under a pressure of 1.3 bars so tha-t the gas entering the C02 saturator 4 through the gas transfer pipe 6 contains only 200 ppm H2S. In the solution running off through -the con-duit 10 the H2S which has been removed is contained as KHS. That solution is fed to the top of the rege~erator 12, in which the H2S is driven off and exits together with C~2 through condui-t 2~.
. :: ;... . , ~
- 1 ~ t5~
In addition -to borax, the regenerated solution running off through conduit 16 contains only 0.58 grams KHS/liter and 68.5 grams KHC0~/liter but 193.0 grams K2C0~/liter.
It is apparent that relative to be composition of the solution in conduit 1, 90% of the KHS conten-t have been decomposed and driven off as H2S but only 67% of the KHC03 content. The solution which has been selectively rege-nerated according to the inven-tion can be used at 140 C
in an absorber under a pressure of 20 bars to desulfurize a gas containing 0.3% H2S by volume to a residual conten-t of 200 ppm, which means that more than 93% of the H2S con-tent have been scrubbed off, whereas only 20 to 30 % of the C2 also contained in the gas are also scrubbed off.
xample 3 Another preferred embodiment of the invention is diagrammatically shown in Fig. 3. A solution which contains 235.8 grams KHC03/liter (= 62% of the -to-tal alkali content), 21,6 grams KHS/liter, 75 grams borax/liter and 51.9 grams K2C03/liter is supplied at a temperature of 120 C and under a pressure of 21 bars through conduit 1. The solution is cooled to g1 C in the indirect cooler 2, and pressure-relieved in valve 3 to the pressure of 1.6 bars which pre-vails in the stripper 4, to which the pressure-relieved solution is supplied. A gas consisting predominantly of water vapor is conducted from the stripper 6 and enters the stripper 4 through the gas transfer pipe 5. The solu-tion is s-tripped at its boiling temperature and after being stripped contains 83.7 grams KHC0~/liter, 4.8 grams KHS/
liter, 75.0 grams borax/liter, and 173.1 grams K2C03/liter.
The solution then flows through conduit 7 into -the indirect cooler 8 and is cooled therein to 85 C and subsequently enters the C02 saturator 9, which is under a pressure of 1.4 bars. A gas consisting predominantly of C02 enters the C2 saturator through a gas transfer pipe 10 and is used to treat the solution at 80 to 90 C. The treated solution - - ....... , ................ . . . :, . .:, .. .
.. :, , , ,' ~ ' ' '~ i ' 3L~S67J~Eii contains 247.2 grams KHC03/liter, 4.4 grams KHS/liter, 75.0 grams borax/liter and 60.6 grams K2C03tliter and is fed in conduit 11 to the second stripper 6. In the latter the solution is regenerated at its boiling tem_ perature under a pressure of 1.6 to 1.7 bars by being stripped with a gas which consists mainly of water vapor and is formed in that the solution circulated in conduits 13 and 14 is indirectly heated in the reheater 12. The regenerated solu-tion running off through conduit 12 con-tains only 0.7 grams KHS/liter as well as 87.5 grams KHC03/liter, 74.8 grams borax/liter and 174.4 grams K2C03/liter. It is apparent that a total of 96.8% of the hydrosulfide KHS and 62.9% of the bicarbonate KHC03 fed through conduit 1 have been decomposed and driven off and that the regeneration has been highly selective for KHS. The regenerated solution can be used to desulfurize gases and need not be cooled before.
A partial stream of the gases rising in the stripper 4 is withdrawn through conduit 16, which in_ corporates a control valve 15, and is cooled to 25 C
in the cooler 17. The resulting aqueous condensate is ~thdrawn from the separator 1-8.and is recycled through conduit 19 to the stripper 4. The cooled gas is fed through the conduit 20 to the H2S scrubber 21 and is scrubbed there under a pressure of 1.4 to 1.5 bars with a partial stream of the alkali carbona-te solution. That partial stream is provided in that part of the solution in condui-t 7 is branched o~f through conduit 22 and cooled in the indirect cooler 23 to 26 C. In the scrubber 21, the H2S is removed from the gas at 26 to 30 C to a residual content of 280 ppm. The scrubbed gas, which consists predominantly of C02, seryes to treat the solution in the C02 saturator 9.
The non-absorbed gas is withdrawn through conduit 24 and is fed by a compressor 25 into the stripper 4O
~. '''',. ' ;'., .
., . ~ - . :
, - , . : " , r : .. :
- 12 ~ ~ 6~6 The high-H2S solution running off from the scrubber 21 is fed through conduit 26 to the top of the stripper 4 for driving off the H2S. The exhaust gas contains water vapor and is withdrawn through conduit 27.
It contains all hydrogen sulfide tha-t has been driven off in the regenera-ting stage and also the C02 which has been driven off.
In the manner which has been described, the solution can be regenerated so that a regenerated solu-tion can be obtained which can be used to desulfurize to a residual content of 200 to 300 ppm H2S a gas which contains 1.0% H2S by volume and 10.2% C02 by volume at 120 C under a pressure of 21 bars, which means tha-t 97%
of the H2S are scrubbed off. Only 25% of the C02 are scrubbed off at the same time, .
, ..
, . :. -- , .
, .... . . .
:, . . .,, - . .
.
~ ,...... . .
5~7~
-pressure relief. After the pressure relief -the solution enters the C02 satura-tor 4, which is under a pressure of 1.2 bars and in which the solu-tion at 85C absorbs C02 from the gas which rises through the gas transfer pipe 6, and consists - 8a -predominantly of C02. As a result, the KHC03 content increases to 235.8 grams/liter and the K2C03 content is decreased. 62% of the total alkali are now present as KHC03. The remaining gas is conducted through a conduit 13 and a compressor 14 into the upper portion of a regenerator 12, which is under a pressure of 1.5 bars-. The solution to be regenerated, which is saturat-ed with C02, flows through conduit 11 into the lower por-tion of the regenerator 12 and is stripped there at its boiling temperature by the gas which rises from the bottom and consists mainly of water vapor. That gas is produced in that the regenerated solution which runs off through conduit 16 is circulated in the conduit 17 and heated by an indirectly heated reheater 15.
Under the inlet 11 for -the solution, part of the gas which rises in the lower portion of the regenera-tor and consists predominantly of water vapor is withdrawn through conduit 18, which incorporates a con-trol valve 19, and the wi-thdrawn gas is cooled in an indlrect cooler 20.
The resulting aqueous condensate is separeted in a separator 21 and is recycled to the regenerator 12 through a conduit 22. The gas leaving the separator 21 consists mainly of C2 and also contains H2S and is conducted through conduit 7 to the H2S scru~ber 5~ A partial stream of the regenerated solution is fed through conduit 8 and the indirect cooler 9 to the scrubber 5, in which a major part of the H2S con-tent of the gas is scrubbed off at 30 C and under a pressure of 1.3 bars so tha-t the gas entering the C02 saturator 4 through the gas transfer pipe 6 contains only 200 ppm H2S. In the solution running off through -the con-duit 10 the H2S which has been removed is contained as KHS. That solution is fed to the top of the rege~erator 12, in which the H2S is driven off and exits together with C~2 through condui-t 2~.
. :: ;... . , ~
- 1 ~ t5~
In addition -to borax, the regenerated solution running off through conduit 16 contains only 0.58 grams KHS/liter and 68.5 grams KHC0~/liter but 193.0 grams K2C0~/liter.
It is apparent that relative to be composition of the solution in conduit 1, 90% of the KHS conten-t have been decomposed and driven off as H2S but only 67% of the KHC03 content. The solution which has been selectively rege-nerated according to the inven-tion can be used at 140 C
in an absorber under a pressure of 20 bars to desulfurize a gas containing 0.3% H2S by volume to a residual conten-t of 200 ppm, which means that more than 93% of the H2S con-tent have been scrubbed off, whereas only 20 to 30 % of the C2 also contained in the gas are also scrubbed off.
xample 3 Another preferred embodiment of the invention is diagrammatically shown in Fig. 3. A solution which contains 235.8 grams KHC03/liter (= 62% of the -to-tal alkali content), 21,6 grams KHS/liter, 75 grams borax/liter and 51.9 grams K2C03/liter is supplied at a temperature of 120 C and under a pressure of 21 bars through conduit 1. The solution is cooled to g1 C in the indirect cooler 2, and pressure-relieved in valve 3 to the pressure of 1.6 bars which pre-vails in the stripper 4, to which the pressure-relieved solution is supplied. A gas consisting predominantly of water vapor is conducted from the stripper 6 and enters the stripper 4 through the gas transfer pipe 5. The solu-tion is s-tripped at its boiling temperature and after being stripped contains 83.7 grams KHC0~/liter, 4.8 grams KHS/
liter, 75.0 grams borax/liter, and 173.1 grams K2C03/liter.
The solution then flows through conduit 7 into -the indirect cooler 8 and is cooled therein to 85 C and subsequently enters the C02 saturator 9, which is under a pressure of 1.4 bars. A gas consisting predominantly of C02 enters the C2 saturator through a gas transfer pipe 10 and is used to treat the solution at 80 to 90 C. The treated solution - - ....... , ................ . . . :, . .:, .. .
.. :, , , ,' ~ ' ' '~ i ' 3L~S67J~Eii contains 247.2 grams KHC03/liter, 4.4 grams KHS/liter, 75.0 grams borax/liter and 60.6 grams K2C03tliter and is fed in conduit 11 to the second stripper 6. In the latter the solution is regenerated at its boiling tem_ perature under a pressure of 1.6 to 1.7 bars by being stripped with a gas which consists mainly of water vapor and is formed in that the solution circulated in conduits 13 and 14 is indirectly heated in the reheater 12. The regenerated solu-tion running off through conduit 12 con-tains only 0.7 grams KHS/liter as well as 87.5 grams KHC03/liter, 74.8 grams borax/liter and 174.4 grams K2C03/liter. It is apparent that a total of 96.8% of the hydrosulfide KHS and 62.9% of the bicarbonate KHC03 fed through conduit 1 have been decomposed and driven off and that the regeneration has been highly selective for KHS. The regenerated solution can be used to desulfurize gases and need not be cooled before.
A partial stream of the gases rising in the stripper 4 is withdrawn through conduit 16, which in_ corporates a control valve 15, and is cooled to 25 C
in the cooler 17. The resulting aqueous condensate is ~thdrawn from the separator 1-8.and is recycled through conduit 19 to the stripper 4. The cooled gas is fed through the conduit 20 to the H2S scrubber 21 and is scrubbed there under a pressure of 1.4 to 1.5 bars with a partial stream of the alkali carbona-te solution. That partial stream is provided in that part of the solution in condui-t 7 is branched o~f through conduit 22 and cooled in the indirect cooler 23 to 26 C. In the scrubber 21, the H2S is removed from the gas at 26 to 30 C to a residual content of 280 ppm. The scrubbed gas, which consists predominantly of C02, seryes to treat the solution in the C02 saturator 9.
The non-absorbed gas is withdrawn through conduit 24 and is fed by a compressor 25 into the stripper 4O
~. '''',. ' ;'., .
., . ~ - . :
, - , . : " , r : .. :
- 12 ~ ~ 6~6 The high-H2S solution running off from the scrubber 21 is fed through conduit 26 to the top of the stripper 4 for driving off the H2S. The exhaust gas contains water vapor and is withdrawn through conduit 27.
It contains all hydrogen sulfide tha-t has been driven off in the regenera-ting stage and also the C02 which has been driven off.
In the manner which has been described, the solution can be regenerated so that a regenerated solu-tion can be obtained which can be used to desulfurize to a residual content of 200 to 300 ppm H2S a gas which contains 1.0% H2S by volume and 10.2% C02 by volume at 120 C under a pressure of 21 bars, which means tha-t 97%
of the H2S are scrubbed off. Only 25% of the C02 are scrubbed off at the same time, .
, ..
, . :. -- , .
, .... . . .
:, . . .,, - . .
.
Claims (14)
1. In a process for selective removal of H2S from hot gases containing H2S and CO2, wherein the gases are contacted at superatmospheric pressure with an alkali metal carbonate solution and the resultant alkali metal carbonate solution which contains hydro-sulfides and bicarbonates is regenerated by pressure relieving the same, the so pressure-relieved solution is thereafter stripped and re-used for selective removal of H2S from hot gases containing the same and carbon dioxide, the improvement which comprises adjusting the alkali metal bicarbonate content of the pressure-relieved solution to be stripped so that it is at least 55% of the total alkali content of the solution and re-using said solution, following stripping, without cooling the same.
2. A process according to claim 1, wherein the solution before being pressure-relieved is cooled below its boiling temperature at the pressure relief end pressure to avoid a decrease of the alkali bicarbonate content of the laden solution before the stripping thereof.
3. A process according to claim 2, wherein the solution is cooled to 80-95° C.
4. A process according to claim 2,wherein the cooled solution is treated under normal or superatmospheric pressure at 50 to 100°C with a gas which consists predomi-nantly of CO2 and the CO2-saturated solution is then stripped at its boiling temperature with a gas which consists predominantly of water vapor.
5. A process according to claim 4, wherein the stripped solution is cooled once more and is stripped at 50 to 100° C with a gas which consists predominantly of CO2 and the solution is subsequently stripped with a gas which consists predominantly of water vapor.
6. A process according to claim 5, wherein the multi-stage treatment of the solution is repeated several times.
7. A process according to claim l, wherein the solution to be regenerated is stripped at its boiling tempera-ture with a gas which consists predominantly of water vapor and has been obtained by an indirect heating of the stripped solution.
8. A process according to claim 1, wherein the exhaust gas formed by the stripping is cooled, the condensate is separated, the cooled gas is scrubbed with a partial stream of the alkali carbonate solution at a temperature below 60°C, and all or part of the scrubbed gas consisting predominantly of CO2 is used to saturate the cooled solution which is to be regenerated.
9. A process according to claim 8, wherein the cooled gas is scrubbed with a partial stream of the alkali carbonate solution at a temperature of 25 to 35° C.
10. A process according to claim 8, wherein all or part of the scrubbed gas which consists predominantly of CO2 is first used to saturate the cooled solution to be rege-nerated and is subsequently used to strip the heated partial stream of the alkali carbonate solution formed by the scrub-bing of the cooled exhaust gas formed by the stripping step and is then discharged and the stripped partial stream is returned to the main cycle of the alkali carbonate solution.
11. A process according to claim 10, wherein a partial stream of the alkali carbonate solution is withdrawn from the scrubber for scrubbing the cooled exhaust gas formed by the stripping step and said partial stream is stripped at its boiling temperature with a gas consisting predominantly of water vapor to remove a predominant portion of hydrogen sulfide from said partial stream, the resulting gas, which contains hydrogen sulfide, is removed and the stripped partial stream is returned to the main cycle of the alkali carbonate solution.
12. A process according to claims or 5, wherein the gas consisting predominantly of CO2 is formed by the regene-ration of a second cycle of alkali carbonate solution used to scrub the desulfurized, CO2-containing hot gas under superat-mospheric pressure.
13. A process according to claim 1, wherein the alkali carbonate solution contains sodium tetraborate in an amount of 1 to 20 mole percent of its total alkali content.
14. A process according to claim 1, wherein the desulfurized hot gas under superatmospheric pressure is used for an ecologically satisfactory production of electric power in a combined power plant process in which energy is recovered from the gas by a pressure relief and/or combustion of the gas and is utilized to drive gas turbines and/or to produce steam.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA306,850A CA1105676A (en) | 1978-07-05 | 1978-07-05 | Process of regenerating alkali carbonate solutions formed by the desulfurization of hot gases |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA306,850A CA1105676A (en) | 1978-07-05 | 1978-07-05 | Process of regenerating alkali carbonate solutions formed by the desulfurization of hot gases |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1105676A true CA1105676A (en) | 1981-07-28 |
Family
ID=4111847
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA306,850A Expired CA1105676A (en) | 1978-07-05 | 1978-07-05 | Process of regenerating alkali carbonate solutions formed by the desulfurization of hot gases |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1105676A (en) |
-
1978
- 1978-07-05 CA CA306,850A patent/CA1105676A/en not_active Expired
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