Sulfur recovery further improved
Over 100 years ago, the original Claus process for the production of elemental sulfur from H 2S and air was introduced to the industry. In 1!", a ma#or improvement was made $y I.%. &ar$en through the intr introd oduc ucti tion on of the the free free'( '(am ame e ther therma mall stag stage. e. )his )his impr improv oved ed or modi*ed Claus +rocess ena$led the application of the original Claus process on a large industrial scale, maing it the ma#or process for producing sulfur in the world. )he recovery performance of the modi*ed Claus process is unf unfort ortunat unatel ely y
limi limite ted d
to -
'
", ",
due due
to ther thermo mody dyn namic amic
e/uili$rium constraints. In 1"", Comprimo%astec announced the S+3C45S6 process, which increased the capa$ility of the Claus e7ciency to typically ".8 ' .! $y introduction of selective o9idation technology to overcome the Claus e/uili$rium limitations. &ourteen years after the successful introduction of S+3C45S6, more than 110 units are under license. 5ll units in operation have met or e9ceeded the guaranteed recovery e7ciency. e7ciency. S+3C45S6 S+3C45S6 is now now a well well prov proven en tech techno nolo logy gy.. Howe Howeve ver, r, to cope cope with with futu future re re/uirements, the sulfur recovery e7ciency needs to $e increased to .8 or higher. 5 detailed review of the process has $een made, and a signi*cant performance increase in the process appears to $e possi$le if the SO2 in the Claus process gas to the selective o9idation reactor could $e converted to S or H2S $y hydrogenation, using the hydrogen present in the Claus process gas. In addition, the control of the selective o9idation reactor has $een improved, to o$tain a higher sulfur yield in the reactor. 5n improved improved S+3C S+3C45S6 45S6 process process has $een introduc introduced, ed, called 3OC45S6. )he improvement is characteri:ed $y the catalytic
reduction of SO2 in Claus process gas, $y hydrogenation to sulfur vapor and hydrogen sul*de prior to selective o9idation. One con*guration, with a small layer of hydrogenation catalyst in the $ottom of the second Claus reactor, is very attractive. Conse/uently a separate reactor stage for hydrogenation is not re/uired with this process con*guration. 3eduction of SO2 increases the performance of the *nal selective o9idation stage, resulting in an overall recovery e7ciency of .8 or $etter with only three catalytic stages. 3OC45S6 is a fully continuous Claus type sulfur recovery process, $ased on $ul sulfur production $y a Claus section, selective hydrogenation of SO 2 and selective o9idation of H 2S. The Claus process
5 typical Claus plant (ow sheet consists of a thermal stage followed $y two or three catalytic stages. In the thermal stage, which consists of a $urner with a com$ustion cham$er and a waste heat $oiler, the H2S present in the Claus feed gas is com$usted to sulfur according to; H2S < 0.8 O2 = 1n Sn < H2O
>1?
+art of the sulfur reacts with the formed H 2O to form H 2S and SO2 according to the reversed Claus e/uili$rium reaction; !n Sn < 2 H2O
2 H2S < SO2
↔
>2?
)he downstream catalytic reactor stages further increase the conversion to sulfur. 5 catalytic Claus stage consists of a reheater, catalytic reactor and sulfur condenser. Condensation of sulfur after each Claus reactor maes further production of sulfur possi$le in the ne9t catalytic stage >&igure 1?. Claus plant limitations
)he theoretical sulfur recovery e7ciency may $e calculated as @ and " for a two and three stage Claus plant respectively. In
reality,
the
long
term
average
recovery
e9perienced
is
appro9imately A ' -. 5part from lower values caused $y deactivated Claus catalyst, the $asic Claus process has three ma#or limitations that hinder the process for meeting higher sulfur recoveries; )hermodynamically limited conversion to sulfur. Increase in water vapor content and a simultaneous decrease of H 2S and SO2 concentrations in the gas. Sensitive air to acid gas control.
Because the Claus reaction is thermodynamically limited, the conversion to sulfur is not complete, and distinct /uantities of H 2S and SO2 remain in the process gas. ater produced $y the Claus reaction increases proportionally with the overall conversion of H 2S. )his process water hinders the conversion to sulfur and limits the total sulfur recovery. In all modi*ed Claus processes, the principal control varia$le is the air to acid gas ratio. )he plant e7ciency is /uite dependent on the two components H 2S and SO2 $eing in the correct ratio for the reaction. )he optimum conversion to sulfur will occur at a H 2S;SO2 ratio of 2;1 . It is clear that, notwithstanding the fact that modern Claus plants are e/uipped with H2SSO2 tail gas analy:ers to control the air demand, the process control is sensitive and easily aDects the sulfur recovery e7ciency. The concept
)he concept applies two main principles to overcome the limitations of the Claus process;
Selective o9idation of H 2S directly into elemental sulfur.
Eore (e9i$le air to acid gas control.
&or selective o9idation of H 2S to sulfur, new catalysts have $een developed $ecause availa$le commercial o9idation catalysts are all sensitive to water and esta$lish the Claus reaction. )he developed catalysts possess some uni/ue properties;
O9idation of more than "8 of H 2S to elemental sulfur in the presence of e9cess airF further o9idation to SO 2 is minor.
Got sensitive to water.
)he introduction of selective o9idation H2S with these catalysts in the Claus process contri$utes to a signi*cant increase in sulfur recovery. Eoreover, the catalysts allow less stringent selective o9idation air control, as the last reactor containing the selective o9idation catalyst can now $e operated with e9cess air. The Process
)he process uses simple conventional Claus plant e/uipment and can $e applied in either e9isting or new plants to increase overall sulfur recovery at low costs. It consists of a thermal stage followed $y two or three catalytic reactor stages >&igure 2?. 3eactors 1 and 2 are loaded with conventional Claus catalyst, while reactor ! is *lled with the selective o9idation catalyst, often called the S+3C45S6 catalyst. In the process, the e9act H 2S;SO2 ratio control of 2;1 has $een a$andoned. )he front of the Claus plant is operated with e9cess H 2S in such a way that the H 2S concentration in the tail gas leaving the last Claus reactor will contain 0." ' 1.2 vol H S. )he e9cess H S results in the additional advantage that the Claus catalyst is re#uvenated continuously, eeping the Claus catalyst very active and the sulfate content low. )he com$ustion air to the unit is split into two streams. )he ma#or part is directed to the H2S $urner of the thermal stage, the remainder is added to the Claus process tail gas.
)he presence of the e9cess H 2S suppresses the SO 2 concentration in the gas. )he tail gas leaving the second or third Claus reactor stage is reheated, mi9ed with e9cess air and passed to the selective o9idation reactor. Here, the non e/uili$rium reaction taes place; H2S + 0.5 O 2
!n Sn + H2O
→
>1? 5s the selective o9idation catalyst does not promote the reverse Claus reaction of the formed sulfur vapor with water vapor, and only minor amounts of SO 2 are formed, the selectivity to sulfur is high. )he catalyst only o9idi:es H2SF other components in the tail gas such as H 2, CO, SO2, COS and CS 2 are not converted. )herefore, COS and CS2 should $e decreased to as low a level as possi$le $efore entering the reactor. )his can $e achieved $y applying the appropriate catalyst and temperature level in the *rst Claus reactor. )he selective o9idation reactor is followed $y a sulfur condenser to remove the sulfur formed in this step. In order to eep sulfur vapor losses low, the last sulfur condenser should run at an outlet temperature of 128 C. ith one thermal stage and two catalytic Claus stages upstream of the selective o9idation stage, the overall recovery is typically .2 for a rich Claus feed gas. ith three catalytic Claus stages upstream, an overall recovery up to .A is achieva$le. S"P#$C%&"S ' catalyst
)he *rst generation S+3C45S6 catalyst has $een developed $ased on a carrier of alpha alumina > α'5I2O!?, which has a very low Claus activity. ery *nely divided iron o9ide >&e 2O!? is coated on the α'5I2O! surface. 5 small /uantity of chromium o9ide >Cr 2O!? is used
to sta$ili:e the iron o9ide. )he advantage of this catalyst is that the thermal sta$ility is e9cellent.
)his catalyst has a very low surface area of appro9imately 10 m 2g, which results in a relatively high JindlingK temperature >reactor inlet temperature? of 2A0 ' 280 C. 5s alumina supports with higher surface areas usually contain gamma alumina > γ '5I2O!?, a material that is very Claus active, other support materials such as silica carriers were investigated. Silicas with the correct pore structure also e9hi$it a very low Claus activity. 5 second generation catalyst, $ased on a silica carrier, was then developed. It transpired that this catalyst, with a surface area of appro9imately 0 m 2g, maintained a high catalytic activity without the re/uirement of an additional /uantity of Cr 2O!. )he reactor inlet temperature
could
$e
lowered
to
200
'
220C,
with
the
corresponding lower utility consumption. )his much lower inlet temperature meant that the average catalyst $ed temperature could $e lowered, which had a very favora$le in(uence on sulfur yield. )he sulfur yield is de*ned as the percentage of incoming H 2S that is converted to elemental sulfur. 5 third generation catalyst is promoted with Ga 2O. )his promoter decreases the SO 2 formation, especially at the higher temperatures in the $ottom part of the catalyst $ed. 5 fourth generation catalyst contains Ln as a promoter to further decrease the SO2 formation at the higher temperatures. )his results in a sulfur yield that is higher, and less sensitive to higher reactor temperatures. Commercial production of this catalyst is now $eing considered, to increase sulfur yields of commercial reactors, this in view of the 3OC45S6 option. Plant limitations
)he conventional S+3C45S6 process can attain recovery e7ciencies up to the range of ".8 ' .A,K depending on the acid gas feed /uality and num$er of catalytic stages.
In the selective o9idation reactor >S+3C45S6 reactor?, H 2S is o9idi:ed to sulfur vapor according to; H2S + 0.52O
!n Sn + H2O
>1? Only H2S is o9idi:ed. )he SO 2 passes through the catalyst unaDected. )his means that the SO 2 concentration in the Claus tail gas >feed to the S+3C45S6 reactor? should $e low, to limit recovery losses. )his can $e achieved $y an increase of H 2S content in the Claus tail gas to appro9imately 1 vol, as is done in the S+3C45S6 process. SO2 is suppressed according to the Claus e/uili$rium; 2 H2S + SO2
(!n Sn + 2 H2O
>!?
$y applying an e9cess of H 2S. Eore than 1 vol of H 2S will not increase the sulfur recovery further, $ecause it will result in too high a $ottom temperature in the reactor, and conse/uently more SO 2 formation and a lower sulfur yield of the incoming H 2S in this reactor. )he SO2 concentration in the Claus tail gas under these conditions still represents signi*cant recovery losses. &or instance, at 0."2 vol H2S inlet concentration, the SO 2 content in the Claus tail gas is still 0.0" vol, corresponding with 0.! recovery loss. The #"$OC%&"S' concept
)o meet the continuously increasing re/uirements for sulfur recovery e7ciency >S3?, a new process should $e capa$le of meeting a S3 ≥.8, which complies with the highest uropean standards >%ermany, )5'4uft?. It is e9pected that most of the uropean countries will move towards this recovery level in the near future. )his means that e9pected S3s should $e $etween .@ ' .-, in order to $e a$le to guarantee .8.
&or o$vious economic reasons, an adaptation of the modi*ed Claus process, $eing the world standard in sulfur recovery, would $e the preferred $asis for a new process. 5 minimum amount of catalytic stages should $e involved, prefera$ly not more than three, as in the standard Claus process. ith two catalytic stages, high recovery levels cannot $e achieved. &our catalytic stages are less attractive with respect to investment costs and increased pressure drop. Of course, low operating and investment costs are of paramount importance for a successful new process. )he desira$le process re/uirements are summari:ed in )a$le 1. )he S+3C45S6 process would $e an e9cellent starting point to meet these process re/uirements. )he sulfur recovery e7ciency limitations of the process should, however, $e overcome to meet the re/uired recovery e7ciency of .8 ≤ S3 ≤ .<. 5 convenient way to decrease SO 2 in the Claus tail gas would $e $y reduction to S >and some H 2S? upstream of the selective H 2S o9idation reactor. )his process step should $e simple and cost eDective. Conversion of SO 2 in the Claus tail gas, a decrease in H 2S content in the e9it from 3'2 Claus from 1.0 down to 0."0 vol, and a simultaneous increase in yield to sulfur from "8 to 0 in the selective o9idation reactor would together result in a very large increase of e7ciency up to .8. )he concept of reduction of SO 2 prior to selective o9idation of H 2S is the $asis of the 3OC45S6 process. The process
)he new 3OC45S6 process, $ased on Claus chemistry, hydrogenation of SO 2 and selective o9idation of H 2S, is an acronym for 9tremely pgraded 3eduction O9idation C45SK.
&igure ! shows an overview of the new process. )he front end is composed of the well nown Claus process, consisting of a thermal stage and a *rst catalytic Claus stage. In the second catalytic reactor, a top layer of normal Claus catalyst is positioned a$ove a layer of another type of catalyst with hydrogenating properties for conversion of SO 2 to S and H 2S. )he last stage is the well nown selective o9idation stage as applied in the S+3C45S6 process. In the 3OC45S6 process, sulfur is produced along three diDerent pathways. Claus reaction
In the Claus section, which also includes the Claus catalyst in the top part of the last Claus reactor, sulfur is made according to the well nown Claus reaction; 2 H2S + SO2
>!?
(!n Sn + 2 H2O
Catalytic reduction of SO 2
SO2 is reduced to sulfur vapor and H 2S in the reduction stage, where H2 and CO have already $een present in the Claus process gas reacts with SO2 according to; SO2 + 2 H2 SO2 + ( H2 SO2 + 2 CO
>A?
!n Sn + 2 H2O H2S + 2 H2O !n Sn + 2 CO 2
>8? >@?
Selective o9idation of H 2S &inally, H2S is o9idi:ed with a high sulfur yield, avoiding the limitations of the Claus e/uili$rium, to sulfur vapor in the selective o9idation stage according to;
H2S + 0.5 O 2
!n Sn + H2O
>1?
In this way, a much improved sulfur recovery e7ciency is made possi$le $y;
Bul removal of sulfur in the Claus section.
Selective hydrogenation of the remaining SO 2.
O9idation of H2S to sulfur.
Catalyst
)he catalytic reduction of SO2 is performed over a sulfur resistant catalyst. Gormal Claus process gas contains su7cient H 2 and CO for the catalytic reduction of small amounts of SO 2. 4oss of sulfur and production of SO 2COS may occur as a result of the following side reactions during the catalytic hydrogenation; reverse Claus reaction; (!n Sn + 2 H2O
>2?
2 H2S + SO2
reduction of sulfur vapour in the feed; !n Sn + H2
>-?
H2S
formation of COS; !n Sn + CO H2S + CO SO2 + (CO
COS
>"? >?
COS + H2 COS + 2CO2
>10?
)he choice of hydrogenation catalyst and process conditions should $e such that the a$ove reactions are minimi:ed. )he type of catalyst and optimum operating conditions have $een determined in la$oratory tests, for a process con*guration with a thin layer of reduction catalyst in the $ottom section of the second catalytic Claus reactor, preceding the selective o9idation stage. )he reduction capa$ilities under proper operating conditions, using an optimi:ed catalyst, are shown in )a$le 2.
)he la$oratory data in )a$le 2 show that $ul SO 2 conversion is achieved. ven with a high content of sulfur vapor and CO in the process gas, a high selectivity towards sulfur is measured. Go reduction of sulfur vapor in the feed occurs. )etter performance of selective o*idation reactor
It has $een e9perienced and con*rmed $y considera$le la$oratory test data, with diDerent types of selective o9idation catalyst, that the yield to sulfur in the o9idation reactor is a function of the catalyst $ed $ottom temperature. In case of too low a temperature, the last 8 ' 10 of the incoming H 2S is not converted to sulfur and a larger
slippage
of
H2S
is
e9perienced.
)oo
high
a
$ottom
temperature will result in more SO 2 formation $y catalytic and gas phase o9idation of H2S and sulfur vapor. &or the second generation type of selective o9idation catalyst, the catalyst $ed $ottom temperature should $e maintained $elow appro9imately 2@0 C for a ma9imum sulfur yield. 5 typical sulfur yield measured in plants is "@ ' 1. Gewly developed selective o9idation catalysts reach A @ yields in a la$oratory reactor. )his indicates that further yield improvements for large commercial reactors may $e achieva$le. 5 selective o9idation reactor inlet temperature of 208 ' 218 C is normally used. )herefore, the accepta$le delta') in the selective o9idation reactor is limited to 80 C for ma9imum yield. )he e9othermic heat of reaction from o9idation of H 2S to sulfur vapor >and some SO 2? results in a temperature rise of appro9imately -0 C per 1 vol H 2S. )herefore, the H 2S inlet concentration is limited to appro9imately 0.- vol H 2S to maintain a ma9imum yield. )his corresponds to an H 2S outlet concentration from the second Claus reactor stage of appro9imately 0." vol, $ecause of dilution $y the inline $urner reheater (ue gas andor selective o9idation air upstream of the o9idation reactor.
&or this reason, the 3OC45S6 process con*guration, with a decreased H2S content of 0." vol in the outlet of the last catalytic Claus stage, *ts $etter in the optimum operating window of the selective o9idation reactor. )his will result in a higher sulfur yield. &igure A shows the temperature pro*le F through the catalyst $ed at a high inlet temperature >curve 1? and at a low inlet temperature >curve 2?. Curve 1 gives too much SO 2 production in the reactor $ottom, whereas curve 2 gives too much H 2S slippage. 5n optimum inlet temperature is important for a high sulfur yield. )he third development is an improved control system of the selective o9idation stage. )he (ow of selective o9idation air is determined $y the plant capacity, the H 2S content in the process gas to the selective o9idation reactor, and the o9ygen concentration in the process gas from the selective o9idation stage. )he correlation $etween these varia$les; Selective O9idation 5ir &low M f >plant capacity, H 2S in, O2 out? provides a tool to maintain an optimum o9ygen concentration in the reactor outlet process gas. )his will also result in a higher sulfur yield. 5 comparison of sulfur recovery e7ciencies of;
5 conventional modi*ed Claus unit with three catalytic reactor stages.
5 S+3C45S6 unit with two Claus reactors and one selective o9idation reactor.
5n 3OC45S6 unit with three reactors.
is presented in &igure 8. ery high recovery e7ciencies of .8 can $e reached with 3OC45S6.
5 further improvement of the S3 may $e achieved with a so'called Kdeep coolerK. )his vertically positioned heat e9changer is cooled with am$ient air and is positioned downstream of the *nal sulfur condenser.
)he
deep
cooler
cools
the
process
gas to
the
solidi*cation temperature of sulfur, which is 11A.8 C. )hus, the sulfur vapor losses are now reduced to very low values. )he deep cooler, which operates continuously without $eing plugged with solid sulfur, may add another 0.1 ' 0.2 recovery e7ciency. "tility consumption
)he S+3C45S6 and 3OC45S6 processes have low utility consumptions. In )a$le !, the utility consumptions have $een compared with the standard Claus process on the $asis of a 100 tpd three stage Claus plant, with a feed gas containing 0 H 2S and catalytic incineration of the oDgases. Conclusion
)he
3OC45S6
conventional
Claus
process increases sulfur process
signi*cantly
recovery of
and
involves
the the
introduction of selective reduction of SO2 followed $y selective o9idation of H2S directly into sulfur in the last reactor stage. Sulfur recoveries of ' .- are achieva$le, depending on feed gas composition. 3OC45S6 is a fundamental improvement of S+3C45S6, maing the process suita$le for the 21sK century >patents are *led?. )he calculated recovery e7ciencies are shown in &igure @. )he recovery
e7ciencies
are
$ased
on
high
COSCS 2 hydrolysis
e7ciencies >8? in the *rst Claus reactor and a tail gas
temperature of 128 C. 5lready *ve commercial units are $eing designed.
igure , The Claus process
igure 2 - The S"P#$C%&"S' process
igure ( - The #"$OC%&"S' process
igure - Temperature pro/les in selective o*idation reactor
igure 5 - Performance of sulfur plants
igure . #"$OC%&"S' capa1ilities
Ta1le - Process reuirements of enhanced S$"
Ta1le 2 - $eduction of SO 2
Ta1le ( ,
