Advanced Sulfur Control Concepts in Hot-Gas Desulfurization Technology

Transcription

1 t DE-AC2Z -94MC32-- Advanced Sulfur Control Concepts in Hot-Gas Desulfurization Technology Quarterly Report January - March 3,998 By: Douglas P Harrison Work Perfored Under Contract No: DE-AC2-94MC32 For US Departent of Energy Office of Fossil Energy Federal Energy Technology Center PO Box 88 Morgantown, West Virginia BY Louisiana State University Departent of Cheical Engineering Batan Rouge, Louisiana 783

2 Disclaier This report was prepared as an account of work sponsored by an agency of the United States Governent Neither the United States Governent nor any agency thereof, nor any of their eployees, akes any warranty, express or iplied, or assues any legal liability or responsibility for the accuracy, copleteness, or usefulness of any inforation, apparatus, product, or process disclosed, or represents that its use would not infringe privately owed rights Reference herein to any specific coercial product, process, or service by trade nae, tradeark, anufacturer, or otherwise does not necessarily constitute or iply its endorseent, recoendation, or favoring by the United States Governent or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Governent or any agency thereof

3 DSCLAMER Portions of this docuent ay be illegible electronic iage products ages are produced fro the best available original docuent

4 EXFCUTVE SUMMARY Professor Harrison, project principe investigator, visited Morgantown for a project review in February 998 Following this review, the project was extended for five onths and $, in increental fbnding was approved The new project copletion date is August 8, 998 On the experiental front, the last six cycles of a 2-cycle sorbent durability test were copleted, final installation of the flae photoetric detector was accoplished, and fifteen tests whose ai was to deterine the iniu prebreakthrough H2S concentration over reduced CeO, were prefored There was a little, if any, evidence of sorbent deteriorationin the durability test For exaple, the tie required for the H2S concentration in the product gas to reach one-half of the feed gas concentration,&,, varied only between 97 inutes in cycle 2 and inutes in cycle 7, with an overall average of 2 inutes There was no apparent deterioration in as the nuber of cycles increased The variation in for regeneration was also sall, ranging fro a iniu of 26 inutes in cycle to a axiu of 23 O inutes in cycle 6 The overall average was 29 inutes and again there was no apparent increase with increasing cycle nuber During the durability test we confired that, when using pre-reduced sorbent and a clean syste, the prebreakthrough H2S concentration was less than ppv, the detection liit of the theral conductivity detector (TCD) Consequently, a ore sensitive flae photoetric detector (FPD) which peritted easureents of H2S concentrations of lppv or less was installed The FPD and TCD were connected in parallel so that, when desired, the entire H2S breakthrough curve could be easured Most of the quarter was devoted to conducting reduction-sulfidation tests to deterine the iniu prebreakthrough H2S concentrations which could be achieved using prereduced CeO, Fifteen runs involving variations in reduction -sulfidation teperature, H2S concentration in the feed gas, and feed gas voluetric flow rate were copleted n all tests the prebreakthrough H,S concentration was less than ppv, and in any of the tests the H,S concentration was equal to or less than lppv for an extended tie period SORBENT DURABLTY TEST The last six cycles of a twenty-five cycle sorbent durability test were copleted in January The initial sorbent charge consisted of 6g Rhone Poulenc CeO, ixed with 3g Al2O3 Reaction conditions were as follows: Reduction at 8 C and at n cycles through the reduction gas contained % H2 in N2at a flow rate of 4 scc n the reaining cycles reduction was carried out in % H, at a flow rate of scc The reduction was generally carried out overnight - -_

5 t 2 Sulfidation at 8 C and at in % H2S/% HJN2 at a rate of 4 scc Each sulfidation cycle was continued until H,S breakthough was coplete, typically about 2 hours 3 Regeneration at 6 C and at in 2% S2/N,at arate of2 scc Regenerationwas continued until the SO, content of the product gas reached 2%, typically about 3 inutes 4Reactor cleaning at 8 C and at using air at scc The purpose of this oxidation step was to reove, to the extent possible, all eleental sulfur deposited downstrea of the sorbent bed during regeneration As fbrther precaution against containation, different reactor exit lines were used during sulfidation and regeneration During sulfidation the product gas flowed through teflon-lined tubing and a 7 p stainless steel filter to the back pressure regulator and then through teflon tubing to the stainless steel chroatograph sapling valve Stainless steel in contact with the sulfidation product gas consisted of three fittings, one filter, the back pressure regulator, the cboatograph sapling valve, and the TCD housing During regeneration the product gas flowed through heat-traced stainless steel lines to the condenser and then through a series of filters and stainless steel tubing to the sapling valve The back pressure regulator was not used during the atosphedc pressure regeneration tests and the only stainless steel parts exposed to both sulfidation and regeneration product gases were one fitting, the sapling valve, and the TCD housing Coplete H2S breakthrough curves for the 2 cycles are shown in Figure Results of a nonreacting H,S tracer tests at the sae conditions are included for coparison All concentrationtie curves were siilar except for the following exceptions n the 2 to 9 inute range tkeh2s concentration fro cycle was clearly larger than fio any of the reaining cycles n the sae tie range the H2S concentrations in cycles 2 through are cosey bunched at an interediate level, while in cyces 6 through 2 H,S concentration was below the TCD detection liit for about 8 inutes These features are ore clearly shown in Figure 2 where the concentration scale is expanded to ephasize results during the early reaction ties The iproveent between cycles and 6 is clearly due to the switch fro % H2to % H2during the reduction step The delayed breakthrough in cycle 6, which is particularly evident in Figure, is attributed to reduced H2S feed rate caused by ass flow controller alhnction The tie required for the product H2S concentration to reach % (one-half of the feed concentration), to, provides a good coparison of sulfidation reproducibility during the active breakthrough stage Figure 3 shows toversus cycle nuber for the twenty-five cycles, excluding cycle 6 because of the ass flow controller probles to,ranged fro a iniu of 97 inutes in cycle 2 to a axiu of inutes in cycle 7 The twenty-four-cycle average was 2 inutes and there was no indication of a decrease in to, with increasing cycle nuber, Cobining the H2S breakthrough curves with the nonreacting tracer results shown in Figure ade it possible to estiate the overall percent conversion of CeO, to C%O,S at the conclusion of each cycle These results are presented in Figure 4 The sallest value of 9% conversion occurred in cycle while the largest value of 6% occurred in cycle 6 f this value 2

6 L rrn = E W - tv 3

7 ( so -S s2 so2 S so3 -s3 -S so4 --4 s24 C -so -~ -s2 -w c6 -+SO6 ----si6 L w S O ~- ~ - ~ 7 c,, -xq) so8 --s8 so9 --e---s9 C S -4D---s2 w - -- s -- L - Run 24 (2 cycles) Sulfidation Conditions: Tep = 8 "C Press = at Total Flowrate: 4 scc Feed Copositions: d % H,S, % H, balance N, - B - - C Q) J st u Tie (in) Figure 2 E a r l y P o r t i o n s of the S u l f i d a t i o n Breakthrough Curves i n the Sorbent D u r a b i l i t y T e s t i

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10 is excluded because of the previously entioned ass flow controller probles, the axiu of 8% conversion occurred in cycle 7 The twenty-five-cycle average (including cycle 6) was 2% These conversion calculations are based on a loss of 9% of the CeO, charge due to foration of volatiles during the ineating period This 9% loss was easured in previous electrobalance tests Regeneration conditions were constant in all cycles and Figure shows that the SO, breakthrough curves were also quite siilar Results fro a nonreacting SO, tracer test are also included for coparison Regeneration results fro cycle 8 are oitted because of eleental sulhr plugging probles Coplete regeneration was achieved, however, as shown by the cycle 9 sulfidation results Plugging also occurred near the end of cycles 6 andl, but the regeneration breakthrough curves were effectively coplete before the proble occurred to, corresponding to 6% SO, in the regeneration product gas (% ofthe feed concentration) is shown for each regeneration cycle in Figure 6 Results fro cycle 8 are oitted During regeneration, to, varied fro a iniu of 26 inutes in cycle to a axiu of 23 inutes in cycle 6 The twenty-four cycle average was 2 9 inutes and, once again, these was no evidence of perforance deterioration in the latter cycles Percent conversion of Ce,,s to CeO, for each cycle based on the SO, breakthrough curves and the nonreacting tracer test are shown in Figure 7 Results ranged fro a low of 9% in cycle to a high of 9% in cycle 6 with a twenty-four cycle average of 983% The scatter in the regeneration results is soewhat larger than in sulfidation because ody 9 or data points collected during a regeneration cycle copared to about 4 data points during sulfidation Nevertheless, the average of 983% for regeneration copared to 2% for sulfidation is felt to be quite good During the entire fifty-eight day duration of the durability test, the sorbent was continually exposed to a teperature of at least 6 C with the teperature being 8 C for approxiately 9% of that tie During that tie the sorbent was alternately exposed to HJN2 during reduction, H2S/HJN2during sulfidation, S 2 / N 2 during regeneration, air during cleaning, and N2purge between each phase The durability of the sorbent is judged to be exceptionally good FLAME PHOTOMETRC DETECTOR (FPD) The ability to reduce H2S concentrations during the prebreakthrough period to below the TCD detection liit led to installation of a FPD having a detection liit of about ppv A great deal of trial-and-error experientation was required before H,S concentrations near the ppv level could be reliably easured For exaple, the reactor and product lines downstrea of the sorbent bed had to be free of residual sulhr HzS produced by the reaction between H, and containant sulhr could easily exceed the H2S in the product gas Stainless steel parts in contact with the product gas were iniized as described in the previous section We also found the syste would not operate reliably with the two detectors arranged in series with the TCD followed by the FPD The 7 -

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14 proble was deterined to be caused by adsorptioddesorption of H2S on the stainless steel surface of the TCD A parallel arrangeent of the detectors as shown in Figure 8 was finally ipleented Colun 2 effluent flows into a reotely actuated three-way solenoid valve which, during the early prebreakthrough period, directs the product to the FPD When the H2S content increases to thefpd saturation level, the solenoid valve is switched and product is directed to the TCD No additional stainless steel is introduced as all wetted parts of the solenoid valve are of teflon Alteration in the analytical syste required that both detectors be recalibrated n addition to installing the solenoid valve and parallel detectors, the volue of the saple loop and the carrier gas flow rate were changed to provide reasonable operating conditions for both detectors As usual, the TCD cdibration for both H2S and SO, was linear through the origin The equations relating concentration and area are: ppv H,S = 287 x Area % SO, = 36 x Area The FPD response to H2S was quadratic and separate equations were deterined for low H2S concentrations ( to 3 ppv), ediu H2S concentrations (3 to ppv) and high H2S concentrations ( -8 ppv) The quadratic equations are: ppv H,S = 62 [( Area) - for ppvh,s 33 ppv H2S = 26 [(l + 42 Area) +] for 3 s ppvh,s s ppv H2S= 6942 [( + 77 Area) -3 for ppv HzS 8 Once again the iniu H2S detection liit for the TCD was near ppv while the axiu and iniu detection liits of the FPD were near ppv and ppv, respectively Results of the first reduction-sulfidation test (Ce2s) using the two detectors in parallel are shown in Figure 9 The standard Ce,-N,O, ixture was prereduced at 8 C and at in % H2/N2overnight Sulfidation was then carried out 8 C and at using % H2S/%H2/N2 H2S concentration is plotted on a logarithic scale so that the entire concentrationrange fro -4 ppv to, ppv (%) is clearly visible The product gas H2S concentration was below ppv, corresponding to 999% H2S reoval, for the first 3 inutes and then increased gradually to

15 Cl 'ent r S -E S 8 N S -2, Saple Loop (a) GC:Operation -Step One --- _ ent 7 -G S - GCOVEN sy -G S / -[- :,,,-,,-,,,,,,-,,,,,--, Hz Saple Loop (b) GC Operation -Step Two Actuator: V Solenoid C,, Ct, Cf:Carrier gases (Heliu) Colun : Water trap colun (PORAPAK Q) Colun 2: Separation colun (CHROMOSL 3) Figure 8 Parallel Arrangeent o f the FPD and TCD Detectors 2

16 r / -*- r - - Ce2sO (FPD) Ce2sO (TCD) Tep = 8 "C Press = at 7 7 ' y O 7 -- / Toatl Flowrate: 4 sccrn Feed Coposition: % H2S; % H;, 89% N2, 8 2 Tie (in) Figure 9 Results of the First Sulfidation Test (Ce2s) Using the Parallel Arrangeent of the FPD and TCD i

17 ppv after about inutes At that tie the three-way solenoid valve was switched to send product gas to the TCD The TCD cannot operate while flow is directed to the FPD and the 3 inute gap in the data was necessary to allow the TCD to war-up and stabilize after being turned on The H2S concentration fro the first TCD saple at 83 inutes was about 2 ppv, and by 6 inutes the H2S concentration increased to near the feed value of, ppv (%) The results fro this test are ir!general agreeent with results fro succeeding runs discussed in the next section except that the FPD breakthrough (in the 3 to 6 inute tie range) was ore gradual than in the succeeding runs As will be shown, the ore noral response was for the H2S concentration to increase fro about ppv to greater than ppv in a single saple interval The reason for this difference is not known REDUCTON-SULFDATON TEST RESULTS A total of reduction-sulfidation tests (including Ce2sO ) were copleted using the FPD and, in soe cases the FPD-TCD, for product analysis Reaction conditions for the tests are suarized in Table The standard reactor charge of 6g Rhone Poulenc C2, and 3g A2, was used in all tests The reduction gas coposition was constant at % H2in N, and reduction and sulfidation pressures were constant at at Reaction paraeters investigated were reductionsulfidationteperature (6 to 8OoC),sulfidation feed gas coposition (2% and % in % H2/N2),and sulfidation gas feed rate (2 to 8scc) The sorbent was pre-reduced in all tests except Cel3sOl H2S reoval in that test was considerably saller than in test Ce2s at equivalent reaction conditions and with pre-reduction No regeneration tests were carried out in order to iniize sulfur containation of the reactor syste Duplicate Test Results Test Ce2s, Ce2s6s, and Ce27s used identical reaction conditions except that Ce27s was terinated iediatey after the product H2S concentration exceeded the FPD saturation value (- OOppv) n both Ce2s and Ce26s product gas flow was shifted to the TCD after FPD saturation so that the entire breakthrough curve was obtained H2S concentrations fro the first 3 inutes of the three tests are copared in Figure The H2S concentration is presented on a linear scale and is liited to 2 ppv Ail early concentrations were in the range of 3 to 8 ppv (corresponding to 9992 to 9997% H2S reoval) However, there were significant differences after 2 inutes n Ce26s the H2S concentration increased between the fifth and sixth saples (-22 and 28 inutes) fro ppv to greater than ppv A siilar increase fro 3 ppv to greater than ppv occurred in Ce27s between the sixth and seventh saples (- 28 to 34 inutes) n contrast, the H2S concentration in Ce2 increased gradually and reained below ppv for inutes (see Figure 9) However, as previously stated, the sharp increase observed in Ce26s and Ce27s is ore coon 4 -_

18 - Table Suary of Reduction SulfidationReaction Conditions (January - March 998) Ce2sO Test Date Ce26sO Ce27sO Ce28sO Ce29s Ce2s Ce2s Ce22s Ce23s Ce2!4s Ce2SsO Ce27s Cc28s Ce29s 33W98 3n7 Ce26s 2/2/98 2/3/98 2/6/98 2//98 2/3/98 2/7/98 2/9/98 2nU98 2/23/98 3//98 3/8/98 3 n RP Rp RP RP RP RP RP RP RP Rp RP RP RP RP RP Tep, 'C Press, a % Ha O y Tep, ' C Mx) Press, at % H2S 2 2,o o 2 2 % Ha O O O O O % Nz source Reducticr Gas Cop % Sulfidaiion Gas Cop Flow W e, scan

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20 Figure shows the coplete H2S breakthrough curves fio Ce2s and Ce26s including both FPD and TCD results Except for the large differencein the tie and slope ofthe FPD breakthrough the results are quite siilar The data gap in both tests is associated with switching product gas flow to the TCD and providing sufficient tie for that detector to equilibrate n both runs there appears to be a second H2S concentration plateau in the to 3 ppv level which lasts for approxiately 8 inutes Majoi- H2S breakthrough then occurred in the 8 to inute tie span in both tests This second concentration plateau and final breakthrough tie correspond, at least qualitatively, to results of earlier runs in which only the TCD was used The Effect of Teperature Tests Ce27s through Ce2 2~ exained the effect of reduction-suifidation teperature in intervals of C between 6 and 8 C Only the FPD was used so that only initial H2S concentrations were obtained Results fro all tests except Ce22s at 6 C are shown in Figure 2 At this low teperature, the H2S concentration in all sapes exceeded the FPD saturation liit n the reaining five tests, all initial concentrations were below ppv and decreased with decreasing teperature fro about 6 pv at 8 C to less than ppv at both 6 and 7 C This decrease in concentration with decreasing teperature is consistent with the sulfidation reaction being exotheric and equilibriu being closey approached At 6"C, the kinetics of the reaction becoes too slow for equilibriuto be approached As previously described, the G -FPD operating conditions are such that the H2S analysis becoes increasingly uncertain as the H2S concentration approaches and goes below ppv t is of interest to note that the prebreakthrough Concentrations shown in Figure 2 copare quite well to previously published results fro Meng and Kay (987) at higher teperature This is illustrated in Figure 3 where H2S concentrationsfro the third saple of each of the Figure 2 tests have been added to previously reported data While the feed gas coposition used by Meng and Kay and in this study are different, and both are different fro the Shell gas copositionused to establish therodynaic liits, the results are quite siilar The iportance of the prereduction step prior to sulfidation ust be ephasized The therodynaicsof the reaction between H2S and substoichioetric CeO, (n<2) are ore favorable than the H2S-CeO, reaction This was proven in test Ce23s at 7" C where the prereduction step was oitted nstead of the approxiately lppv H2S levels fored in Ce2s (SeeFigure 2), all H2S concentrations fro Ce23s were above the FPD saturation liit The Effect of Feed Gas Copositic n tests in which the feed gas contained % H2S such as shown in Figure 2, FPD breakthrough occurred in less than 3 inutes and the H,S concentration increased fio low levels (<lo ppv) to above the FPD saturation liit in a single cycle n order to extend the duration of 7

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23 CeO, A (Meng and Kay, 987) Experiental Data (Meng and Kay, 987) E x p ~ i n e ~ tdata a! (This work) 6 * 8 Teperature (K) 2 Figure 3 Coparison of FPD Prebreakthrough Concentrations Fro This Work With Earlier Results Fro Meng add;kay 4

24 , the FPD prebreakthrough period and, hopefully, to obtain ore inforation on the shape of the breakthrough curve, the concentration of H2S in the feed gas was reduced by a factor of 4 to 2% in tests Ce2 4sO and Ce2 ~Reaction conditions in these tests were the sae except that only the FPD was used in Ce24s and both the FPD and TCD were used in Ce2sOl Also reaction conditions in these tests were the sae as in Ce2OsO except that the feed gas contained % H2S in the latter case H,S breakthrough curves fro these runs are copared in Figure 4 An approxiate correction for the increased H2Sfeed concentration in Ce2s has been ade by ultiplying each saple tie in that run by 4 nitial concentrations fro all tests are at the ppv level or less F'PD breakthrough fro ppv to ppv occurred at approxiately 2 inutes in both Ce24s and Ce2sOl FPD breakthrough appeared to occur at an earlier tie in Ce2s but the factor of 4 used to adjust the reaction tie in this test is at best approxiate - The FPD results fro Ce24s and Ce2~show a secondary plateau in the to 3 ppv level TCD results fro Ce2sOl show that this plateau lasted until about 4 inutes when final breakthrough to 2 ppv HS occurred FPD concentrations above ppv for both Ce24sO and Ce2s ust be treated cautiously as they are above the noinal saturation level and were estiated by extrapolation Siilarly, the TCD concentrations fro Ce2s near ppv should be treated with caution as they are near the lower detection liit ofthe TCD Thus the apparent concentration decrease in Ce2 so following the switch fro the FPD to the TCD ay be attributed to analytic uncertainty The Effect Of Voluetric Feed Rate The effect of voluetric feed rate at 2,4, and 8 scc was investigated at 7 C using feed concentrations of both % and 2% H2S However, prior to these reaction tests a series of nonreacting tracer tests was copleted so that appropriate delay tie corrections as a hnction of flow rate could be ade Delay tie is defined as the tie interval between opening the valve to feed reactive gases and those gases reaching the sorbent bed Results of the tracer tests are presented in Figure The H2S concentrations in the feed and product gases were effectively equal in the third saple (- in) at 2 scc, in the second saple (-7 in) at 4 scc, and in the first saple (-3 in) at 8 scc Delay tie, td, was estiated by nuerically intergrating the area above the breakthrough curve and converting that area to a rectangle of the sae area with height equal to the H2S feed concentration and width equal to td Resultant values of td at the three flow rates are: 2 scc 4 scc 8 scc t, = in t, = 24 in td = 2 in

25 i > n E -A- -T--- Ce24sOl (2% H2S in feed! / Pi Ce2sO (2% H,S in feed N N Tie (in) Figure 4 The Effect of H,S Feed Coposition on FPD Prebreakthrough Concentrations i 8 2

26 2 A- - k=====a=c --- Non-reacting Tracer Tests: 2 Tep = 7 "C Press = at Feed Coposition: 2% H,S; % H;, 897% N, u w Total Flowrate: sccrn -4 sccrn -A8 sccrn / /- 2 3 Tie (in) Figure Nonreacting Tracer Test Results as a Function of Voluetric Feed Rate i 4

27 FPD prebreakthrough concentrations of ppv or less were achieved in all tests with breakthrough generally occurring earlier at the larger feed rates FPD results fro the three tests using 2% H2S in the feed gas (Ce2s, Ce28s, and Ce29s) are shown as afinction of diensionless tie in Figure 6 Diensionless tie, t*, is defined as 2 - td t*= te t, is the delay tie and t, is the theorer:ical tie at which all CeO, would be converted to C%O,S with coplete reoval of H,S When H2S reoval is essentially coplete, as it is for all of the data in Figure 6, the value oft* is effectiveiy equal to the fiactional conversion of CeO, to Cq,S Fro the figure we see that FPD breakthrough begins at t* -7 at 8 scc, 3 at 4, and 32 at 2 scc However, at 2 scc the H2S concentration reained below ppv until t*-4 FPD breakthrough occurred, uch sooner in the tests using % H2S in the feed gas and severely liited the aount of data which could be obtained prior to FPD saturation This was particularly true in test Ce27s at 8 scc where only two saples were obtained prior to breakthrough Resultsfi-o Ce23s, Ce26s, andce27s all at 7 C at at are copared on a diensionless tie basis in Figure 7 Diensionless ties corresponding to the last saple in which the H2S concentration was near ppv decreased fro t*-3 at 2 scc to t*-8 at both 4 and 8 scc There is, however, a reasonably large uncertainty in the last two values because, between saples, at*-9 at 8 scc and at*-4 at 4 scc CONCLUSONS With the ore sensitive F PD, we have shown that initial H2S concentrations in the range of to ppv can be achieved over a teperature range of 6 to 8 C The key to achieving the low prebreakthrough concentration is the prereduction step in which CeO, is reduced to CeO, (62) The prereduction requireent was proven by coparing initial H,S concentrations at 7 C with and without prereduction n the forer case initial H2S concentrations were about ppv and in the latter case all H2S concentrations exceeded the saturation liit of the FPD (- ppv) At 6 C even with a prereduction step the H2S concentration always exceeded FPD saturation This ay be attributed to lack of reduction andhr slow sulfidation kinetics at the lower teperature The low initial H2Sconcentrations were achieved using feed concentrationsranging fio 2 to % H,S and flow rates fro 2 to 8 scc Of course the duration of the FPD prebreakthrough period decreased as both the inlet concentration and feed rate increased n the ost favorable case with a feed rate of 2 scc and 2% H2S, the H2S concentration in the product reained below 2ppv until approxiately 4% of the sorbent was converted to Ce,O,S 24 --

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