Techniques for Investigating SuitorRemoval by Zinc Oxide Absorbents

Radiographie and electron microprobe analyses of pellets used inlaboratory and pilot-plant tests reveal key parameters for anefficient absorbent for use with hydrocarbon feedstocks; thetechniques could be applied to other absorption processes.

P.J.H. Carnell and P.J. DennyImperial Chemical industries PLC, Billingham, Cleveland, England

This paper describes some laboratoryand pilot plant tests which havebeen developed for evaluatingdifferent zinc oxide absorbents.The techniques were developed tostudy the zinc oxide/hydrogensulphide system but could beextended to examine other absorptionprocesses such as the poisoning bysurface absorption of otherheterogeneous catalysts.

The zinc oxide/hydrogen sulphidesystem was chosen because of itswidespread use in the desulphurisa-tion of hydrocarbon feedstocks formodern steam reformers.

The zinc oxide based absorbentsare used in fixed beds and theamount of absorbent used is chosento give a long working life.Plant lives are rarely less thanone year and commonly three to fiveyears. A number of different zincoxide absorbents is availablecommercially and obviously withlong plant lives it is not possibleto carry out rapid evaluations onthe plants. Therefore in order toevaluate different absorbents itis necessary to develop anaccelerated hydrogen sulphideabsorption test which willdistinguish between different

formulations. It is also desirableto determine those chemical andmicromeritic properties which areimportant for good absorptionperformance.

A number of commerciallyavailable absorbents was used forthis study. They had differentphysical forms and used differentbinding agents. Those examinedand their micromeritic propertiesare given in Appendix 1.

Test Methods

The two tests used aresummarised in Table 1. For thefirst test a bed of a singleabsorbent was used and theconditions chosen to simulate plantsusing naphtha feedstocks. Dependingon the efficiency of the absorbent,this test lasted up to ten days.

Table 1. Process Conditions for the Two AbsorptionTests

Temp(°C)Total Pressure (bar)HzS concentration ppmw/wGas CompositionHydrogen (%)GHSVBed volume (mis)Bed diameter (cms)Typical time to breakthrough (days)

TEST1(single bed)

37030

270Naphtha

341900602-5

10-12

TEST2(mixed bed)

37030

2000Natural gas

21000602-52-3

99

Test ConditionsPressure 30 barTemperature 370°CSpace velocity 1900 h"1

Feed Light Naphtha

100Bed depth (%)

10-

30-

20

10

30-

20-

10

Figure 1. Sulfur absorption profiles for absorbentstested separately.

Test conditions

Pressure 30 bar

Temperature 370°C

Space velocity 1000 h ~1

Feed Natural Gas

100 100

Bed depth («/o)

Figure 2. Sulfur absorption profiles for absorbentstested in mixed beds

In the second test an attemptwas made to compare directly twodifferent absorbents in the samebed using a methane feedstock.

Equal volumes of the twoabsorbents were mixed thoroughlyand then charged to the reactor.All of the absorbents listed inAppendix 1 were compared separatelyin this way with the same standardabsorbent. This was taken as sampleD which is ICI catalyst 32-4(Katalco 32-4). This enabled eachabsorbent to be compared directlywith the standard and hence witheach other because each one wasexposed to the same hydrogen sulphideconcentration at each point down thebed for the same length of time.

During all the mixed bed runs,H2S containing radioactive sulphur-35 isotope was used in order tomonitor more closely the sulphurdistribution throughout the bed andwithin individual pellets.

In both cases the test wasstopped when hydrogen sulphide wasdetected in the exit gas.

Examination of Absorbents afterAbsorption Tests

After breakthrough, the bedswere carefully discharged and splitinto six equal parts in the case oftest 1 or four equal parts in thecase of test 2. The particles wereseparated in the mixed bed runs.Each layer was then analysed by X-rayfluorescence for total sulphur andthe absorption profiles shown inFig.l and Fig. 2 constructed forthe different absorbents.

For both tests, the total pick-up of sulphur through the whole bedwas compared with the standard.These relative sulphur pick-ups aregiven in Table 2. Generally it isclear that these breakthrough testscan easily differentiate betweengood and bad absorbents.

Table 2. Comparative Total Sulfur AbsorptionsSAMPLE

Relative Sulphur pick-upon test 2 (mixed beds)

Relative Sulphur pick-upon test 1 (single beds)

A

042

0-11

B

0-25

0-26

C

0-51

XX

D«

1-00

1-00

E

0-69

'XX

F

0-89

1-01

* Sample D taken as standard„ xx Not examined

100

In the samples from the runsusing radioactive H235S, separateparticles from different bed levelswere examined for sulphurdistribution. Polished sections ofthe particles were made afterembedding in an epoxy resin. Inthe case of the granular specimens,hemispherical sections were made»In the case of the cylindricalextrusions axial and circularcross-sections were made*

The sulphur distributions onthese polished sections were thenexamined by two different methods.

(a) Radiographie Method

The sections were placed facedownwards on a photographicfilm and left in the dark for

about 1 week. The weak beta-emission from the ^5g createdan image on the film whichmirrored the sulphurdistribution in the cross-section. Typical results fortwo absorbents are shown inFig.3.

(b) Electron Microprobe Analysis(EMPA)

A Link System Instrument withenergy dispersive X-raymicroanalysis was used. Lowmagnification electron imageswere also made of the samecross-sections studied forsulphur distribution» Typicalresults for two absorbentsare shown in Fig.4.

3. Autoradiographs of sulfur content. Pellets1 and A at bed inlet; pellets 8 and H at bed exit.

4. Electron microprobe photographs ofsulfur concentration.

101

The total pore volumes andpore areas of some dischargedsamples were' also determined bymercury porosimetry for comparisonwith fresh absorbents. Pore sizedistributions are in some doubt asit is possible that there was anunknown change in the mercurycontact angle on changing from anoxide to a sulphide surface. Poresize distribution is thereforenot given but a mean pore radiuswas calculated.

Discussion

The sulphur absorptionsobserved for the different zincoxides do not correlate well withthe total surface areas (see datain Appendix 1 and Table 2). Hence,because the size of the individualzinc oxide crystallites present isinversely proportional to the surfacearea, it follows that the slow steplimiting sulphur absorption is notlattice 4iffusi°n under theconditions used.

The best correlation of therelative sulphur absorptions iswith the mercury densities of theparticles and these areproportional to their total volumeporosities,viz:

pore volume (ml/ml)=l-Hg density ofparticle

Density of ZnO

The correlation with the totalvolume porosities is shown in Fig.5where the relative absorptions fromTest 2 are shown plotted againstthe porosity. Fig.5 shows thatthere is a critical porosity ofabout 0.67 ml/ml. Good absorbentsare those with porosities greaterthan this value.

The reason for the importanceof the total porosity is indicatedby the radiographie and EMPAexaminations of the used absorbents.In the poor absorbents there is asulphur shell around the particlewith little or no indication of anysulphur penetration into the interior.This effect is strikingly illustratedin Fig.3 and Fig.4 which compare agood and a bad absorbent.

to

11'1CDoc

.2 .4 .6 .8

Total porosity (ml/ml)Figure 5. Relationship between relative total ab-sorption and particle porosity.

An explanation of thisobservation is that when zinc oxidecrystallites are converted to zincsulphide, there is a volume increaseof about 37%. However there is nocorresponding increase in the volumeof the whole particle, which meansthat the porosity of the sulphidedregions must decrease.

A plausible explanation of lowsulphur absorption and itscorrelation with porosity, is thatthe sulphidation which initiallytakes place on the outside tends toblock any further access to theinterior.

This theory was confirmed bymercury porosimetry measurementmade on a few discharged samples.After sulphidation, the surfaceareas and porosities both decreaseappreciably. However when thedischarged particles were splitacross the middle and the

102

micromeritics redetermined thenthe measured surface areas andporosities increased, indicatingthat the sulphided outer regionshad tended to block access to thecores.

In general there was goodcorrelation between the relativesulphur uptakes by the two testmethods. However there .was anotable difference between the twotests for sample A. The sulphurpick-up with Test 1 was much lowerthan expected from the results ofTest 2. Repeated runs confirmedthis result. The cause of thedifference could not be the timefactor because Test 1 took longerthan Test 2. A possible explanationwas the difference in feedstocks.In order to test this hypothesis abreakthrough run was done withsample A using 5% hydrogen sulphidein hydrogen alone at a total pressureof 1 atmosphere. Thus whilst thepartial pressure of the hydrogensulphide was roughly the same as inTest 21 the partial pressure andmolecular weight of the accompanyinggas were very much lower.

The results obtained are shown inFig.6. It can be seen that the useof hydrogen in place of naphthagives a marked improvement inperformance. Sample A was different

oo

CO

.Q

O

.Q

CO -̂X2 Q.«'S

30

20

10

oICCo

Hydrogen feedstock.

Naphthafeedstock

100

Bed depth (%)

Figure 6. Effect of feedstock on sulfur capacity oflow porosity absorbent (sample A).

from all of the other absorbents inhaving a low surface area and alarge mean pore radius. It islikely that this type of formulationis more affected by the presence oflarge molecules of hydrocarbon thanthe other absorbents.

Conclusions

The results obtained by thesetests on the absorption of hydrogensulphide by zinc oxide show that itis possible to differentiate betweendifferent absorbents using smallscale reactors. The use ofautoradiography and the electronmicroprobe analyser show that theshape of the absorption profilein the bed is related to theabsorption profile across thepellets. So far these techniqueshave only been applied in the zincoxide/hydrogen system. They couldeasily be extended to othercatalysts and could be used tostudy reactions such as those ofhydrogen sulphide and hydrogenchloride with low temperature shiftcatalysts.

APPENDIX 1. Mlcromerfiic Properties of Zinc Ox-ide Absorbents Studied.

Physical form

Particle size (mm)

Tapped bulk density kg/1

Surface area mz/gm

Helium density : g/ml

Mercury density: g/ml

Accessible porosity1 : ml/ml

Total porosity1: ml/ml

Mean pore radius3: A

A

extr

5

1-18

9-0

5-66

2-38

0-58

0-58

533

B

gran

3-5

1-80

12-7

5-54

3-06

0-46

0-45

235

SAM

C

extr

5

1-15

16-2

5-13

2-05

0-59'

0-63

358

PIE

D

gran

3-5

MO

25-0

4-23

1-79

0-58

0-68

256

E

extr

5

0-90

33-0

4-50

1-83

0-59

0-67

195

F

extr

5

1-06

57-2

4-92

1-79

0-64

0-68

125

1 Calculated from Hg and He densities of particles.2 Calculated from Hg density of particle and density of ZnO single crystals.3 Calculated from surface area and pore volume.

P. J. DENNY P.J.H. CARNELL

103

DISCUSSION

ÂPPL, BASF: Regarding the method you described tocheck the sulfided layer by radiography, it is notsurprising, I guess, that the H2S absorption capacity ofzinc oxide catalyst is not only a function of the serviceconditions but also mainly the porosity that controlsaccessibility to the inner parts. Your interpretation isinteresting that the particles are more or less blockedby sulfide formation in the outer region and that theextent to which this occurs depends on the porosity ofthe catalyst. In yourtesting method No. 2, two differentcatalysts are mixed and after each run are separatedand analyzed separately. I can see no specific advan-tage compared to the simple method No. 1. I cannotsee that additional information can be obtained, andthe operation is also simpler when using separate

reactors. Would you comment on this?

P.J.H. CARNELL, Imperial Chemical Industries: Wewere concerned by the difficulty in obtaining repro-ducible conditions. There is a big problem in moni-toring levels of hydrogen sulfide and in metering it intoa feed and we thought that by using comparative tests,using one material as a standard would enable us toeliminate this kind of error. That was the thinkingbehind using the mixed beds. Factually, you are quitecorrect. When we compare the sets of data from thetwo tests, they are in general good agreements; I thinkthat either method gives a very good indication oflikely performance.

104