Sulfur Removal

Product Bulletin

LiabilityAlthough these instructions have been prepared by experienced technicians and have been basedon the best available information derived from laboratory, pilot plant and commercial experience withthis catalyst, these technicians do not have intimate knowledge of the customers plant andoperation. Therefore, Sd-Chemie Group, in issuing these instructions, cannot assume liability forupsets and damage to either the customers plant or personnel. The customer is urged to reviewthese instructions carefully and to satisfy himself that their application will not be hazardous tohis specific operation. Further, Sd-Chemie Groups Technical Service representatives are presentat plant start-ups in an advisory capacity only and cannot be charged with knowledge andresponsibility for hazardous conditions that might result from the application of the instructions.

Sulfur Removal Product Bulletin

Introduction 4

Applications 4

Product Features 5 G-72D 5 G-72E 5 G-72H 5

Design Parameters 6 Desulfurization of Hydrocarbon Feed to Steam Reformer 6 Desulfurization of Syngas 6 Desulfurization of Raw Natural Gas 7 Desulfurization of CO2 7 Desulfurization of Wet Gases 7

Adsorption Capacity 7

Mechanism and Sulfur Profile 9

Poisons 9

Trouble Shooting 10

Technical Service 11

Guarantees 11

IntroductionZinc Oxide adsorbents for sulfur removal fromhydrocarbon process feeds are the acceptedmethod in industry for pretreatment of feedgas to steam reforming units. The efficiencyand capacity of the current range of ZincOxides allows sulfur to be removed from theprocess feed on a continuous basis andto the levels necessary to protect the down-stream nickel reforming catalysts.Besides the principal application of ZincOxide for sulfur removal in the hydrocarbonsteam reforming plants, Zinc Oxide has awider range of applications in sulfur removal.Zinc Oxides have been developed to a statewhere they are now in effective use attemperatures as low as ambient and cantherefore be directly used in the desulfuriza-tion of raw well head natural gas and in theclean up of a whole range of feedstreamssuch as CO2, acetylene, ethylene, propylene,air etc.

The Sd Chemie Group Zinc Oxide ad-sorbents are designated G-72 and are manu-factured in extruded form. The Sd-ChemieGroup have been supplying Zinc Oxideadsorbents to industry since the 1950s. Thisoperating experience makes SD-CHEMIEuniquely qualified to provide the bestproducts available to industry for this desulfu-rization application as is confirmed by the wellproven operation of G-72 catalysts inhundreds of installations worldwide.

ApplicationsPurification of feed tosteam reforming unitsThe hydrocarbon feed can be in the form of

Natural gas LPG Off gases Naphtha.

The nickel catalysts in the reforming sectionare highly sensitive to sulfur poisoning and thesulfur content of the feed to the reformermust not exceed 0.1 ppm v.

The G-72 Zinc Oxides alone will adsorbdirectly to the required level of purity sulfur inthe form of

H2S COS Methylmercaptan Ethylmercaptan Propylmercaptan.

When the sulfur is in the form of mercaptanshigher than propylmercaptan, disulfides orthiophenes, the Zinc Oxide is normallypreceded by a hydrogenation stage using aCoMo catalyst (C49 catalyst).

Purification of synthesis gas The sulfur in synthesis gases is due to up-stream processes always in the form of H2S.The copper catalysts in the LT CO conversionreactor and the methanol synthesis reactorare very sensitive to sulfur poisoning and thesulfur content of the synthesis gas must bereduced to undetectable levels in order tomaintain satisfactory service life for thesecopper catalysts.

Purification of raw natural gasNormally the sulfur compounds are in theform of H2S and RSH and therefore can beadsorbed directly by the G-72 Zinc Oxide.In this application the G-72 Zinc Oxide isworking at temperatures as low as ambientand retains excellent adsorbent capacity.

As the product purity is not of major concern,rather high space velocity can be applied.

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Purification of CO2G-72 Zinc Oxides are also used to adsorbH2S and COS from CO2 streams in a widerange of operating temperature and pressure.The purified CO2 can be fed to a steamreformer, e.g. in methanol or CO plants, or isused after further clean-up in the foodindustry.

MiscellaneousThe G-72 series of Zinc Oxides is well provenin the purification of olefins, like ethylene andpropylene.Further information about this wide field ofapplication is available upon request.

G-72 Zinc Oxide is even used to removesulfur compounds from air under ambientconditions. This is to protect, for instanceexhibits in museums.

Product FeaturesSd-Chemie Group manufactures threegrades of G-72 Zinc Oxide:

G-72D - for applications at high spacevelocity

G-72E - for high sulfur containing feedstocks

G-72H - for purification of raw natural gas

The physical and chemical properties of aZinc Oxide relate to performance as mea-sured not only by sulfur adsorption efficiencybut also by physical integrity under operatingconditions. These properties in the G-72 ZincOxides are characterized as

large surface area high crush strength low attrition loss good water resistance

These outstanding properties of the G-72Zinc Oxides are achieved through the uniquemanufacturing techniques which incorporatea binder within the zinc oxide. This maxi-mizes sulfur pick-up, which is generallyhigher with G-72 catalysts compared to ZnOadsorbants with 100 % ZnO. The optimumproperties of a zinc oxide can be tailored toa particular application. For example, forfeeds with low sulfur content, the criterion ofdecision on the type of Zinc Oxide to installmay be good physical strength rather thanadsorption capacity, as the Zinc Oxide will bein service for a long time and may never befully sulfur saturated.

All three Zinc Oxides comprise 90% ZnO and10% of a binder. The basic difference ofG-72D, G-72E and G-72H is the bulk density,the internal surface area and porosity,respectively. All G-72 types show excellentside crush strength.

G-72D G-72E G-72H

ZnO [wt %] 90 90 90

Binder [wt %] 10 10 10

Shape extrusions extrusions extrusions

Size [mm] 4.5 4.5 4.5

Bulk density [kg/l] 1.1 1.3 1.6

Surface area [m2/g] 50 30 10

Side crush strength [N] 100 100 100

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R-SH (R=C1-C3) 10 315

R-S-R (R=C1-C3) 10 400

COS 10 270

S compound Max. concentration Min. operatingin HC feed [ppm v] temperature [C]

Design ParametersPurification of feed to a steam reformerThe sulfur sensitive steam reforming catalystsrequire thorough desulfurization of thehydrocarbon feedstock. This is normallydone by a two step system, comprising thehydrogenation of organic sulfur compoundsby Cobalt-Molybdenum (CoMo) or Nickel-Molybdenum (NiMo) catalysts followed by theadsorption of the formed H2S by Zinc Oxide.

The standard catalysts for this wide spreadapplication are G-72D and G-72E.G-72D is used at high space velocities and/orlow sulfur concentrations. The extremely highporosity and surface area make all of the ZnOavailable for the adsorption reaction.

Due to the higher bulk density G-72E canpick up more sulfur expressed in kg S per m3

catalyst. This higher pick up, however, canonly be achieved at space velocitieslower than approx. 3.000 Nm3/m3/h.

The normal operating range for the desulfu-rization of hydrocarbons is:

Temperature: 300 400C Pressure: up to 60 bar Space Velocity: 500 4.000 Nm3/m3/h

Sulfur can be in the form of H2S, COS, shortchain mercaptans and short chain thioethers.

Please note that there are limitations forthe maximum concentrations of organic Scompounds and also for the minimumrequired temperature to achieve the desiredgas purity.

When operating within the following limits,G-72D and G-72E guarantee trouble freeoperation:

In the presence of higher concentrations ofthe a.m. compounds or with thiophens in thefeed we strongly recommend to use aCoMo or NiMo hydrogenation step upstreamzinc oxide.

When the G-72 catalysts are used as re-commended, H2S and other sulfur com-pounds are removed to a concentration lessthan 0.1 ppm v total sulfur.

Purification of synthesis gasesTo protect the sulfur sensitive copper zinccatalyst in the low temperature CO conversi-on reactor or the methanol synthesis reactorit is possible to install a guard bed or a guardreactor of G-72D upstream the copper zinccatalyst.

Typical operating conditions are:

Temperature: 150 250C Pressure: up to 80 bar Space velocity: up to 10.000 Nm3/m3/h

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Purification of wet gasesAt atmospheric pressure above 100C steamhas no adverse effect on the physicalstrength of G-72 zinc oxide catalysts.Contact with high pressure steam, how-ever, can weaken the catalyst unless thetemperature is more than 20C above thecondensation temperature. The catalyst canbe damaged if capillary condensation occurs.

Sulfur removal in the presence of steam isgenerally carried out at temperatures below300C. Above this temperature efficiency canbe limited because the equilibrium of thereaction

ZnS + H2O ZnO + H2S

is reached.At a steam/gas ratio of 0.2 mole/mole thesulfur adsorption capacity will be approx.80% compared to the one of the dry gas.

Adsorption CapacityThe adsorption capacity of the three G-72zinc oxide catalysts will vary with actualprocess conditions with respect to:

Temperature Pressure Space velocity Inlet sulfur content Steam content Reactor design system

Figure 1 can be used as guideline for theinfluence of reaction temperature on thesulfur pick up capacity of G-72D and G-72E.The system with two reactors in seriesinterchangeable results in a much higheroverall sulfur pick up capacity and is there-fore the preferred reactor system.

As it can be seen from figure 1 the optimumsulfur pick up can be achieved at temperatu-res of 350 380C. Temperatures higher than380C will not increase the pick up capacitybut temperatures above 400C will lead tocracking of the hydrocarbon feed over thecatalyst.

Purification of raw natural gasNatural gas with sulfur concentrations ofapprox. 20 30 ppm total sulfur can bepurified directly at the well or platform atambient temperature.As the sulfur pick up capacity of zinc oxidecatalysts are limited at low temperatures azinc oxide with high bulk density has to bechosen to achieve sufficient long catalystlife time. As normally the operating pressureis very high the porosity is not of concern.

G-72H is the catalyst of choice and adsorbsup to 520 kg sulfur per m3 catalyst.

The normal operating conditions are:

Temperature: ambient Pressure: up to 80 bar Space velocity: up to 6.000 Nm3/m3/h

Under these conditions H2S and short chainmercaptans are picked up to result in aproduct purity in the range of 1 5 ppm v H2Sdepending on the operating conditions.

Purification of CO2The removal of H2S and other sulfur com-ponents (mainly COS) from CO2 with ZnOmust not be performed in the temperaturerange of 170C to 280C. At these tempe-ratures zinc oxide is partly converted to zinccarbonate according to

ZnO + CO2 ZnCO3

At temperatures above 280C ZnCO3 de-composes and ZnO is stable in CO2 streams.As normally COS is present in CO2 werecommend to run the desulfurization of CO2at temperatures of min. 300C.

Depending on the desired life time andpick-up capacity per m3 catalyst both G-72Dand G-72E are used in the desulfurizationof CO2.

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The dependence of the sulfur pick up fromthe space velocity is shown in figure 2.

This graph is valid for operating pressurehigher than approx. 25 bar g. At lowerpressure somehow lower space velocitieshave to be applied to achieve the sameperformance.

Figure 2:Sulfur pick up as a functionof space velocity

35

30

25

20

15

10

5

0

1000 2000 3000 4000 5000

Conditions: p = 25 50 bar g,SV = 2.000 Nm3/m3/h per bed

Feed: Natural gas

Conditions: p = 25 50 bar g,T = 380 400C

Feed: Natural gas

Figure 1:Sulfur pick up as a functionof reaction temperature

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35

30

25

20

15

10

5

0

100 200 300 400

Sul

fur

Pic

k U

p [w

t%]

Sul

fur

Pic

k U

p [w

t%]

Temperature [C]

Space Velocity (Nm3/m3/h3)

Mechanism and Sulfur ProfileThe performance of zinc oxide depends onmore than just the percentage of ZnO or thetons of ZnO per cubic meter of catalystbecause not all of the ZnO is immediatelyavailable for gas contact.

As sulfur (H2S) containing gas passes overthe catalyst particles the surface layer of ZnOreacts, with the rate limited only by gasdiffusion. The ZnO inside the pellet reactsmore slowly by solid diffusion until all of theZnO has reacted.

This means that a zinc oxide bed can beviewed as having four zones (see figure 3):

Saturated Zone:all ZnO is converted to ZnS

Solid Diffusion Zone:sulfur migrates to the center of the pellet

Gas Diffusion Zone:where most of the reaction occurs

Fresh Catalyst Zone:where the catalyst is basically new

Figure 3 shows the sulfur adsorbed in thebed (solid line) and the sulfur level in the gas(dashed line). With time on stream this profilemoves down the bed until the bottom of theGas Diffusion Zone reaches the bottom of thecatalyst bed. Sulfur then begins to leak fromthe bed and gradually increases as the profilecontinues to move down the bed.

PoisonsG-72 catalysts are resistant against most ofthe known poisons for catalysts. Carryoverof dust, carbon and/or other solid materialswill physically block the pores of G-72catalysts and therefore reduce the adsorptioncapacity. It also may contribute to an in-creased pressure drop.

HCl has to be avoided at any time as thepresence of HCl leads to the formation ofZnCl2 according to

ZnO + 2HCl ZnCl2 + H2O

ZnCl2 is volatile at operating temperaturesand can be therefore transported to down-stream equipment and may contribute tocorrosion.It is therefore mandatory to install in thepresence of chlorine components a chlorineguard upstream the ZnO catalyst.

Figure 3:Sulfur Profileover G-72 DCatalyst Bed

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Gas Diffusion

Fresh Catalyst

Saturated

Solid DiffusionAdsorbed Sulfur

Sulfur in Gas

G

as F

low

Symptoms Cause Corrective Action

High Pressure Drop Carbon Deposition by cracking of olefins Check CoMo system, Increase H2 recycle

Dust, solid particles on catalyst Remove and/or replace top layer ofcatalyst (approx. 10 % of bed is sufficient)

Catalyst breakage Change-out catalyst and investigatereason for catalyst breakage

Sulfur break-through Insufficient pick-up zone remaining Single bed: Stop plant and replace catalyst 2 beds: Put 2nd bed in first position andreplace used catalyst in former 1st bed

Temperature too low Increase temperature (max. 400 C)

Feed rate too high Reduce feed rate

Channelling Replace catalyst; Catalyst from lower bedmay be reused after screening

Chloride break-through No Chloride Trap Install Chloride Trap

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Trouble ShootingZinc Oxide systems will normally operatetrouble free, the only necessary control beingto maintain a close observation for sulfurbreakthrough (or increase from a previoussteady level) exit the bed.

Installed Zinc Oxide volumes are opti-mally designed to approach full adsorptioncapacity corresponding to scheduled plantshut down cycles. However, this optimisationobviously depends on the sulfur contentin the feed remaining at the projecteddesign level, which is not always the case inpractise.

The only practical problem that may arisewith a Zinc Oxide bed particularly if the ZincOxide is not preceded by a CoMo systemand is therefore the first vessel in the plantstream is a pressure drop increase causedby filtering out any dust or solid particles inthe feed. This dust can also reduce theadsorption capacity of the Zinc Oxide byblocking the pores and additionally lead tochannelling of the feed stream with con-sequent poor desulfurization performance.

Practical recommendation to overcome anypossible, but rare, problems with Zinc Oxideare listed below:

Review of reactor system design

Selection of proper catalyst

Technical reviews of start-up/shut-down procedures

Technical assistance for loading, catalyst start-up and shut-down

Routine performance evaluations

Troubleshooting

Catalyst life projections

On-site training seminars for engineering and operations personnel

Chemical and physical analyses of spent catalyst

Supply of proprietary computer programs for catalyst performance evaluation

Technical ServiceSD-CHEMIE GROUP firmly believes thateven an outstanding catalyst may not providethe best performance without the appro-priate level of assistance during the phases ofselection, installation, start-up and operation. Within our Technical Service Group, we havea wide range of personnel with specificexpertise in the areas of catalyst characte-rization, research, catalyst installation,start-up, plant operation, and performanceevaluations.

This depth of knowledge allows us to providethe following services:

SD-CHEMIE Group also has an extensivelist of state of the art analytical and computersystems to further add to our list of resour-ces.

GuaranteesSd-Chemie Group grants to the user perfor-mance guarantees covering the performanceof its ZnO for desulfurization applications.Before granting such performance guarantees,the design conditions and/or anticipatedoperating conditions must be evaluated andrecommendations or confirmations given onthe basis of Sd-Chemie Groups designsystem.

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