Debottlenecking Claus Sulfur Recovery Units: An Investigation of the application of Zinc Titanates

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Debottlenecking Claus Sulfur Recovery Units: An Investigation of the application of Zinc Titanates

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Contents 1 Executive Summary 2 Claus Process 2.1 Partial Combustion Claus 2.2 Split Flow Claus 2.3 Sulfur Recycle Claus 3 Zinc Titanates 4 Application of Zinc Titanate to Debottleneck Partial Combustion Claus by 10% 4.1 Process 4.2 ASPEN Modeling Results 4.3 Cost of Zinc Titanate Bed Installation 4.3.1 Basis of Costing 4.3.2 Zinc Titanate Beds 4.3.3 Regen Cooler 4.3.4 Blowers 4.3.5 Results 4.4 Alternative Debottlenecking Technology for Partial Combustion Claus 4.5 Cost of 10% Debottlenecking Using COPE Process 5 Debottlenecking Claus Split Flow System by 10% with Zinc Titanates 6 Debottlenecking Claus Sulfur Recycle System With Zinc Titanate 7 Effect of Zinc Titanate Debottlenecking on Existing Tail; Gas Treatment Systems 7.1 Selectox 7.2 SuperClaus99 7.3 Superclaus 99.5 7.4 SCOT Process 7.5 Zinc Titanate as a Claus Tail Gas Treatment 7.6 H2S Removal Efficiency With Zinc Titanate 8 Effects on COS and CS2 Formation 9 Questions for further Investigation



FIGURES Figure 1 Claus Unit and TGCU Figure 2 Claus Process Figure 3 Typical Claus Sulfur Recovery Unit Figure 4 Two-Stage Claus SRU Figure 5 The Super Claus Process Figure 6 SCOT Figure 7 SCOT/BSR-MDEA (or clone) TGCU REFERENCES: PATENTS US4333855_PROMOTED_ZINC_TITANATE_CATALYTIC_AGENT US4394297_ZINC_TITANATE_CATALYST US6338794B1_DESULFURIZATION_ZINC_TITANATE_SORBENTS



Figure 1 Claus Unit and TGCU



An Investigation of the application of Zinc Titanates For Debottlenecking of Claus Sulfur Recovery Units

1. Executive Summary

Zinc titanates do not appear an attractive option for debottlenecking Claus processes by ~10% because the capital cost of the installation is relatively high (10% of Claus plant cost) compared to the cost of the alternative debottlenecking technology of oxygen enrichment (COPE) which equates to 1-2% of plant cost and is much simpler. However zinc titanates may be attractive as a tail gas treatment process for Claus because the H2S emissions will be reduced significantly and the capital cost of zinc titanate bed installation would be less than many alternative tail gas treatment options (SCOT, Selectox, etc.). Indeed if zinc titanate could be demonstrated to remove H2S completely from the tail gas it may then become attractive if legislation on Sulfur emissions tightens. Since most Claus plants in USA and Western Europe already have tail gas treatment plants installed, there is no cost advantage to be gained by debottlenecking with zinc titanates, however, there could be significant benefit in emerging markets, where Claus units haven no Tail Gas Treatment plants installed.

2. Claus Process; Figure 2



2.1 Partial Combustion Claus Claus Processes are used in refineries to convert the H2S recovered from hydrotreaters and hydrocrackers into pure sulfur. The H2S is generally removed from gas streams by absorption using an aqueous solution of alkanolamine. The solution is then heated and stripped to give H2S gas. Generally on a refinery there may be several sources of H2S which will be combined at the stripping stage before being routed to the sulfur recovery stage. The partial combustion Claus process is used for H2S rich gases with H2S concentrations above ~50-60%. It consists of a thermal stage and several catalytic stages for conversion of H2S to Sulfur. In the thermal stage the gas is burnt with sufficient air to convert 1/3rd of the H2S to SO2. This gives a product with an H2S to SO2 ratio of 2:1 and is highly exothermic. A waste heat boiler downstream of the furnace cools the gas by raising steam. The following reaction takes place during the thermal stage producing substantial amounts of Sulfur:

2H2S + SO2 2H2O + 3S The gas stream is then passed through a condenser to condense the elemental sulfur before being reheated and passed to the first catalytic stage. Reheating is carried out to maintain the gas at a higher temperature than the sulfur dew point in the catalyst bed to prevent sulfur condensation which will rapidly deactivate the catalyst. The gas is then passed through 2-3 catalyst beds in series with sulfur condensation stages and reheat between each catalyst bed. Two catalytic stages will generally give 94-95% sulfur recovery while three stages will give 96-97% recovery. A fourth stage is seldom used as it increase recovery by < 1%. After the catalytic stage the gas stream is generally passed to an incinerator where any remaining H2S is converted to SO2 before the gas is vented to atmosphere. It should be noted that the levels of H2S/SO2 remaining in the gas are still significant and with ever tightening emissions constraints it is often necessary to add a Claus tail gas treatment system downstream of the catalytic stage to meet emissions regulations. This will be discussed later. The important thing to note about the Claus process is that the ratio of H2S to SO2 must be as near to two as possible after the combustion stage to attain the best sulfur recovery.



2.2 Split Flow Claus The split flow Claus process is used when the H2S level is 10-50% in the feed gas. The low H2S level means that partial combustion stage would be unstable at the furnace so only 1/3 of the gas is fed to the furnace and combusted completely to SO2 (which is much more stable). This combusted gas is then mixed with the uncombusted gas stream to give H2S: SO2 = 2:1 before being passed to the first catalytic stage. The catalytic stages are similar to the partial combustion process. 2.3 Sulfur Recycle Claus In cases where the H2S level is <10% the SO2 required for the Claus reaction is produced by burning pure sulfur recovered from downstream to produce the SO2 required for the Claus reaction. Another option is to use a Selectox process to recover the sulfur. Selectox is generally used as a Claus tail gas treatment process and will be covered later.

3. Zinc Titanates Zinc Titanate has the properties of absorbing H2S with the production of water if H2S is passed through a bed of zinc titanate.

Zn2TiO4 + 2H2S 2ZnS + TiO2 + 2H2O The zinc titanate bed can be regenerated by air (oxygen) via the following exothermic reactions:

ZnS + 3/2O2 ZnO + SO2

2ZnO + TiO2 Zn2TiO4

Zinc titanates are formed by mixing Zinc Oxide and Titanium Dioxide solids at high temperature. They have been patented in the past for use in sulfur compound hydrolysis and absorption where the bed can be regenerated with air giving off SO2.



4. Application of Zinc Titanate to Debottleneck Partial Combustion

Claus by 10% 4.1 Process By adjusting the H2S to SO2 ratio in the furnace exit gas to greater than 2:1 this will increase the conversion of the Claus reaction in the thermal and catalytic stages due to the higher levels of H2S in the feed gas. The result is that all of the SO2 will be converted but that an excess of H2S will remain. If a zinc titanate bed were added downstream of the catalytic stages the H2S could be removed down to 10-100 ppm. The zinc titanate could be regenerated with air producing SO2 which could be recycled to the beginning of the process. Both on-line and regeneration beds would be necessary for continuous operation. An Aspen model of the process has been constructed to assess whether a 10% increase in throughput could be achieved by the addition of zinc titanate beds downstream of the catalytic stage. 4.2 Aspen Modeling Results As a basis the mass balance for a partial combustion Claus plant with 3 catalytic stages producing 229.1 Te/day sulfur from Gas Conditioning and Processing Chapter 9 p341 was modeled first. Reasonable agreement with the mass balance was achieved by modeling each conversion stage as a kinetic reactor however the predicted temperatures of the model were very inaccurate. This was investigated with Aspen who concluded that the sulfur physical properties were inaccurate within Aspen for this case and that the species of sulfur being produced would have an effect. Literature indicates that S6 and S8 are likely to be formed in the catalytic stages however Aspen doesn't have built in physical properties for these species and I couldn't find any. For the moment the temperatures of the model have been adjusted manually. The results of the model with two zinc titanate beds ( online and regen beds) was that a 10% increase in throughput was achieved for a reduction in air rate to the furnace of 12%. Conversion of SO2 through the thermal and catalytic stages was improved with all the SO2 converted by the end of the 2nd catalytic stage This is consistent with higher conversions predicted from literature for Superclaus 99 where the Claus unit is operated at higher than 2:1 H2S:SO2 ratio.



The H2S level entering the zinc titanate bed was about 6% of the feed to the process. The zinc titanate regeneration was very exothermic and to control the outlet temperature to around 5000C (to avoid refractory lining the vessel) a recycle loop was included around the bed with a cooler to control the outlet temperature and a recycle blower to provide driving force. The configuration chosen was to have one on-line zinc titanate bed and one regenerating bed with the beds switching over after one hour. The net air rate to the process (furnace + regen) was increased by 9%. No purge flow between absorption and regen cycles was included because there should be no hydrocarbons left present after the furnace. Purging is used when hydrocarbons present in the feed to zinc titanate beds in other duties (not Claus processes) may accumulate in online bed and ignite when regen air is introduced. Sulfur recovery was as follows:

Thermal Stage Kmol/h

Cat Stage 1 Kmol/h

Cat Stage 2 Kmol/h

Cat Stage 3 Kmol/h

Base Case 184.6 85.4 23 4.8 Zinc Titanate

Case 208.6 100.5 23.9 0

The condenser duties were not accurately modeled by Aspen but it is apparent that the third catalytic stage is redundant (subject to confirmation) and that the first two condenser duties are significantly larger. This could be important as it is essential that all sulfur is removed before entering the next catalyst bed to prevent sulfur fouling of the catalyst (COPE increases condenser duties but literature doesn't mention needing new condensers after debottlenecking using COPE). The catalyst beds see 10% higher flow but since the H2S to SO2 ratio is now >2:1 this should compensate for the reduced residence time.



4.3 Cost of Zinc Titanate Bed Installation 4.3.1 Basis of Costing A very rough costing for the inclusion of two zinc titanate bed was derived using historical cost information and cost indices. The additional equipment required was as follows:

Item Number Zinc Titanate Vessels 2

Regen Cooler 1 Regen Blower 1

Air Blower Revamp 1 Feed Blower Revamp 1

Assumptions: a. The pressure drop through the plant will increase due to the 10% increase

in flow and also due to the inclusion of two titanate vessels. It has been assumed that the 3rd catalytic stage can be removed saving some pressure drop and that the feed blower can be revamped to produce 10% more flow at 1.8 bara rather than the current 1.55 bara.

b. It has been assumed that there is sufficient margin above the operating pressures of the existing equipment to run at the new higher pressures. (if not then the feed blower would remain untouched and a booster installed between the catalytic stage outlet and the zinc titanate bed inlet)

c. The furnace air and regen air will be supplied from the existing blower uprated to give 10% more air flow at 2 bara instead of 1.55 bara.

d. The furnace, condensers, reheaters and catalyst vessels are capable of the new duties without modification.

4.3.2 Zinc Titanate Beds The zinc titanate bed have been sized based on the process duty as it is the higher gas flow. A GHSV of 2500 has been found in a relevant patent (Dr. Floyd Farha - US4333855) and a Sulfur loading of 150 kgS/m3 has been assumed. A bed volume of 10 m3 was calculated from the GHSV and a life of 2.2h was predicted. Since the bed is once-through, a bed on-line time of 1 hour has been used.



The vessel was sized with an L/D ratio of 0.7 (absolute minimum) to reduce the DP down to < 0.25 bar based upon 3-5 mm catalyst pellets. The vessel is sized based upon a vertical cylindrical vessel 2.6m ID by 2.5 m H. A cost of $1000K per vessel was arrived at by calculating the cost of a CS vessel in 2009 ( correlation ) then multiplying by 3 to allow for using low alloy steel due to the maximum temperature of 5000C and adding a fabrication allowance. The cost was escalated to present day prices using a factor of 1.6. (This cost may be slightly pessimistic but even using a more accurate cost does not change the conclusions significantly.) The cost of the zinc titanate has been excluded. 4.3.3 Regen Cooler The regen cooler has been sized as an air cooler based upon a cost from a recent study. A cost of $54K was used. 4.3.4 Blowers The process is low pressure and so it is believed that blowers rather than compressors can be used. The recycle blower on the titanate regen recycle is a new item. The cost was derived from a correlation which uses the predicted power consumption to give a price and then converted to present day costs. The prices for the air and feed blower upgrades were assumed to be 30% of the cost of new blowers from the same correlation. The costs were corrected by inflation to give present day costs since a cost indice for 2009 couldn't be found. 4.3.5 Results The total equipment cost was calculated to be $624K and an installation factor of 2,48 was used to give installed cost. The total installed cost was estimated at $1,553K installed (excluding zinc titanate costs)



A correlation from Petroleum Refining predicted a Claus plant installed cost of $15.51M for the base case plant and so the 10% debottlenecking using zinc titanate would cost ~10% of the plant installed cost. 4.4 Alternative Debottlenecking Technology For Partial Combustion Claus The main method of debottlenecking partial combustion Claus processes appears to be oxygen enrichment (COPE). The air used to partially combust the H2S is partially enriched with oxygen reducing the inert nitrogen level in the gas passing through the process and allowing more feed to be processed with no pressure drop penalty. The residence time in the catalyst beds remains unchanged because the gas volume doesn't increase however the amount of sulfur condensed in each condenser will increase which doesn't appear to cause any problems. There are three levels of enrichment: Up to 28% enrichment will give an increase in plant capacity of 10-30% for minimal capital cost. The limit of 28% oxygen is the upper limit before the oxygen pipe work metallurgy and cleanliness become a problem. 28-40% oxygen will allow 50-60% capacity increase. In this process the oxygen is not mixed with the air but is inserted into a new burner via a discrete oxygen port. The limit on this system is that above 40% oxygen the temperature limit on refractory lining of the furnace is exceeded. (COPE Phase 1) 40% to 100% enrichment is only possible with feeds above ~65% H2S in the feed gas and requires a special burner and also the introduction of a recycle flow of cool gas from downstream to moderate the furnace temperature. (COPE Phase 2- In about 30 percent of cases using COPE the furnace and waste heat boiler need replacing). An alternative is SURE where a second combustion stage is added upstream of the existing furnace to limit furnace temperatures. For COPE quoted increase in processing rate of up to 150% can be achieved using oxygen enrichment for a capital cost of between 5 and 25% of the capital cost of the original plant for. I do not think that this cost takes account of the cost of supplying oxygen to the system for enrichment. If oxygen is available on-site then the cost is probably reasonable. If a dedicated oxygen supply (imported liquid oxygen) the cost could be substantial.



For SURE the capital cost of a doubling of capacity appears to be about 50% of the original plant cost excluding oxygen supply. 4.5 Cost of 10% Debottlenecking Using COPE Process The capital cost of debottlenecking the plant would be minimal since only an oxygen pipe break-in to the existing furnace air supply is necessary. Assuming that oxygen is not available on-site, for a 10% debottleneck project the oxygen requirement is insufficient to make an on-site air separation plant economical. The oxygen would be supplied as liquid oxygen and storage etc will be required. The plant operator would rent a suitable storage vessel for $1,600/month (installation costs $8,910). For an increase in Claus capacity of 10% (requiring 9Te/day oxygen) oxygen would cost ~$79/Te. The total cost per hour of operation for oxygen and tank rental and delivery would be $32.34/hour. The annual operating cost would be ~$256K(excluding electric drives) and the capital cost might be ~$82K so in comparison with zinc titanate the COPE process would be cheaper and simpler to install. It would take 6 years before the zinc titanate system started to show a saving over COPE.

5. Debottlenecking Claus Split Flow System by 10% With Zinc Titanates

The Claus split flow system is for mid-level H2S concentrations (10-50%) and could be debottlenecked by removing the furnace and waste heat boiler and adding two zinc titanate beds at the outlet. The hot SO2 from the regen could then be recycled to the beginning of the process and the heat recovered from the regen recycle cooler could be used to preheat the feed before the first catalytic stage. The use of a higher H2S to SO2 ratio would improve conversion and eliminate SO2 from the product. The elimination of the waste heat boiler to some degree balances the increase in pressure drop arising from the titanate bed and the increase in capacity (10%) however a regen recycle blower and cooler would be needed. The feed blower and air blower would probably need upgrading. The capital cost of this modification is likely to be in the same order as the partial combustion debottleneck case. (i.e., 10% of original plant cost) The alternatives for debottlenecking split flow Claus are oxygen enrichment (COPE). Mid level enrichment will probably be necessary to get the nitrogen flow savings required. This requires a dedicated oxygen line and a new burner for the furnace. The cost of oxygen would less than previously since the H2S level is lower but the capital cost will be higher as a new burner is necessary.



It is still likely to be significantly cheaper than the zinc titanate option and the lower oxygen usage will reduce operating costs further.

6. Debottlenecking Claus Sulfur Recycle System With Zinc Titanate The Claus Sulfur recycle process is for low level H2S concentrations <10%. To debottleneck using zinc titanate beds would require the elimination of the sulfur burner and the recycle of SO2 from the titanate regen to give a H2S to SO2 ratio > 2 to improve conversion. Heat from the regen recycle could be used to preheat the feed. No saving in pressure drop will be made since the sulfur burner is not in the main process stream and it's elimination does not reduce pressure drop so blowers/compressors will have to be upgraded. Alternative debottlenecking schemes are unclear. I am not sure if COPE is an option here since the amount of air combusted is so low. If it is viable I suspect that the H2S level is so low that oxygen enrichment of the sulfur burner would require higher than 28% oxygen levels so that dedicated oxygen line and a new burner would be necessary. This area needs further investigation.

7. Effect of Zinc Titanate Debottlenecking on Existing Tail Gas Treatment Systems

The advent of tighter emissions consent levels means that Claus plants alone struggle to meet the emissions levels required. Typically Claus plants are required to recover 99.8% of the sulfur feed while most Claus systems achieve 94-96%. To get around this tail gas treatment systems have been added to the end of the Claus processes to polish the tail gas. They will generally reduce the SO2 level to ~250ppm exit the tail gas system. If titanate beds are used upstream to improve throughput what effect will it have on the tail gas treatment systems. The processes are as follows: 7.1 Selectox Selectox takes Claus tail gas (CO2 with low levels H2S and SO2) and initially burns it with sub stoic air to form H2 & CO (reducing gas) which is fed to a CoMo reactor to form H2S then cooled. It is then fed to the Selectox reactor generating SO2 and S after which the S is condensed. 80% of the sulfur in the tail gas is recovered.



By running the upstream Claus at a higher H2S to SO2 ratio to accommodate the zinc titanate beds this will eliminate the SO2 and produce H2S in excess. The H2S will be removed in the titanate beds and recycled as regenerated SO2. The H2S level in the treated gas exiting the zinc titanate will be 10-100ppm(patent) and there should be minimal SO2 so tail gas treatment shouldn't be necessary as long as the gas is passed through an incinerator to convert the H2S to SO2 which will be 10-100 ppm (lower than required by legislation). Thus the Selectox process would be redundant.

7.2 SuperClaus99 Superclaus99 operates at a H2S to SO2 ratio of greater than 2:1 to convert all the SO2 in the catalytic stages and leave H2S to pass into the SuperClaus reactor where it is converted to sulfur by sub stoic combustion. The use of zinc titanate upstream of the Superclaus reactor would remove the H2S making the Superclaus reactor redundant as long as there was a downstream incinerator to convert the H2S (10-100ppm) to SO2. 7.3 Superclaus 99.5 This version runs with H2S to SO2 ratio of 2:1 and uses a catalytic hydrogenation reactor to convert all the remaining SO2 to H2S before passing it forward to the superclaus reactor. This gives slightly better performance. Zinc titanate beds upstream of the hydrogenation reactor could make Superclaus redundant because only low levels of H2S (10-100ppm) would be coming forward anyway which would be converted to SO2 by the downstream incinerator. 7.4 SCOT Process In the SCOT tail gas process a small amount of hydrogen is added to the tail gas which is passed through a hydrogenation convertor to convert CS2, COS etc into H2S. The gas is cooled to ambient and the H2S absorbed by an amine system before being recycled to the Claus process inlet. The effects of zinc titanate beds for 10% increase in throughput are again to make the SCOT process redundant. The low levels of H2S coming forward from the zinc titanate bed will be converted to SO2 in the incinerator.



7.5 Zinc Titanate as a Claus Tail Gas Treatment Zinc Titanate beds will give a much lower SO2 level going up the stack than the current tail gas treatments. It could be installed on a Claus system for ~10% of the Claus system cost and give 10% more throughput with improved emissions levels. The operating costs of the zinc titanate system are associated with electrical drives for blowers and fans. 7.6 H2S Removal Efficiency With Zinc Titanate Typically the capital cost of a SCOT or Selectox tail gas system is 70-100% of the capital cost of the Claus system and significant operating costs in the order of $100/day per 100T sulfur product are incurred. Superclaus can be installed around 10% of the cost of a Claus plant and is thus similar to the projected cost of installing a zinc titanate tail gas treatment system. For a company who already has a tail gas treatment system the savings in operating costs would not be significant enough to justify switching to zinc titanate beds. However for companies with no tail gas treatment there may be a market for zinc titanates as a debottlenecking system cum tail gas treatment for Claus plants. Most refineries in the US and Europe already appear to have tail gas treatment plants installed on their Claus plants so there doesn’t appear to be much of a market for this in the developed world. (Perhaps there is more of a case in emerging markets.) If we could demonstrate that the zinc titanate beds eliminate SO2 emissions altogether (by consuming all SO2 in Claus reactors and reducing the H2S carry-over from the zinc titanate beds to zero) this may be a better driver for replacing conventional tail gas treatments with zinc titanate beds as environmental SO2 emissions limits tighten. If the zinc titanate beds replace an existing tail gas treatment system there should be no issue over pressure drop as the available pressure drop would suffice. (What vessel sizes are used in tail gas treatment systems. Could the vessels be re-used for zinc titanate? The high design temperature required for the titanate vessels would probably prevent their reuse in a titanate system.)



8. Effects on COS and CS2 Formation

COS and CS2 can be formed in partial combustion processes in the furnace. They can be hydrolyzed in the first catalytic stage of a Claus plant by operating it at around 370C. In the split stream Claus process the complete combustion in the furnace prevents formation of CS2 and COS. In Sulfur recycle no CS2/COS should be formed in the sulfur burner. NH3 can be formed in the sour water shift but generally it will be completely combusted in the furnace by routing it to the main burner. It is unclear in split flow/Sulfur recycle Claus systems how it is dealt with. Using zinc titanates should have no impact on COS or CS2 formation based on current information.

9. Questions for further investigation 9.1 How low will the outlet H2S concentration from a zinc titanate bed be? 9.2 What inlet temperature is necessary for the H2S removal bed and for the

regen bed of a zinc titanate bed. 9.3 What size are the vessels in tail gas treatment systems and what design

temperatures are they likely to have. 9.4 Is more detailed information/physical properties available about the

catalytic stages of the Claus process. 9.5 What proportion of Claus plants have tail gas treatment installed. What is

the split of types of tail gas systems used. 9.6 Cost of zinc titanate. 9.7 Life of zinc titanate. 9.8 Zinc titanates patent search for current patents which may restrict using

zinc titanates in Claus plants.



Figure 3 Typical Claus Sulfur Recovery Unit

Figure 4 Two-Stage Claus SRU



Figure 5 The Super Claus Process

The Super Claus process for producing sulfur:9 (1) main burner, (2,4, 6,8) condensers, (3,5) Claus reactors, (7) reactor with selective oxidation catalyst. Figure 6 SCOT



Figure 7 SCOT/BSR-MDEA (or clone) TGCU