PROClaus: The New Standard for Claus Performance

PROClaus: The New Standard for Claus Performance Mahin Rameshni, P.E. Chief Process Engineer WorleyParsons 125 West Huntington Drive Arcadia, CA, USA Phone: 626-294-3549 Fax: 626-294-3311 E-Mail: mahin.rameshni@worleyparsons.com Robin Street Chief Process Engineer WorleyParsons England, U.K. Phone: 44-208-326-5241 Fax: 44-208-710-0220 E-Mail: robin.street@worleyparsons.com Sulfur Recovery Symposium Brimstone Engineering Services, Inc. Canmore, Alberta April 30 th May 4 th, 2001

Table of Contents Page Abstract... i Section 1 Introduction Section 2 2.1 Tail Gas Hydrogenation...2-1 2.2 Direct Oxidation...2-1 2.3 Sub-Dewpoint Claus...2-2 2.4 WorleyParsons Latest Development PROClaus Process...2-3 2.4.1 Process Chemistry...2-4 2.4.2 PROClaus Process...2-6 2.4.3 Demonstration of the First Commercial PROClaus Process...2-9 2.4.4 Applications of 3-Stage & 4-Stage PROClaus Processes...2-10 2.4.5 Key Advantages of the PROClaus Process...2-12 Section 3 Comparison of Tail Gas Clean-Up Processes Section 4 Conclusion Figures Figure 1 Comparison of PROClaus & Other Processes...2-2 Figure 2a Three-State PROClaus Process Flow Diagram...2-7 Figure 2b Four-Stage PROClaus Process Flow Diagram...2-12 Figure 3 Figure 4 Figure 5 Gas Composition at Inlet of 2 nd Converter vs H 2 S Concentration in Feed for Air-Based SRU...4-2 Ratios of Reducing Gases/SO 2 at Inlet of 2 nd Converter vs. H 2 S Concentration in Feed for Air-Based SRU...4-3 Gas Composition at Inlet of 2 nd Converter vs. O 2 Concentration for Oxygen-Enriched SRU...4-4 Figure 6 Ratios of Reducing Gases/SO 2 at Inlet of 2 nd Converter vs. O 2 Concentration for Oxygen-Enriched SRU...4-5 PROClaus1.doc

Table of Contents Page Table Table 1 Comparison of TGCU Processes...3-1 PROClaus1.doc

Abstract With the sulfur content of crude oil and natural gas on the increase and with the ever-tightening sulfur content in fuels, the refiners and gas processors are pushed for additional sulfur recovery capacity. At the same time, environmental regulatory agencies of many countries continue to promulgate more stringent standards for sulfur emissions from oil, gas, and chemical processing facilities. It is necessary to develop and implement reliable and cost effective technologies to cope with the changing requirements. In response to this trend, several new technologies are now emerging to comply with the most stringent regulations. These advances are not only in the process technology but also in the manner in which the traditional modified Claus process is viewed and operated. Expansion demand in sulfur recovery processing capacity for refineries and gas plants worldwide is on the rise due to the expansion activities of overall plant capacity. Over the years, a number of different tail gas clean-up processes have been developed in combination with the basic modified Claus process in order to overcome the thermodynamic equilibrium of the Claus reaction and increase the overall sulfur recovery. The most commonly used tail gas clean-up processes can be divided into three categories: 1. Tail gas hydrogenation/hydrolysis followed by either selective catalytic oxidation (e.g., BSR/Selectox, BSR/Hi-Activity), or selective amine coupled with acid gas recycle (e.g., BSR/MDEA, BSR/Flexsorb, SCOT); 2. Direct oxidation of H 2 S to elemental sulfur (e.g., SuperClaus); 3. Sub-dewpoint Claus (e.g., MCRC, CBA, Sulfreen, Clauspol) As compared to conventional modified Claus units, the above mentioned tail gas clean-up processes require either more processing units, or changing the continuous Claus operation to cyclic or more sensitive/attentive operation (e.g., H 2 S-shifted Claus operation for the direct oxidation process, and cyclic bed switching/regeneration for the sub-dewpoint process). WorleyParsons proprietary PROClaus (WorleyParsons RedOx Claus) process combines three distinct processing steps - conventional Claus reaction, selective reduction of SO 2 and selective oxidation of H 2 S, into one processing scheme. This evolutionary process doesn t rely on tail gas hydrogenation, H 2 S-shifted Claus operation, or cyclic sub-dewpoint operation. Instead, the PROClaus process is a continuous dry catalytic process which operates the reaction furnace and the first Claus stage (or the second stage Claus) just like a conventional modified Claus unit, and the Claus stage is followed by a selective reduction stage and a selective oxidation stage. In a 3-stage or 4-stage configuration, PROClaus can achieve up to 99.5% overall sulfur recovery pending the acid gas compositions. A grass root PROClaus unit is expected to be more cost competitive than any tail gas hydrogenation options, sub-dewpoint Claus and direct oxidation schemes at a i

Abstract comparable sulfur recovery efficiency basis. Any existing 3-stage Claus units can be easily retrofitted to the PROClaus scheme with simple catalysts change out. In addition, the Claus section coupled with any tail gas hydrogenation options can be retrofitted to reduce the total sulfur load feeding the tail gas section. Since two of the three processing steps of the PROClaus process, Claus and selective oxidation, have long been commercially proven, the development efforts were focused on the selective reduction step. The most critical element in the success of the selective reduction step was the development of a highly active and selective catalyst for the reduction of SO 2 to elemental sulfur by reducing gases such as H 2 and CO for Claus operating conditions. This paper will discuss the basic chemistry, process concept, process and catalyst development, key advantages of the PROClaus process, and the comparison of PROClaus to other tail gas clean-up processes. In order to increase the sulfur recovery and meet the new emissions requirements in many countries, the PROClaus technology provides cost effective solutions to this demand, the minimum modifications are required to modify the existing conventional Claus process to convert to PROClaus process. This paper will also discuss the first commercial PROClaus process, which took place at PDVSA s Puerto La Cruz Refining, in Venezuela, April 2001. ii

Section 1 Introduction The most commonly used process for recovering elemental sulfur from acid gas is the modified Claus process. Conventional modified Claus plants typically consist of two to three catalytic stages. Typical overall sulfur recovery ranges from 93-95% and 95-97% for 2-stage Claus and 3-stage Claus plants, respectively, due to the thermodynamic equilibrium limitation of the Claus reaction. Over the years, a number of different methods have been developed in combination with the basic modified Claus process in order to overcome the thermodynamic equilibrium of the Claus reaction and increase the overall sulfur recovery (Figure 1). 1-1

2.1 Tail Gas Hydrogenation 2.2 Direct Oxidation One of the most widely adopted methods involves a Claus tail gas hydrogenation/hydrolysis step followed by either selective catalytic oxidation, or selective amine coupled with recycle of acid gas to the Claus unit. Typically, these processes include two Claus stages, and an hydrogenation/hydrolysis stage which reduces all sulfur species, such as SO 2, COS, CS 2, and sulfur vapor in the Claus tail gas to H 2 S. After the hydrogenation/hydrolysis stage, if the overall sulfur recovery requirement is about 98-99+ %, a selective oxidation catalytic stage can be adopted as the final stage. Commercial processes belong to this category are WorleyParsons/UOP s BSR (Beavon Sulfur Removal)/Selectox, and BSR/Hi- Activity. If the overall sulfur recovery requirement is more than 99.8%, a selective amine step can be adopted as the final step. Selective amine, such as DIPA, MDEA or Flexsorb, is used to absorb most of the H 2 S in the hydrogenated tail gas, and the absorbed H 2 S is thermally regenerated and recycled back to the Claus section for maximal sulfur recovery at the expense of significantly more capital expenditures. Commercial processes such as WorleyParsons/UOP s BSR/Amine or Shell s SCOT are the pioneers of this process concept. The hydrogenation type of processes consist of either four catalytic stages or three catalytic stages followed by a tail gas amine unit. Another method to increase overall sulfur recovery for a modified Claus process is to use a selective catalytic oxidation step as the last stage in conjunction with a 2- stage or 3-stage Claus reactors, such as the SuperClaus process. This approach requires operating an H 2 S-shifted Claus operation (H 2 S to SO 2 ratio higher than 2:1) by reducing the amount of air supplied to the Claus reaction furnace, since any remaining SO 2 leaving the last Claus stage will not be converted to elemental sulfur in the last selective oxidation stage. Instead of controlling the H 2 S/SO 2 ratio in the tail gas at 2:1, direct oxidation scheme requires the control of H 2 S concentration entering the last selective oxidation stage since the direct oxidation reaction of H 2 S is extremely exothermic. Overall sulfur recovery of high-98% can be achieved for a combined 3-stage configuration and up to mid-99% can be achieved for a 4-stage configuration. 2-1

2.3 Sub-Dewpoint Claus Another method employs sub-dewpoint Claus operation. As opposed to the conventional Claus catalytic converters where the produced sulfur remains in the vapor phase, the sub-dewpoint Claus operates the Claus converters at below sulfur dewpoint temperatures. As it is well understood that the Claus equilibrium conversion of H 2 S conversion to sulfur increases with decreasing temperatures in the catalytic operation region. In this operation, produced sulfur is condensed and adsorbed on the catalyst, and subsequently routine bed switching and regeneration is required. A 3-stage sub-dewpoint scheme can achieve about 99% recovery; and a 4-bed system can achieve up to mid-99% recovery. Commercial processes include MCRC, CBA, and Sulfreen. Figure 1 - Comparison of PROClaus & Other Processes Modified Claus 95 % 97 % AC H 2 S / SO 2 Thermal Stage Converter # 1 Claus Converter # 2 Claus Converter # 3 Claus Hydrogenation H 2 S / SO 2 AC 99.9 % Thermal Stage Converter # 1 Claus Converter # 2 Claus Converter # 3 Hydrogenation Water Removal Amine 99.0 % Air Converter # 4 Selectox D irect Oxidation Thermal Stage Converter # 1 Claus Converter # 2 Claus H 2 S AC Air 98.8 % Converter # 3 Selective Oxidation or Claus AC H 2 S 99.3 % Converter # 4 Selective Oxidation 99.5 % Converter # 4 Hi-Activity Air Air 2-2

Sub-Dew point Thermal Stage Converter # 1 Claus Converter # 2 Sub-Dew Point 99.0 % Converter # 3 Sub-Dew Point H 2 S / SO 2 AC 99.5 % Converter # 4 Sub-Dew Point H 2 S / SO 2 AC PROClaus H 2 S / SO 2 99.5 % AC Thermal Stage Converter # 1 Claus Converter # 2 Selective Reduction Converter # 3 Selective O xidation Air 2.4 WorleyParsons Latest Development PROClaus Process The commercially available tail gas clean-up options, as compared to conventional modified Claus process, require either additional processing steps or changing the continuous modified Claus operation to a more sensitive/attentive or cyclic mode of operation. For the tail gas hydrogenation options, the additional processing units include a hydrogenation step, a hydrogenated tail gas quenching and water removal step (this step is not required for the BSR/Hi-Activity option), and a final processing step of either a selective catalytic stage of a selective amine unit. These additional processing units translate to significantly more capital costs as compared to conventional modified Claus units. Direct oxidation scheme, as discussed above, requires an H 2 S-shifted Claus operation by reducing the amount of air supplied to the Claus reaction furnace. Instead of controlling the H 2 S/SO 2 ratio in the tail gas at 2:1, direct oxidation scheme requires operating the H 2 S to SO 2 ratio higher than the normal Claus ratio of 2:1 and the control of H 2 S concentration entering the last selective oxidation stage. H 2 S-shifted Claus operation can lead to potential high H 2 S concentration at the inlet of the direct oxidation reactor. Since the direct oxidation reaction of H 2 S to elemental sulfur is extremely exothermic, potential temperature excursion can occur in the selective oxidation reactor. Precise process air and H 2 S concentration control, and provision of direct oxidation reactor bypass are essential for the direct oxidation scheme. Sub-dewpoint Claus schemes requires additional switching valves and larger converter size than conventional Claus, which results in additional capital costs. In addition, liquid sulfur deposition in the catalyst bed requires cyclic operation, as 2-3

opposed to continuous Claus operation, which involves routine beds switching for regeneration. WorleyParsons latest developed Claus tail gas scheme, PROClaus (WorleyParsons RedOx Claus) process, makes an evolutionary improvement to the current tail gas schemes by eliminating the requirements of additional processing units, or changing the conventional continuous Claus operation to either shifted or cyclic operation. The PROClaus process is a continuous catalytic process that combines Claus reaction, selective reduction of SO 2 to sulfur, and selective oxidation of H 2 S to sulfur into one integrated processing scheme. 2.4.1 Process Chemistry Claus Reaction The basic chemical reactions occurring in a Claus process are represented by the following reactions: Kp at 260 o C H 2 S + 3/2 O 2 -> SO 2 + H 2 O (1) 2 H 2 S + SO 2 <-> 3/x S x + 2 H 2 O (2) 1 x 10 4 In the reaction furnace, one third of the inlet H 2 S is thermally converted to SO 2 in the reaction furnace of the thermal stage according to reaction (1). The remaining two thirds of the H 2 S is then reacted with the thermally produced SO 2 to form elemental sulfur in the thermal stage and the subsequent catalytic stages according to reaction (2). Claus reaction (2) is thermodynamically limited due to the presence of inerts (such as H 2 O) and relatively low equilibrium constant for reaction (2) over the catalytic operation region. Selective Reduction of SO 2 to Sulfur Selective reduction of SO 2 to elemental sulfur by reducing gases (such as H 2 and CO) are represented by the following reactions: Kp at 200 o C SO 2 + 2 H 2 <-> 1/x S x + 2 H 2 O (3) 3.0 x 10 12 SO 2 + 2 CO <-> 1/x S x + 2 CO 2 (4) 4.2 x 10 17 2-4

Reactions (3) and (4) are highly favorable thermodynamically and have very large equilibrium constants, as indicated above, over the conventional Claus catalytic operating temperature range. However, a number of secondary reactions can take place between SO 2 and H 2 /CO producing undesirable by-products such as H 2 S and COS, as represented by the following reactions: Kp at 200 o C SO 2 + 3 H 2 <-> H 2 S + 2 H 2 O (5) 5.8 x 10 19 SO 2 + 3 CO <-> COS + 2 CO 2 (6) 1.1 x 10 24 If elemental sulfur production is the process objective, an effective catalyst must not only be able to achieve high conversion of SO 2, but also achieve high selectivity to elemental sulfur with both H 2 and CO. A few researches had been conducted to develop catalysts for the selective reduction of SO 2 to elemental sulfur by reducing gases such as H 2 and CO. The research efforts had been focused on improving the selectivity and conversion efficiency for the production of elemental sulfur at high space velocity (up to 15,000 hr -1 ) and at a temperature range of 300 to 600 o C, which is much higher than the conventional Claus catalytic operation. In addition, these research efforts had been focused on flue gas cleanup applications based on more concentrated SO 2 feed streams (3 to 30% SO 2 ) derived from regenerable Flue Gas Desulfurization (FGD) processes or advanced integrated gasification combined cycle (IGCC). However, selective reduction of SO 2 to elemental sulfur under Claus catalytic operating conditions had not been explored until WorleyParsons and Lawrence Berkeley National Laboratory started the developmental works a few years ago. Selective Oxidation of H 2 S to Sulfur Selective oxidation of H 2 S to sulfur using H 2 S-shifted operation has been commercialized since the late 1980s (Stork s SuperClaus), and combined hydrogenation/selective oxidation operation has been commercialized since the mid-1990s (WorleyParsons BSR/Hi-Activity). Selective oxidation of H 2 S is represented by the following reaction: Kp at 260 o C H 2 S + 1/2 O 2 <-> 1/x S x + H 2 O (7) 1 x 10 14 2-5

Selective catalysts have been developed and commercialized for the reaction between H 2 S and O 2 for the production of elemental sulfur to minimize the undesirable by-prducts such as SO 2 and SO 3. 2.4.2 PROClaus Process Processing Steps WorleyParsons proprietary patented PROClaus (WorleyParsons RedOx Claus) process, as suggested by its name, consists of three processing steps: Step 1 - a conventional Claus thermal stage and at least one Claus catalytic stage Step 2 - a Selective Reduction stage that converts SO 2 to elemental sulfur Step 3 - a Selective Oxidation stage that converts H 2 S to elemental sulfur The keys to this new process invention are: Combining three distinct processing steps, two being commercially proven, into one fully integrated process. Taking the advantage of the H 2 and CO produced in the Claus reaction furnace as reducing gas for processing Step 2 - selective reduction of SO 2 to elemental sulfur. No external supply of reducing gas is necessary. Develop a highly selective SO 2 reduction catalyst for Claus type process gas (diluted SO 2 stream and lower operating temperatures as compared to previous research efforts focused on FGD applications). Process Description Figure 2a is a simplified process flow diagram of a 3-stage PROClaus process. Acid gas is fed to a Claus thermal stage with controlled amount of air (or oxygen) in which one third of the inlet H 2 S is thermally converted to SO 2 at the burner according to reaction (1). The remaining two thirds of the inlet H 2 S is then reacted with the thermally formed SO 2 to produce elemental sulfur according to reaction (2). Due to the thermodynamic equilibrium of the Claus reaction, only about 60+% of the sulfur is produced in the thermal stage. After waste heat recovery and sulfur condensation, the remaining H 2 S and SO 2 is further reacted in a Claus catalytic converter. The effluent from the 1 st Claus converter is cooled in a sulfur condenser to recover the produced sulfur. A conventional air demand analyzer is located at the outlet of the sulfur condenser for H 2 S/SO 2 ratio control in the thermal stage and 2-6

the Claus converter. The process gas is then reheated prior to entering the second selective reduction converter. In the 2 nd converter, more than 90% of the remaining SO 2 is reduced to elemental sulfur, in the presence of a highly selective SO 2 reduction catalyst developed by Lawrence Berkeley National Laboratory (LBNL catalyst), by reducing gases (H 2 and CO) produced in the reaction furnace as well as by H 2 S. The conversion of H 2 S in the selective reduction converter is equivalent to the Claus equilibrium as in a typical 2 nd stage Claus converter. The effluent from the 2 nd converter is cooled in a sulfur condenser to recover the produced sulfur and reheated to the temperature required for the selective oxidation reaction. The reheated process gas is then mixed with process air prior to entering the last converter. In the 3 rd converter, more than 90% of the remaining H 2 S is reacted with O 2 to form elemental sulfur in the presence of WorleyParsons Hi-Activity selective oxidation catalyst. The converter effluent is cooled in the last sulfur condenser before the tail gas is routed to a thermal oxidizer. An oxygen analyzer located at the outlet of the last sulfur condenser is used to control the process air added to the selective oxidation converter. An overall sulfur recovery of up to 99.5% can be achieved with a 3-stage configuration. Figure 2a, Three-Stage PROClaus Process Flow Diagram Air Acid Gas K.O Drum Reaction Furnace HP Steam Waste Heat Boiler BFW Reheater No. 1 LP Steam Condenser No. 1 Claus Converter LP Steam Condenser No. 2 Reheater No. 2 Selective Reduction Converter H 2 S/SO 2 AC LP Steam Condenser No. 3 Reheater No. 3 Selective Oxidation Converter LP Steam Condenser No. 4 O 2 AC Tail Gas Water Air BFW BFW BFW BFW M Air Blower Sulfur Pit Liquid Sulfur Sulfur Pump 2-7

Reducing Gases Availability One of the key innovations of the PROClaus process is the adaptation of an SO 2 selective reduction step in a Claus environment. According to reactions (3) and (4), two moles of reducing gas are required for each mole of SO 2. In addition, SO 2 is being reduced by H 2 S to the extent that is very similar to the Claus equilibrium in the presence of the LBNL catalyst, and two moles of H 2 S is required for each mole of SO 2 in accordance with the stoichiometry of reaction (2). The Claus thermal stage generates enough reducing gases required for the selective reduction step and no external supply of reducing gases are required for the PROClaus process. A discussion of reducing gases availability for both air-based SRUs and oxygenenriched SRUs, and the impacts of high intensity burners, such as the BOC SURE burner, are followed. Formation of reducing gases, namely H 2 and CO, in Claus reaction furnace and waste heat boiler have been studied extensively by Western Research (now known as Sulphur Experts) since the 1970s. Based on information obtained from Sulphur Experts, empirical plant data from about 500 SRUs have been collected and correlated over the years into their Sulsim simulation program. Different empirical models have been developed for four different types of acid gas feeds: ammonia gas, lean acid gas, rich acid gas, and oxygen enrichment. Based on the Sulsim empirical model predictions, the percentages of gas components, including H 2 S, SO 2, H 2 and CO, after the first stage Claus converter and condenser are plotted in Figure 3 as a function of H 2 S concentration in the acid gas feed for air-based SRU. The ratios of reducing gases (both H 2 +CO and H 2 S+H 2 +CO) over SO 2 are plotted in Figure 4. As indicated in Figure 4, the ratios of reducing gases over SO 2 are fairly constant and always far above the stoichiometric requirement of 2 to 1 for the SO 2 reduction reactions over a wide range of acid gas feeds. It is known that higher flame temperatures promote the dissociation of H 2 S to H 2 and sulfur, and we should expect that oxygen enrichment would produce more reducing gas due to its higher flame temperature. Based on the Sulsim empirical model predictions, the percentages of gas components, including H 2 S, SO 2, H 2 and CO, after the first stage Claus converter and condenser are plotted in Figure 5 as a function of O 2 concentration in the O 2 -enriched air feed for a 90% H 2 S feed. As indicated in Figure 5, the concentrations of both H 2 and CO increase as the O 2 concentration increases. However, H 2 S and SO 2 concentrations also increase because of the O 2 -enriched air operation. As a result of the increases of both the reducing gases and SO 2, the ratios of reducing gases over SO 2 as a function of O 2 concentrations are about the same as in the air-based SRU case. The ratios of reducing gases (both H 2 +CO and H 2 S+H 2 +CO) over SO 2 for O 2 -enriched air operation are plotted in Figure 6. 2-8

Burner design is another major factor in promoting reducing gases generation in the reaction furnace. For example, BOC SURE burner is a tip mix burner, as compared to a premix type, which by design promotes intense mixing and hot zones within the flame, and these hot zones enhance the dissociation reactions. The primary example of these dissociation reactions is H 2 S producing H 2 and sulfur. 2.4.3 Demonstration of the First Commercial PROClaus Process For the first time, the PROClaus Process is demonstrated at one of the existing conventional Claus process at Puerto La Cruz Refinery in Venezuela which was originally designed for about 20MTPD, but it is required to meet the new emissions regulation. An overall guaranteed sulfur recovery efficiency of over 98.9 percent can be achieved with PROClaus, compared to 96-97 percent sulfur recovery with a conventional three-stage Claus unit. The scope of demonstration was the conversion the existing 3-stage Claus sulfur recovery unit to a PROClaus process in order to increase the sulfur recovery efficiency of the unit to meet new Venezuelan environmental regulations. The acid gas feed contains about 67% H 2 S and the remaining are CO 2, H 2 O, and various hydrocarbons. Few changes to the existing three-stage unit were required for the PROClaus conversion. The thermal section, the sulfur condensers and the sulfur reheaters were not required for any changes. The major modifications were: replacement of the second and third stage catalyst beds with direct reduction and direct oxidation catalysts, respectively; relocation of the existing tail gas H 2 S/SO 2 analyzer, a new air line with flow control for the direct oxidation stage; a static mixer to mix the air and process gas feed to the direct oxidation reactor; a heater for presulfiding the direct reduction catalyst; and oxygen analyzer on the outlet of the direct oxidation stage. The H 2 S/SO 2 and H 2 /CO analyzer was installed at the inlet and the outlet to monitor the performance of the direct reduction catalyst. The H2/CO analyzer were installed for evaluation of the reduction catalyst and to record the performance of the catalyst, and it will not be required to install for a new commercial plants. The existing unit was designed for 20 about MTPD, but the demonstration was performed for about 10 MTPD. The Direct Reduction Section is the second catalytic stage of the PROClaus sulfur recovery unit. In the direct reduction section, SO 2 is directly converted to elemental sulfur over a selective catalyst. The produced sulfur is recovered in a sulfur condenser. 2-9

The Hi-Activity Section is the third catalytic stage of the PROClaus sulfur recovery unit. In the Hi-Activity section, H 2 S is directly oxidized to elemental sulfur over a selective catalyst. The yield of the Hi-activity catalyst is about 90%. The produced sulfur is recovered in a sulfur condenser. 2.4.4 Applications of 3-Stage & 4-Stage PROClaus Process Since the emissions requirements are getting tighter more then ever, all the sulfur plants around the world will be affected depends on new requirements. The refiners and gas processors are pushed for additional sulfur recovery capacity, and lower emissions, it is necessary to develop and implement reliable and cost effective technologies to cope with the changing requirements. In response to this trend the following factors should be considered, in order to do the best process selection. Feed gas composition, including H 2 S content, NH 3, CO 2 and hydrocarbons, and other contaminants, & gas pressure and temperature Minimum modifications, & Optimization of the existing equipment Minimum shut downs during modifications Product specification, such as H 2 S, CO 2, H 2 O, hydrocarbons, and mercaptans Required recovery efficiency Concentration of sulphur species in the stack gas Ease of operation Remote location Sulphur product quality Costs (capital and operating) In many countries, the new emissions regulation could be compromised by modifying the conventional existing Claus plant to a three-stage PROClaus Process or for a higher recovery to a four-stage PROClaus Process. The same time the capacity of the sulfur plant could be increased by using WorleyParsons Oxygen enrichment technology at the low, medium and high level of oxygen (28, 45,100 percent) the capacity could be increased up to 25%, 75%, and 150% respectively. 2-10

In most cases no modifications are required by using low level of oxygen, and with burner replacement to BOC SURE burner to increased capacity could be achieved. The same time using PROClaus process will increase the sulfur recovery to meet the requirements. PROClaus offers minimum modifications with low capital and operating cost, and ease of operation for existing plants, as well as for new plants. Three Stage or four-stage PROClaus process could be selected based on new emissions requirements. WorleyParsons proprietary patented 4-Stage PROClaus process, consists of three processing steps: Step 1 - a conventional Claus thermal stage and two- Claus catalytic stage Step 2 - a Selective Reduction stage (the third stage) that converts SO 2 to elemental sulfur Step 3 - a Selective Oxidation stage (the fourth stage) that converts H 2 S to elemental sulfur Figure 2a, and 2b is a simplified process flow diagram of a three-stage and fourstage PROClaus process respectively. 2-11

Figure 2b, Four-Stage PROClaus Process Flow Diagram 2.4.5 Key Advantages of the PROClaus Process The PROClaus process offers the following distinct advantages over other tail gas clean- up processes: 1) The performance of the PROClaus process is not affected by the normal fluctuation of the H 2 S/SO 2 ratio experienced in typical Claus plants. The frontend Claus section of the PROClaus process operates just like a conventional Claus unit. H 2 S/SO 2 ratio of this section is controlled at 2:1 for optimal sulfur recovery efficiency in accordance with the Claus reaction. Even if the Claus section is operated off-ratio, either lower or higher than the preferred 2:1 ratio for the Claus section, the incremental amounts of either SO 2 or H 2 S can be essentially converted to elemental sulfur in the subsequent selective reduction stage or the selective oxidation stage. This process feature offers easy and forgiving control as compared to conventional Claus which requires the control of the H 2 S/SO 2 ratio at 2:1; and direct oxidation process which requires shifted- H 2 S operation in order to control the inlet H 2 S concentration to the selective oxidation reactor. 2) No hydrogenation step is required as in the BSR or SCOT type processes since the remaining SO 2 from the last Claus catalytic stage is directly converted to elemental sulfur in the selective reduction stage. Reducing gases are used for direct sulfur production instead of hydrogenation. This process feature offers a major reduction in capital costs as compared to conventional hydrogenation type tail gas treating processes. 2-12

3) No water removal step is required since both the selective reduction and the selective oxidation catalysts are substantially insensitive to the presence of water vapor in the Claus tail gas. This process feature offers another reduction in capital costs as compared to conventional hydrogenation type tail gas treating processes. 4) Within the control fluctuations experienced under normal operating conditions, the thermal Claus stage produces more than stoichiometric amount of reducing gases for the subsequent selective reduction reactions which convert SO 2 to elemental sulfur. External reducing gas is normally not required. 5) The normal Claus operation and the selective reduction stage reduces the inlet H 2 S concentration to the last selective oxidation stage substantially as compared to direct oxidation processs based on the same number of catalytic reactors adopted. This process feature reduces potential temperature excursion problems across the selective oxidation stage and, subsequently, provides better protection for the direct oxidation catalyst and reduces reactor downtime. 6) A 3-stage or 4-stage PROClaus unit can obtain up to 99.5% overall sulfur recovery, which is higher than other processes, such as a conventional 3- stage Claus, a 3-stage direct oxidation process, or a 3-stage sub-dewpoint Claus. 7) Existing 3-stage Claus plants can be easily modified to 3-stage PROClaus plants by changing out the catalysts in the second and the third converters with the selective catalysts, and minor piping modifications to the last selective oxidation stage. Overall sulfur recovery efficiency can be improved from 95-97% up to 99.5%. Existing 3-stage direct oxidation plants can also be converted to PROClaus by simply changing out the Claus catalyst in the second converter with the selective reduction catalyst and switching the shifted-h 2 S operation to normal Claus operation. Overall sulfur recovery efficiency can be improved from high-98% up to 99.5%. The performance of a 3-stage PROClaus is equivalent to a 4-stage direct oxidation process. 8) Sub-dewpoint processes require cyclic operation, and routine valves switching and catalytic bed regeneration. Unlike sub-dewpoint operation, PROClaus process is a continuous and steady dry catalytic process without routine sulfur emission spikes. 9) Any Claus unit equipped with hydrogenation type of tail gas cleanup processes can be retrofitted to reduce the total sulfur load feeding the TGTU. It involves simply changing out the 2 nd stage Claus catalyst with the LBNL selective reduction catalyst. This type of application is extremely useful in 2-13

debottlenecking the processing capacity of existing BSR/amine type of TGTUs, or reducing the chemical/operating costs of existing BSR/Stretford type of TGTUs. 2-14

Section 3 Comparison of Tail Gas Cleanup Processes As discussed in the above section, PROClaus process offers a number of advantages over other commercially available tail gas clean-up processes. Table 1 summarizes the comparison results between PROClaus and other TGCU processes in (1) number of converters; (2) sulfur recovery; and (3) relative cost as compared to a 3-stage modified Claus plant. Table 1 Comparison of TGCU Processes Process No. of Converters Sulfur Recovery,% Relative Cost Modified Claus 3 97.0 1.00 PROClaus 3 99.2 1.15 PROClaus 4 99.5 1.20 Sub-Dewpoint 3 99.0 1.20 Sub-Dewpoint 4 99.5 1.40 Direct Oxidation 3 98.8 1.15 Direct Oxidation 4 99.3 1.30 BSR/Hi-Activity 4 99.2 1.35 BSR/Amine or SCOT 3 + amine 99.9 1.70 As indicated in Table 1, PROClaus is the most cost-effective processing scheme that can achieve an overall sulfur recovery of up to 99.5%. 3-1

Section 4 Conclusions WorleyParsons proprietary PROClaus process combines the conventional Claus processing step with two selective reaction steps in a 3-stage or 4-stage configuration (depends to acid gas compositions) which enhances the overall sulfur recovery up to 99.5%. The PROClaus process utilizes two highly selective catalysts for direct reduction of SO 2 and direct oxidation of H 2 S to elemental sulfur. This innovative processing scheme overcomes the sulfur yields limitation dictated by the Claus equilibrium. In addition, it offers distinct advantages over other competing technologies: No need to operate the Claus stages off-ratio as in the SuperClaus process, which reduces the recovery efficiency of the Claus stages as well as increasing the inlet H 2 S concentration to the last direct oxidation stage. No need to operate at sub-dewpoint as in the CBA and MCRC processes, which requires routine valves switching and bed regeneration. Do not require a hydrogenation step since SO 2 is converted directly to elemental sulfur in the presence of the highly selective LBNL catalyst. In order to achieve a higher the sulfur recovery and to increase the sulfur capacity by using oxygen in sulfur plants the same time, PROClaus and SURE Burner together will respond to this trend in many countries. A demonstration unit for the PROClaus process was successfully started in April 2001. PROClaus is the latest Revolution in Claus Performance to increase the sulfur recovery in an easiest way. PROClaus process s technological, operational and economic edges over other commercial TGCU processes will certainly revolutionize how an efficient and costeffective SRU/TGCU should be designed and operated in the future. 4-1

Section 4 Conclusions Figure 3 Gas Composition at Inlet of 2 nd Converter vs. H 2 S Concentration in Feed for Air-Based SRU 5 4.5 H2S+H2+CO % Gas Component at Inlet of 2nd Converter 4 3.5 3 2.5 2 1.5 1 0.5 H2+CO H2 H2S SO2 CO 0 50 55 60 65 70 75 80 85 90 95 % H2S in Feed (Dry Basis) Note: 1) Data derived from Sulphur Experts SULSIM simulation empirical model. 4-2

Section 4 Conclusions Figure 4 Ratios of Reducing Gases/SO 2 at Inlet of 2 nd Converter vs. H 2 S Concentration in Feed for Air-Based SRU 6.00 Ratio (Reducing Gases/SO2) at Inlet 2nd Converter 5.00 4.00 3.00 2.00 1.00 (H2S+H2+CO)/SO2 (H2+CO)/SO2 0.00 50 55 60 65 70 75 80 85 90 95 % H2S in Feed (Dry Basis) Note: 1) Data derived from Sulphur Experts SULSIM simulation empirical model. 4-3

Section 4 Conclusions Figure 5 Gas Composition at Inlet of 2 nd Converter vs. O 2 Concentration for Oxygen-Enriched SRU 9 8 H2S+H2+CO % Gas Component at Inlet of 2nd Converter 7 6 5 4 3 2 H2+CO H2S H2 1 SO2 CO 0 20 25 30 35 40 45 50 55 % O2 in Air Note: 1) Based on acid gas feed containing 90% H 2 S (dry basis). 2) Data derived from Sulphur Experts SULSIM simulation empirical model. 4-4

Section 4 Conclusions Figure 6 Ratios of Reducing Gases/SO 2 at Inlet of 2 nd Converter vs. O 2 Concentration for Oxygen-Enriched SRU 6.00 Ratio (Reducing Gases/SO2) at Inlet 2nd Converter 5.00 4.00 3.00 2.00 1.00 (H2S+H2+CO)/SO2 (H2+CO)/SO2 0.00 20 25 30 35 40 45 50 55 % O2 in Air Note: 1) Based on acid gas feed containing 90% H 2 S (dry basis). 2) Data derived from Sulphur Experts SULSIM simulation empirical model. 4-5