The Shell-Paques/THIOPAQ Gas Desulphurisation Process: Successful Start Up First Commercial Unit.

Transcription

1 The Shell-Paques/THIOPAQ Gas Desulphurisation Process: Successful Start Up First Commercial Unit. Alex Benschop and Albert Janssen 1, Alie Hoksberg 2, Munaf Seriwala 3, Ray Abry 4 and Charles Ngai 5 1 Paques B.V., P.O. Box 52, 8560 AB Balk, The Netherlands. A.Janssen@paques.nl 2 Shell Global Solutions Int. B.V., P.O. Box 38000, 1030 BN Amsterdam, The Netherlands. Alie.C.Hoksberg@opc.shell.com 3 UOP, P.O. Box 5017, Des Plaines, IL , USA. mmseriwa@uop.com 4 New Paradigm Gas Processing Ltd., 145, th Ave. S.E., Calgary, AB, Canada, abry@npgas.net 5 Encana Resources, P.O. Box 2850, Calgary, Alberta, Canada T2P 2S5, charles.ngai@encana.com A new process for H 2 S removal from high pressure natural gas, synthesis gas and refinery gas streams has been developed by Shell Global Solutions International BV, UOP and Paques BV. In this process, which already successfully operates in biogas (atmospheric mixture of CH 4, CO 2 and H 2 S) sweetening since 1993, complete removal of H 2 S can be achieved by selective biological conversion of H 2 S into elemental sulphur. A long duration test in a large pilot-plant for treatment of high-pressure natural gas, has demonstrated very smooth operation of the process, showing the high potential of this new breakthrough technology for gas desulphurisation. For the past three years this process was marketed as a very attractive alternative to traditional desulphurisation technologies which resulted in the start up of the first commercial unit in September Start up of the second unit is planned for July 2003 The first commercial unit removes H 2 S from sour high pressure natural gas at the Bantry North Unit of Encana ( Brooks, Alberta, Canada). H 2 S in the sweet gas is guaranteed to be below 4 ppmv, while the total design amount of sulfur to be removed per day is approximately 1 ton. Start up was organized in such a way that the specification for H 2 S in the sweet gas was immediately met and occurred virtually flare free. Sulfur concentrations were typically 0.2 ppmv H 2 S or lower in the treated gas. The second commercial unit is planned to start up from July 1, The unit is in the EPC phase and will remove 13 tons a day of sulfur from a combination of refinery gas streams and spent caustic. Guaranteed maximum H 2 S level in the treated gas is 10 ppmv while the purity of the sulfur powder produced should be higher than 98%. The sulphur produced has a hydrophilic nature, which prevents equipment from fouling or blocking. Moreover, this characteristic makes the product suitable for agricultural use as fertiliser and / or fungicide. Alternatively, the sulphur can be melted to a high purity product meeting international Claus sulphur specifications. In comparison with conventional liquid redox processes significant savings in capital and especially operating costs can be achieved. In addition, for low-pressure applications at low CO 2 -loads, economic savings can be achieved compared to conventional Amine + Claus technology for loadings up to 50 tonnes sulphur/day; this upper limit is gradually increasing. 1

2 1) Introduction H 2 S removal from natural gas with subsequent sulfur recovery is normally performed in amine and Claus plants. These processes are very economical for larger quantities of sulfur (> 15 tonnes/day). If H 2 S removal from smaller gas fields is required, the gas is treated by liquid redox processes, or alternatively, by amine treatment followed by 1) Flaring or, 2) reinjection of the acid gas in an empty well. It is expected that more smaller gas fields with a relatively high H 2 S concentration will be explored in the near future. As a result, the search continues for alternative H 2 S removal technologies, which are safe, cost-effective and easy to operate. Shell-Paques/THIOPAQ 1 is a biotechnological process to remove H 2 S from gaseous streams by absorption into a mild alkaline solution followed by the oxidation of the absorbed sulfide to elemental sulfur by naturally occurring micro-organisms. The Shell-Paques/THIOPAQ process offers: 1) Replacement of amine treating, Claus recovery and tail gas treatment or alternatively liquid redox processes. 2) Minimal chemical consumption. 3) Gas treatment as well as sulfur recovery. 4) 99.99% H 2 S removal for gas streams. 5) 100% conversion of sulfide in the bioreactor with 95-98% selectivity to S. 6) No deactivation of the biological catalyst. 7) H 2 S range from 100 ppmv to 100 vol.% and pressure range from 1 to 75 bar(g). Both direct treatment of natural gas and treatment of amine off-gas are possible. 8) High turndown ratio when gas field starts to be exhausted. Other advantages when compared with more traditional technologies are: 1) There is no free H 2 S available anywhere downstream the scrubber inlet, therefore the unit is very safe and easy to manage. 2) Very little control and supervision required. 3) There are no complex control loops. 4) A relatively large volume of (cheap) solvent is used; therefore changes in solvent composition and performance of the unit are very slow and the process becomes very robust. The first THIOPAQ installation, for removal of H 2 S from biogas, has been taken into operation in In the following years similar installations were installed in Germany, U.K., France and India. Generally speaking, the H 2 S content in these biogases is reduced from about 2.0 vol.% down to 10 to 100 ppm(v), although levels of only a few ppm(v) can also be achieved. The Shell-Paques/THIOPAQ process has been further developed together with UOP LLC and Shell Global Solutions for the desulfurization of H 2 S containing natural gas as well as 1 Shell-Paques process is the name of the technology licensed for applications in natural gas synthesis gas and Claus tail gas. THIOPAQ is the name for the same technology licensed for refinery gas and other applications. 2

3 refinery gasses. The first commercial unit on a high pressure natural gas field in Canada was started up in September The start up was very successful because the sweet gas guarantee of 4 ppmv H 2 S was immediately met. In the autumn of 1999, UOP LLC licensed the first THIOPAQ process for the refining industry for the removal and recovery of sulfur from several streams within the Alexandria Mineral Oils Company (AMOC, Egypt) grass-roots dewaxing complex. A hydrotreater offgas stream, amine regeneration gas and a spent caustic stream will be desulfurized, resulting in the recovery of 13 tons/day of high-purity sulfur. This project resently reached the EPC phase and is expected to start up in July In this paper the working-principle of the Shell-Paques/THIOPAQ technology will be presented while the main focus will be on the start up of the first commercial unit on a natural gas application at the Encana Bantry North unit. 2) Process principle A simplified flow scheme of the Shell-Paques/THIOPAQ process is presented in figure 1. FIG. 1: BLOCK PROCESS DIAGRAM OF THE THIOPAQ PROCESS FOR THE TREATMENT OF H 2 S CONTAINING GAS STREAMS. In the Shell-Paques/THIOPAQ Process, H 2 S containing gas is absorbed in an alkaline solution under full pressure (up to 75 barg) in an absorption step. For hydrocarbon containing gases a tray or packed column is the preferred gas/liquid contactor but gases at low pressures and without hydrocarbons can immediately be directed into the bioreactor. The dissolved sulfide is subsequently oxidized into elemental sulfur in the THIOPAQ reactor: Absorption and hydrolysis of H 2 S: 3

4 H 2 S (g) + OH - HS - + H 2 O (1) Biological sulfur formation: HS - + ½ O 2 S + OH - (2) From the above reactions, it follows that the caustic used to absorb H 2 S-gas, is regenerated during the production of elemental sulfur. Normally, less than 3,5% of the sulfide is oxidized to sulfate. In order to avoid accumulation of sulfate, a continuous bleed stream from the bioreactor is required and make-up water with some caustic is needed. For installations located in dry areas the discharge of sulfate containing aqueous streams into the environment may be unwanted. The bleed stream can be minimized by application of membrane filtration. The membrane-filter separates the sulfate ions from the other ions, leading to a sulfate-rich effluent stream. To avoid any bleeding at all, one can continuously recycle a part of the bioreactor content over a sulfate-reducing stage where sulfate is converted back to hydrogen sulfide. This hydrogen sulfide can be recycled to the biological oxidation step where sulfur is once again produced. In 1999, an installation for the daily production of 4 ton H 2 S has been taken into operation with H 2 gas as the reducing agent. A key feature of the Shell-Paques/THIOPAQ process is its proprietary bioreactor design. Depending on the capacity required, either a fixed-film or gas-lift-loop system is used in this reactor. The gas-lift-loop reactors for sulfide oxidation and sulfate reduction are proprietary Paques designs. In many situations the Shell-Paques/THIOPAQ process can be used for sulfur removal and recovery. When sulfur recovery is desired, the elemental sulfur produced in the aerobic bioreactor will be separated from the aqueous effluent in a separator inside the reactor as a slurry of 20% w/w solids content. There are three options for handling this slurry. 1) The sulfur slurry can be dewatered using a continuous decanter centrifuge resulting in a sulfur cake of 60-65% dry solids content. The water is returned to the bioreactor as process make-up. The sulfur purity is ~ 95-98%. The remaining 2-5% is organic material and trace salts, especially sodium bicarbonate and sodium sulfate. This sulfur cake can be safely land filled as non-hazardous refinery waste. 2) The same decanter centrifuge step described above can be used with the addition of a reslurry vessel to remove dissolved salts attached to the sulfur particles. Then a second separator and centrifuge produces a sulfur cake, again of 60 65% dry solids content, but of 99% sulfur purity. The wash water can be returned to THIOPAQ as process make-up water. This sulfur is suitable for addition to the refinery sulfur pit, or for use in sulfuric acid manufacture where such facilities exist. 3) An alternative to options 1 and 2 is for the sulfur slurry to be directed to a sulfur smelter where molten sulfur with a purity exceeding 99.9% is produced for 4

5 sale. The water is returned to THIOPAQ as process make-up and the solid impurities are landfilled. Options 1 and 2 can be followed by a drying step where a powder is produced that normally can be applied as a fungicide or miticide. The excellent results on the application of biosulfur as a fertilizer were produced by the Alberta Agricultural Research Institute and repeated by the University of Alexandria. 3) Commercial applications Two commercial applications will be detailed further. The first is the first full scale natural gas application. The focus will be on the start up of the unit that occurred in September The second is the first refinery gas application at the AMOC refinery in Egypt which is currently in the EPC phase. 3.1 First Commercial Scale Natural Gas Desulfurization unit Bantry Project Background The first biotechnological purification plant for the desulphurisation of natural gas based on the Shell-Paques process was taken into operation in the second half of September The bacteria are exposed to the gas at high pressure and the plant produces about 0.9 tons of sulphur per day. The gas is extracted from well sites that are on or adjacent to the properties of over forty Canadian landowners located around the Bantry North facility. The plant is built under licence by the company New Paradigm Gas Processing Ltd. This is a subsidiary of the Canadian technology company CCR Technologies Ltd. The plant was built for Encana Resources, a major Canadian gas producer, and is located northeast of Brooks, Alberta. The Shell-Paques biological technology was selected because it was the best available technology for this application. The alternative was acid gas re-injection which was too expensive and therefore not feasible. The required H 2 S out concentration in the product gas was < 4 ppmv which was immediately met at start up. 5

6 Design Basis: Feed, Effluent and Sulfur Product Compositions The Bantry Shell-Paques unit is designed to remove and recover hydrogen sulfide from four natural gas fields. The natural gas stream is scrubbed to remove H 2 S with the loaded scrubbing liquid being regenerated in the bioreactor and recycled to the scrubber ( See flow diagram (Figure 2). The Shell-Paques unit is designed for the following volume and H 2 S concentration. Feed Rate and Composition Specifications Natural gas nm 3 /day 2000 ppmv H 2 S, 2.5 % vol CO NOTE 2 Note: The other components in the feed gas stream are hydrocarbons, methane being the main constituent at 84.4 % vol. The THIOPAQ unit is designed to meet the following specifications for the treated gas, aqueous effluent streams, and sulfur product. Specifications for Treated Gas and Effluent Streams Treated Sour Off-gas: H 2 S < 4 ppmv Bioreactor Off-gas H 2 S 0 ppmv Sulfur recovery specifications Sulfur Throughput 0.9 tonnes/day 6

7 SHELL PAQUES Bantry North Flow Scheme The simplified flow scheme for the Bantry North Shell-Paques unit is shown in Figure 2 below. Clean Gas Vent High pressure natural gas S c r u b b e r To flare Regenerated Scrubbing Solution P o l i s h Compost Filter Reactor Vent Sulfur Slurry Spent Scrubbing Solution F l a s h THIOPAQ Bioreactor Settler Centrifuge Sulfur Cake Sulfur tank Sodium Hydroxide Makeup Nutrient Air Bleed Bleed Aeration tank Bleed tank Air Paques Incoming streams to the bioreactor: The sour natural gas stream is directed to the Shell- Paques scrubber where the H 2 S is extracted into the circulating sodium carbonate / bicarbonate solution. The treated gas, now free of H 2 S, can be sent directly to the customers. The loaded scrubbing liquid flows to the THIOPAQ bioreactor. The bioreactor is supplied with a nutrient stream, make-up water air, and makeup sodium hydroxide to compensate for loss of sodium ions in the effluent streams. The contents of the bioreactor are highly mixed by the action of the incoming air stream due to the gas-lift loop reactor type design. Effluent streams from the bioreactor: The spent air from the bioreactor is discharged to the atmosphere after flowing through a compost filter to remove trace H 2 S. A small spent caustic stream, normally called the bleed stream, is discharged to the bleed tank. Sulfur handling: The sulfur slurry is dewatered in the continuous centrifuge and stored in the sulfur bin. All solid and liquid effluent streams that are produced in the process are stored in dedicated vessels because they all have to be removed from site by road due to the remote location of the facility. 7

8 Realized project schedule The plant started running in the second week of September The project schedule was tight and achieved in 9 months. Basic engineering started in January 2002 while detailed engineering was completed by April. After the selection of the contractor at the end of April, the facility was built in the contractor s facility on two skids. The skids were installed at the end of July and connected to the existing Bantry North infrastructure. Start up was smooth except for some minor mechanical and instrumentation problems. The H 2 S out spec was immediately met as can be seen from the results below. Start up of the unit Customers require a start up period that is as short as possible. For this unit the demand was that start up would be flare free. This means that the H 2 S specification for the treated natural gas of < 4 ppmv had to be met immediately. A perceived disadvantage of biotechnology that is always a concern to customers is the apparent long start up time required. The Shell-Paques technology does not have this disadvantage. As soon as natural gas was introduced the H 2 S specification could be met ( See figure 3 below). Two important measures were taken to ensure a smooth start up of the facility: 1. Sufficient biomass was supplied to the customer to be able to handle the design sulfur load. 2. The scrubbing liquid was conditioned in such a way that important process parameters as conductivity and ph were at their design level. This was achieved by preparing a sodium carbonate solution at the design strength during commissioning before sour natural gas was introduced. Figure 3 clearly shows that the H 2 S out spec for the treated gas was immediately met. Sulfur load at start up was about 40% of the design load because not all sour gas sources had been connected to the facility yet. Pressure in the absorber is 200 PSI(g) ( approx. 13,5 bar(g)). The bacteria can handle pressures up to 75 bar(g). 8

9 Total Gas pressure is 200 PSI ( 13.5 bar) H 2 S concentration H 2 S in ppm H 2 S out ppm Hours Two pictures of the unit are attached to this article as appendix AMOC Middle Distillate Dewaxing Facility AMOC Project Background AMOC is constructing a grassroots middle distillate dewaxing facility scheduled to be on stream in the middle of The H 2 S removal operations for naphtha, LPG, and fuel gas within the complex will generate a hydrotreater sour offgas, an amine unit regenerator acid gas, and a sulfidic caustic stream. The Egyptian environmental regulations require that these streams be desulfurised prior to discharge to air or water. Since there was no existing sulfur recovery facility at the AMOC site, it was necessary to consider various options for sulfur removal and recovery. The following factors were important in determining the best technology to apply to AMOC s desulfurization and sulfur recovery requirements: Limited plot space was available for installation of a sulfur recovery facility Because of the schedule for the dewaxing project, rapid completion of the sulfur recovery section was crucial Stringent environmental specifications needed to be met for the treated streams A saleable sulfur product was required since landfill disposal of sulfur was not permitted. The THIOPAQ process was able to meet all of these requirements. An additional advantage was that it was possible to design a single THIOPAQ unit to handle two gas streams and the spent caustic stream. 9

10 Design Basis: Feed, Effluent and Sulfur Product Compositions The AMOC THIOPAQ unit is designed to remove and recover hydrogen sulfide from three streams simultaneously. The sour off-gas stream is scrubbed to remove H 2 S with the spent scrubbing liquid being regenerated in the bioreactor and recycled to the scrubber. However, the acid gas stream and the spent caustic stream do not require scrubbing and can be admitted directly to the bioreactor for conversion of sulfide to sulfur. This is illustrated in the flow diagram (Figure 2). The THIOPAQ unit is designed to treat these streams with the following throughput capacity and composition. Feed Rate and Composition Specifications Sour Offgas 437 m 3 /hr 3.09 wt% H 2 S 0.1 wt% NH 3 Acid Gas 290 m 3 /hr 93 wt% H 2 S 0.02 wt% NH 3 Spent Caustic m 3 /hr 3.09 wt% sulfide The THIOPAQ unit is designed to meet the following specifications for the treated gas, aqueous effluent streams, and sulfur product. Specifications for Treated Gas and Effluent Streams Treated Sour Off-gas: H 2 S 3 ppmw NH ppmw Bioreactor Off-gas H 2 S 6 ppbw NH 3 1 ppbw Aqueous Bleed Stream Sulfide 3 ppmw Sulfate 3.4 wt% Volume 1.8 m 3 /hr Sulfur recovery specifications Sulfur Throughput 13.5 tonnes/day Sulfur Recovery 95% Sulfur Product purity 99.0 wt% (drier option) 10

11 AMOC THIOPAQ Flow Scheme The simplified flow scheme for the AMOC THIOPAQ unit is shown in Figure 4. Treated Gas Reactor Vent Scrubber Vent Compost Filter Vent Sour Offgas S c r u b b e r Regenerated Scrubbing Solution Waste Water Spent Scrubbing Solution Sulfur Slurry Centrifuge Soft Water Centrifuge Acid Gas Spent Caustic THIOPAQ Bioreactor Sulfur Cake Reslurry Sulfur Cake Dryer Sodium Carbonate Makeup Nutrient Air Bleed Powder Sulfur Waste Water UOP Figure 4: Scheme of the THIOPAQ plant for AMOC Incoming streams to the bioreactor: The sour gas stream is directed to the THIOPAQ scrubber where the H 2 S is extracted into the circulating sodium carbonate solution. The treated sour gas, now free of H 2 S, is directed to the fuel gas header. The spent scrubbing liquid flows to the THIOPAQ bioreactor. Because the acid gas stream is predominantly H 2 S (92.2%), scrubbing is not necessary so the stream is admitted directly to the bioreactor where H 2 S dissolves in the aqueous medium. The spent caustic stream is also admitted directly to the bioreactor. The bioreactor is supplied with a nutrient stream, air, and makeup sodium carbonate to compensate for loss of sodium ions in the effluent streams. The contents of the bioreactor are highly mixed by the action of the incoming air stream due to the gas-lift loop reactor type design. Effluent streams from the bioreactor: The spent air from the bioreactor is scrubbed with caustic to remove traces of H 2 S or NH 3 ; the spent caustic is bled off to wastewater treating. The scrubber vent air is discharged to the atmosphere after flowing through a compost filter to remove trace H 2 S and odors. 11

12 Sulfur handling: The sulfur slurry is dewatered in the continuous centrifuge. The water is returned to the bioreactor and the 60% dry solids cake is reslurried and centrifuged a second time to increase sulfur purity. The water from the second centrifuge operation is split between makeup water to the bioreactor and a bleed stream to wastewater treating. The purified 60% solids cake is dried to a powder containing 10% moisture and bagged. Status of the project Although engineering was finalized in 2000, it took a while to get the project to the next phase. Only recently the EPC contractor was selected by AMOC. Civil work has started and mechanical completion will be achieved in July Start up will follow immediately afterwards. 8) Conclusions/Summary The following experience with Shell-Paques/THIOPAQ has been achieved: 1) H 2 S load from kg/day. Bench-marking studies show that Shell- Paques/THIOPAQ is cost effective up to 50 tons per day. 2) H 2 S concentration in sour gas ranging from 50 ppm(v) up to 84 vol.%. 3) H 2 S concentration in sweet gas 1 ppm(v) 4) H 2 S specification for treated gas can be achieved immediately at start up. 5) Pressures of the sour gas up to 75 bar(g) 6) Reactors ranging from m 3. 7) Highly flexible in responding to H 2 S peak loads. 8) Formation of hydrophilic sulfur. So far plugging problems have not been encountered and little operator attendance is needed. 9) Sulfur re-used for the production of sulfuric acid, hydrogen sulfide and agricultural applications. 10) Start up of the unit is simple and sweet gas specs can be achieved immediately. 12

13 Appendix 1 : Color pictures of the Encana Shell-Paques facility