NEW DEVELOPMENTS OF THE THIOPAQ PROCESS

Transcription

1 NEW DEVELOPMENTS OF THE THIOPAQ PROCESS FOR THE REMOVAL OF H 2 S FROM GASEOUS STREAMS. A.J.H. Janssen (Paques Natural Solutions), P.O.Box 52, 8560 AB Balk The Netherlands B.J. Arena (UOP) 25 East Algonquin Road Des Plaines Il USA Sjoerd Kijlstra (Shell Global Solutions) P.O. Box BN Amsterdam The Netherlands Abstract THIOPAQ is a biotechnological process to remove H 2 S from refinery gases to produce elemental sulfur. In the autumn of 1999, the first THIOPAQ process was licensed to 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 highpurity sulfur. Several laboratory and pilot plant studies were performed to find an outlet for the bio-sulfur. The results of these studies are presented in this paper.

2 Introduction The consistent global trend toward tighter regulations on petroleum refinery products is driving sulfur levels down. Mandated lower sulfur levels are already in place for diesel fuel in many countries, and strong incentives for decreasing sulfur in gasoline are becoming common. The possibility of lower sulfur levels in jet fuel is also being considered for the future. This drive toward deeper desulfurization must therefore result in a corresponding increase in sulfur levels in refinery and petrochemical waste and by-product streams. However, because tighter sulfur regulations are also bearing down on these wastes, the petroleum refiner and petrochemical producer must manage sulfur-containing streams with the most effective technology possible. For a variety of H 2 S-containing liquid and gaseous streams the THIOPAQ process is a suitable answer for the refiner s need to desulfurize these streams. THIOPAQ is a biotechnological process to remove H2S from gaseous streams by absorption in a mild alkaline solutions and the oxidation of the absorbed sulfide to elemental sulfur by naturally occurring micro-organisms 1. The THIOPAQ process offers: 1) Replacement for amine treating, Claus recovery and tail gas treatment or alternatively caustic scrubbing or liquid redox processes. 2) Minimal chemical consumption 3) Gas treatment as well as sulfur recovery 4) 99.99% H2S removal for gas streams 5) 100% conversion of sulfide in the bioreactor with 95-98% selectivity to S. 6) No deactivation 7) Applies to H 2 S concentrations of 100 ppmv to 100 vol.% 8) High turn down ration In comparison to traditional removal processes such as the Amine/Claus process the following advantages are recognized: 1) There is no free H 2 S available anywhere downstream the scrubber, 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 more robust. 5) The solvent is not sensitive to oxygen, which means that oxygen containing waste gas streams can be treated, e.g. vacuum offgas. The first THIOPAQ installation, for removal of H 2 S from biogas, has been taken into operation in In the following years similar installations have been installed in

3 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 desulfurisation down to a few ppm(v) is possible. The THIOPAQ process has been further developed together with UOP LLC and Shell Global Solutions for the desulfurization of H 2 S containing gases in the refining and petrochemical industry. Because of its ability to regenerate gas scrubbing solutions, this process maximizes resource use, minimizes waste, and in most situations, is the lowest-cost option for treating inorganic sulfur in waste streams. The THIOPAQ process has been commercialized for sulfur removal applications in several industries including the pulp and paper, chemical and mining industry. 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. In this paper the working-principle of the THIOPAQ technology will be presented and options for handling the sulfur product are discussed. Process principle In the THIOPAQ 1 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: 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 1 The Shell-Paques process refers to the specific application of THIOPAQ technology to natural gas desulfurisation.

4 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 is needed. For installations located in dry areas the discharge of sulfate containing aqueous steams 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. The occurrence of any effluent stream at all, can be prevented by continuously recycling a part of the bioreactor content over a reductive stage to form back hydrogen sulfide. A sulfate-reducing reactor is a suitable process unit to carry out this step. 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 simplified flow scheme of the 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.

5 BIOREACTOR DESIGN A key feature of the 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. In either case, the living microorganisms are attached to a support media to ensure that they remain in the reactor and do not become part of the waste stream. The gas-lift-loop reactors for sulfide oxidation and sulfate reduction are proprietary Paques designs (Figure 2). Vent Air Effluent Downcomer Riser Feed Air FIG. 2: DESIGN OF BIOREACTOR The bioreactor design has the following benefits: Excellent mixing. Gas-lift operation ensures excellent mixing. The reactors are vertical (25 to 90 ft) cylindrical vessels with a riser cylinder inside. The reaction gas, air or hydrogen, is passed through the riser, and a circulation stream is created around the riser to ensure excellent gas-solid contact. Effective phase separation. The top of the reactor is designed to act as a three-phase separator, where liquid, solid carrier material, and gas are separated. The separation of liquid from biological sludge takes place so effectively that the biomass is almost completely retained in the reactor. Consequently, the age of the biomass sludge is high, making the production of surplus sludge low (about 5 to 10%).

6 No odor. The CIRCOX reactors are closed systems, and so emissions can easily and completely be controlled. Small plot size. The reactors are designed to require a small plot area. When refinery space is at a premium, this design offers important advantages. Stable operation. Because the life of the colony of microorganisms present in the reactor can be infinite, with proper operation, replacement of the microorganisms may never be required. In addition, the microorganisms are adaptive, thereby making the system flexible to process or feed upsets. Inexpensive materials of construction. Ambient operating conditions allow extensive use of noncorroding construction materials (such as polypropylene and polyethylene) for piping, valves, and other equipment. Using these materials results in long equipment life. For example, the THIOPAQ bioreactors at a commercial zinc refinery have been in service for more than five years with no downtime required for maintenance. Sulfur Handling Options In many situations the 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 bioreactor as process make-up. The sulfur purity is ~ 95-98%. The remaining 2-5% is organic material and trace salts, especially sodiumbicarbonate and sodiumsulfate. This sulfur cake can be safely landfilled 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. In the latter cases, a dry sulfur powder is required.

7 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 sale. The water is returned to THIOPAQ as process make-up and the solid impurities are landfilled. In the following section, the results from washing, drying and smelting the sulfur cake from a continuous decanter centrifuge are presented. The sulfur samples were taken from a THIOPAQ unit for the removal of H 2 S from biogas. Sulfur Wash The most notable aspects of the sulfur containing samples was that they contained a large amount of sodium (3.5 wt.%) and significant quantities of sulfate (1.5 wt.%) and thiosulfate (0.9 wt.%).the sodium originates from the sodium hydroxide added in the process and sulfate and thiosulfate is from some over-oxidation of H 2 S. The ash-content of the paper filtered sulfur sample was 7.5 wt.% but this could easily be reduced down to 0.09 wt.% by a water wash. The washing procedure consisted of washing with water (twice the sample volume). In the dried sulfur the carbon content was only lowered from 1.37 wt.% to 0.72 wt.% for the washed material. FTIR spectroscopy measurements show that presence of aliphatic hydrocarbons, which most likely stem from the lipids of the cell wall of the bacteria. Since only half of the hydrocarbon content could be removed by the water wash, it seems very likely that the cell wall lipids interact strongly with the sulfur. These observations are in agreement with the strongly hydrophilic nature of the raw bio-sulfur. Drying of Sulfur cake The full-scale applicability of a dryer for the production of a dried sulfur product has been investigated. The 40 wt.% moisture containing wet bio-sulfur cake from a decanter/centrifuge was continuously fed to a paddle dryer. The rotor/paddle assembly conveyed the wet cake through the dryer in a spiral fashion. The housing of the dryer was equipped with a heat transfer jacket for the use of water or steam as the heat transfer media. For drying, hot nitrogen flowed counter current to the direction of the solids flow and carries the evaporated water to the condensor. We were perfectly able to operate the dryer in such a way that the dried sulfur product had a moisture content of 10 wt.%. Particle size measurements show that 40 wt% of the total solid content was below 400 µm while 60 wt% ranged from 400 to 1000 µm.

8 Sulfur smelting by pressure liquefaction At our request, Alberta Sulphur Research Ltd (ASRL) studied the effect of smelting the bio-sulfur. In an autoclave, sulfur slurry was heated to 130 C while the gas pressure continuously increased. At 120 C a clear distinct layer of liquid sulfur settled at the bottom of the autoclave. This was easily removed via a bottom valve and solidified to yellow sulfur, the color being not much different to premium grade sulfur. The upper phase was the aqueous phase, containing the dissolved sodium salts present in the raw solution. The sulfur product had an ash content of 70 ppmw and a carbon level of ppmw (by carsul test). These contents are within the commercial specifications for export grade sulfur. Application of biologically produced sulfur as a soil fertilizer Sulfur is a major nutrient which ranks fifth or sixth in quantity of macronutrients taken up by plants, comparable to the demand of phosphorus. Sulfur can be taken up by plant leaves from the atmosphere as very reduced (COS, CS 2 and H 2 S) up to highly oxidized compounds (SO 2 ). However, most of the sulfur is taken up by plant roots as water soluble sulfate. 2 Because of the adoption of pollution control measures in industrial countries the decrease in SO2 emissions between 1980 and 1987 varied from 22% in Poland to 64% in France. Several countries switched from coal-fired to gas-fired industries at the end of the 1960s. 2 The improvement in air quality was beneficial to natural ecosystems but from the late 1980s onwards, the decreased S supply resulted in a widespread S deficiency in the soils used for cultivation of several highly S-demanding crops, especially oilseed rape and cereals in Denmark, England, F.R.G. and Scotland. 2 It is clear that in those circumstances additional S feeding is required. The hydrophilic properties of the bio-sulfur and its small particle size (less than 100 µm) should offer agronomic advantages for short season climates affected by sulfur deficiency. In greenhouse experiments the applicability of THIOPAQ bio-sulfur as a soil fertilizer was evaluated by the Alberta Agricultural Research Institute, University of Edmonton. For this purpose, the effect of biologically produced sulphur on the Canola yield was compared with the effects of several commercially available sulphur fertilisers. The first year results look very good for the biologically produced sulphur. However, a complete review will be published at the end of the year 2000 when a two-year study will be finished.

9 H 2 S Removal and Sulfur Recovery from Refinery Offgas Streams (AMOC) A THIOPAQ system similar to that described in the preceding section can also be applied to the removal of H 2 S from various refinery gas streams such as hydrotreater offgas, amine regeneration gas, fuel gas, etc. Normally, H 2 S containing gas streams are treated in an amine unit; the amine regeneration off-gas is then sent to the Claus unit for sulfur recovery. However, in certain situations, where amine and/or Claus capacity is limited, treating the original streams and/or the amine off gas and recovering sulfur with THIOPAQ may be desirable. This version of the THIOPAQ process has recently been licensed by UOP to AMOC refinery for a 13 tons per day installation. In addition, the THIOPAQ process has been applied commercially to a petrochemical plant vent air stream in England. This commercial unit treats 2000 Nm 3 /hr of vent air containing ppm of H 2 S which is converted to elemental sulfur. In addition, periodic shock loads of more than 1800 ppm H 2 S are common in this stream and the THIOPAQ unit was designed to meet these demands. Figure 3 is a plot of H 2 S concentration in the feed and treated effluent during a 43 day period for this operating unit. Complete H 2 S removal was maintained over this period and the THIOPAQ bioreactor was able to adapt to the periodic high loads of H 2 S. This unit operated continuously from 1997 to 1999 when it was shut down due to changes in the petrochemical complex.

10 H 2 S Concentration, ppm H2S in H2S out Days on stream FIG.3: RESULTS FROM COMMERCIAL THIOPAQ UNIT FOR TREATMENT OF VENT AIR AT A PETROCHEMICAL PLANT Conclusions/Summary 1) The following experience with THIOPAQ has been achieved: 2) H 2 S load from kg/day. Bench-marking studies show that THIOPAQ is cost effective up to 40 tons per day. 3) H 2 S concentration in sour gas from ranging 50 ppm(v) up to 84 vol.%. 4) H 2 S concentration in sweet gas 1 ppm(v) 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, agricultural applications.

11 Literature References 1) Janssen A.J.H. et al., Demonstration of the Shell-Paques process for high pressure natural gas desulfurisation at BEB Erdgas und Erdöl GmbH. Proceedings of the Sulphur98, Tucson, ) Ernst W.H.O., Agricultural aspects of sulfur. In: Environmental technologies to treat sulfur pollution, edited by P. Lens and L. Hulshoff Pol, IWA Publishing, page 355, 2000