A SELECTIVE TREATMENT OF VARIOUS OILS AND GAS CONDENSATES TO REMOVE LIGHT MERCAPTANS AND HYDROGEN SULFIDE 1   A.M. Mazgarov, Director, VNIIUS (Russia); A.F. Vildanov, Laboratory Manager, VNIIUS (Russia); S.F. Sciamanna, Process Engineering Manager, Tengizchevroil (Kazakhstan); L.W. Jossens, Staff Research Engineer, Chevron Research & Technology Co. (USA). Abstract A significant increase in the availability of mercaptan-containing crudes and gas condensates has made crude oil demercaptanization (DMC) an important issue. For example, the light crudes of Tengiz and Douglas, the gas condensates of Qatar and the heavy crudes of the Ural-Volga area have a high mercaptan content. In addition, the content of mercaptan sulfur in the crudes of Tatarstan are nearly 1000 ppm (including up to 100 ppm of toxic C1-C3 mercaptans). Hydrogen sulfide and C1-C3 mercaptans have an objectionable odour, high toxicity and are corrosive. A highly effective DMC process was developed for selective cleaning of light crudes and gas condensates. The main principle of the process is the low temperature oxidation of C1-C3 mercaptans by oxygen (air) to disulfides in the presence of a catalyst (IVKAZ) in an aqueous alkaline solution. The design capacity of the two units in Tengiz are 4 MM tonnes/year each. The first unit was started in March 1995 and the second in August 1996. The C1+ C2 mercaptan sulfur content before treatment is 250-350 ppm and after is 0.5-6 ppm. The consumption of IVKAZ catalyst is 0.05 g/tonne and caustic is 50 g/tonne. For the specific treatment of heavy oils containing mercaptans but which exhibit a tendency to form emulsions, chemical scavengers were also developed. Chevron Mercaptan Scavengers (CMS) were developed for TATNEFT oil and Qatar gas condensate. Scavenger consumption is 1.5-3.0 kg/tonne of crude. CMS type scavengers are much less expensive than other commercially available scavengers, are selective for low molecular weight mercaptans and are non-toxic.   INTRODUCTION The production volumes and processing of mercaptan-containing oils and gas condensates are growing steadily in both the CIS countries and elsewhere in the world. Fifteen to twenty years ago, mercaptan-containing oils and gas condensates were produced only in the Pre-Caspian Lowlands (condensates from Orenburg, Astrakhan and Karachaganak; oils from Tengiz and Zhanazhol). Since that time there has been a significant increase in the number of fields which produce mercaptan-containing oils and gas condensates. In the 1980’s, production increased of gas condensate (SRSH = 0.185 Wt. %) from the Carter Creek Field (Wyoming, U.S.A.). In the 1990’s, production began of Markov oil (SRSH = 0.4 Wt.%) from the Irkutsk region (Russia), of Qatar condensate (SRSH = 0.17 Wt. %) from the Arabian Peninsula, of Douglas oil (SRSH = 0.13 Wt. %) from Liverpool Bay (UK). In addition, production is rapidly increasing of heavy oils (from carboniferous zone) containing up to 30-70 ppmw (as sulfur) of methyl- and ethyl- mercaptans in the region between the Volga and Ural Rivers (Tatarstan, Bashkortostan, Samara, Ulyanovsk, Orenburg, and Perm regions). Table I shows total and mercaptan sulfur concentrations in oils and gas condensates of selected fields. Light (or low molecular-weight) mercaptans are volatile, highly toxic, corrosive, and have an objectionable odor.2 Table II presents maximum permissible concentrations of these mercaptans in Russia. High toxicity and volatility of these light mercaptans cause serious environmental problems during the storage and transportation of the crude oil and condensates which contain them.3   TENGIZ OIL FIELD Among the fields with high mercaptans, oil from the Tengiz Field (Kazakstan) is the largest and most prominent. Unlike Orenburg and Astrakhan gas-processing plants where these condensates are stabilized in the de-butanization regime, oil stabilization at the Tengiz gas processing plant is carried out in the de-propanization regime. Therefore, the majority of methyl-mercaptan (the most toxic) remains in the stabilized oil. Tengiz oil is stored in non-hermetically sealed (or fixed roof) tanks. The oil is then transported by pipeline or rail along densely populated regions of Russia to Novorossiysk and could pose serious environmental problems.   It should be noted that at the 1991 startup of the Tengiz Processing Plant by the company Tengizmuniagaz (TMG), the light mercaptan problem had not been solved. In April 1993 when the Joint Venture Company "Tengizchevroil" (TCO) was formed between TMG and Chevron, the problem of light mercaptan treatment became critical because the mercaptan-related issues limited production. Any subsequent increase in the plant production capacity required reducing the light mercaptan content in stabilized crude oil. The expansion of TCO was, in part, only possible if the reduction of methyl- and ethyl-mercaptans in oil was achieved. Certain Western companies proposed a technology scheme based on distillation of IBP-62oC fraction from stabilized oil, subsequent treatment of mercaptans in this fraction by the Meroxâ process, and then recombination of the treated fraction with the oil. Implementation of this technology required high capital and operating costs. The low thermal stability of other sulfur-containing compounds in Tengiz oil further complicates this scheme. These sulfur compounds prevent a complete distillation of light mercaptans and hydrogen sulfide. The thermal destruction of organic sulfur compounds into more light mercaptans and hydrogen sulfide begins at a temperature as low as 180-190° C.   FUNDAMENTAL INVESTIGATIONS VNIIUS proposed to TCO a direct oxidation process of oil DeMerCaptanization (DMC) over the phthalocyanine catalyst "IVKAZ". The efficiency of IVKAZ exceeds that of well-known catalysts by three to four times (Table III).4 Fundamental investigations of the kinetics and mechanism of mercaptan oxidation in a two-phase system (alkaline solution and hydrocarbons, [1-3]) and the kinetics of destruction by oxidation of phthalocyanine catalysts [3, 4] formed the scientific and technological basis for the selective crude oil demercaptanization process (DMC-1). The kinetic equation of mercaptide oxidation in an alkaline medium by molecular oxygen is shown in Equation (1). (1) The constants in Equation (1) are as follows: K1Kp = 2.07 x10-2 m3 / [Pa-mole-s] Kp = 1.1 x 10-4 Pa -1 Kr = 950 m3/mole The concentration of mercaptide ion [RS-], catalyst [Kt], and disulfides [RSSR] are specified in mole/m3. The concentration of oxygen [O2] is specified in Pa. The term [RSSR] reflects the interfacial mass transfer effects. It is determined from the experimental data of [RSH]oil vs. time and is calculated by subtracting the [RSH]oil at a particular time from the initial concentration. The kinetic equation for the oxidative destruction of cobalt phthalocyanine in an aqueous alkaline medium was extensively investigated [4].   DMC-1 PROCESS TECHNOLOGY Figure I shows a flow scheme of the DMC-1 process. Stabilized oil at 50-60° C is supplied to the bottom of pre-wash apparatus V-1, where selective removal of hydrogen sulfide and naphthenic acids from oil takes place by 1 wt. % aqueous solution of sodium hydroxide. After treatment for hydrogen sulfide and naphthenic acids, oil is pre-mixed with a catalyst complex (CC) and compressed air in static mixer M-1. The mixture then passes to the bottom of a reactor R-1. The quantity of air supplied is 50% excess of the amount determined by the stoichiometry given in Equation (2).   2 RSH + 1/2 O2 ¾ ® RSSR + H2O. (2)   A pressure of 1.2 MPa in the reactor provides complete dissolution of air into liquid phase. The mixture of oil, CC, and dissolved air passes through a distributor located at the bottom of the reactor. In the reactor, oxidation of mercaptans to disulfides takes place at 50-60° C. The reactor column is equipped with perforated trays. Intensive mixing of oil and CC is achieved by the high velocity of liquid flow through tray perforations. The reaction mixture from the top of the reactor column enters gravity settler V-4, where oil is separated from CC. The catalyst complex from the bottom of V-4 is supplied to the reactor R-1, by pump P-1. Demercaptanized oil from the top of V-4 enters the coalescer, V-5, for separation of entrained CC droplets from the oil. Oil from V-5 is transported into storage tanks. Alkaline solution and a catalyst complex solution are prepared in Vessels V-2 and V-3. Periodically an amount of spent CC is added directly into V-1 from the pump, P-1. The CC consists of a 10-20% aqueous solution of sodium hydroxide and 0.005% IVKAZ catalyst.   DEVELOPMENT & COMMERCIALIZATION In June 1993, TCO decided to build an integrated pilot plant to test the VNIIUS DMC-1 technology. The plant, with a 1 m3/hr capacity, was built by Jordan Engineering Company (UK) , supplied to Tengiz in October 1993, and was operated during November-December 1993. The tests were successful and the level of was less than 6 ppmw treatment (residual C1 + C2 mercaptan content, measured as sulfur) and with a catalyst consumption of less than 0.1 g/tonne of oil. On the basis of these pilot test results and the VNIIUS Technological Reglament (a design document) , Bechtel Engineering Company (UK) developed the detailed process design package for the commercial Tengiz oil DMC-1 facility. The facility consists of two DMC-1 process plants each capable of treating 4 MM t/year5 . Brown and Root Company was awarded the contracted for detailed engineering, equipment procurement and construction. Design and construction of the plants was completed in 9 months. In October 1994, the plants were constructed in modules and delivered to Tengiz via the Volga-Don Canal. One of the plants was successfully commissioned in March 1995 and the second plant in August 1996.   PROCESS PERFORMANCE The operation of both plants is stable and each plant has achieved a capacity 4.6 MM t/yr. The total content of methyl- and ethyl-mercaptan after treatment does not exceed 5 ppmv.6 Actual catalyst consumption has been less than 0.05 g/ton of treated crude, and sodium hydroxide consumption (as calculated for dry sodium hydroxide) has been less than 50 g/ton. Suprisingly, these values are lower than those achieved on DMC treatment of other light hydrocarbon materials (e.g. LPG). Table IV shows light mercaptan (C1-C4) content in oil before and after treatment by the first Tengiz DMC-1 process plant. The samples were taken on June 15, 1995 and analysed according to the "Gas Chromatography with Flame Photometric Detector" [5] procedure. As seen in Table IV, the DMC-1 process provided removal of essentially all of methyl- and ethyl-mercaptans, 70% of propylmercaptans and 20% of butylmercaptans. From the beginning of operation of the DMC-1 process, mercaptan odor disappeared near storage tanks of the Tengiz facility and the pump station in Atyrau from which oil is transported to Samara via pipeline. Trial shipments of Tengiz oil via tank cars in Atyrau showed complete absence of mercaptans in the environmental air near the place of shipment. The DMC-1 process plants in Tengiz were the first in the world for the treatment of mercaptans in crude oil. The ongoing analysis of its overall operation and the operation of its separate units has resulted in many process modifications and improvements.   SCAVENGERS The DMC-1 process is applicable to light oils and gas condensates. However, it is not applicable for heavy oils that tend to form emulsions with alkaline solutions. For such cases, scavengers were developed for the treatment of hydrogen sulfide and mercaptans. Chemical scavengers neutralize mercaptans by either oxidation or by molecular coupling to reduce their voltality. Chevron Mercaptan Scavenger (CMS-8) was successfully tested on oil fields of the Joint Stock Company "Tatneft." A different scavenger, CMS-10, was tested on oil from the Markov field and on gas condensate from Qatar. Scavengers are mixed with oil in a static mixer, or a pump, at 30-60° C and then reaction with hydrogen sulfide and mercaptans proceeds in a pipe. CMS-8 and CMS-10 oxidize hydrogen sulfide to elemental sulfur and mercaptans to disulfides. Elemental sulfur also reacts with mercaptans converting them to disulfides. H2S + CMS ¾ ¾ ® So + H2O (3) 2 RSH + CMS ¾ ¾ ® RSSR + H2O (4) So + 2 RSH ¾ ¾ ® RSSR (5) Scavenger consumption is 1.5-3.0 kg/t of oil. CMS scavengers cost three to four times less expensive than other commercially-available and are highly selective towards light mercaptans and are non-toxic.   CONCLUSIONS Transportation and storage limitations sometimes necessitate treatment of oils and gas condensates because of odour and safety concerns. The DMC-1 process effectively eliminated light mercaptans from Tengiz crude oil. Plant capacity and treatment performance have been exceptional. In addition, chemical scavengers were developed to control light mercaptans in both light and heavy crudes and gas condensates. Their use is determined by laboratory screening and economics.   ACKNOWLEDGEMENTS The authors would like to thank the Production Operations Department of Tengizchevroil whose support and efforts made the successful development of the DMC-1 process possible. References V. A. Fomin, A. M. Mazgarov, N. N. Lebedev/Neftekhimia, 1978, 18, No. 2, pp 298-303. A. M. Mazgarov, A. F. Vildanov, V. V. Medem, et al., "Chemistry and Technology of Fuels and Oils," 1987, No. 11, p 21. S. A. Gorokhova, dissertation of a candidate of technical science, "Liquid-Phase Catalytic, Demercaptanization of Light Oil Fractions Over Cobalt Phthalocyanines," Kazan, 1989, 133. A. F. Vildanov, I. A. Arkhireeva, S. A. Gorokhova, et al., Vestnik MGU, Series 2. GOST P 50802-95 Oil, "Methods of Determination of Hydrogen Sulfide, Methyl- and Ethylmercaptans."   Key Word Summary 1. Selective treatment, light or low molecular-weight mercaptans 2. Volatile, toxic, corrosive, objectionable odor 3. Environmental problems, transportation and storage 4. Direct oxidation, catalytic oxidation, demercaptanization 5. Commercial plant, pilot test results 6. Methyl-, ethyl-mercaptan content, after treatment.   Table I Total Sulfur and Mercaptan Content in Various Oils and Gas Condensates Raw Material Total Sulfur Content Wt. % Total Mercaptan Content Wt. % (as sulfur) CH3SH ppmw (as sulfur) C2H5SH ppmw (as sulfur) Astrakhan Condensate 1.38 0.19 17 82 Orenburg Condensate 1.25 0.84 4 206 Karachaganak Condensate 0.67 0.16 15 208 Qatar Condensate 0.26 0.17 17 313 Tengiz Oil (Kazakstan) 0.66 0.08 100 103 Oil of Yamashi Field (Tatarstan) 3.16 0.14 1.5 34 Zhanazhol Oil 0.47 0.18 8 32 Novolabit Oil (Ulyanovsk) 4.58 0.35 17 11 Radaevsk Oil (Samara Region) 3.05 0.078 7 28 Shelkanov Oil (Bashkortostan) 4.45 0.054 4 25 Noshov Oil (Perm Region) 3.40 0.067 5 25 Douglas Oil (Great Britain) 0.40 0.13 3 25 Markov Oil (Irkutsk Region) 1.00 0.41 23 42 Carter Creek (U.S.A.) 0.64 0.185 67 100     Table II Russian Standards for Maximum Permissible Concentration (MPC) of Mercaptans and Hydrogen Sulfide Component Boiling Point T (oC) Working Zone MPC mg/m3 Populated Zone* MPC mg/m3 (x 10-6) Odor Threshold, mg/m3 (x 10-6) Methylmercaptan +6 0.8 9 20 Ethylmercaptan +36 1.0 30 5 i-Propylmercaptan +60 1.5 50 200 Hydrogen Sulfide -61 10 8000 11   * Maximum Instant Concentration in a Populated Zone   Table III Activity and Stability of Metal-Phthalocyanines Metal-Phthalocyanines (MePc) Activity Oxidation of PrSNa Keff * 104 (s-1) Stability Oxidation of MePc Keff * 105 (s-1) Without Catalyst 0.22 - PcMn (SO3H)4 0.23 8.68 PcZn (SO3H)4 0.24 7.46 PcAlCl (SO3H)2 0.25 10.20 PcSbCl (SO3H)2 0.26 12.60 PcCrCl (SO3H)2 0.24 2.93 PcFe (SO3H)4 0.34 11.40 PcNi (SO3Na)2 0.29 1.44 PcCu (SO3H)2 0.52 0.22 PcCo (SO3Na)2 5.35 9.84 PcCo (COOH)8 25.40 39.40 PcCo (NO2)4 (SO3H)4 20.71 7.55 PcCoNH (oct) CH2COOH 42.52 8.42 PcCo Sulfamoil 14.00 9.42 PcCo [SO2N (PhCH2) CH2COOH]2 58.03 8.65 PcCoBr7 (OH)8 73.81 2.04 Meroxâ 2 14.20 7.51 PcCo (OH)4 (SO3H)2 37.52 8.71 MOSKAZ-1 25.40 0.18 MOSKAZ-2 124.00 1.39 IVKAZ-2 187.00 4.54 IVKAZ 65.42 2.05   Table IV Light Mercaptan Content Before and After Treatment   Concentration (ppmw as sulfur) Component Before DMC-1 After DMC-1 Hydrogen Sulfide 1.63 0 Methylmercaptan 57.0 0.26 Ethylmercaptan 74.32 1.39 i-Propylmercaptan 42.72 13.28 n-Propylmercaptan 16.46 4.26 tert-Butylmercaptan 5.82 5.44 sec-Butylmercaptan 39.4 31.40 Dimethylsulfide 1.86 1.80 Methylethylsulfide 1.27 1.27 Dimethyldisulfide 21.47 33.06 Methylethyldisulfide 17.38 82.08 Diethyldisulfide 2.96 26.5