NEW PROCESS AND CATALYST FOR CRUDE, FUEL

AND WASTE WATER DESULFURIZATION

A.M.Mazgarov, A.F.Vildanov

GUP "VNIIUS", Kazan, Russia

Received 15.07.01 Cited 19.08.01

Low molecular-weight organic sulfur compounds (H₂S, RSH, COS, CS₂) contained in crude, gases and gas condensate are one of the major environmental pollutants.

The production volumes and processing of sour, mercaptan-containing crude and gas condensate are growing steadily in the CIS countries as well as in other countries of the world. Fifteen to twenty years ago mercaptan-containing crudes and gas condensates were produced only in the Pricaspian Lowlands (Orenburg, Astrakhan, Karachaganak condensates, and Tengiz and Zhanazhol crudes); however, in the 1990s, the geography of such fields extended significantly. Production began of Qatar condensate (S_RSH= 0.17% mass.) on the Arabian Peninsula, Douglas crude in the Irish Sea, gas condensate of Carter-Creek Field (USA), Markov crude in Irkutsk region (S_RSH= 0.4% mass.), etc.

Besides, production of heavy carboniferous crudes containing up to 50-80 ppm of methyl- and ethyl-mercaptans is increasing rapidly in the region between the Volga-river and Urals. Total and mercaptan sulfur concentration in crudes and gas condensates produced in some fields are shown in Table 1.

Table 1 Total and mercaptan sulfur content in various crudes and gas condensates

Low molecular weight mercaptans are volatile, highly toxic, corrosive, and have an objectionable odour. High toxicity and volatility of these mercaptans cause serious environmental problems during storage and transportation of such hydrocarbon raw materials.

Among the crudes with such high mercaptan content the oil of the Tengiz Field takes a special place because this field is the largest and most perspective. To treat Tengiz crude for low molecular weight mercaptans and hydrogen sulfide, VNIIUS developed and proposed to Chevron a process of direct oxidational demercaptanization of the crude over highly efficient phthalocyanine catalyst "IVKAZ", the activity and stability of which exceed activity and stability of well known Merox catalyst by 3 to 4 times (Table 2). The catalyst activity was determined in the reaction of n-propyl mercaptan oxidation with molecular oxygen at a temperature of +30⁰C. The catalyst stability was determined in the reaction of their oxidation with molecular oxygen in 20% aqueous solution of sodium hydroxide at a temperature of +30⁰C.

Table 2  Activity and stability of metal phthalocyanines

Fundamental investigations of the kinetics and mechanism of mercaptan oxidation in a two-phase system "alkaline solution – hydrocarbons" [1-3] and the kinetics of oxidational destruction of phthalocyanine catalyst [3, 4] provided a scientific and technological basis for the process of selective crude oil demercaptanization (the DMC-1 process).

On the basis of pilot test results and VNIIUS technological reglament, Bechtel  Engineering Company developed a project of a commercial plant for Tengiz crude demercaptanization consisting of two process lines for 4 MM t/yr capacity.

A flowsheet of the plant is shown in Fig.1.  Stabilized crude at a temperature of 50-60⁰C is supplied to the bottom of prewash apparatus (V-1) where selective removal of hydrogen sulfide and naphthenic acids from the crude by 1% aqueous solution of sodium hydroxide takes place. The treated crude is mixed with a catalyst complex (CC) and air supplied by a compressor in mixer M-1 and is sent to the bottom of reactor R-1. The quantity of the air supplied is determined by equation stoichiometry:

2RSNa + 0.5O₂ + H₂O ® RSSR + 2NaOH                    (1)

and is adopted in 1.5-fold exceed. A mixture of the crude, CC and dissolved air passes through a distributing collector into the reactor where oxidation of mercaptans to disulfides takes place at a temperature of 50-60⁰C and a pressure of 1.2 MPa following reaction (1).

From the top of the column the reaction mixture flows to gravitational settler V-4, in which the crude is settled from the CC. From the bottom of V-4 the catalyst complex is supplied to reactor R-1 by pump P-1. The demercaptanized crude from the top of V-4  is sent to coalescer V-5 to be separated from entrained drops of the CC. Then the crude is sent from V-5 to storage tanks. The alkaline and catalyst complex solutions are prepared in vessels V-2 and V-3.

The catalyst complex is a 5-20% aqueous solution of sodium hydroxide and 0.005% IVKAZ catalyst.

The DMC-1 plant was commercialized in March 1995 and the DMC-2 plant in August 1996. Both plants work sufficiently stably. Each plant reached a capacity of 6 MM t/yr. Summary methyl- and ethyl-mercaptan content after treatment does not exceed 5 MM^-1. The actual catalyst consumption was below 0.05 g/t of the treated crude and sodium hydroxide content (calculated on dry sodium hydroxide) was below 40 g/t. These values are lower as compared to analogous values during demercaptanization of light hydrocarbon raw materials.

Table 3 shows low molecular weight mercaptan (C₁-C₄) composition before and after treatment in the DMC plant. The mercaptans were analyzed using a gas chromatograph with a flame-photometric detector [5].

Table 3

 As is seen from Table 3 the DMC-1 plant provides removal of all ethyl mercaptans, 70% of propyl mercaptans and 20% of butyl mercaptans.

After the start-up of the DMC-1 plants mercaptan odour disappeared on the whole Tengiz refinery area and near its storage tanks. Shipment of the Tengiz crude in tank cars to European countries via Russian territory became environmentally safe.

The DMC plant in Tengiz was the first plant of crude demercaptanization in the world practice. Experience of operation of this plant and analysis of operation of its separate units allowed to develop more perfect modifications of the process: DMC-1M and DMC-3.

The DMC-1M process as compared to the DMC-1 process allows to remove hydrogen sulfide and mercaptans C₁-C₂  not only from light crudes and gas condensates but from heavy highly viscous crudes on oil-fields with minimal capital costs as well. Figure 2 shows a flowsheet of the process.
The crude to be treated, stoichiometric quantity of air for oxidation of hydrogen sulfide and mercaptans C₁-C₂ and an aqueous-alkaline solution of IVKAZ catalyst in a quantity not exceeding 5 l per ton of the crude (mercaptan scavenger VMS-1) are mixed in tubular mixer M-1 and in orifice mixer-reactor R-1. Intensive mixing of the reactants and selective oxidation of mercaptans     C₁-C₂ to disulfides and oxidation of hydrogen sulfide to elemental sulfur and sodium thiosulfate take place in reactor R-1 at a temperature of 20-50⁰C and a pressure of 0.4-0.5 MPa.

The elemental sulfur in its turn also oxidizes mercaptans to disulfides. Formation of an emulsion provides better oxidation of the mercaptans and hydrogen sulfide. Due to small quantity of the alkaline mixed with the crude, possible formation of an emulsion with heavy crudes is not hazardous for transportation, storage and processing of the crude. Afterwards the reaction proceeds in a pipe and storage tanks. A major portion of the alkaline solution settles in the tanks and is re-used for catalyst complex preparation in vessel T-2. Proportioning pump H-2 provides continuous supply of the catalyst complex from T-2 to mixer M-1.

A small quantity of the alkaline remains in the crude and acts as a corrosion inhibitor in a pipeline and refinery equipment.

The DMC-1M process was tested on heavy crudes of oil fields of IS "Tatex" and Tengiz crudes and revealed a possibility of deep treatment of the crudes for low molecular weight mercaptans C₁-C₂ and hydrogen sulfide at minimal consumption of the alkaline and the catalyst.

The DMC-3 process provides removal of mercaptans C₁-C₄ and hydrogen sulfide from crudes and gas condensates with high sulfur content. The process is carried out in two stages. On the first stage hydrogen sulfide and mercaptans C₁-C₂ are removed from the crude  using a 2-10 % aqueous caustic solution. Then the caustic solution is regenerated by oxidation of sodium sulfide and sodium mercaptide with air oxygen over heterogeneous UVKO catalyst following reactions 1 and 2:

                        3Na₂S + 4O₂ + H₂O ® Na₂SO₄ + Na₂S₂O₃ + 2NaOH           (2)

On the second stage mercaptans C₃₊ are oxidized to disulfides with air oxygen over IVKAZ catalyst dissolved in a 2-10 % caustic solution.

A flowsheet of the DMC-3 process is shown in Fig.3.

The stabilized crude at a temperature of 20-50⁰C is fed into mixer M-1 where it is mixed with a circulating caustic solution and then to separator V-1, in which it is separated into oil and alkaline phases.

The caustic solution saturated with the mercaptans and hydrogen sulfide is mixed with air and enters regenerator R-1. The caustic regeneration takes place in the regenerator at a temperature of 50-60⁰C and a pressure of 4-5 bar over UVKO catalyst following reactions 3, 4.

Then the reaction mixture flows from the regenerator to air separator V-2, in which separation of air and disulfides from the caustic solution takes place. The caustic solution is then returned from the bottom of separator V-2 by pump P-1 to mixer M-1.

The crude settled from the caustic solution is sent from separator V-1 to the second stage of  treatment to mixer M-2, in which it is mixed with air and the catalyst complex (a solution of the caustic with homogeneous IVKAZ catalyst). A mixture of the crude, catalyst complex and dissolved air is fed into reactor R-2, in which oxidation of mercaptans C₃₊ to disulfides takes place at a temperature of 50-60⁰C and a pressure of 6-12 bar. From the top of reactor R-2 the reaction mixture is sent to separator V-3 to be separated for the crude and alkaline phases. The crude containing the dissolved disulfides C₃ is sent to a storage tank and the catalyst complex is supplied to mixer M-2 by pump H-2.

The DMC-3 process was commercialized on Orenburg gas refinery for treating Karachaganak condensate for hydrogen sulfide and mercaptans in October 2000. A capacity of the plant is 2 MM t/yr (35000 bbl/day). The mercaptan C₁-C₄ content before treatment was 1600 ppm, after treatment – below 30  ppm.

An efficient alkaline absorptive DMD-2K process with oxidational catalytic regeneration of the alkaline solution was developed for treating a broad fraction of light hydrocarbons C₂-C₆ for sulfur compounds (H₂S + RSH + COS + CS₂). The process is based on alkaline of COS + CS₂ at a temperature of 50-70⁰C and on absorption of CO₂ + H₂S + RSH  with the alkaline with the subsequent oxidation of mercaptides to disulfides, of toxic sodium sulfide to non-toxic sodium sulfite and thiosulfate with air over UVKO-2 catalyst. To increase extractive and hydrolyzing ability of the alkaline solution, up to 20% of polar organic solvent is added.

A flowsheet of the process is shown in Fig.4. A hydrocarbon fraction at a temperature of 40-50⁰C and a pressure of 0.5-2.0 MPa is fed into the bottom of extractor V-1. A 10-20% aqueous solution of caustic soda at a temperature of 50-70⁰C is supplied to the top of the extractor using pump P-1. Hydrolysis of COS and CS₂ following reactions:

COS + H₂O ® CO₂ + H₂S     (3)

CS₂ + 2H₂O ® CO₂ + 2H₂S   (4)                      

and absorption of H₂S, CO₂ and RSH with the alkaline following reactions:

H₂S + 2NaOH ® Na₂S + 2H₂O                      (5)

CO₂ + 2NaOH ® Na₂CO₃ + H₂O                   (6)

RSH + NaOH ® RSNa + H₂O                       (7)

take place in the extractor. The treated hydrocarbon fraction is fed from the top of V-1 to separator-demister V-2 and then to a tank farm. The alkaline solution saturated with sodium mercaptides and sulfide is heated in heat exchanger T-1 to a temperature of 60-70⁰C and is sent to the bottom of reactor R-1. An estimated quantity of air at a pressure of 0.4-0.5 MPa is also supplied to the reactor bottom. Oxidation of the mercaptide and generation of the alkaline proceed according to reaction (1) and oxidation of toxic sodium sulfide to neutral sulfate and thiosulfate take place in reactor R-1 over UVKO-2 catalyst.

The alkaline solution and air enter separator V-3. Then the air is supplied from the top of the separator to the nearest burner for processing. The alkaline and the disulfides are sent from the bottom of V-3 to separator V-4, in which the disulfides are separated from the regenerated alkali. The upper layer (the disulfides) is sent from separator V-4 to a tank farm and the lower layer (the alkali) is pumped into extractor V-1 by pump P-1.

As the alkaline solution becomes diluted with the reaction water and saturated with salts, a portion of the alkaline discharged periodically from the system and can be used for treating hydrocarbons for H₂S or for neutralization of acidic waste water. The DMD-2M process operates successfully since 2000 at Perm gas refinery for treating broad fraction of light hydrocarbons. A capacity of the plant is 300 000 t/yr.

Heterogeneous UVKO catalyst are carbon-fibrous materials produced by pyrolysis (t = 800-900⁰C) of cellulase fibre in an atmosphere of CO₂. A specific surface of such activated materials is from 300 to 600 g/m². These materials contain up to 0.3 % of metal oxides, mainly ferrum oxide, which provide their high catalytic activity in a reaction of sodium sulfide oxidation in an aqueous-alkaline medium. A new commercial catalyst (UVKO-1) for treating sulfurous-alkaline waste waters for toxic sodium sulfide and sulfite (Serox-W process) was developed on the basis of this carbon-fibrous material. A commercial form of the catalyst is fabricated as rolls of carbon fibrous cloth with a metal Rabitza sieve. The rolls may be of various size depending on the reactor diameter: 200, 300, 400, 600, 800 mm in a diameter and 0.5, 1.0, 1.5 m of height.

It was determined experimentally that application of cobalt phthalocyanines to the surface of carbon-fibrous materials increases the catalyst activity in the reaction of sodium sulfide oxidation insignificantly and mercaptide oxidation rate by 4-5 times. In this connection regeneration of mercaptide-containing  alkaline solutions is carried out using UVKO-2 catalyst, which is prepared by application of cobalt phthalocyanine from aqueous-alkaline solution to UVKO-1. Optimal concentration of cobalt phthalocyanine (IVKAZ) on the surface of UVKO-2 catalyst is 0.02-0.05 % of the support mass.

Fig.5 shows a flowsheet of the Serox-W process.

The waste waters treated for petroleum products mechanical impurities and heated in heat exchanger T-1 to a temperature of 60-80⁰C are sent via filters F-1 and F-2 to reactor R-1 charged with UVKO-1 catalyst. Oxidation of sulfide and mercaptide sulfur by air oxygen, supplied to the bottom of the reactor, takes place in the reactor at a temperature of 60-80⁰C and a pressure of 0.4-0.5 MPa on the catalyst surface following reactions 1 and 2.

From the top of R-1 the treated waste waters and the air are sent to air separator V-1, and the spent air is sent to the nearest burner for calcination. After cooling in cooler C-1 to a temperature of 50⁰C the treated waste waters are sent to biological treatment facilities. A degree of treatment for mercaptide and sulfide sulfur in the process is up to 99.5 %.

The Serox-W process is being used on Ryazan oil refinery since 1990 and on Kuibyshev, Ufa, Yaroslavl oil refineries since 1995-1996 for treating process condensate from catalytic cracking plants. A capasity of the plants is up to         30 m³/hr, residual sodium sulfide content does not exceed 20 mg/l. Catalyst life is at least 5 years.

However,  UVKO-1 catalyst as well as all other heterogeneous catalysts is not applicable for treating sulfurous-alkaline waste waters from plants of ethylene production by hydrocarbon pyrolysis. The matter is that these waste waters contain significant amounts of olefins, dienes and acetylide hydrocarbons, which polymerize quickly in an alkaline medium at a temperature about +40⁰C and block up a catalyst and a reactor completely. Therefore, only homogeneous catalyst should be used for oxidation of sulfurous-alkaline waste waters from pyrolysis plants as was done by us at Burgas petrochemical plant. Due to high activity of the phthalocyanine catalyst its consumption is very low (0.01 g/t) and accordingly, catalyst concentration in the oxidized sulfurous alkaline waste waters is insignificant (£ 10 ppm). Besides, molecules of metalphthalocyanines on plants of biologic treatment of waste waters accelerate a reaction of organic and nonorganic compound oxidation.

References

1. V.A.Fomin, A.M.Mazgarov, N.N.Lebedev. Reactivity of sodium mercaptides during their oxidation with oxygen over cobalt disulfophthalocyanine. /Neftekhimia. 1978. 18. No.2, pp. 298-303.

2. A.M.Mazgarov, A.F.Vildanov, V.V.Medem et. al. A complex demercaptanization scheme of light oil fractions and gas condensates of the Pricaspian Lowlands. //Khimia i tekhnologia topliv i masel. 1987. No.11, p.21.

3. S.A.Gorokhova. Dissertation of a candidate of technical science "Liquid phase catalytic oxidational demercaptanization of light oil fractions over cobalt phthalocyanines ". Kazan, 1989, p.133.

4. A.F.Vildanov, I.A,Arkhireeva, S.A.Gorokhova et. al. "Stability of metal cyanine solutions. 1. Oxidation of Co disulfophthalocyanine with oxygen in an alkaline medium. – Vestnik MGU, Ser.2. Khimia. 1988, vol.29. No. 6, p.614.

5. GOST P 50802-95. Oil. Method of determination of hydrogen sulfide, methyl- and ethyl mercaptans.

6. A.M.Mazgarov, A.M.Fakhriev, I.Kh.Gizatullina et.al. "Hydrolysis of aliphatic sodium mercaptides" // Neftekhimia. 1979. Vol.19. No.6, p.897.

7. V.A.Shevchuk, V.B.Leshinski, L.K.Tatevosyan "Efficiency of odorant production from hydrocarbon raw material. Collected abstracts of VNIIEGasprom  "Preparation and proceeding of gas and gas condensate". M., 1980. Vyp.3, p.10.