Ind. Eng. Chem. Process Des. Dev. 1984,
88
23,88-93
Desulfurization by in Situ Hydrogen Generation through Water Gas Shift Reaction Meyyappan Kumar,+ Aydln Akgerman,'t and Rayford G. Anthony Kinetics, Catalysis, and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
Sulfur removal from benzothiiphene was realized by in situ hydrogen generation through the water gas shift reaction. Benzothiophene dissolved in decalin (2% S by weight) was fed to a trickle-bed reactor cocurrent with carbon monoxide and water. A commercial Ni-Mo/A1203 catalyst was employed after presulfiding. Reaction conditions were 1000 psia and 310-370 OC, which is within the operating range of industrial hydrotreaters. The reactions taking place were CO H20 e COP H, and C8H8S 3H20 C8H,, H2S. I t was assumed that the desulfwizatiin reaction was rate controlling with the water gas shift reaction in dynamic equilibrium. A second-order irreversible reaction was assumed for desulfurization: r = kCsrCn2.A novel approach was taken in data analysis and it was found that the modified rate constant at 623 K was k , = 8.51 f 0.59 X g-mol/(g of cat. min) and the activation energy was E = 20.35 f 2.6 kcai/g-mol.
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Introduction Hydrotreating of crude oil is a major process in refining. Sulfur removal from crude is necessary because of the catalyst poisoning activity of sulfur compounds, SO2 emission standards and restrictions, final product quality, etc. Due to continued depletion of petroleum resources and pricing policies, today more and more high sulfur heavy crude or bottoms are being used. In addition, development of synthetic fuels from coal or shale oil is inevitable within the next decades. Some heavy crude, bottoms, and synthetics all contain considerable amounts of sulfur and the costs associated with hydrogen production for hydrotreating may be prohibitive in the future. This study was undertaken to investigate the possibility of hydrodesulfurization by in situ hydrogen generation. If successful, the reforming gases can directly be fed into the desulfurizer without the necessity of separation of hydrogen. In addition, CO and H 2 0 can be produced by partial combustion and fed into the reactor without first converting them to H2 by water gas shift reaction and separating H2 from COz and unreacted gases. In the past much research was directed toward development of hydrodesulfurization catalysts and reaction networks. Since thiophenic sulfur is the hardest to desulfurize (Gates and Schuit, 1973), most of the research was on desulfurization of thiophenic sulfur compounds. Therefore in this study hydrodesulfurization of benzothiophene is investigated. Benzothiophene hydrodesulfurization was studied by various investigators by using hydrogen. Basically two mechanisms are proposed for this reaction. The first one is sulfur ring hydrogenation giving dihydrobenzothiophene followed by sulfur extrusion resulting in ethylbenzene and H2S. The second one is direct sulfur removal resulting in HzS and styrene and styrene reacting with H2S to give phenylethanethiol followed by desulfurization to ethylbenzene. Guin et al. (1980) proposed a reaction network which involves both mechanisms. Their mechanism is also in agreement with other investigators (Givens and Venuto, 1970). They have detected both dihydrobenzothiophene and 1-phenylethanethiol but not styrene, which was probably too short lived. The formation of phenylAmoco Oil Co. (Refinery), Texas City, TX 77590.
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ethanethiol is expected resulting from the Markonikov addition of H2S to styrene. However, Daly (1978) and Furimsky and Amberg (1976) detected styrene. The mechanism proposed by Daly (1978) postulates that dihydrobenzothiophene either directly desulfurizes to ethylbenzene or the ring opens to form 2-phenylethanethiol which desulfurizes to form ethylbenzene. Direct sulfur removal resulting in styrene is also postulated. Styrene further reacts to form 1- or 2-phenylethanethiol. The Furimsky and Amberg (1976) mechanism is basically the same with the exception of formation of ethylbenzenefrom dihydrobenzothiophene through the intermediate 2phenylethanethiol. In general, as an organic sulfur molecule gets more and more substituted hydrogeneration step becomes more pronounced (Givens and Venuto, 1970) probably because of the inacessibility of the sulfur atom due to steric hindrance. Based on these mechanisms, various rate expressions were proposed taking into consideration the rate-inhibiting effect of H2S as well as hydrogen partial pressure (Frye and Mosby, 1967; Gates and Schuit, 1973; Metcalfe, 1969; Cecil et al., 1968; Morooka and Hamrin, 1977). However, a pseudo-second-order rate expression of the form r = kPsPH usually gives a satisfactory fit. In industry, normally, trickle-bed reactors are employed for hydrodesulfurization. Although there is widespread use of trickle bed reactors on a very large scale in the petroleum industry, it is surprising that so little information has been published concerning design of these reactors. In trickle bed reactors liquid and gas phases flow cocurrent down the reactor filled with a fixed bed of catalyst. The liquid trickles over the catalyst bed forming a laminar film over the catalyst surface or flows in rivulets. The gas phase is essentially continuous and flows through the voids in the bed (Satterfield, 1975; Bischoff and Froment, 1979). The performance of tricle bed reactors is significantly affected by the transport processes and partial wetting of the catalyst by the liquid. Criteria for partial wetting and negligible transport effects have been developed in terms of dimensionless parameters (Lee and Smith, 1982). In this study desulfurization of benzothiophene by in situ hydrogen generation through the water gas shift re1983 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984 89
action is studied at 1000 psia pressure and 310-370 O C since these are the normal operating conditions for HDS. Since Ni-Mo/A1203 catalysts in sulfided form are good catalysts for the water gas shift reaction as well (Newsome, 1980), no second catalyst was employed. Overall reaction stoichiometry is given by H2O CO G C02 + H2
+ BT + 3Hz
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EB
HHXH, = PYH,
ndecalin =I!f. nHzO = nb,O - nb,Oxw
YH2
and r2 can be restated as k1CT2PnL& r2 = k?H6(1 - 6)nT2
nkT - n&TX
r2 =
nEB = n k ~ ~ nH2S
=
~ B T ~ & T ~ P ~ H ~ B T
--~ B
where
r1 =
=
kl
+ nhzo + nlo + nbT
nhnk
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nknh
=o
- constant = @
then r2 =
~ B T ~ B T ~ H . ~ B = T
nT2
CH20CC0 - kl'CCOzCH2= 0
k1C&nh20nko k1'CMo,nh2
T
6(1 - 6)
The reaction rate for the water gas shift reaction would be r1
H H ( 1 - 6)nTnT
It was shown (Kumar, 1982) that 0.187 < ( 6 0 - 6 ) ) < 0.247 and (YBTvaries the same way as 6 ( 1 - 6). Therefore, as a first approximation it was assumed that
&TX
nT = n$ - 2n&Tx n$ = nb
= nfr/nb
nbT = (YBTnBT; nh = (1 - aH)nH Since most of the .hydrogen is in the gas phase aH
