Hydrotreatment of spent lube oil: Catalysts and reactor performance

© 7P97 Elsevier Science B. V. All rights reserved. Hydrotreatment and hydrocracking of oil fractions G.F. Froment, B. Delmon and P. Grange, editors

323

Hydrotreatment of spent lube oil: Catalysts and Reactor Performance C. Yiokari^ S. Morphi^ A. Siokou^ F. Satra^ S.Bebelis^ and C.G. Vayenas^ C. Karavassilis^ and G. Deligiorgis^ ^Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece ^LPC Hellas, GR-15125 Athens, Greece

The hydrotreatment of various spent lube oil fractions (light distillate) was investigated in a batch reactor at 880 psi and 330°C using a variety of laboratory prepared and commercial Ni/Mo, Co/Mo, Ni/W and Co/W catalysts supported on y-AbOs , SiOi, Zr02 or ZrOi (8% Y2O3) (YSZ) extrudates. The reactor operating time was chosen to be similar to the residence time of the commercial unit of the refinery of LPC Hellas, S.A.. The hydrodesulfrirization activity of the home-made catalysts is within 10% of the best commercial ones. The quality (viscosity index) of the product is also very good. The catalysts are compared regarding their relative hydrodesulfurization performance as well as the hydrogen consumption in parallel hydrogenolysis reactions.

1. INTRODUCTION One of the key processes in the recycling of lube oils is desulfurization, as lube oils for recycling contain high contents (0.5-1.5%) of sulfur. The removal of sulfur is dictated both by environmental concerns and by the requirements for upgrading of the lube oil, as during the desulfurization nitrogen removal, saturation of aromatics and olefins and hydrocracking is also taking place, leading to products with properties within the specifications required by the market. Many hydrodesulfurization (HDS) processes have been developed [1], while most studies on hydrodesulfurization concern model sulfur compounds such as thiophene, benzothiophene and dibenzothiophene [2-4] as well as of their alkyl-substituted derivatives [5-7]. Fewer studies on hydrodesulfurization have been carried out using feedstocks derived from refmeries [8,9] where the reaction environment is much more complex and coexisting aromatic species as well as various types of sulfur compounds compete for the same active sites on the HDS catalysts surface [8,9]. This results in different reactivities of various sulfur compounds in the feedstock compared to the corresponding ones in the case of model compounds in a pure solvent and also to hydrogen consumption in excess of the stoichiometric requirements of sulfur removal as hydrogen sulfide. The latter increases for feedstocks with high levels of complex sulfur compounds.

324 The objective of this study was to test a variety of Mo-Ni, Mo-Co, W-Ni and W-Co catalysts on different supports (y-Al203, Si02, Zr02 and YSZ) regarding their abiUty for hydrodesulfurization of vacuum distillates obtained in the process of recycling of lube oils. A commercial NiMo/y-Al203, catalyst was also tested for comparison. The above catalysts covered a wide range of active phases and supports as most of the hydrodesulfurization studies in the literature concern molybdenum or tungsten based catalysts promoted by nickel or cobalt and supported on y-Al203 or Si02 [10-12], while few reports concern W-Ni/yAI2O3, [10] or W-Ni/y-Al203 [10]. In addition to testing the above catalysts regarding their hydrodesulfurization activity, an estimation of the corresponding relative hydrogen consumption in hydrogenolysis and hydrodesulfurization reactions is also reported.

2. EXPERIIVIENTAL 2.1 Feed The lube oil feed used in the present investigation was a vacuum distillate obtained from L.P.C. Hellas with a sulfur content of 0.615% wt. Its properties are summarized in Table 1. Table 1 Properties of lube oil total sulfur (wt%) density(15°C)(g/ml) pour point C Q viscosity index

0.615 0.86 -6 102

2.2 Catalysts The catalysts that were used covered a wide range of homemade Ni/Mo, Co/Mo, Ni/W and CoAV catalysts supported on y-Al203, Zr02, Zr02 (8 mol % Y2O3) or Si02 extrudates. The catalysts were prepared by succesive incipient wetness impregnation of supports with solutions of (NH4)Mo7024-4H20, Co(N03)2H20, (NH4)io(Wi204i)-5H20 and Ni(N03)2-6H20 in distilled water. The two-phase systems were dried in a rotary evaporator at 50 °C under reduced pressure and then were succesively calcined at 110 °C overnight and at 500 °C for 5h. The homemade catalysts were compared with the commercial Ni/Mo catalyst supported on y-A^Os which is used by L.P.C Hellas and supplied by AKZO. 2.3 Presulfation and hydrotreatment Both the presulfation of the catalysts and the hydrogenation of the lube oil took place in the experimental setup shown in Figure 1. By proper adjustment of the valves either presulfation of the catalysts or hydrogenation of the lube oil is carried out.

325

> vent

H2 S scrubber

He N, H2S Ho Figure 1 : Experimental setup

During presulfation the reactor was operated as a continuous flow reactor since the valves at the entrance and exit of the reactor are kept open. The presulfation procedure comprised of the following steps: Initially a stream of He was passed through the reactor at atmospheric pressure, while the reactor was heated to 360 °C at a heating rate of 5.5 °C/min. The flow of He was continued at 360 °C for Ih and then a stream of 15% H2S and 85% H2 (vol.%) was supplied to the reactor under atmospheric pressure. The presulfation time was adjusted accordingly so that the total amount of H2S passed over the catalyst contained approximately 6 times more sulfur than the stoichiometric amount needed for the sulfation of the Mo, W, Co and Ni oxides. After completing the presulfation, a flow of He was passed through the reactor for Ih at 360°C and then the reactor was cooled to room temperature at a cooling rate of 4°C/min. The hydrogenation of the lube oil was carried out in the same 300ml reactor operating as a stirred batch autoclave (Fig. 1). The catalyst to oil mass ratio in the mixture charged in the reactor was equal to 1:3. The batch reactor was pressurized with N2 and tested for leaks before each run. Each run consisted of the following steps: Pressurization of the reactor with hydrogen at room temperature up to about 600psi, setting of the stirrer speed at 120rpm, heating of the reaction mixture to 330 °C at a rate equal to 6 °C/min and fixing of the pressure in the reactor ( 850 to 950 psi) to 880 psi by H2 addition or release to or from the reactor. The reaction time was counted from the moment the temperature in the reactor had reached 330°C and the pressure was fixed to 880psi. The reactor operating time was chosen to simulate the residence time of the reactants in the industrial desulfurization reactor of the refinery of L.P.C Hellas and was equal to 3h. The consumption of the gaseous hydrogen

326 during the reaction was followed by monitoring the total pressure (Figure 2) using a pressure gauge. At the end of the run the reactor was cooled to room temperature, the gas in the reactor was removed by opening a needle valve at the exit of the reactor and the reactor was purged with He to remove the remaing and dissolved gaseous products. The liquid product was filtered to assure complete removal of suspended fines. 2.4 Analysis The sulfur content of the feed and product oils was determined using an ASOMA sulfur analyzer (200T-series) based on X-ray fluorescence analysis. The viscocity index was measured according to the ASTM D-2270 method.

3. RESULTS Since there is a variety of complex sulfur compounds in lube oils, the mechanism of the HDS reaction is quite complex . The HDS reaction that takes place can be written in a simple form as RHS + H2^H2S+R-H

(1)

For small variation in PH2 the global hydrodesulfiirization kinetics can be approximated by: dCs •=Kndt

Cs"

(2)

where Cs is the total sulfur concentration, Kn is a rate constant and n the reaction order with respect to the sulfur compounds. If n=l, integrating eq. 2 results in hi(l-xs) = Ki -t

(3)

while if n=2, integrating eq.2,results in ""^ - = K , . C o t 1-

(4)

where xs is the sulfur conversion,Co is the initial sulfur content and Ki, K2 are rate constants for first and second order HDS reaction respectively. Figure 3 shows the dependence of the sulfur concentration on reaction time when the reaction time is varied between 0 and 360 min in the case of a Mo/Ni on y-A^Os catalyst. As shown in Figures 4a and 4b the discrimination between first and second order kinetics is not easy as the corresponding fitting is equally good in both cases. As shown in Figure 4 at reaction time equal to zero the sulfur conversion, based on the sulfur concentration in the feed lube oil before heating the reactor, is not zero as during the heating period preceding the reaction time t a significant amount (almost 55%) of the sulfur is removed because the HDS

327

reaction already starts at temperatures and pressures lower than 330°C and 880psi respectively. In order to take this into account the reaction time, t, was corrected by adding the value, -t*, where t* is the time difference between the time corresponding to Xs=0 and t=0. The value of-t* is equal to 16 min regardless of the exact kinetics (Fig.4). The corrected time tc = t -1* is used throughout the present work for catalyst comparison. 900

850 800h

i-

750

0.08.

O 15%Mo03-4.5%NiO/y-Ab03 15% M0O3 - 4.5% CoO / Zr02 |- •15%W03-4.5%CoO/YSZ

700 h 650

J

40

I

I

1

I

80 120 t,inin

I

L

160

200

250

Figure 2: Dependence of pressure on reaction Figure 3: Dependence of sulfiir concentration time for three different catalysts on reaction time t and corrected reaction time tc for catalyst 15%Mo03-4.5%NiO/ y- AI2O3 (prepared) ; see text for discussion.

B

-2H

250

Figure 4a: Dependence of sulfur conversion on reaction time. The solid line corresponds to fitting according to first order kinetics (Eq. 3)

Figure 4b : Dependence of sulfur conversion on reaction time. The solid line corresponds to fitting according to second order kinetics (Eq. 4).

328 The H2 pressure in the reactor was not kept constant during the reaction time so the H2 consumption due to the hydrogenation and hydrogenolysis reactions was followed by monitoring the pressure in the reactor. Figure 5 shows the pressure corresponding to the H2 consumed by both the hydrogenation, hydrogenolysis and the HDS reactions. The latter can be calculated by measuring the sulfur conversion corresponding to each reaction time tc. The figure also shows the H2 pressure corresponding to the H2 dissolved in the lube oil. The dissolution of H2 in the lube oil was practically completed during the heating period preceding the reaction. The quantity of H2 which is dissolved in the lube oil was estimated with blank experiments, where no catalyst was used, from the difference between the measured pressure in the reactor and the pressure calculated from the ideal gas law if no H2 were dissolved in the lube oil. This difference corresponds to a H2 pressure equal to 200psi. The H2 consumed for hydrogenation-hydrogenolysis during the heating period was calculated from the difference between the pressure in the reactor in the presence of catalyst and the pressure expected from the ideal gas equation, after subtracting the H2 pressure corresponding to the amount of dissolved hydrogen. t,inm 100

150

100 150 t^ ,min

200

250

Figure 5: Dependence of the hydrogen pressure corresponding to the HDS and hydrogenolysis reactions and to hydrogen dissolution in the oil on reaction time for the catalyst 15%Mo03 - 4.5%NiO / y-Al203. A useful parameter for comparing the HDS catalysts regarding the hydrogen consumption in the hydrogenation-hydrogenolysis reactions is the ratio a defined from: a=

mole of H2 consumed due to hydrogenation and hydrogenolysis mole of H2 consumed due to HDS

(5)

329 Figure 6 shows the dependence of a on the corrected reaction time time tc for the Mo/Ni on Y-AI2O3 catalyst as calculated from the data in Figure 5. It is clear that for long reaction times the extent of the hydrogenation and hydrogenolysis reactions becomes more significant. Li a similar way the value of the parameter a for all catalysts has been calculated for a reaction time t equal to 180 min and is presented in Table 2. In the same Table we present the values of the BET surface area of the catalysts, determined by N2 adsorption at -196 °C, the viscocity index of the final product and the % conversion of the sulfiir compounds . Figure 7 shows the sulfur conversion when using the laboratory prepared Mo/Ni on y-AliOs catalyst versus LHSV"\ The LHSV (Liquid Hour Space Velocity) is calculated from the equation: LHSV = gr oil fed / (gr catalyst reaction time in hr)

(6)

The same figure compares all catalysts tested, regarding their HDS activity by comparing the sulfiir conversion at LHSV"^=1.26 h.

100 150 t^,min

Figure 6 : Dependence of the parameter a ( Eq. 5 ) on corrected reaction time for the lab prepared catalyst 15%Mo03-4.5%NiO/Y-Al203 and comparison of the a values for the various catalysts for tc equal to 196 min (LHSV •^= 1.26 h). Symbols designatmg the various catalysts are defmed in Table 2.

0.0

0.4

0.8

1.2

( gr oil feed / gr catalyst h)'^ Figure 7: Dependence of the sulfiir conversion on LHSV "^ for the lab-prepared catalyst 15%Mo03-4.5%NiO/Y-Al203 and comparison of the sulfiir conversion values for the various catalysts for LHSV •^= 1.26 h . Symbols designating the various catalysts are defined in Table 2.

4. DISCUSSION Among the various catalysts tested the one corresponding to the largest sulfiir conversion for LHSV"^=1.26 h is the commercial catalyst 15% Mo03-4.5%NiO on Y-AI2O3.

330 The homemade 15% Mo03-4.5%NiO catalyst on Y-AI2O3 results in comparable sulfur conversions. The catalysts that are supported on Y-AI2O3 exhibit the highest performance for HDS and the ones supported on YSZ the lowest. The catalysts supported on Zr02 present also good performance for HDS conversion compared to the other catalysts. Molybdenum containing catalysts give higher conversions than those containing tungsten, while nickel is a better promoter than cobalt. These comparisons, however, are based on the mass of the active component and not on their BET surface area or, better, on their active surface area. The latter can be measured via NO chemisorption and a comparison of all tested catalysts on the basis of their active surface area will be published elsewhere. Regarding hydrogen consumption the smaller a factors for LHSV"^=1.26 h correspond to the 15% W03-4.5%NiO on Y-AI2O3 catalyst. This catalyst also gives a satisfactory sulfur conversion and a very high viscosity index (Table 2). Further work is currently underway in order to optimize the catalyst composition for enhanced reactivity, HDS selectivity and final product quality.

Table 2 Catalyst BET surface area. Viscosity index, parameter a (Eq. 5) and % sulfur conversion for LHSV"^=1.26 h for the various catalysts tested. Catalysts

(El) 15%Mo03-4.5%NiO/Y-Al203 (commercial) (•)15%Mo03-4.5%NiO/Y-Al203 (A)l 5%MO03-4.5%COO/YA1203

(Q)15%W03-4.5%NiO/Y-Al203 (V)l 5%W03-4.5%CoO/Y-Al203 (•)15%Mo03-4.5%NiO/Si02 (A)l 5%Mo03-4.5%CoO/Si02 ( ^ )15%W03-4.5%NiO/Si02 (4^)15%W03-4.5%CoO/Si02 (•)15%Mo03-4.5%NiO/Zr02 (1^)15%Mo03-4.5%CoO/Zr02 (A) 15%W03-4.5%NiO/Zr02 (O) 15%W03-4.5%CoO/Zr02 (+)15%Mo03-4.5%NiOA^SZ (n)15%Mo03-4.5%CoOA^SZ (0)15%W03-4.5%NiOA^SZ (e)15%W03-4.5%CoOA^SZ

BET surface (m^/g)

Viscocity Index

a

135.3 203.9 244.4 183.7 212.9 214.3 256.5 228.2 231.2 46.4 42.7 48.7 41.9 15.1 13.8 11.3 10.4

111 114 103 132 109 106 110 108 110 103 109 106 105 106 108 108 112

6.95 6.92 7.04 2.71 7.35 4.55 4.10 4.25 6.29 6.12 6.30 3.82 4.23 5.27 5.03 5.04 5.15

% Sulfur Com

97.22 96.55 87.53 77.50 52.03 67.52 48.81 44.85 19.60 84.45 81.70 51.65 52.37 48.06 42.52 52.23 27.27

331

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