Ind. Eng. Chem. Res. 1992,31, 1283-1288
1283
Industrial Regeneration of Naphtha Reforming Catalysts Contaminated by Sulfate Ions: Effect of Sulfate Level Teresita F. Garetto, Armando Borgna, and Carlos R. Apesteguia* Instituto de Investigaciones en Catdlisis y Petroquimica (INCAPE), FIQ- UNL-CONICET, Santiago del Estero 2654, 3000 Santa Fe, Argentina
Jean Claude Lavalley Laboratoire de Spectrochimie, U.A. 414, I.S.M.Ra, Universitt? de Caen, 14032, Caen C&dex,France
When naphtha reforming catalysts are sulfur contaminated, a special procedure is employed during catalyst regeneration to diminish the sulfation of the support. Thus, from an industrial point of view it is important to know the effect of the sulfate level on the naphtha reforming process. In this paper, most of the results were obtained on a Pt-Re/A1203 catalyst taken from commercial reforming reactors. Fresh Pt-Re/A1203 catalyst was also employed. The effect of the sulfate level on naphtha reforming was studied through an accelerated deactivation run at reforming conditions using a naphtha feed. Correlations were established through the analysis of parameters such as octane number, volumetric liquid yield, percentage of gases, percentage of aromatics in the liquid, and coke formation. The results showed that for sulfate levels higher than 0.08-0.1% S the rate of deactivation is increased as a consequence of enhanced coke formation. Such an increase of the coke formation results from (a) the difficulty of restoring the chlorine level on the support because of the blockage of the Lewis acid sites of the alumina by sulfate ions and (b) excessive sulfidation of the metallic fraction by H2S produced during the reduction of the sulfated catalysts with H2.
Introduction Naphtha reforming catalysts are sensitive to the presence of sulfur compounds which are present as impurities in the hydrocarbon feed. Catalyst poisoning typically results from strong chemisorption of sulfur compounds on the surface of the metal. During commercial practice feed is previously treated in a hydrodesulfurization (HDS)unit to reduce the sulfur concentration before reaching the reforming unit. The maximum sulfur level admissible in the feed depends upon the catalysts used. For standard Pt/Al2O3 catalysts, the preferred sulfur levels have been outlined (Ciapetta, 1971). When a conventional Pt-Re (atomic ratio Re/Pt = 1)catalyst is employed, the sulfur level must be as low as 1 ppm (Menon et al., 1982). New %kewed" catalysts containing higher Re/Pt ratios are even more sensitive to the sulfur compounds. For these catalysts, a maximum sulfur level of 0.5 ppm is allowed (Mc Lung, 1988). Because of these high-severity limits regarding sulfur poisoning, an additional sulfur-guard unit is often incorporated before the industrial reforming reactors to eliminate almost completely sulfur compounds in the feed. The loss of catalytic activity caused by sulfur poisoning occurs through two main mechanisms: (i) during the reducing conditions of the operation cycle, sulfur can be slowly incorporated to the catalyst by adsorption of H2S onto metallic sites; (ii) during catalyst regeneration, oxidized sulfur species can be adsorbed onto the support poisoning the acid sites (Franck and Martino, 1982). Since during industrial operation sulfur concentration in the feed is maintained at low values, this first mechanism is unlikely to occur. However, during the operation cycle sulfur is accumulated as iron sulfide on the reactors walls, in hot spots, and mainly in granulated metal produced by corrosion. When coke burning is carried out, the iron sulfide is oxidized to gaseous SO2and/or SO3 which can be fixed as sulfate ions by the support. If a high *Towhom correspondence should be addressed. Present address: Exxon Research & Engineering Co., Route 22 East, Annandale, NJ, 08801.
amount of sulfates is accumulated, the catalytic performance is adversely affected. In spite of the large bibliography regarding sulfur poisoning of naphtha reforming catalysts, few investigations have been specifically focused on the poisoning by sulfate under industrial conditions and on the special regeneration procedures needed to eliminate it. From an industrial point of view it is important to determine the maximum sulfate content allowable on the catalyst without interfering in a successful regeneration. In this paper we report the effecta that the sulfate level has upon catalyst chlorine regulation and catalytic activity during naphtha reforming.
Experimental Section A set of fresh (BF) and used (BU) samples of a commercial Pt-Re/A1203-C1catalyst of 0.31% Pt and 0.32% Re was employed. Fresh samples have 0.85% C1,80 ppm S,170 m2 g-l BET specific area (Sg),and 0.45 cm3g-l pore volume. Used samples were obtained after industrial use in a reforming unit composed of four endothermic reactors in series. Samples were removed from reactors during the fifth industrial regeneration. The length of the fifth operating cycle was 185 days. A high-purity A 1 2 0 3 powder (Cyanamid Ketjen CK 300) was also employed. Alumina CK 300 has 180 m2g-' BET specific area and 0.49 cm3 g-l pore volume and contains 50 ppm sulfur. Catalytic reforming activity was measured by meam of an accelerated deactivation test using bench-scale equipment (Apesteguia et al., 1984). The standard run utilizes three test periods. Initial operation was carried out under industrial reforming conditions (P= 15 atm; H2:naphtha molar ratio = 4)during 8 h (period I). Then, pressure was decreased during 8 h to 1 atm (period 11). In period I11 the conditions of period I were restored. During all the periods temperature, weight hourly space velocity (WHSV) and H2:naphthamolar ratio were kept at 505 OC, 5 h-l, and 4,respectively. The severe conditions of period 11produce a deactivation similar to long reaction time under the conditions of period I. The commercial hydrotreated naphtha contained less than 1 ppm sulfur and had the following properties: re-
0888-5885f 92f 263l-l283$03.Oo/O 0 1992 American Chemical Society
1284 Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 Table I. Iron and Sulfur Contamination of Industrial Catalysts; Fifth Regeneration in the Reforming UnitD reactor no. 1 2 3 4 1 1 sample BU-1 BU-2 BU-3 BU-4 BU-I BU-I(0x) 0.24b % Fe 0.057 0.028 0.050 0.026 0.24b 0.093' %S 0.087 0.056 0.075 0.050 0.036' 0.09' %C 0.12 0.02 0.10 0.050 9.8' 0.15 0.39 1.5 1.9 S/Fe 1.5 2.0 Samples BU-1 to BU-4 were taken after coke burning. Sample BU-I was taken before coke burning. Sample BU-I(0x) was sample BU-I treated with Oz/Nz, 5% 02,at 480 'C for 8 h. bThe Fe level corresponded to the whole sample composed of two fractions: a pellet fraction and a Fe-containing powder fraction (samples BU-I and BU-I(0x) without screening). 'The % S and % C values were determined for the pellet fraction (samples BU-I and BU-I(Ox) after screening).
search octane number (RON), 59; density, 0.736 g/mL; mean molecular weight, 109; boiling point range, 62-147 "C; composition (wt %): paraffins 64.2, naphthenes 24.2, aromatics 11.6. Research octane number, volumetric liquid yield (&+, where C5+represents hydrocarbons with five or more carbon atoms), percentage of gases (% G), and percentage of aromatics in the liquid (% ArJ were calculated from chromatographic analysis data. The fixed-bed, single-pass tubular reactor consisted of a 17-cm stainless-steel tube, with a useful height of 8 cm and an annular section of 2.1 cm2,which gives a volume of 16-17 cm3. It corresponds approximately 10 g of catalyst with an alumina base. The catalyst bed temperature could be controlled to within 1"C. The reactor exit gases were bifurcated. A small part was sent to a gas chromatograph through a sampling valve. The other part was condensed in a water refrigerated condenser. On-line chromatographic analysis was performed using a flame ionization detector and a capillary column coated with squalene. Details of the experimental procedures and of the reproducibility of the data obtained with this test unit are reported in Sad et al. (1980). The data presented in the present paper are average values of two replicate runs. In all cases, catalyst pellets were crushed and the 4080-mesh fraction was separated and loaded to the reactor. Sulfur levels on the catalysts were measured by chemical analysis and X-ray fluorescence. Chlorine and iron contents were determined by chemical analysis, using conventional colorimetric techniques. Carbon content of the catalysts after the runs was determined by combustion volumetry.
Results 1. Sulfate Contamination of the Catalysts. The industrial reforming unit was a Magnaforming type unit, with four reactors in series (Ciapetta and Wallace, 1971). In the fifth industrial regeneration, the sample BU-I was taken from reactor 1before coke burning. Samples BU-1, BU-2, BU-3, and BU-4 were taken after coke burning from reactors 1,2,3, and 4, respectively. Iron and sulfur contents of these samples are given in Table I. Regarding samples BU-1 to BU-4, higher Fe levels were detected in samples BU-1 and BU-3. This can be explained by taking into account that the presence of Fe on the catalysts is caused by a corrosion phenomenon of the reforming unit. Corrosion is usually higher in reactor 1 and 3 because in the Magnaforming process hydrogen and chlorine compounds are introduced through these reactors. It is significant to note that the sulfur level of the samples is directly related to the Fe content: the higher the % Fe, the higher the % S content. As shown in Table I, the S/Fe weight ratio for samples BU-1 to BU-4 did not change
Table 11. Preparation of the Three Series of Samples with Different Sulfate Level catalyst BF catalyst BU-3 alumina CK 300 sample % S" sample 5% S" sample % s" BF-a 0.008b BU-3-a 0.075' A-a 0.005c BF-b 0.044 BU-3-b 0.145 A-b 0.070 BF-c 0.120 BU-3-c 0.180 A-c 0.125 BF-d 0.315 BU-3-d 0.244 A-d 0.220 BF-e 0.480 BU-3-e 0.340 A-e 0.320 "Sulfur level of the samples after performing step I11 of the preparation procedure (see text, section 2). bSulfur level after performing step I of the preparation procedure. 'Sulfur level before performing any treatment of the preparation procedure.
significantly, varying between 1.5 and 2. In order to obtain more insight into the formation of sulfate on the catalyst during industrial regeneration, the sample BU-I was taken from reactor 1before coke burning. Sample BU-I consisted of two fractions: the regular pellet fraction and an additional powder fraction. The latter was chosen because its reddish color suggested that it was highly contaminated with Fe. This was confirmed by chemical analysis as a value of 0.24% Fe was determined for the whole sample BU-I (Table I). The 90 S was measured for the pellet fraction after screening, obtaining a value of 0.036%. Sample BU-I (pellet + powder fractions) was treated with 02/N2,5% 02,at 480 "C. The resulting sample was named BU-I(0x). The % S was again measured for the pellet fraction after screening, and a value of 0.093% was found. Such a value was clearly greater than that obtained before the oxidizing treatment (0.036% S), thereby indicating that during the coke burning an additional quantity of sulfur was incorporated into the pellet fraction of sample BU-I. The source of this additional sulfur was the Fe-containing powder fraction. 2. Effect of the Sulfate Level on Catalytic Activity. Coke burning during the industrial regeneration of reforming catalysts causes the concomitant elimination of chlorine and the sulfation of the support. In order to reproduce the state of the catalysts after coke burning, a sample of catalyst BF (0.85% C1) was submitted to the following treatments: Step I. Partial removal of chlorine. The chlorine content was lowered using a gaseous mixture HC1/H20/air at 500 "C and atmospheric pressure. Following this treatment, chlorine on the catalyst was decreased to 0.35%. Step 11. Sulfidation at 300 "C with H2S highly diluted in H2. Step 111. Oxidation of sulfided samples to incorporate sulfate ions to the support. A gaseous mixture of 02/N2, 5% 02,at 480 "C was employed. After step I11 and in order to reproduce the final regeneration steps usually performed in the industrial unit, the samples were treated according to the procedure given in steps IV to VI: Step IV. Chlorine regulation. Samples were treated using a gaseous mixture of HC1/H20/air, with H20/HCl = 60 and 600 ppm HC1 at 500 "C for 4 h. Under these conditions, an equilibrium chlorine concentration of ca. 0.75480% C1 is obtained when the same samples are not contaminated by sulfur (Castro et al., 1981). Step V. Reduction in H2 at 500 "C for 4 h. Step VI. Presulfidation with H2S/H2at 480 "C for 2 h. Then the samples were purged with a dry N2 stream for 30 min. By changing the H2S concentration of the H2S/H2 mixture in step 11, a set of sulfated catalysts with different final amounts of sulfur was prepared (Table 11, catalyst BF). Sample BF-a was taken as reference and was not
Ind. Eng. Chem. Res., Vol. 31, No. 5,1992 1285 Table 111. Naphtha Reforming Tests RON Arl sample 1' 2b 1 2 BF-a 91.8 85.9 59.2 51.0 BF-b 91.6 85.6 58.8 50.8 51.2 85.4 62.0 BF-c 93.4 48.5 82.9 60.9 BF-d 92.4 59.8 46.4 BF-e 92.0 82.8 1: values obtained at the end of period I.
%G
&E,+
1
78.0 78.3 78.6 79.6 79.0
* 2:
2 84.5 84.7 85.4 86.2 86.8
1 13.30 13.21 11.62 10.80 10.54
2 6.71 6.87 5.99 5.90 5.38
ARON' -5.9 -6.0 -8.0 -9.5 -9.2
A(% ArJc -8.2 -8.0 -10.8 -12.4 -13.4
6.5 6.4 6.8 6.6 7.8
A(% G)c -6.59 -6.34 -5.63 -4.90 -5.16
%C
1.92 1.88 2.35 3.15 3.80
values obtained at the end of period 111. Difference values between periods I11 and I.
045
0 40 00
I
,
I
o,4Yo s Figure 1. Period I. C1/Cs and C1/G ratios as a function of the sulfate level. 0.0
0.2
sulfated through steps I and 111; i.e., sample BF-a was prepared according to the procedure given in step I and steps IV-VI. It was verified that the surface area and pore volume of catalyst BF were not changed by the treatment involving steps I-VI. The catalytic activity, selectivity, and stability of the samples for naphtha reforming were measured under the operational conditions outlined in the Experimental Section. Table I11 presents RON, qC5+, 90Arl, and % G values obtained at the end of periods I and I11 and also the differences between the periods I11 and I values. In the following analysis, RON was taken as measure of activity and qC5+as a measure of selectivity. It must also be taken into account that RON and 9% Arl will have similar variation trends because of the high octane number of the aromatics and that 9% G and qC5+are related when one increases, the other decreases. Period I. The values given in Table I11 were obtained at the end of period I, where catalyst activity is stable after the initial rapid coke deposition. The amount of coke formed in period I is quite low due to the high H,pressure employed and to the short cycle length (8 h). No specific trend for the RON and % Arl values was observed. In contrast, vC5+and % G varied with the amount of sulfur on the catalysts. Because of the low deactivation rate, the changes of the selectivity in period I are most affected by the hydrogenolysis activity of the metallic fraction. The methane/propane ratio is used to compare hydrogenolysis and hydrocracking. Hydrocracking proceeds on acid sites, and the C-C bond scission occurs mainly with formation of propane and butane whereas methane is formed via hydrogenolysis on metal sites only. Thus, the changes in the C1/C3 ratio can be related to metallic and acidic functions. The percentages of C1 and C3 were calculated, and in Figure 1the C1/C3and C1/Gratios are plotted as a function of the sulfur level of the samples. Both parameters diminished when the amount of sulfur was increased. Besides, the similar behavior of these parameters suggested that the % G changes listed in Table I11 were mainly caused by the modification of the metallic function. Demethylation of aromatics is also a typical hydrogenolysis reaction. In Figure 2 the values of the benzene/
02
04 Yo s Figure 2. Aromatics in period I. B/Tand B/Ar ratios as a function of the sulfate level.
0
--
-
00
Benzene i
O
n
02
O4 O/O s Figure 3. Aromatics in period 111. Representation of S.A. and the production of benzene, toluene, and xylenes as a function of the sulfate level. S.A., the selectivity ta aromatics, is defined in the text.
toluene (B/T) and benzene/aromatics (B/Ar) ratios were plotted as a function of sulfate level on the catalysts. Both parameters diminished with increasing % S, thereby confirming that the hydrogenolysis activity was passivated by increasing the sulfate level of the samples. Period 111. Period I1 is carried out at high severity to produce a rapid deactivation by coking. Therefore, the catalyst activity exhibited in period I11 is directly related to the amount of coke deposited during period 11. Table I11 showed that the carbon levels of samples BF-a (nonsulfated) and BF-b (0.044% s)were similar,but when the amount of SO4" ions on the samples was increased further, coke formation increased significantly. As a consequence, the values of ARON and A ( % Arl) exhibited a similar decay trend (Table 111). On the contrary, the liquid yield values (7C5+)determined at the end of period I11 slightly increased with the 7% S of the catalysts. Regarding aromatics, the selectivity to aromatics (S.A.), defined as the ratio of mass of aromatics produced per mass of paraffins converted, was plotted as a function of % S in Figure 3. The S.A. values corresponding to samples BF-d and BF-e (high sulfur level) were clearly lower than those obtained for samples BF-a, BF-b, and BF-c. In the same figure, the concentrations of benzene, toluene, and xylenes in the liquid fraction were also represented. In all cases, a rather linear decrease with the % S was found. 3. Effect of Sulfate Level on Chlorine Regulation. In order to study the effect of the sulfate level on the
1286 Ind. Eng. Chem. Res., Vol. 31, No. 5,1992
03
-
00
02
O4 % S Figure 4. Effect of the sulfate level of the samples on the chlorine regulation.
chlorine adjustment, three sets of samples were prepared (Table 11). The preparation procedure consisted of steps I-IV given under section 2. The sulfate levels shown in Table I1 were measured after step 111. The set corresponding to catalyst BF was the same employed in the catalytic activity testa. The second set of samples was prepared using industrial catalyst BU-3 (Table I). Catalyst BU-3 was taken from reactor 3 after the coke burning during the fifth industrial regeneration. Then, it was submitted to the same preparation treatment involved in steps II-IV. Step I was avoided because catalyst BU-3 already had a low amount of residual chlorine (0.17% Cl). Five samples with different sulfate levels were prepared (Table 11). Sample BU-3-a was submitted only to the chlorine regulation treatment (step IV). Finally, the third set of samples was prepared by using the high-purity CK 300 alumina. Such an alumina was sulfated according to steps I1 and I11 (step I was not performed). After that, the chlorine regulation (step IV) was carried out by using a H20/HC1 molar ratio of 10 instead of 60, which was the value employed for catalysts BF and BU-3. When a value of H,O/HCl = 10 is used, an equilibrium chlorine concentration of ca. 1.1% C1 is to be expected (Castro et al., 1981). Five samples with increasing amounts of sulfur were prepared (Table 11). Sample A-a was not sulfated and therefore was only submitted to step IV. In Figure 4 the amounts of chlorine deposited on the samples upon step IV are plotted against sulfate level. Although for catalysts BF and BU-3 the chlorine regulation step was performed by using the same molar ratio H20/ HCl = 60, the amount of chlorine retained on catalyst BU-3 is lower than that of catalyst BF. This is attributed to the decrease in the Sg value of the BU-3 catalyst after five industrial regenerations (Sg = 150 m2g-'). The diminution of AP+ Lewis sites diminishes the chlorine retention of the support (Santacesaria et al., 1977; Castro et al., 1983). For the three sets of samples, the amount of chlorine diminished when the sulfate level was increased. This effect was significant for samples containing above ca. 0.0&0.1% s. Discussion The conditions used during the industrial operation of the reforming unit (high temperature, reducing atmosphere, presence of chlorine compounds) cause the corrosion of reactors made of ordinary carbon steel. Our resulta indicated that in the Magnaforming process iron contamination is most pronounced in the first and third reactors. Although the level of sulfate exiting HDS units is quite small, thermodynamic data (Mc Carty and Wise, 1982;
Bartholomew et al., 1982) indicate that the formation of stable surface iron sulfide is favored under industrial conditions. Thus, sulfur accumulates as FeS during the operating cycle. As a consequence, the amount of sulfur on the catalyst at the end of the cycle is related to the Fe level as suggested by the results obtained for samples BU-1 to BU-4 in Table I. The formation of iron sulfide does not influence the catalytic performance during the operation cycle. Strictly, the contamination of the catalyst occurs when an oxidizing mixture is used to burn coke during the regeneration step. The iron sulfide particles are oxidized and decomposed into Fe203and gaseous SO2and/or SO3 which can be fixed by the support and eventually transformed to surface sulfate ions in a finaloxidation step. The formation of oxidized species on alumina follows a rather complex mechanism (Chang, 1978; Lavalley et al., 1981; Saussey et al., 19831, and the amount of sulfate formed depends on the temperature and on oxygen partial pressure in the oxidation mixture. At industrial conditions low po2 is used to prevent a too rapid and energetic coke burning. Temperature and pot are carefully increased reaching ca. 480 "C and 5 % 02,respectively, at the end of the coke burning step. Our results show that under these operational conditions sulfate contamination is effectively incorporated into the catalyst (Table I, sample BU-I(0x)). Although the reduction of sulfates on alumina occurs at temperatures above 600 "C, the presence of Pt catalyzes the reduction of the sulfate ions. Treatment by Hzat 500 "C reduces the sulfate ions in the immediate surrounding of the metal particles (Apesteguia et al., 1987). The reduction-oxidation of the sulfur compounds into the catalyst is interpreted by the redox reaction
SO~-8upport + metal
H2, 500 ' C 02,480 O C
S-metal
+ 02-support (1)
From a fundamental point of view, validity of the redox reaction 1 has been well established and adequately explains the role sulfates play in the metal poisoning under reducing conditions (Maurel et al., 1975; Mathieu and Primet, 1984). However, only a few studies relating the effect sulfate has on the catalytic performance of commercial reforming catalysts has been published in the open literature (Franck and Martino, 1982). From an industrial point of view a major question is: What is the maximum sulfate level on the catalyst that can be tolerated without affecting the catalytic performance? Our catalytic testa suggested that during the initial period of the standard run the RON and % Arl valuea were not significantly changed, but the 9% G diminished when the sulfate level on the catalyst was increased. This was because the main effect of sulfur in this period was to diminish hydrogenolysis reactions. The C1/C3 ratio and demethylation of aromatics were significantly reduced indicating that sulfur affected mainly the metal function. The change in selectivity caused by sflidation is a very well-known phenomenon and has been rationalized in terms of the ensemble effect (Ponec, 1983). Adsorbed sulfur diminishes the number of larger Pt ensembles which are required for hydrogenolysis, and as a consequence, the selectivity to isomerization, cyclization, and dehydrogenation is enhanced. In the present case, the catalysta were presulfided before the catalytic tests. As presulfidation was a common step for all the catalysts, the diminution of CI/C3 and B/T ratios with increasing sulfur (Figures 1 and 2) is attributed to additional sulfidation of the metals caused by the reduction of Sod2ions (see reaction 1). An important effect of the sulfate level was noted after the accelerated deactivation period. The formation of coke
Ind. Eng. Chem. Res., Vol. 31, No. 5, 1992 1287 on the catalyst increased when the amount of sulfate on the support was increased. As a consequence, the values of % Aq, RON, and S.A. in the third period of the standard run diminished for the samples containing higher sulfate levels (A( % hl)and ARON values in Table 111, and S.A. plot in Figure 3). There are two possible explanations that can account for such an increase in the formation of coke with the amount of sulfates. The first explanation concerns sulfidation of the metal fraction. It has been reported that presulfidation increases the quantity of coke deposited on the whole catalyst (Parera et al., 1986). Although presulfidation inhibits coke formation on the metal preserving their dehydrogenation activity (Wilde et al., 1987), coke deposition on the whole catalyst is enhanced on sulfided catalysts (Barbier and Marecot, 1986). The second and most probable explanation relates the deactivation rate to the imbalance of metallic and acid sites. It is well-known that reforming catalysts are bifunctional catalysta and need an adequate balance between acid and metal sites to obtain a minimum coke deactivation. Regarding acid sites, it was found (Parera et al., 1980) that the optimum chlorine content for maximum hydrogen spillover corresponds to a surface where nearly half of hydroxyl groups are substituted by chlorine. For a typical reforming alumina of 18WOOm2/g surface area this value is about 0.749% C1. Maximum hydrogen spillover diminishes the concentration of coke precursors on the catalyst surface. Our results showed that the chlorine adjustment is affected by the presence of sulfates on the support (Figure 4). For values of sulfates higher than ca. 0.08-0.1% S the expected equilibrium concentration of chlorine diminished, thereby producing the imbalance between acid and metal sites. This suggests that the presence of sulfates impedes the regulation of the chlorine level because of the competition for the adsorption sites on the A1,0, support. The adsorption of HC1 on alumina presumably occurs on Lewis acid sites producing new OH groups of Bronsted acid character (Peri, 1966; Tanaka and Ogasawara, 1970). In a simple representation, the adsorption of HC1 can be considered as an exchange between OH and HC1 surface groups: OH 0I I A(O,A(o,AIt
OH OH I I
+ HCI Z= A(o,A(o,AI
71 Sr CI
0I A(
A(: 0'
&, ' 0
+
H20
(2)
The number of adsorption sites (which is the sum of OH and HC1 surface concentrations) and the equilibrium constant of eq 2 depend on the temperature. For a given temperature, the equilibrium chlorine concentration depends on the H20/HC1molar ratio of the gaseous mixture used in the chlorination step. In the case of commercial alumina CK-300, the total number of sites at 500 "C is about 1.1 X l O I 4 sites cm-2 (Castro et al., 1980, which corresponds to a saturation chlorine concentration of ca. 1.2% C1. We obtained a value of 1.15% C1 for sample A-a (Table 11,pure alumina) by using a H,O/HCl molar ratio of 10 at 500 "C. The amount of chlorine on commercial catalyst BF was 0.73% C1 when a value of H20/HC1 = 60 was employed (Table 11, sample BF-a). The support of catalyst BF was also a CK-300 type alumina. The oxidation of adsorbed H2Sor SO2on alumina leads to the sulfate formation via different mechanisms (Saur et al., 1981). In our case the initially adsorbed molecule was SO2. The SO2sulfation depends on the temperature, on the alumina hydroxyl content, and on the gaseous ox-
ygen concentration. In a previous paper, Saussey et al. (1983) determined the value of 1.3 X 1014S042-ions cm-2 for complete surface coverage of sulfates on a Degussa C alumina above ca. 500 "C. Such a value is close to that corresponding to the saturation chlorine concentration on alumina at 500 "C (1.1x 1014sites cm-2). It represents 3.75% SO4" by weight (or 1.25% expressed as % S) for our alumina CK-300. Like HCl adsorption, the adsorption and sulfation of SO2 is primarily regulated by the number of Lewis acid sites on the alumina (Rosynek and Strey, 1976; Fiedorow et al., 1978). Also, as in HCl adsorption, sulfate formation enhances Bronsted acidity, possibly via surface inductive effects on neighboring hydroxyl groups (Przystajko et al., 1985). The formation of a sulfate-like structure from SO2 sulfation can be pictured as follows: O*"@ SO2
+
,AC
0- OH 1 1 ' 0
AI,
0 '
AI
1
OY3\O AI,
' '0
OH
I
I
AI,
AI
0 '
The observation that sulfate formation impedes chlorine regulation may be due to simple steric blockage of Lewis acid sites on alumina. Our results showed that this effect is quite specific. In fact, when the amount of SO4%is only ca. 8% of the complete surface coverage (0.1% S referred to 1.25% S) the chlorine adjustment on alumina was affected (Figure 4). Thus,it seems that the maximum sulfate level that can be admitted on industrial reforming catalysts should not be higher than that corresponding to about &lo% of the total adsorption sites for sulfates on the support.
Conclusions Under industrial conditions, sulfate contamination is effectively incorporated into the catalyst during coke burning. The iron sulfide particles are decomposed, and gaseous SO2 is fixed by the support as sulfate ions. For sulfate amounts higher than 0.0&0.1% S coke formation is enhanced. The presence of sulfates impedes the regulation of the chlorine level by blocking Lewis acid sites on the alumina support. As a consequence of the imbalance between metallic and acid sites, the rate of deactivation is increased. The effect of sulfate ions is quite specific since the amount of 0.1% S represents only ca. 8% of the complete sulfate coverage on the support. Acknowledgment We acknowledge the lfinancial support of the Consejo de Investigacjones Cientificas y TBcnicas (CONICET) and the Secretaria de Estado de Ciencia y Tecnologia (SECYT), Argentina. Registry No. Pt, 7440-06-4; Re, 7440-15-5; C, 7440-44-0.
Literature Cited Apesteguia, C. R.; Garetto, T. F.; Beltramini, J. N.; Parera, J. M. Effect of the metal loading and the metal particle size on naphtha reforming: Pt/A1203-C1catalysts. In Catalysis on the Energy Scene; Kaliaguine, S., Mahay, A., Eds.; Elsevier, Amsterdam, 1984;pp 381-388. Apesteguia, C. R.; Garetto, T. F.; Borgna, A. On the Sulfur-Aided Metal Support Interaction in Pt/A1203-C1Catalysts. J. Catal. 1987,106, 73. Barbier, J.; Marecot, P. Effect of Presulfurization on the Formation of Coke on Supported Metal Catalysts. J. Catal. 1986,102,21. Bartholomew, C. H.;Agrawal, P.; Katzer, J. Sulfur Poisoning of Metals. Adu. Catal. 1982,31, 135. Castro, A.; Scelza, 0.;Benvenuto, E.; Baronetti, G.; Parera, J. M. Regulation of the Chlorine Content on Pt/A1,03 Catalysts. J. Catal. 1981,69,222.
Ind. Eng. Chem. Res. 1992, 31, 1288-1292
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Received for review June 21, 1991 Revised manuscript received January 16, 1992 Accepted February 20, 1992
Mixing Characteristics of a Micro-Berty Catalytic Reactor Hasan Hamount and John R. Regalbuto* Department of Chemical Engineering, University of Illinois at Chicago, P.O. Box 4348, Chicago, Illinois 60607
The bulk and internal gas-phase mixing characteristics of a new, smallest commercial Berty-type reactor (Autoclave Engineers), for use with gas-solid catalytic reactions, have been evaluated over a range of impeller speeds, pressures, flow rates, and catalyst particle sizes. Mixing in the bulk gas phase has been investigated using step change tracer experiments. These results show no measurable dead volume and good mixing for impeller speeds above 1000 rpm, at pressures above 100 psi and for all catalyst forms. The operating regime free from internal concentration gradients has been determined over pelleted, chunked, and powdered Ni/A1203catalysts utilizing methanation of carbon monoxide as a probe reaction with a reactant flow rate of 8500 cm3(STP)/(g of catalystoh). Performance in each case is summarized as a function of pressure, catalyst particle size, and impeller speed. The results show good internal mixing at or above a pressure of 80 psi and impeller speeds greater than or equal to 1000 rpm. Poor bulk and internal mixing occurs at atmospheric pressure. Introduction The measurement of chemical kinetics in the absence of physically controlling factors is of prime importance in the study of gas-solid catalytic reactions. A satisfactory bench-scale reactor will possess a number of characteristics (Carberry, 1976): 1. First is a well-mixed operation or, in other words, operation in the absence of temperature and concentration gradients, which ensures measurement of intrinsic reaction rates and facilitates scale-up. 2. A satisfactory bench-scale reactor also has a small size, consistent with the production of accurate data in +Present address: Commonwealth Edison, P.O. Box 767, Chicago, IL 60690. 0888-5885/92/2631-1288$03.00/0
lab-scale research. This reduces hazards and safety requirements and ensures a short system time constant, minimizing transient disruptions when the experimental conditions are changed. 3. Next, the reactor has the capability of handling actual industrial catalyst. 4. Last, it has a standard design, as far as possible, allowing immediate availability for a wide variety of chemical systems. A recently produced 1-in. micro-Berty reactor (Autoclave Engineers) would appear to fulfill these requirements. It is currently the smallest volume Berty-type reactor in commercial production. This study was undertaken to evaluate its performance as a perfectly mixed, continually stirred tank reactor (CSTR) for heterogeneous gas-solid catalyzed reactions. Two regimes of mixing were inves0 1992 American Chemical Society
