R&D on New, Low-Temperature, Light Naphtha Isomerization Catalyst and Process

Transcription

1 2000M1.1.2 R&D on New, Low-Temperature, Light Naphtha Isomerization Catalyst and Process (Low-temperature isomerization catalyst technology group) Takao Kimura, Masahiko Dota, Kazuhiko Hagiwara, Nobuyasu Oshio, Koji Baba 1. Contents of Empirical Research As environmental regulations on motor gasoline become stricter throughout the world, isomerate consisting mainly of isoparaffin can be regarded especially as promising candidate substitutes for conventional gasoline blendstock through Reid vapor pressure, aromatics and olefin regulations. In the current naphtha isomerization process, however, pretreatment by hydrodesulfurization of light naphtha is required. What is more, in part of the process, dehydration of feedstock and of gas is imperative, so that huge capital investments in new equipment are required. The target of the present R/D is to develop a catalyst of higher poisoning resistance against moisture and sulfur content than the conventional light naphtha isomerization industry catalyst, and to develop processing for the same. More specifically, the purpose is to develop a new isomerization catalyst having low-temperature activity and higher poisoning resistance for feedstock in which the moisture content is saturated and for feedstock containing roughly the amount of sulfur in pretreated MEROX process. Using this developed catalyst, the purpose is also to develop a new isomerization process by such means as combining simplified pretreatment systems in which the pretreatment conditions of light naphtha are eased. Specific target values of development are presented below. Intermediate targets Final targets (1) Feedstock sulfur content 150 massppm or below 150 to 700 massppm (2) Feedstock moisture content Saturation levels (room temperature) Saturation levels (room temperature) (3) Reaction temperature 200 C or below 140 to 180 C (4) Product oil octane number 78 or above 79 or above (RON) In line with the overall plan given in Table 1-1, research has continued to advance in 1999, mainly on catalyst development and process development. As a result of R&D in the previous year, a highly activity, new catalyst system drastically superior to the conventional catalyst in sulfur resistance was discovered. Yet the search continues for catalyst of even higher activity, and an economical process that includes catalyst regeneration must be developed, so the original four-year research plan was extended by one year to a five-year plan. 1

2 Table 1-1 Overall Plan Year Item Catalyst development Process development Industrialization research 2. Empirical Research Results and Analysis Thereof 2.1 Catalyst development (1) Low-temperature isomerization activity of MEROX naphtha by CAT-A catalyst Using the catalyst (CAT-A catalyst) of high poisoning resistance against organic sulfur, discovered last year in feedstock, the low-temperature isomerization activity of light naphtha treated MEROX process (MEROX naptha) was evaluated. The sulfur content of the MEROX naphtha used in the reaction was 220 massppm, and the moisture content was at saturation level. When the reaction temperature is lowered from 200 C to 180 C, the i-c5/total-c5 ratio drops 10 to 15%, but the ratio of catalyst deactivation over time are roughly equivalent (Figure 2-1). Given this fact, at the reaction temperature of 180 C or below, the final target, optimization of LHSV and other reaction conditions is required along with search for catalyst of higher low-temperature activity. Reaction time (h) Reaction conditions: Pressure 3MPa, LHSV 2.9/h, H 2/Oil 2 mol/mol Feedstock: MEROX naphtha (sulfur content: 220 massppm) Figure 2-1 Low-temperature activity of MEROX naphtha by CAT-A catalyst 2

3 (2) Search for catalyst of high low-temperature activity With CAT-A catalyst as a base, an investigation was made effect of active metal species and of the amount of active metal loaded. In isomerization reaction the feedstoks used n-pentane containing 300 massppm of sulfur (addition an organic sulfur compound ((n-c 3 ) 2 S 2 )) (Figure 2-2). A number of bimetallic catalysts were found which exhibit higher isomerization activity than that of CAT-A catalyst at 180 C. From an investigation of the effects of amount of active metal loaded, it was noted that there is an effect with Pt/SO 4 /ZrO 2 catalyst, but that the isomerization activity was roughly equivalent to that of CAT-A catalyst (Figure 2-3). In the Pt/SO 4 /ZrO 2 catalyst, a deactivation was admitted catalytic poising by sulfur in feedstock, especially organic sulfur, but by increasing the amount of active metal loaded, resistance against sulfur poisoning improved dramatically. In the CAT-A catalyst, on the other hand, no special effects were noted under the current reaction conditions since the resistance of sulfur poisoning is high originally, but respecting catalyst life, it is believed that higher active metal content is preferable. As a result of further screening of these catalysts, using MEROX naphtha (sulfur content: 260 massppm), it was found that bimetallic C catalyst based on CAT-A catalyst indicated higher low-temperature activity. Temperature: 200 C Temperature: 180 C i-c5/total-c5 ratio (%) Reaction conditions: Temperature 180 C or 200 C, LHSV 5/h, Pressure 1.47 MPa, H 2/Oil ratio 2 mol/mol Feedstock: n - C5 + (n - C 3) 2S 2 (S = 300 massppm) i-c5/total-c5 ratio: Average value from 4-hour to 8-hour reaction time Figure 2-2 Effect of Active Metal Species on CAT-A Catalyst 3

4 Temperature: 200 C Temperature: 180 C i-c5/total-c5 ratio (%) BASE Double BASE Double metal quantity metal quantity metal quantity metal quantity CAT-A Pt/SZ Reaction conditions: Temperature 180 C, LHSV 5/h, Pressure 1.47 MPa, H 2/Oil ratio 2 mol/mol Feedstock: n - C5 + (n - C 3) 2S 2 (S = 300 massppm) i-c5/total-c5 ratio: Average value from 4-hour to 8-hour reaction time Figure 2-3 Effect of Amount of Active Metal Loaded on Catalyst 2.2 Investigation of process development (1) Effect of sulfur content in feedstock with CAT-A catalyst Shown in Figure 2-4 and Figure 2-5 are the results of an investigation of the effect of sulfur content in feedstock on isomerization activity by CAT-A catalyst. Although the poisoning resistance against sulfur in feedstock is high with this catalyst, catalytic activity drops with increases in sulfur content and catalyst deactivation over time progresses sharply. From these results, it is conjectured that the sulfur content in feedstock for long-term continuous operation at present should be 100 massppm or less. i-c5/total-c5 ratio (%) Figure 2-4 Sulfur content (ppm) Reaction conditions: Temperature 190 C, Pressure 3MPa, LHSV 2.9/h, H 2/Oil 2 mol/mol Feedstock: Mixtures of MEROX napththa and desulfurized naphtha Correlation between sulfur content and isomerization activity of light naphtha with CAT-A catalyst 4

5 Deactivation (%/h) Figure 2-5 Sulfur content (ppm) Reaction conditions: Temperature 190 C, Pressure 3MPa, LHSV 2.9/h, H 2/Oil 2 mol/mol Feedstock: Mixtures of MEROX naphtha and desulfurized naphtha Effect of sulfur content in light naphtha on deactivation of CAT-A catalyst (2) Results of long-term continuous operation by bench plant using CAT-A catalyst Using 40ml of CAT-A catalyst, experiments were done on long-term continuous operation by bench plant with two types of MEROX naphtha, one containing 72 massppm sulfur and another containing 241 massppm. Table 2-1 presents the results of analysis of these MEROX naphtha used in the experiments. The reaction temperature was varied to compensate for the drop in reaction activity that comes with deactivation of the catalyst so that the octane number (RON) of C5 + fraction in the products oil is kept at 78 or above in the experiments, Table 2-1 Properties of feedstock for long-term continuous operation by bench plant MEROX naphtha CR-M/X MEROX naphtha YR-M/X GC-RON Sulfur content (massppm) Moisture content (massppm) Saturation 32 (Saturation) Density (g/cm 3 ) C1-C C Bz content (mass %) C7+ content (mass %) The trends in reaction temperature and in i-c5/total-c5 ratio are indicated in Figure 2-6. Yet over the 4089 hours during which the reaction temperature rose from its initial 185 C to 215 C, operation took place in feedstock of 72 massppm sulfur. Then the feedstock was switched to that with 241 massppm sulfur and operation was continued for 5600 hours until the final reaction temperature reached 270 C. 5

6 The trends in the C5+yields and C1-C4 gas yields obtained long-term continuous operation are shown in Figure 2-7 and estimations of catalyst life are presented in Figure 2-8. In estimations of catalyst life, reaction test data in which feedstock of 72 massppm sulfur was taken as the base, and corrections were made after removing data on drops in catalytic activity following sudden stops and start-ups during long-term continuous operation, and data on drops in activity after switching to feedstock of high-concentration sulfur content. As a result, from an initial reaction temperature of 185 C up to 240 C, the trend in product yield selectivity remained roughly constant and the catalytic life up to the final reaction temperature of 240 C could be estimated at approximately 330 days. From these results, it is conjectured that with a light naphtha (sulfur content: 72 massppm) obtained by MEROX pretreatment, using the developed CAT-A catalyst, industrialization of a new isomerization process is possible. i-c5/total-c5 ratio (%), Octane number (RON) Octane number i-c5/total-c5 ratio Reaction temperature Sudden shut down, restart up Feedstock change (CR-M/X YR-M/X) Reaction temperature ( C) Reaction time (h) Feedstock: MEROX naphtha (Sulfur content CR-Mx: 72 ppm,yr-mx: 241 ppm) Reaction conditions: Pressure 3.1MPa, LHSV 2.9/h, H 2/Oil 360NL/L Figure 2-6 Results of long-term continuous operation by CAT-A catalyst Octane number (RON), C5+ yield (%) (C5+ yield) (Octane number) Feedstock (C1-C4 gas) Feedstock change (CR-M/X YR-M/X) Feedstock C1-C4 gas (%) Figure 2-7 Reaction time (h) Trend in reaction yield by CAT-A catalyst 6

7 Reaction temperature ( C) Deactivation: 0.14 C/day Figure 2-8 Reaction time (days) Assumption: It is assumed that CR-M/X (sulfur content: 72 massppm) is the feedstock, that the initial reaction temperature is 185 C and that the final reaction temperature reaches 240 C (Octane number of product oil is held at 78.) Estimated service life of CAT-A catalyst (3) New process concept design A design concept for the new process was formulated based on data from long-term continuous operation using the CAT-A catalyst. Here balance sheets on material balance and on heat balance for each piece of equipment in the flow of processing were compiled based on material balance data of SOR (193 hours after starting) and MOR (after 3180 hours) obtained in reaction tests. The utility consumption of each equipment piece, equipment piece specifications and equipment costs were then calculated. The design concept for the process was formulated based on the following assumptions. Operation days: Isomerate product: 8,000 hours (333 days) 5,000 BPSD Product specifications: As suitable gasoline base material property, Hydrogen sulfide content 1 massppm, RVP 110 kpa Each case of hydrogen gas once through and recycle Process flow and calculations are presented in Figure 2-8. These results indicate that in terms of process construction costs, recycle is lower than hydrogen gas once through, and when hydrogen gas and other costs are also factored in, the later option is even more advantageous. What is more, with the newly developed process, the cost of hydrogen desulfurization equipment for feedstock pretreatment, which is imperative in the conventional process, can be reduced. For this reason, it is believed that the new process will be much more economical than the conventional light naphtha isomerization process. 7

8 Figure 2-9 Concept Design Process Flow Diagram 2.3 Analysis of spent catalyst An investigation was made of the factors behind catalytic deactivation due to sulfur, notably organic sulfur, in feedstock through an analysis of CAT-A catalyst and Pt/SO 4 /ZrO 2 catalyst after the catalysts have been used for a long-term continuous operation. Each sample for 989h of operation with MEROX naphtha containing 220 massppm of sulfur and for 2843h of operation of each with desulfurized naphtha containing 2 massppm of sulfur was analyzed. Deactivation was much more conspicuous in the former catalyst than in the later. Test samples washed with acetone solvent and dried were used for analysis of each catalyst. Results of analysis (Table 2-2) of upper, middle and lower of spent catalyst indicated that carbon content of about 1.5 mass% can be observed in the upper of each catalyst, but there was slightly less carbon content in the Pt/SO 4 /ZrO 2 catalyst, where it was 1 mass% or less from the middle to the lower (Figure 2-10). The 13 C-CP/MAS-NMR spectra of the carbon component produced on these catalysts are shown in Figure In the upper s of both catalyst samples, the peak of aromatic carbon was observed, but the lower s was not observed it. In view of the fact that catalyst deactivation progresses gradually from the catalytic bed upper to lower, this can be regarded as a major contributor to the catalyst deactivation. Furthermore, the sulfur content in all the s of both catalysts was held at approximately 80 to 90% of the sulfur content in the new catalyst, so the factors behind the differences in catalytic activity of the two catalysts are unclear. 8

9 Table 2-2 Results of Analysis of Spent Catalyst Catalyst Pt/SZ CAT-A Feedstock Desulfurized naptha MEROX naptha Sulfur content /massppm Reaction time /h i-c5/total-c5 ratio /% Analysis of spent catalyst Catalyst bed position Upper Middle Lower Upper Middle Lower Carbon content / mass% Sulfur content / versus Fresh Surface area / versus Fresh Pore volume / versus Fresh Reaction temperature 185 to 210 C, LHSV 2.9/h, Pressure 3.1 MPa, H 2 /Oil 2 mol/mol Carbon content (mass %) Upper Middle Lower Reaction conditions: Temperature 185 to 210 C, LHSV 2.9/h, Pressure 3.1 MPa, H 2/Oil ratio 2 mol/mol Figure 2-10 Distribution of carbon content on spent catalyst (a) Pt/SZ spent catalyst Upper (b) CAT-A spent catalyst Upper Lower Lower Measurement conditions: MAS speed, 3.5 khz; TOSS pulse method used Figure C-CP/MAS-NMR spectra of spent catalyst 9

10 2.4 Evaluation of product oil Tests were performed to assess the gasoline properties of product oil obtained in long-term continuous operation by bench plant. Used as test samples were product oil recovered from 2,100 h to 2,300 h (mean 2,200 h), and the results of analysis are presented in Table 2-3. These results indicate that octane number and other targeted properties can serve adequately as the properties of gasoline blendstock. Table 2-3 Results of Analysis of Properties of Product Oil MEROX naphtha (M/X-A) Product oil (isomerate) (2200 h) RON (JIS K 2280) MON(JIS K 2280) RON (GC calculation) MON (GC calculation) Density (g/cm 3 ) RVP (kpa) DIST ( C) IBP % % EP Composition (PONA) (vol %) Aroma Olefin Naphtene Paraffin ic nc ic Sulfur content (coulometric titration method) 72 ppm 1 ppm or below 3. Results of Empirical Research The following results were obtained from R&D conducted in (1) A catalytic candidate composition of high low-temperature activity was discovered which is superior to the promising catalyst of high resistance to sulfur poisoning discovered in the previous year. (2) Continuous reaction operation for 5600 hours was achieved through bench plant isomerization reaction experiments in which practical equipment naphtha feedstock (MEROX naphtha of 72 massppm sulfur content) was used. (3) From the results of evaluation on product oil obtained in long-term continuous operation, it was found that the targeted octane number of 78 or above could be attained and that other properties were also adequately suitable for gasoline blendstock. (4) A process design concept was formulated based on data on long-term continuous operation and the economy of the process in question was investigated. 10

11 (5) From analysis of spent catalyst used in reaction, findings were obtained on the factors behind catalyst deactivation. 4. Summary From the results of experiments in long-term continuous operation, using catalyst discovered in the present R&D, it was confirmed that octane number of 78 or above, set as the intermediate target for catalytic performance, could be roughly achieved using light naphtha with 150 massppm sulfur content or less and with the saturation levels of moisture at room temperature. In the future, plans call for more work on the development of catalyst and processing, aimed at final targets, and on research into industrialization. Copyright 2000 Petroleum Energy Center all rights reserved. 11