HYDROISOMERIZATION OF 1-PENTENE TO ISO- PENTANE IN A SINGLE REACTOR

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1 Chemical Engineering Communications ISSN: (Print) (Online) Journal homepage: HYDROISOMERIZATION OF 1-PENTENE TO ISO- PENTANE IN A SINGLE REACTOR Delanie Lamprecht & Arno De Klerk To cite this article: Delanie Lamprecht & Arno De Klerk (2009) HYDROISOMERIZATION OF 1- PENTENE TO ISO-PENTANE IN A SINGLE REACTOR, Chemical Engineering Communications, 196:10, , DOI: / To link to this article: Published online: 12 May Submit your article to this journal Article views: 1887 Citing articles: 7 View citing articles Full Terms & Conditions of access and use can be found at

2 Chem. Eng. Comm., 196: , 2009 Copyright # Taylor & Francis Group, LLC ISSN: print= online DOI: / Hydroisomerization of 1-Pentene to Iso-Pentane in a Single Reactor DELANIE LAMPRECHT AND ARNO DE KLERK Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and Development, Sasolburg, South Africa It has been shown that hydroisomerization of an olefinic feed to an iso-paraffinic product can be performed in a single reactor using a platinum on mordenite (Pt-MOR) zeolite catalyst. This conversion finds application in a Fischer-Tropsch refinery where the syncrude is rich in olefins and conventional hydroisomerization would require hydrogenation of the feed as a feed pretreatment step. In order to demonstrate that the proposed process configuration is viable from a catalysis point of view, the hydroisomerization of 1-pentene over Pt-MOR has been investigated experimentally in the range C, 2 MPa, WHSV 1 3 h 1, and H 2 :1-pentene molar ratio of 3:1 5:1. The catalyst was stable during one week of continuous operation. The highest iso-pentane yield (69%) was obtained at 250 C. Side reactions (dimerization and cracking) increased with increasing temperature, decreasing H 2 :1-pentene ratio, and increasing space velocity. Keywords 1-Pentene; Fischer-Tropsch; Hydroisomerization; Iso-pentane; Mordenite; Refining Introduction Hydroisomerization of low-octane linear C 5 -C 6 paraffins, such as light straight-run naphtha, to higher octane branched paraffins is a well-known refining technology (Travers, 2001). However, it finds limited application in Fischer-Tropsch refining, since the C 5 -C 6 fraction of Fischer-Tropsch derived naphtha typically contains more than 70% olefins, which are mainly linear alpha-olefins (Dry, 2004). In current commercial Fischer-Tropsch refineries, different refining strategies are being followed, among others the skeletal isomerization of n-pentenes to iso-pentenes (Cowley, 2006) to serve as feed for etherification to produce the high-octane fuel ether tertiary amyl methyl ether (TAME). Nevertheless, from a future fuel quality perspective, such as the reduction in the specified benzene, ether, and olefin content in gasoline, the hydroisomerization of straight-run Fischer-Tropsch naphtha to highoctane iso-paraffinic naphtha remains an attractive refining option to obtain a high octane number blending component. Application of commercial C 5 -C 6 hydroisomerization technology to olefinic Fischer-Tropsch derived naphtha requires olefin hydrogenation as a feed pretreatment Address correspondence to Delanie Lamprecht, Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and Development, P.O. Box 1, Sasolburg 1947, South Africa. delanie.lamprecht@sasol.com 1206

3 Hydroisomerization of 1-Pentene to Iso-Pentane 1207 Figure 1. Hydroisomerization mechanism of n-pentane to iso-pentane over a bifunctional metal promoted solid acid catalyst. step. This is not only costly, due to the additional hydrotreater, but from a mechanistic point of view unnecessary. Hydroisomerization takes place by dehydrogenation of the paraffin to produce an olefin, acid-catalyzed skeletal isomerization of the olefin, and subsequent re-hydrogenation of the olefin to produce an iso-paraffin (Figure 1). It should therefore in principle be possible to use an olefinic feed, as long as side reactions due to a high initial olefin partial pressure can be overcome. This is the opposite of n- pentane dehydroisomerization to iso-pentene (Herrera et al., 2001), which employs the same principle, but with the aim of producing an olefin from a paraffin. The objective of our study was the hydroisomerization of 1-pentene to isopentane in a single reactor. The investigation provides proof of the technical feasibility of this concept. This is of industrial interest not only due to the olefinic nature of the Fischer-Tropsch naphtha, but also due to process intensification opportunities and possible reduction in the environmental footprint of the process. Experimental Section Materials and Feed A commercially available platinum on mordenite (Pt-MOR) hydroisomerization catalyst, Hysopar, was used in the study. The catalyst was supplied by Süd-Chemie Catalysts (Richards Bay, South Africa). The chemical composition and physical properties of the catalyst, as provided by the supplier, are shown in Table I. The catalyst was activated (reduced) at 290 C under H 2 pressure. The hydrocarbon feed material, 1-pentene, was selected as a representative model compound and was obtained from the Sasol Synfuels Fischer-Tropsch refineries (Secunda, South Africa). The 1-pentene feed used in the study had a purity of 99.5 mass %. Hydrogen was supplied from the Sasol 1, Sasolburg utility infrastructure. Table I. Chemical and physical properties of the commercially available Pt on mordenite (Pt-MOR) catalyst employed for hydroisomerization Property Value Platinum loading (mass %) 0.3 Particle shape extradites Bulk density (kg m 3 ) Porosity (cm 3 g 1 ) 0.4 Average length (mm) 4.9 Average crush strength (N mm 1 ) 6.3

4 1208 D. Lamprecht and A. de Klerk Equipment and Reaction Procedure All experiments were conducted in a stainless steel plug-flow reactor system, 600 mm in length and with an internal diameter of 10 mm. Two thermocouples were spaced along the catalyst bed, with another thermocouple in the preheating section and a fourth below the catalyst bed. The rest of the reactor was filled with 0.7 mm carborundum (SiC) particles. The feed pot was kept under a nitrogen pressure of 0.7 MPa to ensure that 1-pentene feed remained in the liquid phase. The 1-pentene and hydrogen were co-fed at the top of the reactor and the reaction was studied under near isothermal conditions. (Due to the exothermic nature of the hydrogenation reaction, it was not possible to achieve a constant temperature in the catalyst bed.) In all experiments, except the high space velocity study, the reactor was loaded with 18 g of uncrushed catalyst, which resulted in a catalyst bed length of 150 mm. During the high space velocity study the reactor was loaded with only 6 g of uncrushed catalyst, which was diluted with 18 g carborundum, which resulted in a catalyst bed length of 123 mm. The operating range investigated was C, H 2 :1-pentene molar ratio of 3:1 5:1, and weight hourly space velocity (WHSV) of 1 3 h 1. The pressure was kept constant at 2 MPa during the investigation. Inertness of the reactor was verified by conducting an experiment without catalyst, during which no conversion was observed. Analyses The feed material and products were analyzed qualitatively and quantitatively after steady-state conditions were reached using an Agilent 6890 gas chromatograph with a flame ionization detector (GC-FID). The liquid products were separated on an HP-Pona 50 m 200 mm ID 0.5 mm methyl siloxane column, and the tail gas products were separated on a Plot Alumina=KCl 50 m 320 mm ID 10 mm column. Results and Discussion Catalyst Selection The C 5 -C 6 hydroisomerization catalysts that are employed in industry today are mainly bifunctional metal supported solid acids. The selectivity and catalyst life of the solid acid isomerization catalysts are greatly improved by loading a hydrogendissociative metal on the solid acid and conducting the reaction in the presence of hydrogen (Sinfelt et al., 1962; Zhang et al., 1995; Chu et al. 1998; Demuth et al., 2003; Ono, 2003). In all instances platinum is the preferred metal, while three different solid acid supports are commonly used, namely chlorided alumina, sulphated metal oxides, and zeolites (Kuchar et al., 1999; Weyda and Köhler, 2003). We selected a Pt-MOR catalyst due to its reported high water tolerance (Floyd et al., 1998; Weyda and Köhler, 2003), despite its higher operating temperature and resultant lower equilibrium conversion to the branched isomers. This decision is related to the composition of industrial Fischer-Tropsch derived C 5 -C 6 feed material, which typically contains dissolved water and oxygenates, such as alcohols and carbonyls (Mashapa et al., 2007). During hydroisomerization oxygenates will be hydrogenated to produce hydrocarbons and water. Water tolerance is consequently a key consideration for catalyst selection in order to avoid the cost associated with feed dryers and oxygenate removal in the presence of reactive olefins.

5 Hydroisomerization of 1-Pentene to Iso-Pentane 1209 It has also been shown that a benzene concentration in the feed of up to 4.9% can be tolerated by Pt-MOR catalysts (Hancsok et al., 1999). Hydroisomerization of C 5 -C 6 using Pt-MOR zeolite catalysts may therefore also be employed to reduce the benzene content in refinery streams. However, benzene has an inhibiting effect on skeletal isomerization due to diffusion hindrance in the one-dimensional channels of mordenite. These features make technology based on a Pt-MOR catalyst attractive in terms of versatility, simplicity, and robustness. Proof of Concept The two most important differences between the direct hydroisomerization of an olefinic feed and hydroisomerization of a paraffinic feed are: (a) the high olefin partial pressure at the reactor inlet, and (b) the consumption of hydrogen during olefin hydrogenation, with its concomitant heat release. Under conditions of high temperature and high olefin partial pressure, side reactions are favored that can lead to product degradation and catalyst deactivation. Side reactions include dimerization to produce heavier hydrocarbons, as well as b-scission of the heavier hydrocarbons to produce light gases. The heavier hydrocarbons on the catalyst surface are not easily desorbed and may accumulate as carbonaceous deposits. In order to demonstrate that the hydoisomerization of an olefinic feed has a chance of being developed for industrial application (proof of concept), it was necessary to show that close to equilibrium conversion of 1-pentene to iso-pentane could be obtained without excessive side product formation and catalyst deactivation. It was realized that the reactive nature of the olefinic feed may require a lower operating temperature, which will reduce side-product formation and catalyst deactivation. The proof of concept study therefore focused on the temperaturedependent conversion of 1-pentene to determine the minimum practical operating temperature, the extent of side-product formation, and an indication of catalyst deactivation. Hydroisomerization of 1-pentene over Pt-MOR at 2 MPa, WHSV of 1 h 1, and ah 2 :1-pentene ratio of 5:1 approached equilibrium conversion to iso-pentane at 250 C as the temperature was increased from 200 to 270 C (Figure 2). No traces of 1-pentene were detected in the product and a 9 10 C exotherm was observed in the first part of the catalyst bed. The 1-pentene feed was completely hydrogenated over the Pt-MOR catalyst. Side-product formation increased with increasing temperature (Table II), with some dimerization and acid cracking being observed. Nevertheless, the overall side-product formation was less than 5% at 270 C, which is the highest temperature that was investigated. At the end of the run, during which the temperature was increased from 200 to 270 C, the temperature was again decreased to 200 C. The iso-pentane yield at start-of-run conditions (19.9%) was slightly higher than the iso-pentane yield after one week on stream (19.4%), which may indicate some catalyst deactivation. The experiment was not designed to quantitatively evaluate catalyst deactivation, but it qualitatively suggests that deactivation is not excessive. A proper study to determine the rate of catalyst deactivation is of industrial relevance but has not been conducted during the present investigation.

6 1210 D. Lamprecht and A. de Klerk Figure 2. Ratio of iso-pentane to total C 5 hydrocarbons at different temperatures during the hydroisomerization of 1-pentene over Pt-MOR in a fixed bed flow reactor at C, 2 MPa, WHSV of 1 h 1, and H 2 :1-pentene molar ratio of 5:1. Hydrogen-to-Feed Ratio The hydrogen-to-feed ratio is an important operational parameter, since it has an impact on equipment sizing and processes economics. One would like to operate at the lowest possible hydrogen-to-feed ratio, but there is a trade-off involved, since side-product formation and the rate of catalyst deactivation increase with decreasing hydrogen-to-feed ratio. This becomes even more important in the hydroisomerization of an olefinic feed, since rapid olefin hydrogenation may result in hydrogen starvation, formation of coke precursors, and eventual catalyst deactivation if the hydrogen-to-feed ratio is too low. The influence of this parameter was therefore investigated. The formation of side products decreases as the H 2 :1-pentene molar feed ratio is increased from 3:1 to 5:1 (Table III). This is as expected, since hydrogen has a diluting effect, reducing bimolecular reactions, such as dimerization, and the higher hydrogen partial pressure increases olefin hydrogenation, which suppresses Table II. Side-product yield at different temperatures during the hydroisomerization of 1-pentene over Pt-MOR in a fixed bed flow reactor at C, 2 MPa, WHSV of 1 h 1,andH 2 :1-pentene molar ratio of 5:1 Compound Yield (C-atom %) 200 C 225 C 250 C 260 C 270 C Methane < Ethane < Propane Iso-butane n-butane C 6 and heavier

7 Hydroisomerization of 1-Pentene to Iso-Pentane 1211 Table III. Side-product yield at different hydrogen-to-1-pentene feed ratios during the hydroisomerization of 1-pentene over Pt-MOR in a fixed bed flow reactor at 250 C, 2 MPa, WHSV of 1 h 1,andH 2 :1-pentene molar ratio of 3:1 5:1 Compound Yield (C-atom %) H 2 :C 5 ¼ 3:1 H 2 :C 5 ¼ 4:1 H 2 :C 5 ¼ 5:1 Methane Ethane Propane Iso-butane n-butane C 6 and heavier monomolecular side reactions, such as cracking. Similar findings have previously been reported (Fujimoto et al., 1992; Zhang et al., 1995). The percentage branching in the C 5 hydrocarbon fraction (not shown) remained constant, because thermodynamic equilibrium had been reached. The decrease in methane and ethane formation with increasing H 2 :1-pentene ratio indicates that methane and ethane are both formed by acid cracking, rather than by hydrogenolysis. The concentration of these compounds in the product should be limited, since the buildup of light hydrocarbons in the tail gas determines the hydrogen purge rate and thereby affects overall hydrogen consumption. The experiments have been found to have good reproducibility, as can be seen from a comparison of the results obtained for the different experiments conducted at 250 C and H 2 :1-pentene feed ratio of 5:1 (Tables II and III). Space Velocity From the preceding experiments it was clear that all olefins in the feed had been hydrogenated to paraffins and that C 5 isomerization equilibrium conversion was achieved at 250 C, 2 MPa, and a weight hourly space velocity of 1 h 1. Apart from the economic benefits of operating at a higher space velocity, it was also of interest to determine what fraction of the catalyst bed was exposed to a higher olefin partial pressure than would be found when operating with a paraffinic feed. The yield of iso-pentane decreased by 15% (absolute) as the WHSV was increased from 1 to 3 h 1, while the other operating conditions were kept constant at 250 C, 2 MPa, and H 2 :1-pentene ratio of 5:1. The change in space velocity from 1to2h 1 did not have a significant influence on side-product formation, but a further increase in space velocity to 3 h 1 resulted in more side products being formed (Figure 3). With increasing space velocity a larger fraction of the catalyst bed is exposed to an olefin concentration that is higher than the equilibrium olefin concentration. Since side-product formation is related to the olefin partial pressure, the observed increase in side-product formation with increasing space velocity is understandable. Despite the increase in space velocity, no traces of 1-pentene were detected in the reactor effluent. It should be noted that for this experiment the catalyst bed had been loaded differently. A catalyst-to-inert dilution ratio of 1:3 had been used, rather than an

8 1212 D. Lamprecht and A. de Klerk Figure 3. Influence of space velocity on side-product formation during hydroisomerization of 1-pentene over Pt-MOR at 250 C, 2 MPa, and H 2 :1-pentene molar ratio of 5:1. The main side products are methane and ethane (&), propane (^), butanes (.), and C 6 and heavier hydrocarbons (~). undiluted catalyst bed. This resulted in a lower conversion, which may be due to either a temperature effect or some bypassing caused by inhomogeneity in the catalyst loading. The results can therefore not be directly compared with those in Tables II and III. Commercial Implications Hydroisomerization of n-pentane to iso-pentane is mildly exothermic ðdh r;25 C ¼ 8kJ mol 1 Þ. The heat release during hydroisomerization of a paraffinic feed is very little, with an adiabatic temperature rise in the order of 10 C. However, hydrogenation of 1-pentene to n-pentane is very exothermic, and it is accompanied by a high heat release ðdh r;25 C ¼ 126 kj mol 1 Þ. The calculated adiabatic temperature rise of the hydroisomerization of 1-pentene to iso-pentane, shown in Figure 2, is around 270 C. Heat management is consequently central to further engineering development of the proposed olefin hydroisomerization process before it can be considered for industrial application. It should be noted that the proof of concept test work with 1-pentene constituted an extreme case, which was deliberately selected to stress-test the viability of the proposal from a catalysis point of view. In practice, typical Fischer-Tropsch refinery streams will consist of mixtures of pentenes and pentanes. Aspects that were considered during the development of a realistic flow scheme for the hydroisomerization of Fischer-Tropsch derived naphtha are: (a) A typical straight-run high-temperature Fischer-Tropsch C 5 -cut contains around 85% olefins and 15% paraffins. A low-temperature Fischer-Tropsch naphtha contains even less olefins. The fresh feed olefin content will therefore be at most 85%. In a Fischer-Tropsch C 5 -raffinate (Cowley, 2006), which is a more likely hydroisomerization feed, the olefin content is 70 80%.

9 Hydroisomerization of 1-Pentene to Iso-Pentane 1213 (b) The hydroisomerization reaction is equilibrium limited, and unconverted n-pentanes can therefore be recycled to dilute the fresh feed. Further, when a mixed C 5 -C 6 naphtha feed is employed, the bottoms fraction of a deisopentanizer column will contain n-pentane and the C 6 hydrocarbons, and only part of this bottoms fraction can be recycled. Although this may argue for a more elaborate separation (Hunter, 2003), it is worthwhile noting that the C 5 material reaches equilibrium faster than the C 6 material. The conversion of C 5 is equilibrium constrained, but the conversion of C 6 material is kinetically constrained. By recycling the partially converted C 6 material, the octane number of the product is increased without the cost normally associated with C 6 separation and recycling. (c) Considering that heat release is the key constraint, rather than hydroisomerization, reactor sizing may be optimized to balance iso-pentane yield with recycle requirements for heat management. In this way, the overall productivity of the design can be improved. (d) The fresh feed temperature ( C) can be lower than that of normal hydroisomerization processes, since the hydrogenation reaction will proceed at much lower temperatures. The heat of hydrogenation can then be beneficially used to preheat the feed to hydroisomerization conditions (around 260 C). (e) Hydrogenation of some unsaturated hydrocarbons during hydroisomerization is not an uncommon practice, since crude oil derived light straight-run naphtha contains some benzene (typically 1 2%) that is hydrogenated to cyclohexane over the hydroisomerization catalyst (Oliver and Hashimoto, 1974). The hydroisomerization process may therefore be employed as benzene removal technology too, although this would increase the total heat release in the process. There is consequently no need to remove benzene from the naphtha feed. (f) The efficiency of feed-product heat exchangers can be significantly increased due to the larger temperature differential. Furthermore, during normal operation there is no need for a final feed heater, since the heat of hydrogenation is used to preheat the feed. (g) No feed pretreatment is required, because the Pt-MOR catalyst is water tolerant and the Fischer-Tropsch feed is sulfur-free (<1 mg g 1 ). However, provision must be made for decanting water in the product. (h) Hydrogen consumption will be determined by the olefin content of the fresh feed, as well as the reaction selectivity to light hydrocarbon gases (C 1 -C 2 ) that can build up in the hydrogen recycle loop. The proposed hydroisomerization process (Figure 4) is less complex than a typical hydroisomerization process with n-pentane recycle. There is no need for a fired heater to preheat the feed, since the heat of olefin hydrogenation is employed to heat the feed to the required reaction temperature. The product separator must make provision for two liquid phases and is used to remove water produced from oxygenate hydrogenation. Light hydrocarbons (C 4 and lighter) are removed in the stabilizer column. The deisopentanizer column separates the iso-pentane from the heavier hydrocarbons. The composition of the bottoms product from the deisopentanizer column is dependent on the fresh feed composition and if a mixed C 5 -C 6 feed is used, only part of the heavier hydrocarbon product can be recycled. Heat management determines the reactor size and the catalyst volume to fresh feed ratio (Figure 5). One way of approaching the design is to reduce the per-pass n-pentane to iso-pentane conversion and thereby regulate the ratio between the fresh

10 1214 D. Lamprecht and A. de Klerk Figure 4. Process flow diagram of a hydroisomerization process for the conversion of olefinic Fischer-Tropsch naphtha to an iso-paraffinic product over a Pt-MOR catalyst. feed and the paraffinic recycle. This will yield the smallest reactor size. Another approach is to manipulate the recycle directly, which will yield a higher octane final product if a mixed C 5 -C 6 naphtha is used as feed material. Environmental Aspects The proposed direct hydroisomerization of an olefinic Fischer-Tropsch naphtha to an iso-paraffinic product in a single reactor over a Pt-MOR zeolite catalyst has a number of advantages from an environmental point of view: (a) It is an example of process intensification, where the heat of olefin hydrogenation is beneficially used for preheating while hydroisomerization takes place. This reduces the overall energy requirements of the process and thereby indirectly lowers the CO 2 footprint of the technology. (b) The selection of a Pt-MOR catalyst for hydroisomerization has environmental advantages too. It does not require the addition of chloro-alkanes to keep the catalyst active and it does not require feed pretreatment to remove water (or oxygenates). (c) The higher operating temperature required by Pt-MOR compared to the other catalyst types is not detrimental to the process when used with a Fischer- Tropsch feed. On account of the olefins in the feed, a fired heater is not required for preheating (see first point) and the lower equilibrium conversion at higher temperature is not an issue, since paraffin recycle is required for heat management in any case. (d) The combination of olefin hydrotreating and hydroisomerization in a single process unit reduces the hardware requirements compared to having two separate processing units, namely a hydrotreating unit and a hydroisomerization unit. This reduces the environmental footprint from a materials, manufacturing, and construction perspective.

11 Hydroisomerization of 1-Pentene to Iso-Pentane 1215 Figure 5. Calculated adiabatic temperature rise for different fresh feed-(85% 1-pentene, 15% n-pentane)-to-recycle ratios during hydroisomerization over Pt-MOR at a feed inlet temperature of 100 C and H 2 :fresh feed molar ratio of 5:1. Conclusions A process is proposed for the direct hydroisomerization of an olefinic feed to isoparaffins over a Pt-MOR zeolite catalyst. The catalyst selection has been based on its water tolerance, which allows oxygenate and olefin-rich Fischer-Tropsch derived naphtha to be processed directly. The process configuration can beneficially employ the heat of olefin hydrogenation to preheat the feed in parallel with hydroisomerization. Significant process intensification is possible, and the process has a smaller environmental footprint than conventional crude oil feed based hydroisomerization processes. In order to demonstrate that such a process configuration is viable from a catalysis point of view, the concept has been investigated experimentally with a 1-pentene feed. The following conclusions could be drawn from the experimental investigation, which covered the operating range C, 2 MPa, WHSV 1 3 h 1, and 3:1 5:1 molar ratio of hydrogen to 1-pentene: (a) The hydroisomerization of 1-pentene to iso-pentane is feasible using a Pt-MOR zeolite catalyst in a single reactor. (b) With the formation of iso-pentane being thermodynamically more favorable at low temperatures, the highest iso-pentane yield (69%) was obtained at 250 C. (c) Total conversion of 1-pentene was observed. No traces of 1-pentene were detected in the reactor effluent during any of the experiments. (d) Side reactions increased with increasing temperature, decreasing H 2 :1-pentene ratio, and increasing space velocity. (e) The side reactions were mainly dimerization and cracking, leading to the formation of heavier and lighter products than the feed. These reactions were promoted by a higher olefin concentration over the catalyst. (f) The catalyst was stable during one week of continuous operation. The results indicated that there may have been some deactivation, but the rate of catalyst deactivation has not been quantified.

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