PROCESS AND REACTOR SYSTEM FOR OLIGOMERIZATION OF OLEFINS

Abstract: The invention concerns a process and a system for oligomerization of olefins. According to the process, feedstock containing olefins from C3 to C20, or a mixture thereof, is introduced to a catalytic distillation reactor system comprising a distillation column (27) and at least one side reactor (31, 32), where the olefins of the feedstock are contacted with a catalyst (30) for manufacturing a product containing oligomers. According to the invention, a solid oligomerization catalyst such as a zeolite is used, and the olefins are contacted with the catalyst at a temperature sufficiently high to attain oligomerization. The present process is employed in the manufacture of, e.g., intermediate distillates and lubricants. By combining the reaction and distillation steps in a single system, the energy balance of the oligomerization process can be improved substantially.

Description: Process and reactor system for oligomerization of olefins The present invention concerns a process according to the preamble of claim 1 for oligomerization of olefins. According to the present process, a feedstock containing C 3 - C 20 olefins, or a mixture thereof, is fed to a distillation reactor system, where said olefins of the feedstock are contacted with a catalyst in order to produce an oligomer- containing produc . The invention also concerns a reactor system according to the preamble of claim 25 for oligomerization of olefins. Oligomerization of olefins aims at converting light olefins into heavier olefins . This reaction is carried out in the presence of a catalyst, which comprises either an acid or a base. Conventionally, a heterogeneous catalyst is used. Thus, a mixture of light olefins in the liquid phase, gas phase or a mixture thereof, or in supercritical phase are usually contacted with a solid phase catalyst. Then, the olefins combine with each other forming dimers, trimers, tetramers and even heavier oligomers. The degree of oligomerization depends on the pressure and temperature employed. If the reaction temperature used is high (above 200 °C) , depending on the catalyst type, cracking products are produced by side reactions, and disproportionation reactions will give rise to other products different from the oligomers contained in the olefin stream used as feedstock. Thus, the properties of the catalyst employed have an extremely strong influence on the composition of the products obtained. Heavier olefins from the oligomerization processes can be utilized as, for instance, gasoline and diesel fuel blending stock, solvents and lubricant stock, all of which can further be hvdrogenated to improve their stability, or alternatively, as feedstocks for various chemical processes using heavy olefins. Also in the latter case, the mixture of the olefins must be separated by, e.g., distillation into proper fractions. Thus, the product being manufactured via oligomerization can be controlled by an appropriate choice of reaction conditions, catalyst used and the mixture of olefins employed as feedstock. In many of the above-listed applications, a benefit of the products manufactured by oligomerization is their freedom from sulfur (namely, sulfur compounds can be removed to a high accuracy from light hydrocarbons) and low content of aromatics, in many cases far below one percent. Processes intended for oligomerization of light olefins have been developed by, e.g., Mobil (MOGD process) , Shell (SPGK process) , IFP (Dimersol, Selectopol and Polynaphtha processes) , UOP (Hexall process) ; catalytic polymerization (using phosphoric acid as a liquid-phase catalyst or impregnated on a carrier) ; and further, by Bayer, Hύls/UOP (Octol process) . Catalytic polymerization mentioned above, which uses a liquid catalyst, is complicated in that waste acid is produced. By contrast, most of the other processes listed by reference employ a solid phase catalyst, and they are run in a tubular reactor or a configuration of multiple tubular reactors . The reason for the use of multiple-reactor configurations lies in the exothermic nature of the oligomerization reaction. To control the temperature in a system of multiple reactors, intermediate cooling of the reaction mixture can be arranged between the reactors . Another frequently employed arrangement is to recycle the product back to the reactor feed after passing a possible distillation unit or flash drum unit. The latter system produces heavier olefins and simultaneously achieves temperature control for the reactor, because the oligomerization of heavier olefins releases less heat per mass unit than the oligomerization of lighter olefins. A third alternative employed for solving the energy balance problems is the recycling of already processed, inert paraffinic components in order to control the temperature in the reactors. An example of embodiments based on intermediate cooling and reflux of inert paraffins is provided by the process disclosed in EP Patent Application No. 0,397,273. According to said publication, a hydrocarbon flow containing light olefins is taken via heat exchangers and a boiler to two series-conn ' ected reactors in which the olefins are contacted with a zeolite catalyst. The hydrocarbon flow is subjected to intermediate cooling between the reactors and the product flow from the second reactor is subjected to additional cooling prior to further processing. During further pro¬ cessing, the butane-containing fraction is separated from the product in a distillation column. The oligomerized hydrocarbons are removed as the bottom product from the dis- tillation column. The distillate from the distillation column is condensed and a portion thereof is recycled back to the column, while the remaining portion containing mostly C 3 and C 4 alkanes is recycled to the reactors. A specific problem of processes based on intermediate cooling is the inefficient recovery of reaction heat, which results in a greater number of equipment required. The feedstock and product flows in these processes must be alternatingly first heated and then cooled. Moreover, heat produced separately must be introduced to the fractionation step of the products. Neither is the recycling of inert paraffins an ideal solution in terms of process energy utilization. It is an object of the present invention to overcome the drawbacks of the above-described conventional techniques and to achieve an entirely novel process and reactor system for the oligomerization of olefins. presence of a solid phosphoric acid catalyst, water vapor being used to maintain the catalyst coposite at the desired degree of hydration. The process is implemented in an apparatus comprising a centrally located polymerization reactor, a monomer stripping section positioned above the reactor and a polymer stripping section situated below said reactor. According to an alternative embodiment, the reactor and the monomer and polymer stripping sections can be combined to form a single continuous unit of apparatus maintained at a uniform pressure throughout. The present invention differs from the above-described prior- art techniques in several respects. The present invention utilizes a solid phase catalyst suited for oligomerization of olefins, i.e. a "solid oligomerization catalyst". Only such solid catalysts are employed which will function without any separate adjuvants, such as water (US 2,486,533) or BF 3 (US 4,935,577) , which have to be separately removed from the catalytic system. Said catalyst can be placed directly in the distillation apparatus. However, the present invention particularly concerns a process and an reactor system in which the solid catalyst is placed in a separate reactor con¬ nected to said distillation column in a manner that permits recycling of the sidestream flow obtained from the column back to the column after being subject to a conversion reaction in said reactor. The olefins are contacted with the catalyst at a temperature sufficiently high to attain oligomerization, and at least a portion of the product flow leaving the catalyst zone is fractionated in the same distillation column. A further difference to the prior art is that it is an object of the present invention to oligomerize all olefins, not only alpha-olefins . More specifically, the process in accordance with the invention is principally characterized by what is stated in the characterizing part of claim 1. The system in accordance with the invention is characterized by what is stated in the characterizing part of claim 25. In the context of the present application the term "catalytic distillation" is used for denoting a combination of a reaction with the fractionation of products by distillation so that — an essential portion, preferably at least approx. 20 %, of the ' internal liquid flow through the distillation column is routed through the catalyst zone and — the portion of liquid flow routed through the catalyst zone is returned to the distillation column at a point which is most desirable for the distillation process. The above-mentioned characteristics are important to the end of achieving the goals of the invention. Consequently, such an arrangement in which a relatively small sidestream flow is taken from the distillation column intended for the fractionation of the products of the oligomerization process and routed to prereactors fails to utilize the heat released by the exothermic reaction in the improvement of the energy balance of the distillation process. Accordingly, such an alternative does not represent "catalytic distillation" in the meaning of the present application. It should further be noticed that the side reactor according to the present invention preferably comprises a flow reactor, such as a tubular reactor or equivalent, which is operated in such a way that no fractionation of the products will take place therein; the reactants and the reaction products flow in the same direction through the side reactor. Although there usually is only one inlet in the side reactor, i.e. the one for the feed drawn off from the distillation column, it is in principle possible to feed two or more streams into the side reactor by using one or several additional inlets. All the reaction product is, however, withdrawn from the side reactor from one point and the fractionation of the product is carried out in a distillation column or in a reactive distillation column. In the following description, the terms "catalyst zone" and "catalyst layer" will be used interchangeably. The catalysts suited for use according to the invention can be, e.g., zeolite catalysts or their ion-exchanged variations. Typical zeolites incorporate crystalline alumino- silicate zeolites such as, e.g., ZSM-5 zeolite presented in US patent publication 3,702,886 and variants thereof such as ZSM-11, ZSM-12, ZSM-23, ZSM-35, and ZSM-38 for instance. Other suitable catalysts are, e.g., borosilicates, ferrosilicates and/or aluminosilicates. As catalysts, HSi and other hetero polyacid -type catalysts can also be used. However, the invention is not limited to the above-named catalyst grades alone, but rather, all such solid phase catalysts that catalyze the oligomerization reactions of olefins are suited for use according to the invention. The catalyst is preferably a zeolite, and the reaction is carried out at a minimum temperature of 150 °C. If several catalyst layers are used in embodiments of the invention, be it inside the distillation column, in the side reactors or in both the distillation column and the side reactors, the various catalyst layers may contain catalysts of different types. This makes it possible to use for each respective location of the layer the catalyst type that works best. As a result, some heating and cooling phases may become superfluous and can therefore be avoided. As an additional benefit of using different types of catalysts in the same reaction system, it may be pointed out that, working in different temperature ranges, different catalyst have different selectivity and will provide products with different product distributions and various amounts of sideproducts . It is therefore possible to modify the properties of the products by the selection of the catalyst types. The use of different catalysts can be illustrated by a process in which the upper side reactor of a conventional distillation column contains ZSM-5 zeolite, which will catalyze oligomerization of light olefins, and the lower side reactor of said column contains a Ni modified zeolite which only catalyzes dimerization. This embodiment makes it possible to change both the ratio between the diesel oil and lubricant oil fractions and the quality of said fractions. Another example of the use of different catalysts is indicated in Example 5 below, wherein a ZSM-5 zeolite and a HSiW (hetero polyacid) -type catalyst are employed. The catalyst for the reaction is placed in either the distillation apparatus, or alternatively, in at least one separate reactor (later called the "side reactor") connected to said distillation column, the sidestream flow from the distillation column to said side reactor (s) being returned from the reaction zone of said side reactor(s) back to the column after being converted in the reaction. An embodiment according to the invention incorporates at least one catalyst layer which is arranged in conjunction with the trays located above the feed tray of the dis¬ tillation column, a reaction consuming light olefins being achieved. According to the invention the minimum number of sidestream flows/reactors is one, while the maximum number thereof can be as high as the number of trays in the distillation column, depending on the desired conversion or economical aspects. The maximum number is preferably determined by the number of side reactors required for obtaining optimum distillation conditions. Preferably, an embodiment according to the invention has 1 to 20, in particular about 2 to 10 side reactor units. In exemplifying embodiment 3 discussed later, a distillation column/reactor system is described having the catalyst placed entirely in- the side reactor. The side reactor unit may comprise only one reactor, or alternatively, it can be configured from multiple smaller reactors connected in series, parallel, or both series and parallel . According to a preferred alternative embodiment of the invention, the catalyst charge required in the distillation column/reactor system is entirely placed in the side reactors, whereby significant benefits are obtained in terms of system maintainability, for instance. The scope of the invention also covers systems in which at least one catalyst zone is placed in the distillation column in a conventional manner. The sidestream flow taken from the column to the side reactor can be equal to the internal liquid reflux flow of the column (total drawoff) or smaller, however, at least approx. 20 % of the total flow. The total drawoff scheme can be applied in cases where the column is connected to multiple reactors, whereby the sidestream flow is returned to a tray below the drawoff tray. In the latter case — which is applicable in a single-reactor configuration — the drawoff flow is approx. 40 to 90 %, typically approx. 60 to 70 %, of the reflux flow, thus making possible its return to a suitable tray below the drawoff tray. In this case the .choice of a suitable tray is determined by the conventional design rules for the optimum feed tray of a distillation column. The sidestream flow from the column can be routed via the side reactor either from bottom to top, or alternatively, from top to bottom. For a tubular reactor and a fixed-bed reactor, both flow directions are possible, while a fluidized-bed reactor requires a flow upward from the reactor bottom. The sidestream circulation is implemented either as forced circulation by means of a pump or as natural circulation by the thermosiphon effect, whereby the reaction heat boils the liquid causing said effect. If pumped circulation of the hydrocarbon mixture is employed, the reactor can be a fixed-bed reactor, a tubular reactor or a fluidized-bed reactor, or any combination thereof, or a configuration of multiple reactors in series. In a system based on the thermosiphon effect, only a fixed-bed or tubular reactor is suitable, and further, of these the latter represents here an inferior alternative as the gas imparting the recirculation is condensed therein. The volume of the catalyst charge in the side reactor can be dimensioned for 0.01 to 100 times the liquid hourly space velocity which means the volume rate of liquid throughput per hour through the reactor. A typical value is approx. 0.1 to 10 times the space volume. Expressed in engineering units, the reactor LHSV is 0.01 to 100 m 3 liquid/m 3 cat. /h, a typical value being 0.1 to 10 m 3 liquid/m 3 cat . /h. If the reactor is of the fixed-bed type, the fluid super¬ ficial velocity in the reactor is dimensioned according to the catalyst manufacturer specifications, typically being in the range 5 to 30 m/h for the liquid flow. In a fluidized-bed reactor the fluid superficial velocity is 1 to 10 times the minimum fluidization velocity, typically 2 to 6 times the minimum fluidization velocity. The operating pressure of the reactors is determined by the column operating pressure and the reactor type. When a side reactor is used in connection with heat exchangers, a pump and a back-pressure regulator, the operating conditions of the side reactor can be different from those of the distillation column. Thus, the side reactors can be operated at pressures' above the distillation column pressure, a condition which is frequently advantageous in terms of reaction yield. The distillation column pressure is then, e.g., approx. 1 to 10 bar, while the side reactor pressure is in the range from 10 to 100 bar. The operating pressure of the side reactors can also be in the same order of range as the distillation column pressure (in practice, equal to column pressure plus the pressure loss in the piping) . Then, the operating temperature of the reactors is determined by the boiling point of the hydrocarbon mixture, thus preventing the occurrence of overheated zones in the reactor. In a circulation based on the thermosiphon action, a part of the liquid flow through the side reactor is vaporized. Then, the side reactor can be run without the need for the pump, back-pressure regulator and heat exchangers . This is possible by virtue of the energy released in the exothermic reaction that provides a portion of the vaporizing heat. Vaporization can occur in the side reactor, whereby the preheater is arranged prior to the side reactor, or alternatively, in a boiler arranged after the reactor. The latter approach is preferred. When the number of the side reactor units in the oligomeriza¬ tion process is greater than one, the distillation column/reactor configuration can be complemented by at least one heat exchanger connected so that the outlet flow of one side reactor is used for heating the feed flow of another side reactor. Such a heat exchanger can be formed by, e.g., tubes placed inside the catalyst bed of the side reactor. The distillation column side drawoffs can be used for achieving different olefin and inert component flows. Accord¬ ing to a preferred embodiment of the invention, the oligomer- rich product is withdrawn from the above-described columns from a tray, which is located below the feed tray and which is selected ' according to the desired composition of the product . A drawoff can also be used for withdrawal of paraffinic component streams containing, e.g., C 4 alkanes . A system according to the invention can be connected to product fractionation and hydrogenation equipment, whereby the invention becomes suited for manufacture of intermediate distillates and lubricant stock. Example 4 below includes the analysis values of a lubricant stock distillate manufactured according to the invention. The invention provides significant benefits. Thus, the process concept can combine the reaction step with the distillation step in a single system. This approach offers an improvement of the process energy balance. A significant benefit of the reactor system described in the present patent application is the utilization of the relatively high reaction heat from the oligomerization of olefins in the distillation step. The product obtained from the oligomerization reaction that is a mixture of hydrocarbons with different carbon chain lengths (plus abundant isomers) must always be separated into suitable fractions by, e.g., distillation. The process concept according to the present invention makes it possible to simultaneously perform out the reaction and separate the desired product from the nonreacting inert hydrocarbons (paraffins) possibly carried over from the feed. As mentioned above, the side reactors and the distillation columns can be run under different conditions. In other words, the optimal operational ranges can be chosen independently for the reactors and the column, respectively. This makes it possible for instance to improve the activity of an ageing catalyst by increasing the temperature without there being any need for a change of the operational temperature of the column. An additional benefit of the process is its flexibility in product grade changes. Further notice must be made on the characterizing feature that the present reactor system permits control of product composition and qualities by virtue of column pressure, temperature and degree of fractionation. The thermal stability properties of the lubricant stock ob¬ tained after the fractionation and hydrogenation steps are excellent . In the following the invention is examined closer with the help of a detailed description and application examples. Alternative embodiments of the invention are illustrated in the annexed drawings in which Fig. 1 shows the simplified process diagram of a system in accordance with the general concept of the invention, in which alternative the catalyst beds are located in the distillation column, Fig. 2 shows the process diagram of another process in accordance with the general concept of the invention which alternative differs from that shown in Fig. 1 in that a part of the oligomerization product is withdrawn from a point above the column bottom and a part of the inert paraffins are withdrawn from a tray located in the upper section of the column, Fig. 3 shows the process diagram of a third process in accordance with the general concept of the invention having a catalyst-filled prereactor placed preceding the distillation column shown in Fig. 1, Fig. 4 shows the process diagram of a first embodiment of the invention having the catalyst beds placed in two side reactors so that the drawoff point from the distillation column to the first reactor is situated above the feed point of the olefin mixture, Fig. 5 shows a second embodiment of the invention analogous to the alternative described above with the exception that the olefin mixture is fed to the second side reactor, Fig. 6 shows the process diagram of a third embodiment of the invention having two side reactors arranged so that the feed of one reactor is heated in a heat exchanger by energy recovered from the product flow of the other reactor, Fig. 7 shows the process diagram of a fourth embodiment of the invention having the catalyst placed in both the distillation column and two side reactors, Fig. 8 shows the process diagram of an fifth embodiment of the invention having the catalyst placed in a side reactor and having the recirculation between the distillation column and the side reactor implemented by virtue of thermosiphon action, Fig. 9 shows the process diagram of a sixth embodiment of the invention having the catalyst placed in side reactor and having the recirculation between the distillation column and the side reactor implemented as forced circulation, Fig. 10 shows the process diagram of a seventh embodiment of the invention having two series connected distillation columns with multiple catalyst beds in both of them. In the diagrams the reference numerals 1, 9, 18, 27, 41, 55, 70, 84, 95, 110 and 111 denote a column, 2, 10, 19, 28, 42, 56, 71, 85, 96 and 112 denote a feed point of the feedstock, 3, 11, 20, 29, 43, 57, 72, 86 and 97 denote a column tray, 4, 12, 21, 30, 44, 58, 73, 87, 98 and 113 denote a catalyst bed, 5, 13, 22, 33, 47, 61, 76, 89, 100, 114 and 119 denote a distillate condenser, 7, 15, 24, 35, 49, 63, 78, 91, 102, 116 and 121 denote a return point of the distillate, 6, 14, 23, 34, 48, 62, 77, 90, 101, 115 and 120 denote a boiler of the bottom product, and 8, 16, 25, 36, 50, 64, 79, 92, 103, 117 and 122 denote a return point of the vapor obtained from the boiler. The meanings for the rest of the reference numerals will be evident from the following description. The general process principle is shown in Fig. 1. With reference to the diagram, an olefin-containing hydrocarbon mixture is fed directly to a distillation column 1 at an appropriately selected point (feed nozzle 2) , possibly via a preheating stage. The column is provided with trays 3, e.g., valve trays between which catalyst layers 4 are adapted containing a zeolite catalyst. In the embodiment illustrated in the diagram, the feedstock is fed in the space remaining between the upper and middle layers of catalyst. In the column light olefins are oligomerized in a zeolite-catalyzed reaction forming primarily containing C X1 and heavier hydrocarbons, most of them unsaturated. Oligomerization is also accompanied by cracking reactions in which light hydrocarbons are formed. The column top product flow contains light paraffinic hydrocarbons and possible light cracking products of the reaction. The column bottom product flow contains heavier hydrocarbons (oligomers) that are taken to further process¬ ing. These can be routed to, e.g., a separate hydrogenation bottom boiler 6, thus recycling the lighter hydrocarbons back to the column. The olefinic hydrocarbon streams obtained from the catalytic distillation system are not as such directly suited for use as gasoline, diesel fuel or solvent. The octane number of the olefinic gasoline components is high, while due to their instability they can be hydrogenated into paraffinic components of a lower octane number. Also the instability of the diesel fuel and solvent fractions is removed in hydrogenation. As the hydrogenation inevitably causes changes in the carbon chain lengths to some extent, separation equipment such as distillation columns must be placed after the hydrogenation to obtain the desired product fractions. Hydrogenation can also be performed separately for each olefin fraction. In both cases the products are different kinds of paraffinic solvents as well as blending stock for diesel fuel, gasoline and possibly for lubricants. The proportions of said components in the final product are dependent on the oligomerization conditions. In conjunction with all embodiments of the invention, it is possible to arrange drawoffs for both the oligomerized olefin streams and the paraffin-containing streams. The number of such streams can be one or greater. If the drawoff is located above the catalyst layers, a stream rich in light paraffins is obtained. Placement of the paraffins drawoff at a tray situated in the top section of the column facilitates purposeful design of the fractionation process of the cracking products and the paraffins. Correspondingly, a drawoff located below at least one catalyst layer provides a stream rich in oligomerized olefins, whereby at least two product fractions are obtained for hydrogenation: the drawoffs and the bottom product. Fig. 2 elucidates the latter alternative arrangement. The column 9 with its trays 11 and reaction zones 12 is equi- benefit over the embodiment shown in Fig. 1 is that the catalyst contained in the reactor can be regenerated in a suitable order. For instance, three side reactors can be in operation concurrently, while the fourth is being regenerated. With reference to the embodiment shown in Fig. 4, the olefins 28 are fed to a conventional distillation column 27 operated at a temperature of approx. 300 to 350 °C in the bottom (tray N) and at a temperature of, e.g., approx. 10 to 70 °C in the top (tray 1) . The light olefins of the feedstock are vaporized in the column 27, rise upward and a portion of them is routed via a drawoff to a first side reactor 31, where they are contacted with the catalyst. The reactor is maintained at a temperature of, e.g., approx. 200 to 250 °C. The reaction product is removed from the reactor and returned to the column onto a tray below the initial drawoff tray. The column drawoff and the return point of the flow recycled from the side reactor are both located above the feed tray of the column feedstock. A side reactor connected in the above- described manner is hereinafter called the "upper side reactor" . The column also has another side reactor 32 connected to it. Then, the flow to be routed to the latter reactor, is taken from a drawoff located below the feed tray 28. Sirice the stream passing through the latter side reactor 32 is returned below the feed tray, the side reactor 32 can be called the "lower side reactor" analogously with the upper side reactor 31. For other parts of its system configuration, the second side reactor is fully equivalent to the first side reactor. The side reactors 31, 32 can be operated at a pressure above that of the column 28 (e.g., 50 bar vs. 5 bar) . In the case shown in Fig. 4, the side reactors are fixed-bed reactors, whereby the liquid flow is arranged from top to bottom. With reference to the embodiment of Fig. 5, the system arrangement is equivalent to that shown in Fig. 4 with the exception that the feed flow to the oligomerization process is routed partially — or as shown in Fig. 5 — entirely to side reactors 45, 46. With reference to Fig. 6, the use of a heat exchanger is shown for transferring heat from the product flow of one side reactor to the feed of another side reactor. Then, the lower section of a column 55 is connected to a side reactor 60 (that is, a lower side reactor) , whose product flow is taken via a heat exchanger 65 prior to returning the flow back to a distillation column 55. The colder side of the heat exchanger 65 is fed with a flow taken from a drawoff 66 and performing as feed for a side reactor 59 (upper side reactor) which is connected to the upper section of the column. By virtue of the heat exchanger 65 the feed flow can be heated to a desired temperature prior to routing it to the upper side reactor. This approach offers an optimal utilization of the reaction heats in a system based on the combination of a dis¬ tillation column with side reactors. Instead of a separate heat exchanger 65, cooling pipes can be installed inside the catalyst layer of the side reactors 59, 60, the feed flow of one reactor being routed via said pipes into another reactor. Additionally, Fig. 6 shows an approach to feeding a portion of the feedstock to the first side reactor 59. Further shown in the diagram is the arrangement of routing the liquid flows which rise from the bottom upward through the side reactors . The reactors in the illustrated case can be tubular reactors, for instance. With reference to Fig. 7, a combination of the arrangements according to the diagrams of Figs. 1 and 4 is shown. In this system a part of the catalyst charge 73 is placed in the column 70, while the remaining part is placed in the side reactors 74, 75. The drawoff and return points 80, 81 of the first side reactor 74 are located above the reaction zone of the column, while the corresponding points of the second side reactor are situated below said reaction zone. The olefins back to the column 95 is taken via a back-pressure regulator 103. Such a combination of the pump 106, heat exchangers 107, 108 and back-pressure regulator 109 can be employed to run the reactor 99 under operating conditions different from those of the distillation column 95. With reference to Fig. 10, an alternative embodiment is shown having the catalytic column divided in two. The first column 110 is operated at a higher pressure than the second column 111. The product composition can be altered by varying the column pressures. Such a two-column system can be complemented with a pre-reactors in accordance with Fig. 3, and the columns can be provided with drawoffs . The catalyst charge 113 can be placed either in the column (s) , in the side reactors alone or in both the column(s) and in the side reactors . If a heavier product is to be manufactured using any arrangement in accordance with the above-described embodiments, the bottom product of the column can be taken to a postreactor when needed. During the optimization design of the column and the connected prereactors, side reactors and postreactors, catalysts of different operating temperatures can be desiredly placed to multiple locations in the system. Concomitantly, a disproportionate increase in column or reactor size is avoided. Next, a few examples are given with test results. Example 1 Feed of a mixture of C 4 olefins and C 4 paraffins directly to a catalytic distillation reactor system A catalytic distillation reactor system according to Fig. 1 including three catalyst layers I, II and III was fed with a mixture of C 4 olefins and C 4 paraffins having a composition of i-butane 26 % n-butane 14 % 1-butene 21 % 2-butene 39 % The catalyst was conventional ZSM-5 zeolite. The reaction beds were maintained at different temperatures so that the column lower section was at a temperature higher than that of the column upper section. The catalyst bed temperatures were as follows: Catalyst I: 265-280 °C Catalyst II: 255-260 °C Catalyst III: 180-220 °C The distillation reactor pressure was 20 bar. The bottom product composition (as olefins) obtained after the oligomerization step was: Example 2 Feed of C 4 olefins through a prereactor The oligomerization reaction was performed using an arrangement in accordance with Fig. 3 by introducing a feed stream of pure 1-butene via a prereactor to a catalytic dis¬ tillation reactor. A catalyst bed comprised of ZSM-5 zeolite was contained in both the prereactor and the catalytic dis- tillation reactor. (The prereactor temperature was 240 °C and pressure 50 bar. The product composition (olefins) after the prereactor was: The product stream was next introduced to the catalytic distillation reactor, where it was oligomerized to obtain a product rich in lubricant blending stock fractions . The distillation reactor was in accordance with Example 1 provided with three reaction beds maintained at different temperatures so that the lower section of the distillation reactor was at a temperature higher than the reactor upper section. The temperatures in the reactor were as follows: Catalyst I: 265-280 °C Catalyst II: 255-260 °C Catalyst III: 180-220 °C The distillation reactor pressure was 7 bar. Categorized by boiling point range, the product composition obtained from the distillation reactor was: As is evident from the above data, the catalytic distillation system according to the invention is well suited to the manufacture of heavy fractions. By reference it must be noted that a raffinate twice processed through a conventional tubular reactor contains less heavy fractions than the product obtained by virtue of the catalytic distillation system. Example 3 Oligomerization of mixture of C 4 olefins and C 4 paraffins in a catalytic distillation system including side reactors A mixture of C 4 olefins and paraffins was oligomerized using a distillation reactor system in accordance with Fig. 5 in which arrangement the fractions obtained from the distillation column are recycled via two catalytic side reac- tors back to the distillation column. The C 4 feed was introduced to the inlet of the upper side reactor in the manner shown in Fig. 5. The feed composition was: The side reactors had fixed catalyst beds in which the catalyst was ZSM-5 type zeolite. The distillation reactor system was operated with a tempera- ture differential between the parts of the system. Thus, the temperature at the distillation column bottom was 330 °C, while the column top temperature was 20 °C. The inlet flow temperature to the higher side reactor was 70 °C and to the lower side reactor 200 °C. Both catalytic side reactors were maintained at 230 °C. The distillation column pressure was approx. 5 bar, while the side reactors were run at 50 bar. The product was fractionated by the following boiling point ranges : Example 4 Separation of lubricant blending stock fraction The lubricant blending stock fraction was separated from the product obtained from Example 2 by distillation (at 410 °C and above) and said fraction was hydrogenated. Analysis results: Viscosity index 105 Pour point -36 °C The thermal stability of lubricant blending stock fractions manufactured in the described manner was excellent . The thermal stability was even better than that of poly-alpha- olefins and, contrary to these, the oligomer products manufactured according to the present invention were entirely free from catalyst residues. Example 5 Oligomerization of a mixture of C 4 olefins, C 4 paraffines, propene and propane in a catalytic distillation system equipped with a side reactor, the reaction beds being provided with different catalysts A mixture of C 4 olefins and paraffines (Raffinate II) together with propene was oligomerized in a distillation reaction system corresponding to Figure 5. Distillation fractions withdrawn from the distillation column were recirculated via two catalytic side reactors back to the column. The C 4 hydrocarbons as well as the C 3 hydrocarbons were fed into the feed flow of the upper side reactor as depicted in Figure 5. The feed of the lower side reactor only comprised a stream withdrawn from the distillation column. The composition of the C 4 feed was the same as in Example 3. The C 3 feed contained about 80 % of propene, the rest being propane. The mass flow ratio of the C 3 feed to the C 4 feed was 23:103. Fixed catalyst beds were used in the side reactors . The upper side reactor contained a ZSM-5 type zeolite as a catalyst, whereas the lower side reactor contained a HSiW (hetero polyacid) catalyst. The different parts of the distillation reactor system were kept at different temperatures. Thus, the temperature at the bottom of the distillation column was 330 °C, whereas the temperature at the top was 20 °C. The temperature of the inlet flow to the upper side reactor was 70 °C and to the lower side reactor 200 °C. The upper side reactor, which contained the ZSM-5 catalyst, was maintained at 235 °C, whereas the lower side reactor, which contained the HSiW catalyst, was kept at 220 °C. The distillation column pressure was approx. 2.5 bar, while the side reactors were operated at 50 bar. The product was fractionated by the following boiling point ranges : IBP - 200 °C = 31 % 200 - 350 °C = 60 % 350 - 410 °C = 6 % 410 - FBP °C = 3 % The cetane number of the distillate fraction of 200 to 350 °( was 50. Both the 350 to 410 °C and the 410+ °C fractions were, subject to hydrogenation, ' suitable for use as lubricants. This Example clearly shows that different oligomerization catalyst can be used in the reactors of the process. Furthermore, the Example indicates that the system according to the invention can be run on a mixture of propene and butenes as feedstock. Example 6 Oligomerization of propene in a catalytic distillation reactor system Propene containing about 20 % of propane as an impurity was oligomerized in a distillation reactor system depicted in Figure 5. Distillation fractions withdrawn from the distillation column were recirculated via two catalytic side reactors back to the column. The propene was fed into the feed flows of both side reactors, as shown for the upper side reactor in Figure 5. The propene feed of the upper side reactor was twice the amount of the propene feed of the lower side reactor. The side reactors contained fixed reaction beds, whose catalyst comprised a ZSM-5 type catalyst. The conditions of the distillation reaction system were approximately the same as in Example 5, with the exception for the temperatures of the side reactors: both reactors were maintained at a temperature in the range from 220 to 230 °C. The product was fractionated by the following boiling point ranges : IBF - 200 °C = 15 % 200 - 350 °C = 72 .% 350 - 410 °C = 10 % 410 - FBP °C = 3 % The lubricant 410+ °C fraction was hydrogenated and its viscosity index was determined. It turned out that the described reactor system produced a lubricant fraction having a viscosity index of 123, i.e. a better viscosity index than that obtained in Example 4. The reason for this improvement would appear ' to be that an oligomer product produced frorr, propene is branched in a different way than an oligomer croduct produced from butenes. The pour point of the lubricant fraction was -54 °C.

Claims: Claims : 1. A process for oligomerization of olefins, according to which process C 3 to C 20 olefins contained in the feedstock are subjected to an oligomerization reaction in the presence of a catalyst in order to produce a product containing oligomers, c h a r a c t e r i z e d in that - the oligomerization reaction is carried out in a cata¬ lytic distillation system comprising a distillation column (27; 41; 55; 70; 84; 95) for product fractionation, and connected thereto, at least one side reactor (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) , in which an essential part of the catalyst of the catalytic distillation system is placed, said catalyst comprising a solid oligomerization catalyst which forms a catalyst layer (30; 44; 58; 69; 73; 87; 98) , - at least a portion of the internal liquid flow of the distillation column is routed through said catalyst layer (30; 44; 58; 69; 73; 87; 98) , and - the portion of internal liquid flow of the column routed through said catalyst layer is recycled back to the distillation column (27; 41; 55; 70; 84; 95) . 2. The process according to claim 1, wherein there are 1 to 20, preferably about 2 to 10 side reactors (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) . 3. The process according to claim 1, wherein at least approx. 20 % of the internal flow of the distillation column is routed through said catalyst layer (30; 44; 58; 69; 73; 87; 98) . 4. The process according to claim 1, wherein 40 to 90 % of the internal liquid flow of the column (27; 41; 55; 70; 84; 95) is routed through said catalyst layer (30; 44; 58; 69; 73; 87; 98) . 5. The process according to claim 1, wherein the catalyst used is of the ZSM-5 type. 6. The process according to claim 5, wherein the oligomerization reaction is performed at a temperature in the range from approx. 150 to approx. 350 °C. 7. The process according to claim 1, wherein the recycled portion of the internal liquid flow is returned to the distillation column (27; 41; 55; 70; 84; 95) to a point advantageous for the distillation step. 8. The process according to claim 7, wherein, having passed through said catalyst layer, said liquid flow is recycled back to said column to a return tray below the drawoff tray of said liquid flow, whereby the location of said return tray is chosen so as to attain desired fractionation of the components contained in the recycled flow. 9. The process according to claim 7, wherein the liquid flow withdrawn from a drawoff point (37; 66; 80) above the column feed tray (28; 56; 71) is routed to a side reactor (31; 59; 74) , and having passed through the catalyst layer, it is recycled back to the column to a point above said feed tray, while yet below said drawoff point (38; 67; 81) of said liquid flow. 10. The process according to claim 7, wherein the liquid flow withdrawn from a drawoff point (39; 68; 82) below the feed tray (28; 56; 71) of the column is routed to a side reactor (32; 60; 75) , and having passed through the catalyst layer, it is recycled back to the column to a point below said drawoff point (40; 69; 83) of said liquid flow. 11. The process according to claim 1, based on the use of at least two side reactors, wherein the sidestream to the first reactor (31; 59; 74) is withdrawn from above the feed tray of the reactants, while the sidestream to a second reactor (32; 60; 75) is withdrawn from below said feed tray of the reactants. 12. The process according to claim 11, wherein the product stream of said first reactor (31; 59; 74) is recycled back to the column to a point above the feed tray (28; 56; 71) of reactants, while the product stream of said second reactor (32; 60; 75) is recycled back to the column to a point below said feed tray of reactants. 13. The process according to any one of the previous claims, wherein the sidestream withdrawn from said column (95) is recycled by thermosiphon action via said reactor (99) back to said distillation column. 14. The process according to claim 1, wherein at least a portion of the reactant feedstock is fed directly to at least one side reactor (45) . 15. The process according to claim 14, wherein a substantial portion of the reactant feedstock is fed to such a side reactor (45) which during the startup of the system or change of product grade is entirely disconnected from receiving a sidestream from the distillation column (41) . 16. The process according to 1, wherein at least one of the reactors (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) is operated at temperature and/or pressure conditions different from those of the distillation column (27; 41; 55; 70; 84; 95) . 17. The process according to claim 16, wherein the pressure of at least one of said reactors (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) is maintained at 10 to 100 bar, while the pressure of said distillation column (27; 41; 55; 70; 84; 95) is maintained at 1 to 10 bar. 18. The process according to claim 1, based on the use of at least two side reactors, wherein at least a portion of the heat contained in the outlet flow of one side reactor (60) is transferred to the feed flow of the other side reactor (59) . 19. The process according to claim 1, wherein the oligomer- containing product is withdrawn from the distillation column/reactor system (9, 12) from a tray (17) situated below the feed tray (10) , said withdrawal tray being selected so as to render a desired product composition. 20. The process according to any one of the previous claims, wherein the product obtained from oligomerization reaction is routed to a postreactor (111) to the end of obtaining conversion into a yet heavier product. 21. The process according to claim 1, wherein the distilla¬ tion column (70) includes at least one catalyst layer (73) . 22. The process according to claim 1, which comprises using a catalytic distillation system which includes at least two catalyst layers (30; 44; 58; 69; 73) , wherein at least one of the catalyst layers contains a different type of a catalyst than the other layers. 23. The process according to any one of the previous claims, wherein the side reactor comprises a flow reactor, in which the reactants and the reaction products flow in the same direction. 24. The process according to any one of the previous claims, wherein all the reaction products are withdrawn from the side reactor from one point. 25. A system for oligomerization of olefins, said system being suited for subjecting olefins contained in a feedstock to an oligomerization reaction in the presence of a catalyst in order to produce a product containing oligomers, c h a r a c t e r i z e d in that it comprises a distillation column/reactor system including a distillation column (27; 41; 55; 70; 84; 95) suited for fractionation of the product, and connected thereto, at least one side reactor (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) , in which an essential part of the catalyst charge (30; 44; 58; 69; 73; 87; 98) of the catalytic distillation column/reactor system is placed. 26. The system according to claim 25, wherein at least half of the catalyst charge (30; 44; 58; 69; 73; 87; 98) is placed in at least one of said side reactors (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) . 27. The system as defined in claim 25, wherein the catalyst charge (30; 44; 58; 69; 73; 87; 98) is entirely placed in at least one of said side reactors (31, 32; 45, 46; 59, 60; 88; 99) . 28. The system according to claim 25, wherein said system includes 1 to 20, preferably 2 to 10 side reactors (31, 32; 45, 46; 59, 60; 74, 75; 88; 99) . 29. The system according to claim 25, including at least two side reactors, wherein the first reactor (31; 59; 74) is connected to the column section situated above the column feed nozzle (28; 56; 71) , while the other reactor (32; 60; 75) is connected to the column section situated below the column feed nozzle. 30. The system according to claim 25, wherein the feed nozzle (42) of the olefin stream is adapted to at least one of said side reactors (45) . 31. The system according to claim 25, including at least two side reactors, wherein the inside of the catalyst bed of the first side reactor is provided with piping via which the feed flow of the second side reactor is routed. 32. The system according to claim 25, including at least two side reactors, wherein the outlet of the first side reactor (60) is connected to a heat exchanger (65) suited for heating the inlet flow of the second side reactor (59) . 33. The system according to claim 25, wherein the outlet of a side reactor (99) is connected to a heat exchanger (108) suited for at least partially vaporizing the recycle flow back to said column (95) from said side reactor to the end of achieving recirculation by virtue of thermosiphon action. 34. The system according to claim 25, wherein said system has equipment required for product fractionation and hydrogenation connected to it. 35. Use of the system defined in claim 34 in manufacturing lubricant blending stocks.

Inventors: KESKINEN KARI I, KESKINEN, KARI, I.
Applicants: KESKINEN KARI I, NESTE OY, KESKINEN, KARI, I.
Application year: 1993
Main classification: C07C2
IPC codes: B01D3, C07C2, C10G50