NO123009B - - Google Patents

Description

Process for the production and purification of polymers.

This invention relates to polymerization. In one aspect, it relates to an improved process for recovering polymers.

It is an object of this invention to provide an improved process for the recovery of polymers. Another purpose is to recover valuable by-products from a polymerization process. It is also an aim to increase the speed of polymer extraction. It is also a purpose to increase the filtration speed. Furthermore, it is a purpose, in a process of the kind described, to utilize as a filter aid or cover a material that is already available in the process, thereby avoiding the introduction of extra materials in the various steps of the process. Other purposes and advantages will be apparent from this presentation for professionals in the field.

Excellent polymers of olefins can be prepared by polymerization of a 1-olefin with a maximum chain length of 8 carbon atoms and no branching closer to the double bond than in the 4-position, by contact with a catalyst comprising chromium oxide, in which at least part of the chromium is hexavalent, associated with another oxide, i.e. at least one oxide selected from the group consisting of silicon, aluminium, zirconium and thorium. Suitable olefins are ethylene, propylene, 1-butene, 1-pentene, 1-hexene, and diolefins such as 1,3-butadiene. The catalyst can be prepared by depositing the chromium oxide, usually in the form of a solution of a compound that can be easily annealed, on the oxide of silicon, aluminum, zircon or thorium, dry and activate in a substantially anhydrous oxygen-containing gas at a temperature in the range of 400 to 820° C, for a time from 1 to 20 hours. The polymerization is generally carried out at a temperature in the range of 65 to 235° C. It is often desirable to carry out the polymerization in the presence of a solvent or a reaction medium which is generally a cycloparaffinic or paraffinic coal hydrogen which can be liquefied under the conditions of the process, i.e. a temperature of from 65 to 235° C and a pressure in the range atmospheric to 70 kg/cm2. Suitable solvents for this purpose are such C3 to C12 hydrocarbons as n-pentane, isopentane, n-hexane, 2-methylpentane, 2,2,4-trimethylpentane, cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, dimethylcyclopentanes, and dim -ethylcyclohexanes, or mixtures of any of the substances mentioned. The reaction can advantageously be carried out in a pressure reactor with a suitable mechanical stirring device, the catalyst being used in finely divided form in suspension in the solvent or reaction medium. The product mixture consists of solvent, polymer and suspended solid catalyst. The reaction mixture can advantageously be heated to a temperature higher than the polymerization temperature for a period of time and with sufficient stirring to obtain maximum dissolution of the polymer in the solvent. The heated mixture is then filtered to remove the suspended solid catalyst. The filtration is generally carried out at approximately the same temperature and pressure as the aforementioned heating step in which maximum dissolution of the polymer in the solvent has been achieved. It is desirable to vent unreacted olefin or other gas from the reactor fluid, and this can conveniently be done during the heating and dissolution step. Thus, the filtration will generally be carried out at a temperature above the polymerization temperature, but at a somewhat lower pressure, which is however still higher than the atmospheric pressure and is determined by the vapor pressure of the mixture at the dissolution and filtration temperature. The polymer substance is recovered from the resulting filtrate.

"According to the present invention, a method for the extraction and purification of polymers is provided where a mixture comprising a polymer substance, solvent for said polymer and a normally solid catalyst suspended in said solvent is filtered to remove said catalyst, whereby a certain amount of the catalyst is placed on the filter surface and the surface of said catalyst is essentially free of polymer.

It has been found that the filtration of the catalyst can be carried out at an increased rate by using, either as a cover over the filter or as a filter aid that can be mixed into the filter feed, a certain amount of the catalyst whose surface has been made virtually free of polymer with extraction with a solvent for such polymer. It has been observed that after the reactor fluid has been heated as described above to achieve maximum solubility of the polymer, a relatively small amount of polymer with a very high molecular weight remains on the surface of the catalyst. It is believed that this adherent polymeric substance tends to delay the removal of catalyst by filtration, although recovery of the polymeric substance is not hindered.

If, prior to filtration, the suspended catalyst is mixed with a catalyst whose surface is free of polymer; it appears that the adhering catalyst is diluted or distributed over a larger volume, and its delaying effect on the filtration is actually reduced or eliminated'. -

, The material used as a solvent for the polymer which adheres to the catalyst's surface after the dissolution step is preferably a carbon hydrogen f solvent. Since the polymer that remains on the catalyst has a higher average molecular weight than the amount of the produced polymer that is dissolved from the catalyst and is less soluble

soluble in the lighter coal water substances, we prefer to use a solvent that has a higher solubility for the remaining heavy polymer than the solvent used as the reaction medium in the polymerization step. As a preferred class of solvents for use in accordance with this invention, we use a liquid hydrocarbon f or a mixture of liquid hydrocarbons (liquid under the conditions prevailing in contact with the solvent) which has at least 6 carbon atoms per molecule and preferably at least 10 carbon atoms per molecule, usually from 12 to 30 carbon atoms per molecule. The hydrocarbon solvent should be one which does not undergo destructive chemical changes during the treatment or extraction of the filtered catalyst. Preferred coal hydrosubstances are those which, within the indicated molecular range, are of a paraffinic, cycloparaffinic or aromatic nature. When the solvent used for the wash contains less than 10 carbon atoms per molecule, prefer

that it is aromatic, because aromatic coal hydrosubstances have a higher dissolving power for the heavy polymer than non-aromatic coal hydrosubstances in this molecular weight range. Certain monoolefinic coal hydrocarbons, i.e. C10 and higher polymers obtained as residual fractions in processes where lower boiling olefins such as isobutylene and/or propylene are polymerized in the presence of such catalysts as silicic acid, alumina or phosphoric acid on a carrier to produce gasoline coal hydrocarbons may be used; thus, fractions known within the industry as "triisobutylene" are usable for this purpose. These fractions are recovered as C10+. or C12+ fractions that are obtained as residues in processes where propylene and/or butylene are polymerized to hydrocarbons with boiling points in the petrol range that are recovered as distillates. We prefer, however, to use a kerosene, a cycliparaffin, or an aromatic hydrocarbon, or a mixture of any of these, because of the greater resistance of these compounds to chemical changes under

process, compared to olefinic ones

connections. Examples of such solvents are the decanes, the indecenes, the dodecenes, the tetradecenes, and higher-boiling paraffin coal water substances. A very suitable solvent is a fraction known as "heavy alkylate" which is obtained from processes where low-boiling isoparaffins (e.g. isobutane or

isopentane) is alkylated with low-boiling olefins (e.g. propylene, butylenes, or pentenes) and hydrocarbons with boiling points in the gasoline range are extracted from the total alkylate, leaving a residue that begins to boil at 150° C or higher, and which generally boils in the range 150 to 260° C. This is a mixture of relatively high-boiling isoparaffins, and can be used as an extraction liquid according to the present invention. Such fractions are described in U.S. Pat. patent 2,442,011, issued May 25, 1948. A suitable isoparaffinic heavy alkylate fraction of the type described which is suitable for use in the present invention is a mixture of isoparaffins sold under the trade name "Soltrol". Another type of liquid isoparaffinic fraction which can be used is one which can be obtained by a non-destructive hydrogenation of a triisobutylene fraction, of a C12+ liquid isobutylene polymer fraction, or of a G12+ fraction of a copolymer of propylene and isobutylene, prepared in presence of phosphoric acid or silicon-aluminium catalysts in polymerization as described above. Such fractions are liquid and very rich in isoparaffins. Other examples of suitable solvents are normally liquid aromatic hydrocarbons such as toluene, xylenes, trimethylbenzenes, naphthalenes and alkylnaphthalenes (which are liquefied by dissolution or pressure), and cyclic hydrocarbons such as the decalins (decahydronaphthalenes) and tetralin (tetrahydronaphthalene).

The amount of solvent to be used to wash the spent catalyst is sufficient to produce a separate liquid phase which can be separated from the washed catalyst by decantation and is generally in the range from 1 to 20 parts by weight per weight part catalyst.

In accordance with the invention, a catalyst which is obtained as a filter cake from the filtration process can be washed or extracted with a solvent of the type described, preferably at elevated temperature and atmospheric pressure. However, higher pressures can be used if desired, particularly if the solvent is one such as toluene, which has a relatively low boiling point. Where the extraction or washing is carried out at a temperature below the boiling point of the solvent, but high enough for the solvent to have a considerable vapor pressure, a reflux condenser can be connected to the extraction vessel to prevent loss of solvent, and the extraction can still be carried out at atmospheric pressure without undue risk of fire or loss of solvent. A suitable temperature for this extraction is usually at least 94°C, preferably 150 to 260°C, and preferably no higher than the temperature at which the solvent begins to boil. However, higher temperatures can be used if desired, although equipment to work under pressure will be required in such cases. It is desirable that the solvent can essentially be kept in liquid form during the contact: Temperatures lower than 94° C can also be used; however, the extraction is then somewhat less efficient and requires longer treatment periods. The extracted catalyst, which is thereby freed from significant amounts of remaining adherent high-molecular polymer material, is sent back to the filtration step to serve as a filter aid and/or covering material. If desired, the filter cake can be processed before or after the extraction step with a selective sedimentation or another method to remove the fine particles.

In this preparation, the term "solvent" is used, when it stands alone, to refer to the hydrocarbon solvent used as a reaction medium or solvent in the polymerization, while the term "washing solvent" refers to the solvent used to washing or extracting residual high molecular polymer material from the filtered catalyst: The catalyst added prior to the filtration has already been described. This catalyst can be a freshly prepared catalyst which has not yet been subjected to the final activation where the catalyst is heated to a temperature in the range of 400 to 820° C in the presence of a substantially anhydrous oxidizing gas. Or the added catalyst can be one part of the catalyst that has previously been used in the polymerization process and from which the adhering polymer is derived. been removed, e.g. by oxidation.

According to another embodiment of the invention, fresh catalyst, which has been prepared for example by impregnating a suitable carrier, such as a co-precipitated macroporous composition of silica-aluminium gel and then preferably one that has been treated with steam and air at elevated temperature-as briefly described above, with an aqueous solution of a chromium salt which can be annealed to oxide, followed by a drying of the resulting impregnated mixture, is added to the mixture of catalyst and solution, prior to filtration, as a filter aid, or it can be placed on the surface of the filter as a filter cover. After filtration, the filter cake is removed from the filter, it is subjected to treatment with an oxygen-containing gas to remove adhering organic matter by combustion, and is then heated in an oxygen-containing gas, which is preferably substantially free of water vapor, at a temperature in the range 400 to 820° C, preferably 480 to 595° C, for a period of time between 1 and 10 hours. It is preferable that the final activation step is carried out with an oxygen-containing gas, such as air, which is essentially free of water vapour, i.e. with a dew point below -f- 18° C. Since the deposits on the catalyst generally contain hydrogen , water is formed as a by-product during combustion. It is therefore desirable, although not absolutely necessary, to carry out the regeneration in two stages, where the first adhering organic matter is removed by combustion, and where in the second the catalyst is activated by heating to an elevated temperature in a stream of oxygen-containing gas which previously described. The activated or reactivated catalyst can be fed to the filtration step as mentioned above, or it can be fed directly to the polymerization reactor. Alternatively, part of the catalyst can be fed to the filtration step to serve as a filter aid, and the remainder can be fed to the polymerization step to serve as a regular catalyst therein.

In yet another embodiment of the invention, freshly prepared and activated catalyst can be supplied only to the polymerization step, the filter cake from the filtration step can be regenerated and reactivated as described above and sent back into circulation, partly to the polymerization step and partly to the filtration step. In such a system, filter aid or cover is only obtained from the regeneration step.

The use of a suspended catalyst in a system of a type described here eventually leads to the formation of fine particles, which also act to delay filtration. According to one embodiment of the invention, the regeneration step can be conducted with the catalyst in the form of a fluidized bed, and the gas for regeneration or activation can be passed through the catalyst bed at a sufficient speed to swirl up the fine particles from it, and the thus swirled fine particles can be drawn out of the system in the effluent liquid or activation gas. During actual operation, the amount of fine particles removed in this way can amount to from 10 to 60 per cent of the catalyst based on the weight of the catalyst supplied to the regeneration system.

The accompanying drawings are schematic fabrication diagrams illustrating embodiments of the present invention.

As shown in figure 1 of the drawings, fresh flea solvent (i.e. cyclohexane, here only referred to as "solvent" as opposed to "washing solvent" which is described below) enters the system through the inlet 2 and mixes with it finely divided catalyst which is supplied from the storage device through the star valve 4. The resulting catalyst slurry passes through the line 5 to the reactor 6. An olefin, such as ethylene, is supplied to the reactor through the inlet 7. In the reactor 6, the olefin is brought into contact with solvent and catalyst at a temperature of f. e.g. 150° C, and a pressure of approx. 35 kg/cm2 in. a time of approximately 2 hours.

Liquid phase is continuously withdrawn from the reactor 6 through the pipe line 8. Additional solvent can be added to the liquid phase through the pipe 9 at a suitable elevated temperature which is achieved using the heating device 10. The addition of solvent at this point can be used to adjust the visco -sity of the reactor liquid to any desired value. The resulting mixture passes through the line 8 and the heating device 11, where it is heated to a sufficiently high temperature to ensure a substantially complete dissolution of the polymer substance in the solvent. The resulting mixture proceeds to a dissolution zone 12 where the heated mixture is maintained in a state of agitation for a suitable period of time and a suitable temperature (eg 162° C) to ensure dissolution of practically all polymer material in the solvent. A heating time of approx. 30 minutes at a temperature of approx. 162° C is generally sufficient when the solvent is cyclohexane and the polymer is a polymer of ethylene. Any unreacted ethylene or other gas is taken out through the outlet 13. The resulting suspension of catalyst in solution is led through the pipeline 14 to a filter zone which is generally discharged at the broken line 15. This filter zone may comprise two or more filter units 16 and 17 , which are preferably adapted so that they are essentially kept at the same pressure and temperature as exist in the dissolution zone 12. Several filter units can be used, one being used for filtration while the others are covered with a filter cover by a method to be described in the following. The filtrate, which is practically free of suspended catalyst, is passed through pipelines 18 or 19 to the polymer recovery zone 20. This zone may include any desired apparatus for removing solvent from the polymer, for example by cooling and fining the polymer, or by evaporation of the solvent. The polymer substance can be extracted and carried away via line 21 as a product of the process. Any material with an intermediate boiling range or with a lower boiling point below that of the solvent can be recovered in zone 20 and removed using devices that are not shown. Recovered solvent can be sent back into the process through lines 22 and 7 or 23. Part of the recovered solvent can be used as washing liquid and supplied to the filter unit 16 via line 25, the washing liquid being supplied at any desired temperature (generally the filtration temperature ). 30 which is provided with a suitable agitator and heating coil 31. In the tank 30 the catalyst, which contains some adherent polymer material which has a tendency to delay filtration, is treated with a washing solvent supplied through the inlet 60 and the line 61. This solvent, which has already are described, have a higher dissolving ability for adherent polymer matter than those t solvent which is used in the polymerization zone, and is led to the tank 30 and there brought into contact with the filtered catalyst under such conditions that the adhering polymer is extracted or dissolved from the surface of the catalyst. A moderate temperature increase can be achieved by means of the heating coil 31. The resulting mixture passes to the filtration zone 33 which is designed and constructed in a similar way to the filter zone 15. The filtrate from the filter zone 33 is passed through the conduit 34 to the polymer recovery zone 35, which may be of similar execution as the polymer extraction zone 20. Polymer substance can be carried away as a product through the outlet 36. From the preceding description it will be obvious to informed persons that the polymer extraction zones 20 and 35 can be combined into a single zone if desired. However, since it is preferable in zone 33 to use a solvent different from that used in filtration zone 15, separate recovery of the solvents can often more conveniently be used using separate recovery zones as shown in the drawing. Part of the washing solvent recovered in zone 35 and carried away through line 61 can be used to wash the filter cake, as indicated by line 62 and heating device 63. Recovered and washed catalyst is removed from filtration zone 33 and passed through lines 37, 38 and 39 for use as described below. If desired, a portion of the catalyst can be sent to the dryer 40, which is constructed in any suitable manner, and any remaining washing liquid can be recovered through the outlet 41. Drying at elevated temperature and/or reduced pressure can be used. Part or all of the catalyst sent to the dryer can be led out of the system through the outlet 42 or recovered if desired. Alternatively, the catalyst can be swirled up by means of a stream of inert gas to remove fine particles, which are taken away through outlet 42 or recycled if desired. The details of this particular work operation are not shown in the drawing, but can be easily obtained by those skilled in the art by studying the present embodiment. Washed catalyst can be sent back to the filter zone 15 through lines 38 and 39. The catalyst can easily be transferred by means of a carrier gas, or preferably by means of a liquid carbon hydrogen diluent, such as cyclohexane (not shown in the drawing). A portion of the recovered catalyst can be fed through line 48 to a sludge tank for filter deck 43 if desired. The operation of this tank is described below.

A suitable filter cover can be fitted to any of the filter units 16 or 17. A filter cover material such as diatomaceous earth can be supplied to the system through line 44 and stirred in a tank 43 to form a slurry in solvent which is supplied through line 23. The resulting sludge can be supplied to filter unit 16 or 17 through line 46 or 47 respectively. One of the filter units 16 or 17 can be used for filtration while the other is provided with a cover. Generally, in the practice of the invention, it is not necessary to add any additional filter cover material after the operation has begun and has reached a satisfactory stable state. Recovered catalyst can be fed through lines 39 and 48 to tank 43 to provide a suitable cover material. However, it is often desirable, when starting the process, to use a filter cover material from an outside source, at least temporarily.

The reaction and transfer of solid catalyst produces a certain amount of spent material and fine particles. It often becomes necessary to remove fine particles, i.e. catalyst finer than 100 mesh, from time to time or continuously during the process. This can be done by swirling the dried catalyst as previously indicated under the description of the drying 40. Another method comprises passing the catalyst, preferably in the form of a slurry in such a solvent as cyclohexane, through the conduit 49 to the settling or sedimentation zone 50. In in the zone 50, the coarser material settles and is recovered in the form of a thickened sludge, which can be fed back through lines 51 or 23, for example. The suspension of fine particles can be passed through line 52 to the zone for removing fine particles 53. Zone 53 can consist of a filter and an evaporation device that evaporates the solvent and leaves the catalyst fine particles as a residue or a sludge concentrate. The fine particles may be removed from the system through outlet 54 and discarded, or be treated for recovery of one or more of the components, by means not shown. Solvent that is recovered in zone 53 can be fed back through lines 55 and 23.

As shown in Figure 2 of the drawings, a solvent such as cyclohexane enters the system through the inlet 102 and is led to the cleaning zone 103 where impurities such as oxygen and other low-boiling components that exert a destructive effect on the catalyst are removed, e.g. by distillation.

Purified solvent passes through the line 104 to the storage for solvent 105. Solvent is taken out from the storage zone 105 through the line 106 and led to the reactor 110. Fresh catalyst is supplied from the storage zone 107 through a valve 108, which can be a star valve. Pressure or air supply required to cause the catalyst to flow from the reservoir 107 can be supplied by pressurized air or gas supplied through the inlet 109. The resulting slurry of catalyst in solvent is fed to the polymerization zone 110, which is generally in form of a draft container provided with a mechanical stirring device. An olefin, such as ethylene, is added through inlet 111. The olefin is polymerized by contact with the catalyst in reactor 110 at a temperature in the range of 94 to 178° C. and a pressure of about 28 to 49.2 kg/cm 2 . A suitable residence time in the reactor 110 is about 2 hours, although longer or shorter times can be used. Liquid is withdrawn from reactor 110 through line 112 and can be diluted with solvent supplied through line 113 to provide the desired viscosity and ensure the dissolution of most of the polymer material in the subsequent dissolution steps carried out in zone 114. In zone 114 the mixture is heated to a temperature which is usually 5.5 to 14, and often 28° C higher than in the polymerization zone 110. The mixture is stirred for a time, generally around half an hour, as the necessary heat is supplied through the heating means 115 , which can be an electric heating cable or a steam coil. Any gas, including unreacted ethylene, can be removed through vent valve 116.

The resulting solution of polymer in solvent containing suspended catalyst is passed through line 117. Regenerated catalyst is added from storage device 119, the necessary pressure or aeration being carried out by means of a suitable inert gas such as nitrogen, which is introduced through inlet 120. The regenerated catalyst is supplied through the star valve 121 and the line 122, and the resulting mixture goes to the mixing zone 118 where thorough mixing of the materials is achieved by means of a suitable stirring device. The resulting mixture passes through conduit 125 to filtration zone 126, which may include a filter such as a rotary filter, adapted to operate at the relatively high temperatures and pressures prevailing in dissolution zone 114 and mixing zone 118. The resulting filtrate is passed through conduit 127 to the separation zone 128. Zone 128 comprises any suitable equipment to separate solvent from polymer, for example cooling and a filter device which causes precipitation of polymer from the solution and subsequent filtration of the polymer; or a series of evaporation devices, at least some of which can be operated under vacuum. Recovered polymer product of polyethylene or other polymer material, depending on which particular olefin or olefins have been used, is taken out through outlet 129. Recovered solvent is fed back via line 130. The filter cake obtained in filter zone 126 is fed through a suitable line 121, which can be a screw conveyor, to the drying zone 132, where any adhering solvent can be evaporated and sent through the cooler 133 and the line 134 to the storage zone for solvent 105. Dried catalyst is sent through the line 135 and star valve 136 to the regeneration zone 137. In zone 137, the catalyst is brought into contact with a stream of oxygen-containing gas, e.g. an oxygen-nitrogen mixture containing 1-2 percent oxygen, which enters through the inlet 138. The oxygen-containing gas is brought into contact with the catalyst, which may be in the form of a compact fluidized bed in zone 137 under conditions which provide oxygen the dation of possibly adhering organic deposits on the catalyst. The regeneration is usually carried out at a temperature of from 480 to 540° C. The activation preferably takes place by continuing the contact after the carbonaceous material has been burned away from the surface of the catalyst. This continued contact can be carried out at a temperature in the range of 480 to 595°C for a period of from 1 to 20 hours, usually from 2 to 15 hours. The gas used for the contact is preferably essentially anhydrous and may have a content of oxygen which is different from that used as combustion gas (air or pure oxygen may be used in the activation step). As previously indicated, the combustion of deposits may be carried out in a zone separate from the final heating or activation step which is carried out in the presence of a substantially anhydrous, oxygen-containing gas. However, the entire regeneration and activation zone is indicated in the drawing as regeneration zone 137, and regeneration and activation can be performed in a single zone or vessel. Gas for regeneration and/or activation is removed from zone 137 through outlet 139. The contact process in zone 137 can, if desired, be conducted with such a gas velocity that material finer than a predetermined particle size, e.g. 200 mesh, is led out of zone 137 with the flowing gas that is drawn off through the outlet 139. The gas that is drawn off can be sent through a cyclone or other suitable separator 140, with any coarse material being sent back through line 141. Outflowing gas containing fine particles is drawn off through outlet 140A.

Regenerated and activated catalyst is taken out of zone 137 through pipe 142 and star-

the nose valve 143 and is sent to the cooling zone 144 where it is brought into contact with an inert gas such as essentially anhydrous nitrogen which is supplied through the inlet 145. Blowing away fine particles can be carried out, in whole or in part, in the cooling zone 144 with the help of the cooling gas. Outflowing gas is taken out through pipe 146. The cooled catalyst, generally at a temperature in the order of 94 to 150° C, is taken out via line 147 and star valve 148, and is transported, at substantially the same temperature, by means of an inert water-free, oxygen-free gas such as nitrogen which is supplied through the inlet 149 to a cyclone or other suitable separator 150, where the catalyst is freed from gas. The gas is drawn off through the outlet 151, and the catalyst is delivered to the storage zone for regenerated catalyst 119. From zone 119, part of the catalyst can be passed through the star valve 124A and line 124 to the polymerization zone 110 as a catalyst for the polymerization reaction.

In yet another embodiment of the invention, non-activated fresh catalyst can be added from the storage zone 152 through the pipeline 153 and the star valve 154 as a filter aid, together with or without the regenerated catalyst from zone 119, to the material passing through line 117 to the mixing zone 118 and the filtration zone 126. Suitable gas for pressure or aeration can be supplied to the catalyst through the inlet

155. In this embodiment of the invention, a catalyst can be used which has been prepared according to the method briefly described above, except that

has omitted the last activation step with

heating in the presence of an oxygen-containing gas at a temperature in the range of 400 to 820° C. Thus, the partially ready-made catalyst can be used as a filter aid prior to the final filtration step. The activation then takes place in the regeneration zone 137, as previously described, and the activated catalyst is led to the polymerization zone 110 via the storage zone 119.

It will be clear to those skilled in the art that the transfer of solids in the described system can be accomplished by means known in the art, e.g. suspension in a liquid, gravity, or when using lock chambers where rather large pressure differences occur. These technical aids are well known in the industry and have therefore not been described in detail here.

Regenerated or fresh, non-activated catalyst can also be used as a filter cover. In this embodiment of the invention, a slurry of the catalyst can be made, for example in a hydrocarbon such as cyclohexane or isooctane, and applied to the filter surface prior to the catalyst being removed from the polymerization reactor liquid by filtering through the layer of solids thus placed on the filter.

Example 1.

In the following experiment, the catalyst was prepared by using as a component a commercial cracking catalyst comprising a composition of silicon-alumina made by simultaneous precipitation of both, so they formed a gel containing about 90% by weight silicon oxide and about 10% by weight alumina. This gel is activated by heating at an elevated temperature in a mixture of steam and air as described in patent no. 89 517. The activated, macroporous composition is impregnated with an aqueous solution of chromium trioxide. The excess of the solution is allowed to run off the solids. The remaining solids are dried by shaking and by being exposed to infrared radiation. The catalyst is activated by heating in the presence of air that has been dried to a dew point below 18° C, the activation being carried out at a temperature of 508° C for a period of 5 hours. This catalyst, which is stored in an atmosphere of anhydrous nitrogen prior to use, is suspended in cyclohexane and fed to a reactor equipped with mechanical stirring. The ethylene is also supplied to this reactor, which is maintained at a temperature of 144° C and a pressure of 35 kg/cm.2. The amount of catalyst in the reaction mixture is kept at approximately 0.5% by weight and the rate of ethylene supply is regulated so that a polymer concentration of approximately 8% is achieved in the liquid flowing from the reactor. This liquid is removed from the reactor and heated to a temperature of 158° C for 30 min. after an equal volume of cyclohexene has been added. At the same time, the pressure is reduced to 7.03 kg/cms by venting unreacted ethylene. Washed catalyst is added to the remaining mixture as described below. The resulting mixture is fed to a filter which is essentially maintained at the same pressure and temperature as in the dissolution step (158° C., 7.03 kg/cm 2 ). The filter surface is a paper filter element that rests on a perforated metal disk placed inside a pressure chamber. Polymer is recovered from the filtrate by cooling to room temperature and filtration. After heating after filtration in a vacuum oven for several hours, the polymer product obtained has a molecular weight of approx. 37,000, a specific gravity of about 0.96, and a melting point of about 120°C.

The catalyst recovered from the filter is mixed with five times its weight of an isoparaffin fraction having the following properties, which is obtained by alkylating isobutane with a mixture of butylenes in the presence of anhydrous hydrogen fluoride at approx. 38° C, and a molar ratio of isobutane : olefin of about 5:1 at sufficient pressure to keep the reacting substances in the liquid phase:

The mixture of catalyst and solvent is stirred for approximately 30 minutes at 150° C. in a tank equipped with a stirring device and reflux cooler. The resulting mixture is filtered through a filter of the type described above and washed with several fresh portions of the isoparaffinic solvent. The recovered washed catalyst is then used as a filter aid in the filtration of a new amount of liquid flowing from the reactor as described previously. The amount of washed catalyst that is added is approximately equal to the weight of catalyst in the flowing liquid from the heating and dissolution step. This gives an increased filtration rate compared to filtration in the absence of the washed catalyst.

The filtrate from the catalyst washing step is distilled to recover the isoparaffinic solvent. A residue corresponding to 40 to 60 percent of the filtered catalyst (disregarding the catalyst added as a filter aid) is obtained, and this primarily comprises a polyethylene of a special grade (type) with a molecular weight above 80,000 (based on intrinsic viscosity). Results of a similar nature to the preceding are obtained by using tetralin or decalin as washing solvent instead of the heavy alkylate fraction.

Example 2.

In the following experiment, the catalyst was made by using as a component a commercial cracking catalyst comprising a silicon-alumina composition made by simultaneous melting of silicon and aluminum as a gel with a content of approximately 90% by weight of silicon and 10% by weight of aluminum oxide. This gel is activated by heating at an elevated temperature in a mixture of steam and air as described in patent no. 89 517. The activated, macroporous composition is impregnated with an aqueous solution of chromium trioxide. The excess of the solution is allowed to run off from the solids. The remaining solids are dried by shaking and by exposure to infrared radiation. Part of the catalyst is activated by heating in the presence of air that has been dried to a dew point below -18° C, the activation being carried out at a temperature of 508° C, for a period of 5 hours. This catalyst, which is stored in an anhydrous nitrogen atmosphere prior to use, is suspended in cyclohexane and fed to the reactor, which is equipped with mechanical stirring. The ethylene is also supplied to this reactor, which is maintained at a temperature of 144° C. and a pressure of 35.15 kg/cm 2 . The amount of catalyst in the reaction mixture is kept at approximately 1 percent by weight, and the feed rate for the ethylene is regulated so that a polymer concentration of about 3 percent is obtained in the liquid that is carried away from the reactor. This liquid is led away from the reactor and heated to a temperature of 158° C for 30 min. after addition of an equal volume of cyclohexane. At the same time, reduce

the pressure is raised to 7.03 kg/cm.2 by blowing out unreacted ethylene. To the result

mixture, a catalyst is introduced

posed as described above, but not active

host. The amount of added activator is practically

roughly equal by weight to that already present in the mixture prior to the addition of unactivated catalyst.

The resulting mixture is carefully stirred

gelled up and fed onto a pressure filter which is maintained at approximately 7.03 kg/cm2 and 161° C. The filter element is a very paper laid over

a perforated metal plate which is placed in the filter frame.

The resulting filtrate is cooled to approximately room temperature, and there

is a polyethylene with a molecular weight of approximately 37,000 and a melting point of 120°C

as well as a specific gravity of 0.96 in the case of filters

ring and removal of adhering solvent by evaporation.

As a general rule, the amount of catalyst added to the feed to the felt

ret as filter aid from 14 tJ1 5 times wec-

ten of the catalyst originally present in said feed. Larger quantities can of course be used if desired. When the polymer-free catalyst is used as a filter cover, the amount used will depend on the particular filter surface used.

tes, and can be chosen in accordance with the practice that is well known to those who are informed in the area.

It will appear from the foregoing that

a process has been provided in which solid catalyst recovered from a polymerization process can be washed and reused as a filter aid or filter cover to remove new amounts of catalyst.

It will also be seen that provision has been made

an improved process comprising the use of polymer-free catalyst as a filter aid or filter cover for a polymer solution to be filtered to remove polymerization catalyst from it. In addition to this, the high molecular weight residual polymer that is deposited on the catalyst can be recovered as a special high molecular weight polymer pro-

skill. Thus, after washing with the washing solvent as described, the washing solvent containing extracted residual heavy polymer material can be separated from the washed, used catalyst by decantation or centrifugation.

miter. It is also understood that extra filter aid and/or filter cover material can be added to the system in addition to the catalyst that has been treated with solvent and sent back into the system.

met. Examples of such extra material are diatomaceous earth, clay soil and cellulose.

Claims (8)

1. Procedure for the extraction and purification of olefin polymers where a mix- ding comprising a polymer, solvent for said polymer and a normally solid polymerization catalyst suspended in a solution of said polymer in said solvent is filtered to remove said catalyst, characterized in that a certain amount of said catalyst is placed on the filter surface, the surface of which in everything essential is polymer-free. 2. Method as stated in claim 1, characterized in that the solid catalyst residues from the filtration are washed with a solvent which has a greater dissolving power for the polymer attached to said solids than the solvent in said mixture and at least part of the resulting essentially polymer-free catalyst is recycled to the filtration step as a filter aid. 3. Method as stated in claim 2, characterized in that said residue of said solids is brought into contact with the solvent at a temperature of at least 94° C and preferably in the range from 150-260° C, and said solvent is present essentially in liquid form. 4. Method as stated in one of the preceding claims, characterized in that the polymer-free catalyst is placed on the filter surface so that a filter cover is formed before the filtration of said mixture. 5. Method as set forth in one of claims 2, 3 or 4, characterized in that said residue of the solids is brought into contact with a solvent comprising a liquid carbon hydrogen with at least 6 carbon atoms per molecule, and this is carried out under sufficient pressure to keep the solvent in the liquid phase. 6. Method as stated in one of the preceding claims, characterized in that the polymer-free catalyst is added to the mixture before filtration. 7. Method as stated in one of the preceding claims, characterized in that the ratio between added polymer-free catalyst and the catalyst already present in the mixture which is fed to the filtration step is within the range 1:4 to 5:1. 8. Method as stated in one of the preceding claims, characterized in that said mixture comprises a normally solid polymer of an aliphatic 1-olefin, a solvent for said polymer and a normally solid catalyst comprising chromium oxide combined with at least one oxide selected from the group silicon, aluminium, zirconium and thorium.