FIELD OF INVENTION

The present invention relates to a method for the purification of spent polymerization solvent using adsorbents. The present invention provides a method for removing impurities such as aromatic, olefinic, moisture from the polymerization solvent using a fixed adsorbent bed. The present invention further provides a process for removing multiple impurities such as aromatic, olefinic and moisture from the spent polymerization solvent under the same set of process conditions. The present invention discloses an adsorbent and adsorptive process for purifying the spent saturated paraffmic solvent through removing impurities like aromatics, olefinic and moisture generated during making of polymers like high density polyethylene (HDPE) and ultrahigh molecular polyethylene (UHWMPE).

BACKGROUND OF THE INVENTION

During polymerization process impurities like spent catalyst residues, aromatic compounds, olefins and moisture are generated. The spent catalyst residues like magnesium ethoxide and TiCl ₄ and other organic contaminants are removed by treating with pure saturated paraffmic solvent comprised of C ₅ -Ci ₆ carbon number range followed by alkali washing to remove inorganic impurities. Remaining spent paraffinic solvent contains moisture, unsaturates and aromatics as major impurities. It is extremely important to purify a polymerization solvent prior to its reuse for the polymerization of an olefin in the presence of a Ziegler-Natta catalyst, because the Ziegler-Natta catalyst is deactivated by various poisonous components such as compounds with polar groups contained therein.

Various methods of removing catalysts residues from polymers are known. For example, U.S. Pat. No. 3,271,372 discloses a process wherein an alcohol solvent is used to disperse the granular particles of the polymer which is separated from the slurry. U.S. Pat. No. 3,337,514 requires contacting a solution of an polymer with steam, followed with aqueous mineral acid treatment, then with water wash under turbulent conditions and finally separating the polymer solution from the aqueous phase. U.S. Pat. No. 2,950,336 discloses the separation of aromatic compounds and olefins from hydrocarbon mixtures that may also include paraffins, using a zeolitic molecular sieve.

U.S. Pat. No. 4,725,338 claims a process for purifying an olefin polymerization solvent suitable for use in the presence of a Ziegler-Natta catalyst, through usage of a multi-stage distillation column without usage of adsorbent.

U.S. Pat. No. 4,433,194 discloses a method of purification of cyclohexane solvent with titanium tetrachloride, followed by adsorbent treatment like silica gel and distillation in the presence of alkali metal hydrides.

JP2000143718A discloses a process for removing olefins from a

"polymerization solvent" by circulating through a fixed column packed with an acid- treated clay (apparently means fixed bed!) at ambient conditions. The polymerization solvent includes C ₅ -C ₁₂ aliphatic hydrocarbons, C ₆ and C ₇ alicyclic hydrocarbons and aromatic hydrocarbons such as benzene, toluene and xylene. This document does not disclose removal of moisture or aromatics.

In the prior art olefin removal is performed at 120 to 250°C temperature; and aromatics removal is achieved at ambient temperatures provided it is moisture free. Olefinic removal by clay is a catalytic action taking place between olefin molecules and active acid sites plus free acid available on clay surface whereas moisture and aromatics adsorption is physical adsorption related to polarity of water and aromatic molecules, surface area and porosity of acid activated clay.

None of the prior art methods provide a process for simultaneously removing aromatics, olefins, and moisture from spent paraffinic solvent system. There is a need for a simple and efficient process for removing aromatics, olefins, and moisture from spent paraffinic solvent system. The present invention relates to a method for the purification of spent polymerization solvent using adsorbents. More specifically, the invention is concerned with a fixed bed adsorptive process for the purification of spent paraffinic solvent whereby undesirable impurities aromatics, olefins, and moisture which cause a detrimental effect in the polymerization of olefins are effectively removed.

OBJECTS OF THE INVENTION

One of the important objects of the present invention is to provide a simple and efficient method for the purification of solvents which are utilized in ethylene polymerization reactions. Another object of the present invention is to provide a method for simultaneously removing impurities such as aromatic, olefinic and moisture from spent polymerization solvents.

Still another object of the present invention is to remove impurities such as aromatic, olefinic and moisture from spent polymerization solvents under the same set of process conditions.

Yet, another object of the present invention is to provide an adsorbent based method for the purification of spent paraffinic solvents.

Another object of the present invention is to provide a simple method for the purification of spent polymerization solvents comprising treating with suitable adsorbents such as silica gel, zeolite molecular sieve, activated alumina and acid activated montmorillonite to remove contaminants therefrom, and recovering the purified saturated paraffinic solvent.

SUMMARY OF THE INVENTION

The above and other objects of the invention are achieved by the present invention which relates to a method for the purification of spent polymerization solvent using adsorbents. More specifically, the invention is concerned with a fixed bed adsorptive process for the purification of spent paraffinic solvent whereby undesirable impurities aromatics, olefins, and moisture which cause detrimental effect in the polymerization of olefins are effectively removed.

Accordingly, the present invention provides a process for purifying spent polymerization solvent comprising: (a) feeding said spent polymerization solvent to a fixed adsorbent bed wherein the solvent is substantially free of polymerization catalyst and monomers; and (b) subjecting said fixed adsorbent bed to a adsorbent bed temperature of 10 to 120°C; adsorbent bed pressure of 1 to 10 atmospheres; thereby removing aromatic impurities, olefinic impurities and moisture from said spent polymerization solvent to obtain purified spent polymerization solvent.

In another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the spent polymerization solvent is separated and recovered from a Ziegler-Natta olefin polymerization reaction mixture.

In yet another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the spent polymerization solvent recovered comprises aromatic impurities upto 600 ppm, bromine index of upto 60 ppm and moisture content of upto 200 ppm. In still another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the spent polymerization solvent is any one from C ₅ to Ci ₆ saturated paraffins or mixture thereof and preferably any one from C9 to Co saturated paraffins or mixture thereof.

In another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the spent polymerization solvent has a boiling point in the range of 35°C-320°C preferably the boiling point is in the range of-152°C- 302°C.

In yet another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the aromatic impurities are aromatic compounds selected from the group comprising benzene, toluene, ethyl benzene, trimethyl benzenes, naphthalenes, Cu to Ci ₆ aromatics, unsaturates, oxygenates, water and sulfur containing aromatic compounds and mixtures thereof and wherein the monomer is ethylene.

In still another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the adsorbent is selected from the group comprising of zeolite, silica gel, activated alumina, activated bentonite clay and attapulgite clay preferably acid activated bentonite clay most preferably sulfuric acid activated calcium bentonite clay.

In another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the acid activated bentonite clay is substantially in the form of crushed particles and particle size of said crushed particles is in the range of 0.4 mm to 1.0 mm.

In yet another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the pore volume of said acid activated clay is in the range of 0.45 to 0.55 cc/gm and Brunauer, Emmett and Teller (BET) surface area is 250 to 350 mVgm.

In still another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the adsorbent bed temperature is preferably 30 to 60°C; adsorbent bed pressure is preferably 2 to 4 atmospheres; and liquid hourly space velocity of said adsorbent bed is from about 0.1 to about 2.0 per hour preferably 0.5 to 1.0 per hour.

In another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the purified spent polymerization solvent comprises upto 20ppm aromatic impurities; 2ppm bromine index; and upto lOppm of moisture.

In another embodiment the present invention provides a process for purifying spent polymerization solvent wherein the purified spent polymerization solvent has a purity of at least 99.99wt%.

In yet another embodiment the present invention provides a purified spent polymerization solvent prepared by the process as disclosed herein.

In still another embodiment the present invention provides the use of purified spent polymerization solvent prepared by the process as disclosed herein in polymerization reactions.

BRIEF DESCRIPTION OF DRAWINGS

Figure-1: Aromatics adsorption kinetics of zeolite 13X at 30°C: Adsorption kinetics of aromatics impurities present in spent solvent is measured on zeolite 13X. Figure-2: Aromatics adsorption kinetics of acid activated clay at 30°C: Adsorption kinetics of aromatics impurities present in spent solvent is measured on acid activated montmorillonite clay.

Figure-3: Fixed bed adsorption breakthrough of aromatics on acid activated clay: This figure shows the aromatics breakthrough curve of aromatics wherein aromatic content in the feed and treated samples is determined by UV method.

Figure-4: Fixed bed adsorption breakthrough of aromatics on zeolite 13X and acid activated clay: This figure shows the aromatic content in the feed and treated samples. The aromatic content is determined by UV method.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an adsorbent and adsorptive process for purifying the spent hydrocarbon solvent involving C ₅ to C] ₆ paraffin's used for polymerization reactions involving olefinic ethylene monomer and polymerization catalyst of Ziegler-Natta type to produce high density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHWMPE). Ziegler-Natta catalysts is well described, in "Ziegler-Natta Catalysts and Polymerization" by John Boor, Jr. (Academic Press) as well as Journal of Macromolecular Science— Reviews in Macromolecular Chemistry and Physics, C24(3) 355-385 (1984) and ibid., C25 (1), 57-97 (1985). Olefins which can be polymerized by such Ziegler-Natta catalysts are those having preferably 2-4 carbon atoms, such as ethylene, propylene and butene-1. It is well known to those skilled in the art that polymerization reactions of olefinic hydrocarbons, such as the Zieglar-Natta type, are sensitive to various types of impurities present in the solvent which have a significant and deleterious effect regarding the degree of polymerization of the olefinic hydrocarbon as well as the structure of the polymer which is prepared. During catalyst preparation inert paraffmic solvent is used as solvent which is being utilized in large quantities^ recycled and reused. During the process of polymerization catalyst preparation typical impurities accumulated in the paraffmic solvent post polymerization reactions are aromatic compounds like benzene, toluene, ethyl benzene, trimethyl benzenes, naphthalene's, Cn-Ci ₆ aromatics, olefinic unsaturates, oxygenates, water and sulfur- containing compounds. Impurities such as aromatics, water, oxygen-and sulfur- containing compounds have a tendency to react with the catalyst which is employed for the polymerization process and therefore these impurities deactivate the catalyst faster thereby reducing the output and efficiency of the catalyst. Molecular characteristic of typical aromatics molecules present in spent polymerization solvents are given below in table 1.

Table 1: Molecular characteristics of aromatic molecules

More specifically during process of ethylene polymerization magnesium ethoxide and TiCI ₄ are used for catalyst preparation in the polymerization unit. Initially, magnesium ethoxide is dissolved in paraffmic solvent and temperature of vessel is maintained to 70-100 ^° C. Addition of TiCI ₄ is done in 2-5 stages to maintain Mg: Ti ratio of 1 :1 to 1 :3. The solution is kept at 100°C to 140°C for 40-80 hrs. The unreacted TiCLt and other contaminants are removed by washing with pure paraffmic solvent. The mixture is again maintained at 70-100°C for 3-8 hrs. The decanted paraffmic solvent contains inorganic impurities and aromatics as major contaminants. Paraffinic solvent is treated with NaOH solution followed by water wash to remove the inorganic impurities and is termed as spent paraffinic solvent. The spent paraffinic solvent contains moisture, unsaturates and aromatics as major impurities. In the case of spent polymerization solvents recovered after polymerization systems, it is more preferable to purify them by the process of this invention after low boiling-point compounds such as unreacted monomers, e.g., ethylene, propylene, buten-1 and/or the like are removed beforehand.

Due to treatment with NaOH treatment and subsequent water wash the moisture content in spent solvent increases. The olefins present in the paraffinic solvent of CI ions (from TiCI ₄ ) and O ions present in ethoxide and temperature of 100°C-140°C for 40-80 hrs results in the formation of aromatic molecules. Inorganic impurities like Mg and Ti and excess chloride and oxide ions are removed by NaOH wash.

One method of reducing the aforementioned impurities which may be present in the solvent system is distillation but this method can lead to change in solvent composition and can subsequently affect the catalyst solubility in the solvent. In contradiction to these prior methods of purifying a solvent system, it has now been surprisingly found that the impurities which are present in a solvent system such as paraffinic solvent can be effectively removed by a simple treatment hereinafter with acid activated montmorillonite clay and the solvent system may be successfully employed in a polymerization of specific olefins such as ethylene to make UHWMPE and HDPE using Zeigler-Natta catalyst.

It is ifnportant to use a suitable low cost adsorbent for spent solvent purification because the adsorbent like zeolite molecular sieves generally adsorb low amount of polymerization-inhibiting components which are present in high concentrations in spent solvent and the polymerization solvent is used in large volumes on an industrial scale. Hence it is always desirable to develop a low cost adsorptive purification process for spent polymerization solvent. Typical acid activated montmorillonite clays are available at 20-30% of the cost of a zeolite molecular sieves normally used in the solvent purification process. The purification of the spent solvent system is effected by treating a solvent with low cost acid activated montmorillonite clay,

The presence of unsaturate' s in paraffinic solvents is measured by Bromine Index as per ASTM method D1491 , which signifies the presence of unsaturated olefins in paraffin's. Bromine Index is expressed as milligrams of the bromine available to react with 100 gm of the paraffin sample.

In the preferred embodiment of the present invention, the treatment step is affected at ambient temperatures and atmospheric pressure; although elevated temperatures and pressures may be used without deviating from the scope of the invention. Aromatic content in spent paraffinic solvent post catalyst preparation is estimated using UOP method 495-75 (Aromatics in molex n-paraffin products by Ultraviolet spectroscopy" published in UOP Laboratory test methods for petroleum and its products- 1972). This method determines the average amount of weight percent naphthalene and alkyl benzene present in spent solvent by measuring absorbance at 285 and 270 nm.

Preferred paraffinic solvents are saturated hydrocarbons which include alkanes, cycloalkanes and alkylcycloalkanes or mixtures thereof. Typical solvents include hexane, cyclohexane, methyl cyclohexane and other hydrocarbons of these classes. These and other similar hydrocarbons are well known to those skilled in art of polymerization of ethylene. The solvent used for polymerization catalyst preparation in the present invention is a mixture of C ₅ to Ci ₆ paraffin's with C ₉ to CH paraffinic hydrocarbons predominantly and comprises 98-99 vol% as given in Table 2 below: Table 2: Mixture composition of fresh paraffinic solvent

Zeolite 13X is well known for selective adsorption of aromatics molecules, over paraffins which is well described by Denaer et. al, in Microporous and Mesoporous Materials, 96, 149 - 156, 2006. An important feature of the present invention is use of acid activated montmorillonite clay which possesses a combination of cation exchange intercalation and swelling properties which makes it unique for adsorption of aromatics molecules which have typical kinetic diameter of 5-7 A. The montmorillonite clays have layer lattice structures in which inter lamellar/channel spaces is available for adsorption of aromatic molecules depending on the size and shape and is well explained in "The chemistry of clay organic reactions" by BKG Theng, published by Halsted Press, Johnson Willy & Sons Inc, New York, 1974 and YS Bhat et al in Journal of Porous material, ISSN 1380-2224, 2009.

Sorption kinetics of aromatics impurities present in spent solvent is measured on acid activated montmorillonite clay and zeolite 13X which are given in Figure- 1 and Figure-2 and diffusion coefficients calculated are given in Table 3. Respective selectivity's of aromatic molecules over C ₉ paraffin's are given in Table 4 to know suitability of adsorbent for aromatics removal from paraffin stream.

Table 3: Sorption capacity and diffusion of n-nonane and aromatics on 13X at room temperature

Table 4: Aromatics selectivity over C9 paraffin

Acid activated montmorillonite clay is prepared by extruding (6-8 mm cylindrical size) calcium rich montmorillonite granules with 30-40% of moisture content, and again re-extruded (3-5 mm). Extrudates are subjected to 40% concentrated sulfuric acid activation treatment at 85°C for 6-8 hours. Acid activated extrudates are finally water washed, air dried finally oven-dried and sized to 0.42 to 1 mm granules. Final acid activated clay has total acidity 22 mg KOH/g, with pH of 3.5 and moisture content of 1-2 wt%. The prepared acid activated clay has surface area of 250-350 m ² /g and pore volume of 0.5 cc/g. Thus prepared acid activated clay adsorbent is activated in a muffle furnace at 300 to 400°C under dry nitrogen flow for 2-4 hrs before loading in to the fixed adsorbent bed for purification of spent paraffinic solvent. The adsorbent bed is further activated at 250 °C under the flow of UHP nitrogen for 8 hours. The Bed is then cooled to ambient (25 to 30°C) temperature. The adsorber effluent is cooled to 5-10°C in the condenser. Spent solvent is passed through activated clay adsorber to remove aromatics, olefins and moisture.

The solvent thus purified can be utilized for polymerization reaction in which ethylene is treated at polymerization conditions in the presence of polymerization catalyst.

The following examples are given for purposes of illustrating the process of the present invention in which a spent saturated paraffinic solvent suitable for use as a medium in a polymerization reaction is purified to remove undesirable contaminants there from. However, these are merely representative examples and optimization details and are¾iot intended to restrict the scope of the present invention in any way. Example 1

50 gm of activated clay of the size of 1 to 1.5 mm granular sized is activated in furnace at 250°C under nitrogen atmosphere and is charged in a stainless steel tubular column of the dimension of 8 inch length and ½ inch internal diameter. The adsorbent is further regenerated in column to remove any air and moisture ingress during loading of the adsorbent in flowing nitrogen heated from near ambient temperature to 220°C at the heating rate of 2°C/minute then held at 220°C for another 2 hrs. The nitrogen pressure during regeneration was maintained at 2 psig while the nitrogen flow rate is varied between 60 to 120 ml/minute. Finally the regenerated activated clay adsorbent is cooled to ambient temperature with dry nitrogen in the tubular column under nitrogen atmosphere. Post nitrogen regeneration of adsorbent bed in column spent saturated paraffinic solvent having 600 ppm of aromatic content is fed in the column. During the adsorption cycle, adsorbent bed temperature is maintained at 30°C, liquid hourly space velocity (LHSV) of 1 hr ¹ (v/v hr) and pressure of 8 bar. Samples at the outlet of the column are collected at regular intervals.

The aromatic content in the feed and treated samples is determined by _* UV method with aromatics breakthrough curve as shown in Figure-3. 20 ppm of aromatics concentration is decided as breakthrough point.

Adsorbent prepared as disclosed in Example 1 can adsorb 0.65 wt% of the aromatics as estimated from adsorption breakthrough point. Moisture content is reduced to <10 ppm and Bromine Index <2 PPM.

Example 2

Example 1 is repeated except that instead of maintaining 30°C adsorbent bed temperature of 40°C is maintained. The aromatic content in the feed and treated samples is determined by UV method with aromatics breakthrough curve as shown in Figure-3. 20 ppm of aromatics concentration is decided as breakthrough point. Adsorbent prepared as in Example 1 can adsorb 0.35 wt% of the aromatics as estimated from adsorption breakthrough point. Moisture content is reduced to <8 ppm and Bromine Index <2 PPM.

Example 3

The process disclosed in Example 1 is repeated except that instead of maintaining 30°C adsorbent bed temperature, 50°C was maintained. The aromatic content in the leed and treated samples is determined by UV method with aromatics breakthrough curve as shown in Figure-3. 20 ppm of aromatics concentration is decided as breakthrough point. Adsorbent prepared as in Example 1 can adsorb 0.22 wt% of the aromatics as estimated from adsorption breakthrough point. Moisture content is reduced to <5 ppm and Bromine Index <2PPM.

Example 4

In a manner similar to that set forth in Example 1, 50 gm of zeolite 13X molecular sieve of the size of 2 to 3 mm spherical beads previously activated in furnace at 250°C under nitrogen atmosphere is charged in adsorption breakthrough setu (as explained in Example 1 ). During the adsorption-cycle, the spent saturated paraffinic solvent containing 600 ppm of aromatics sent to column at 120°C, liquid hourly space velocity (LHSV) of lhr ^'1 (v/v/hr) and pressure of 6 bar. Samples at the outlet of the column are collected at regular intervals.

The aromatic content in the feed and treated samples is determined by UV method with aromatics breakthrough curve as shown in Figure-4. 20 ppm of aromatics concentration is decided as breakthrough point. Moisture content is reduced to <2 ppm and Bromine Index <5ppm.

Adsorbent prepared as in Example 1 can adsorb 0.52 wt% of the aromatics as estimated from adsorption breakthrough point.

Example 5

This example illustrates the utility of solvent purified using activated clay in accordance with the method set forth in Example 1 to act as a solvent for a polymerization reaction. Purified solvent is tested for polymerization of ethylene is carried out in 1 L Buchi glasuster polyclave reactor which is heated at 75-105°C under N ₂ flow for about 3 - 4 hrs to remove oxygen and moisture followed by cooling to ambient temperature. Subsequently a calculated amount of THB black catalyst slurry containing 25% Ti ³⁺ dispersed in 500 ml of treated cyclohexane solvent containing the requisite quantity of co-catalyst (TIPRA) so as to maintain desired AI/Ti molar ratio's is added to the above conditioned reactor under N ₂ atmosphere. After boxing up the reactor, the occluded gas in the medium is vented out gently under agitation. After stopping the agitator, requisite quantity of hydrogen (minimum possible was 0.1 bar) is transferred to the reactor where molecular weight regulation is required - if not, this step is not performed. Ethylene is then introduced into the system at desired pressure unde! agitation (500 rpm). Ethylene pressure is maintained at 1.5 to 3 bars throughout the run (2 hr). Simultaneously the hot water circulation is started and the temperature is maintained at 75° C; thus controlling the exothermic nature of the polymerization. After two hours the residual ethylene is vented out the contents to ambient temperature. The polymer formed is in the form of uniform powder. The polymer is washed with acidic methanol, maehanol and acetone. The polymer is filtered and dried under vacuum at around 60 °C. The weight of polymer is recorded to calculate the productivity of the catalyst in terms of g polymer per g of catalyst and g polymer per mmole of Ti. The productivity is based on a 2 hrs period. The reactor is cleaned, boxed up and then baked under nitrogen for the next reaction.

Post clay treatment productivity of catalyst is improved to 131 gm from 21 gm without clay treatment at 30°C for UHMWPE at AI/Ti ratio of 4 and maintained pressure of ethylene and hydrogen 2.5 bar and 0.1 bar respectively. Fresh solvent yields 140g of UHMWPE polymer. Similarly HDPE yield increases to 182 gm at AI/Ti ratio of 11 and maintaining pressure of 2 bar and 0.5 bar for ethylene and hydrogen respectively with clay treated solvent at 30C compared to 29 gm without any treatment. Fresh solvent yields 190g of HDPE polymer.

Advantages of the present invention:

1. The present invention provides a simple and economical method of simultaneous removal of olefinic impurities, along with other impurities such as aromatic and moisture from spent polymerization solvent.

2. The method disclosed in the present invention uses activated clay as adsorbent to remove impurities without subjecting the spent polymerization solvent to any pre-treatment such as distillation/decantation etc.

3. Moisture removal ^"* in the method disclosed on the present invention is upto ppm level using acid activated clay and without resorting to decantation/distillation.

4. The method disclosed in the present invention removes multiple impurities such as aromatic, olefinic and moisture from the spent polymerization solvent under the same set of process conditions.