USRE27432E - Solvent cleaning system - Google Patents

Abstract

PROCESS FOR REMOVING POLYMERIC MATERIALS FROM PROCESSING EQUIPMENT INVOLVING CONTACTING SUCH MATERIALS WITH A TETRAHYDROFURAN SOLVENT WHILE AGITATING SO AS TO

EFFECT SOLUTION OF THE POLYMERIC MATERIALS IN THE SOLVENT.

Description

BEST AVAILABLE COPY July l1, 1972 A. P. T oRRl-:NANo 'ETAL Re. 27,432

SOLVENT CLEANING SYSTEM Uriginal Filed Nov. 8. 1965 ARCH/E F? TOEEENZANO s nw? E@ Cm M WN am mm 74T .1. .N m A H Y e e July 11, 1972 riginal 'med Nov. a. 1965 PoLYv/A/YLcHLoe/DE DISSOLU T/oA/ EA TE, L15/HE. FT a BEST AVAILABLE COPY A. P. ToRRENzANo ET AL Re 27.432

soLvENT CLEANING SYSTEM s sneetsgsheeliz EFF-'ECT 0F CoNTAM/NANT CONCENTEA TION /N THE SoU/ENT ON PVC HOMoPoLYMe-E D/ssoLU-r/o/v EA TE JS/N6' A 95 PEE CENT soLuv-/QN or: TsTPAHYDeoF-UPAN, AND wA-rse AT Soft:

c a 4 Y c; 8 lo la /4 CQNCENTEAT/ON oF PoLYv/N YLcHLoe/DE N THE SLVENY; GR//OO @E SOLA/ENT RECH/E F? TOEENZQNO i BMU-w Fie. a

A BEST AVAILABLE COPY A. P. 'roRRENzANo ET AL July ll, 1972 s'oLvEN'r CLEANING SYSTEM original Fired Nov. a, 1965 MAX/MUM SOLDE/UTY of: PVC: EES/Ns /N TETEAHYDEO PUPA/v( THF) WA TEE Mix 'fuese Les END (l) Low/v/OLE'IIL/Lf/ea WEIGHT Pvc HoMoPoLYMER (a) His/l lMcm/.E(iL/,611? WEIGHT Pvc: HoMoPoLY/wae (3) v/Nvz. cHLoe/DE WNYL AcETAv-e coPoLYME/e (4) GEAFT coPoLYMEe oF' VIN YL. cHLoe/DE oN To cHLomNATf-:D Pours THYLENE' vom gmac 42,0

.lIh WTQ@ GQNCO@ WQ@ PLLazmDQOW U l ZDQQ THF CONCEN TPATION) GPAMS THF/GERM THF' 'fh/ATE@ AEC/41E PvToeEENZAA/o EICHHHED TT 'MNCINCE F' G I l fNvEm-oes Re. 27,432 Reissued July l1, 1972 Matter enclosed in heavy brackets [l appears in the original patent but forms no part of this reisme specilication; matter printed in ltall indicates the additions made by reissue.

ABSTRACT oF 'ma DISCLOSURE Process for removing polymeric materials from processing equipment involving contacting such materials with a tetrahydrofuran solvent while agitating so as to effect solution of the polymeric materials in the solvent.

This invention relates to a novel system for the removal of residual polymeric contaminants, from vinyl halide polymerization equipment. More specifically, this invention is directed to a solvent system for the removal of accumulated polymeric contaminants in polymerization reactors, reflux condensers and analogous processing equipment associated therewith.

The chemical industry has long been plagued with continuing build-up of residual polymeric materials in polymerization reactors and associated equipment after the removal of the charge from the reactors. These residual contaminants produce problems in quality and overall production. The quality problems materialize when contaminants ake oil the side of the equipment surfaces and become intermixed with a present polymerization charge whereby the final polymerized product quality is inferior due to the oil-grade contaminant material. Additionally, the buildup of residual contaminants in the process equipment reduces heat transfer capability, process control and emergency venting capability. Hence, frequent equipment cleaning is necessary, thus leading to reduced operating etliciency and overall production.

Historically, this type of equipment has been cleaned manually by removing the reactor from service and sending a cleaning crew into the reactors with a jackhammer or the like to manually chip the residual polymeric contaminants from the inner surfaces of the vessels. Such procedure is quite undesirable for the following reasons:

First, manual chipping is excessively time consuming and causes further decreases in production capacity; ergo, a loss of profits. Secondly, additional manpower is required for chipping thereby adding to overhead and pro ducing a still further reduction in profits. Thirdly, the use of such an archaic cleaning method always leads to the possibility of defects on the inner surfaces of the reactors, and these defects, by reducing the smoothness 'of the reactor surfaces, compound the problem of residual polymer build up.

Various attempt have been made to resolve the aforedescribed problem of polymeric build up in the polymerization equipment. One such attempt is described in U.S. Patent 3,070,548, to Brooke. Brooke denes a system for the removal of polymeric materials from processing equipment, especially in the petroleum retining field. Lower' diolen hydrocarbons and vinyl compounds are cited as typical examples of polymers that leave residual build up which he removes with a solution of methylene dichloride and kerosine. The methylene dichloride-kerosine solution does not dissolve the polymer to a greater extent, but loosens the bond at the equipment surface whereby the contaminants fall oil and/or are easily removed vbymechanical means.

A further attempt for the removal of polymer build up is taught in the Thompson U.S. Patent 2,880,121. Thompson directs his efforts to removal of-polypropylene residue by oxidizing the polypropylene and then dissolving the oxidized polypropylene in an organic solvent having a boiling point of over 200 F.

None of the aforedescribed procedures for polymeric build up removal have been foundsatisfactory. It is therefore an object of this invention to provide a suc cesstul and feasible means for removal of residual pol- Vymeric materials from polymerization vessels.

A major object of the present invention is to provide a method for removing polymeric build up from vinyl halide polymerization vessels.

A further object of this invention is the removal of polymeric build up in vinyl chloride reaction vessels by contacting said polymeric build up with a solvent, removingl the solvent contaminant solution from the reactor, repurifying the solvent and recycling for further cleaning use.

A further object of this invention is to provide a commercially feasible and economical solvent cleaning process for vinyl halide polymerization vessels which employs as the reusable solvent system a tetrahydrofuran/ water mixiure.

In the drawings, FIGURE l is a graphical representation of the effect of agitator speed on the dissolution rate of various contaminants when subjected to a tetrahydrofuran solvent.

FIGURE 2 is a graphical representation of theeifect of contaminant concentration in the solvent on polyvinyl chloride homopolymer dissolution rate using a 95% solution of tetrahydrofuran and water at- 50 C.

FIGURE 3 is a graphical representation of maximum solubility of various vinyl chloride resins in several different tetrahydrofuran-water solutions.

The present invention generally involves filling a vinyl halide reaction` vessel and/or retlux condenser with a tetrahydrofuran solvent solution; holding the solution in the vessel for a time sufficient to dissolve polymer build up on the inner walls of the reactor while subjected to continued agitation; discharging the solvent containing dissolved contaminants from the reactor; subjecting said solvent to distillation to remove the dissolved contaminants, and recycling the solvent to the cleaning system.

As is apparent in the following descrz'ptlom'THF conrains .up ro about l0 percent water without seriously irn pairing the capacity of the product solvent system to dissolve vinyl halide polymer when practicing the teachings of the present invention. Also, such a solvent system can advantageously bc-employed to clean vnyl halide polym erzaron equipment by applying such a solvent system to such equipment while simultaneously subjecting such solvent system to agitation and heat. Further, the resulting solution of vinyl halide polymer in such solvent system can be readily distilled so as to separate excess water and dissolved vinyl halide polymer and provide the sorepurz` fied tetrahydrofuran solvent system for re-utse in a subsequent solvent cleaning cycle.

Tetrahydrofuran is a known solvent for vinyl halide polymers and copolymers and especially for vinyl chloride polymers and copolymers of vinyl chloride and other monomer copolymerizable therewith. While the group of monomers copolymerizable with vinyl chloride is not so specifically limited, vinyl acetate, vinyl laurate, alkyl acrylates, alkyl methacrylates, alkyl maleates, alkyl fumarates, vinylidene chloride, acrylonitrile, vinyl cetyl ether, vinyl lauryl ether, vinyl myristyl ether, are suitable.v Also, however, the copolymers may be a graft copolymer in which case the one of the starting constituents will be polymeric and the other monomeric. Exemplary are polyethylene, chlorinated polyethylene, chlorosulfonated polyethylene, copolymers of ethylene and propylene, copolymers of ethylene and vinyl acetate and others too numerous to mention. The selection of tetrahydrofuran as a solvent for the present invention was not, however, attributed to the knowledge of the prior art. Numerous solvents are known for dissolving vinyl halides such as for example, trimethylene oxide, tetramethyl urea, dimethyl acetamide, tetrahydrofuran, cyclohexanone, cyclopentanone, cyclohexene oxide, diethyl acetamide, diethylformamide, dimethyl formamide, tetrahydropyran, ethylene oxide, methyl ethyl ketone, dioctyl phthalate, dioxane and many others. These solvents vary, however, in dissolution strength with the molecular weight of the compound to be dissolved. By virtue of the use of tetrahydrofuran, the present invention is successful for removing both homopolymer and copolymer contaminants having low or high molecular weights. All the solvents listed were not tested for various reasons, e.g. availability, cost, ease of handling, recovery of the solvent, etc. Particular solvents that were tested include dioctyl phthalate, methyl ethyl ketone, cyclohexanone, dimethyl formamide, dimethyl acetamide, tetrahydrofuran, dioxane and combinations thereof. Among these solvents, tetrahydrofuran, methyl ethyl ketone and cyclohexanone were considered superior with tetrahydrouran by far being the superior of these three.

The present invention is accomplished by pumping a tetrahydrofuran solvent into [the] a polymerization reactor. The tetrahydrofuran must, of necessity, be of at least 90% purity (with approximately 10% water) and prior to its ingress into the reactor is preferably preheated to approximately 80-100 C. A suicient amount of solvent .is pumped into the reactor to completely fill the reactor and when a reflux condenser is associated therewith to fill the condenser to the point where the solvent level just covers the top tube sheet. The solvent is then agitated to attain a turbulent flow within the reactor for a period of from one and one-half to two and one-half hours and preferably two hours. When a reux condenser is employed, the solvent solution is continuously recycled through the reactor and condenser during the cleaning operation. During this stewing or agitation period, the polymeric contaminants within the polymerization reactor are dissolved by the tetrahydrofuran soluton. The tetrahydrofuran-contaminant solution is then transferred to a recovery operation wherein the contaminants are removed from the solvent and the solvent is readied for additional cleaning use. In general, repurication, of the solvent conraining dissolved vinyl halide polymeric materials may be accomplished by a variety of conventional distillation procedures. Prior to solvent recovery, however, a single charge of solvent generally has sufficient dissolution potency for two or even more cleaning operations.

In solvent recovery, a problem confronted was the distillation of sufficiently pure tetrahydrofuran suitable for further cleaning and precipitation of polymer. The distillation [is] may be conducted under vacuum and [is] may be carried out in two steps. [A] For example, when a ash distillation is the tirst step, [and] approximately 75% of the tetrahydrofuran from the contaminant containing solution is recovered. The overhead product is an 94-96% azeotrope of tetrahydrofuran and water.

Live steam [is] is then directly injected into the remaining contaminant containing solution. During the steam injection, a point is attained where sutiicient water is mixed with the solvent solution to precipitate the polymeric constituents dissolved therein. Continued distillation recovers substantially all the remaining tetraliydrofuran, which is combined with the tetrahydrofuran porooo . 4 tion removed during the flash distillation operation. By this process, [The] the combined purity of recovered solvent is typically i12-94%.V Residual materials in the still,

i.e. water, a very minor amount of tetrahydrofuran'and precipitated contaminants are discarded to waste.

Other suitable repurificalion procedures include total recycle (or a side stream thereof) of a polymer rich solvent solulion to a distillation column to recover overhead a THF-water azeorrope (containing a water content corresponding to that obtained in conventional distillation equipment operated in a conventional manner).

In the above description of the process of the present invention, several factors are critical. The three most important factors are tetmhydrofuran-water solution concentration, agitation, and dissolution rate of contaminants. The degree of agitation of the 'solvent during the solution of contaminants from within the processing equipment greatly inuences the rate at which the contaminants are dissolved by the solvent. It has been determined that the critical point about which the efficiency of dissolution revolves is the transition point from laminar to turbulent flow. Very interestingly, over a range of temperatures from 20 C. to 50 C. the dissolution rate reported as pounds/hour/sq. ft. increased almost exponentially immediately upon attaining turbulent ow of solvent within the procesing equipment. This eiect is reported pictorially in FIGURE 1, and is set forth in more detail in the following example.

EXAMPLE I Experiments were conducted on a laboratory scale to correlate the effect of agitation on the dissolution rate on polyvinyl chloride homopolymer and copolymer. A solvent consisting of a tetrahydrofuran-water solution was agitated in a 2000 milliliter beaker by means of a variable speed stirrer with (l) a 2", 3blade marine impeller and (2) a 2", 2paddle agitator. In conducting the experiments to determine the various dissolution rates, both agitator speed and solvent temperature were varied. Results of these experiments are reported numerically in Tables I, 2 and 3 and plotted .graphically on FIGURE 1. Broken lines on the graph indicate extrapolation back to the vicinity of the beginning of turbulent tiow within the reaction vessel.

TABLE L EFFECT OF BOLVENT AGITATIO l B-BLADE MARINE IMPELLER AGIIATOR INwlDTllSSLl':

TION RATE 0F PVC-VINYL CONTAMINANTS l ACETATE COPOLYMER Sol'yent Dissolution m ra ure,

Agltator speed, r.p.m. pe C. lbafhnrlll' Solvent Dissolution tem erature rat Agitator speed, r.p.m. p C.' lbs/hallal;

TABLE 3.-EFFECT OF SOLVENT AGITATION WiTH A 2",

3-BLADE MARNE IMPELLER AGITATOR ON DISSOLU- TION RATE OF POLYVINYLCHLORIDE CHLORINATED POLYETHYLENE GRAFT COPOLYMER CONTAMINANTS Solvent Dissolution temperatura, ra

.agita-tor speed, r.p.m. C. lbs./hr./it.I

20 Negligible 20 Negligible The second important variable is contaminant concentration in the solvent. From the general mass transfer equation,

M=K(AC) where :mass iiux, mass per unit time per unit area AC=concentration difference, mass per unit volume K=overall mass transfer coeiiicient, length per unit time it is expected that an increase of the contaminant concentration in the solvent would decrease the dissolution that up to a maximum of l0 percent water dilution did not appreciably atleet the-maximum solubility of vinyl chlo ride polymers in the tetrahydrofuran solution.

EXAMPLE III Various vinyl chloride polymers, as set forth in Table 5, were dissolved in pure tetrahydrofuran until a. grainy structure appeared throughout the solution. This point represents maximum solubility of the polymer in tetrahydrofuran. Equal portions of this saturated solution were then titrated with water to various tetrahydrofuran-water concentrations. The addition of water precipitated polymer and the precipitated polymer was removed, weighed and deducted from the dissolved polymer in the pure solvent to provide the maximum polymer solubility in the various concentration tetrahydrofuran. solutions. Results for low molecular weight polyvinyl chloride homopolymer, high molecular weight polyvinyl chloride homopolymer, a copolymer of vinyl chloride and vinyl acetate, and a copolymer of vinyl chloride grafted onto chlorinated polyethylene are numerically reported in Table 5 and pictorially represented in FIGURE 3.

TABLE 5.-SOLUBILITY OF VARIOUS VINYL CHLO RIDE RESINS IN TET- RAHYD ROFURAN-WATER SOLVENT rate of contaminant. This decrease is attributable to a lowering of the molecular diiusivity and an increase in the solvent viscosity. The act-ual effects of contaminant concentration were discerned as per the procedures set forth in Example II.

EXAMPLE Il RATE AT 150 C. SOLVENT TEMPE RATURE AND 850 R.P.M. AGITATOR SPEED PVC hornopolyrner dissolution rate, lbs./hr./1't.2

Concentration o1 PVC in solvent,

gm./100 gm. solvent:l

This third important variable is the concentration of the tetrahydrofuran-water solution and its eiiect on maximum solubility of the polymeric contaminants. Normally, the addition of water to a solvent decreases the maximum solubility of materials therein directly. It has been discovered, however, that the water addition to tetrahydrofuran does not so affect the maximum solubility of vinyl chloride polymers directly. Instead as set forth in the following example and FIGURE 3, it was surprising to nd Solvent temperature must likewise be considered but only from the standpoint of economics. An increase in solvent tem-perature increases the dissolution rate but creates problems both in handling and solvent recovery. It is, therefore, necessary that the solvent temperature be optimized to take advantage of the dissolution rate while simultaneously affording case of handling and processability of the solvent and repurication. An optimum range of solvent temperature has been found to be from -100" C. The present invention is practiced by malntanng elevated solvent temperatures below abou! C., and by simultaneously employing agitation sucievnr to achieve turbulent flow 0f solvent in processing equipment, during a cleaning operation.

The teachings of Example Il and FIGURE 2, and of Example III and FIGURE 3 illustrate the significance of maintaining a control both on :he amount of vinyl halide polymer solute materials dissolved in the tetrahydrofuran/ water cleaning solvent and on the wafer content of such cleaning solvent. Water build up in the tetrahydrofuran cleaning solvent inherently takes place since polymcrzalion of vinyl halide monomers in reactors is commonly accomplished under aqueous liquid phase conditions, so that residual, though small, amounts of water are commonly present in vinyl chloride polymerization equipment a! the beginning of a solvent cleaning operation n accordance with this invention. Consequently, although one can commence a first solvent cleaning and recovery cycle of this invention using anhydrous or (conveniently) commercially available tetrahydrofuran (the latter contains less than about V2 percent water), the water content of the solvent cleaning solution builds up during repeated cleaning and recovery cycles. When, during a solvent recovery operation by distillation n accordance with the teaching of this invention, the water content of the cleaning solvent is reduced below about 10 percent to as low as thewafer content of a tetralzydrofuran/water azeotrope depending upon the conditions used in distillation (preferably under vacuum), a solvent system becomes reusable. Dissolved vinyl halide polymer is readily removed from a cleaning solvent during a solvent recovery operation in accordance with the teachings of this invention and so is easily mantained under percent.

The present invention has alleviated the disadvantages known heretofore and is specifically described in the following example.

EXAMPLE IV A 94% tetrahydrofuran-water solution was pumped from storage into a previously vacuum-purged polyvinyl chloride reaction kettle. The solvent prior to entry into the reactor is ltered and passed through a solvent heat exchanger where it was heated to a temperature of 90 C. The solvent Was pumped into the reactor via a spray nozzle located in the dome of a reux condenser. Suflicient solvent was pumped in until the reactor was completely -filled and the solvent level in the redux condenser was above the upper tube sheet. Shortly after the start of pumping of solvent into the reactor, the reactor agitator was started and was operated at speeds suicient to provide turbulent flow withinthe reactor. While agitation continued, the solvent was continuously recycled through the reactor and relluxv condenser for a period of two hours during which time the polyvinyl chloride contaminants were dissolved.

At the end of the two-hour period, the solvent having the polyvinyl chloride contaminants dissolved therein was discharged from the reactor and passed through a series of two heat exchangers which respectively reduced the temperature of the 'solvent to 38 and 24 C. As the solvent was being discharged from the reactor, low pressure nitrogen was added to the reactor to maintain a slight positive pressure. Following complete emptying of the reactor, the reactor surfaces were sprayed with cold uncontaminated tetrahydrofuran solvent to wash down residual contaminants left on the reactor walls. A tine mist of water was then sprayed into the reactor to collect any residual solvent vapor. All of the solvents previously discharged from the reactor were returned to storage and were reused for a second cleaning operation.

After the second cleaning operation, which was carried out identically with the one previously described, the discharged solvent was transported to solvent recovery for repurilication ofthe tetrahydrofuran. In the solvent recovery, the tetrahydrofuran-water-polyvinyl chloride contarninants were preheated to 160 C. prior to entry into the still. As the heated tetrahydrofuran-water-polyvinyl chloride contaminant solution entered the still via a passed pressure reducing valve, a large portion of the tetra'hydrofuran ashed off as a tetrahydrofuran-water azeotrope. In fact, 75% of the tetrahydrofuran (SM-96% purity) was removed at this point as the azeotrope. The remaining solution collected in the bottom of the still. Overhead vapors from the continuous llash distillation were condensed in a vertil barometric condenser and were transported to a rinse solvent storage tank. The remaining solution in the still was then stripped by means of direct steam injection and the overhead vapors from the strip distillation (approximately 85% terahydrofuran) were condensed and transferred to solvent storage where the combined product was mixed with the overhead product from the ash distillation to produce a reuseable tetrahydrofuran solution of approximately 90% purity. Subsequent to the steam stripping, cold water was added to the still which cooled down the remaining water-polymer slurry. This slurry of water was precipitated polymer left in the still was then subjected to a separation process whereby the water was separated from the polymer contaminants and both constituents were discarded as waste.

lt is envisioned that various modifications will readily occur to those skilled in the art which will be well within the scope of the present invention as defined in. the appended claims.

What is claimed is:

furan and up to about vl0 percent water during continued agitation until said polymeric materials are dissolved in said solvent; discharging the solvent containing said dissolved polymeric materials from said processing equipment; and repurifying said tetrahydrofuran solvent] [2. The process of claim 1 wherein repuricaton of the tetrahydrofuran solvent is accomplished by means of vacuum distillation] [3. A process for removing vinyl halide residual polymeric materials from the internal surfaces of polymerization processing equipment comprising substantially filling said processing equipment with a solvent solution comprsing on a 100 weight percent basis at least 90 percent tetrahydrofuran and up to about -10 percent water; agitating said solvent solution in the range of turbulent ow for a period suicient to dissolve the said residual polymeric contaminants; discharging the said solvent solution having the said dissolved polymeric'materials contained therein from said processing equipment; and subjecting said discharged solvent solution to vacuum distillation whereby said 'solvent solution is repuried to contain at least 90 percent tetrahydrofuran] [4. A process for cleaning internal surfaces of vinyl halide polymerization reactor vessels comprising substantially filling the reaction vessels with a solvent solution comprising on a 100 weight percent basis at least 90 percent tetrahydrofuran and up to about 10 percent Water; agitating said solvent in the range of turbulent ow and continuing said agitation for a period of approximately 2 hours while maintaining said solvent solution at a temperature in the range of from S50-100 C., whereby residual vinyl halide polymeric materials from the inner surfaces of the reaction vessels are dissolved in the tctrahydrofuran solvent; emptying said reaction vessel; distilling the solution emptied from the reaction vessel by preheating said solution; ilash distilling a portion of said solution and strip distilling the remaining portion of said solution by a direct steam injection method, whereby overhead vapors from said liash and said strip distillation steps contain at least percent tetrahydrofuran] 5. A process which employs repurifiable solvent for removing vinyl halide polymeric materials from the interna! surfaces of polymerization processing equipment comprising a: plurality of reactors and equipment operatively associated therewith for the polymerization of vinyl halide monomer, said process comprising:

(A) sequentially firs! performing at leas! once the (I) substantially completely filling one of said reactors with a solvent comprising on a weight percent basis at least 90 weight percent tetrahydrofuran and ilp to about 10 percent water,

(2) agitating said solvent in said one reactor in the range of turbulent flow at an elevated tem perature below about 100 C.,

(3) discharging thc resulting solvent mixture from said one reactor (B) and, then after so sequentially performing the steps in A, at least once, thereafter recovering a solvent suitable for reuse in the sequence of steps in A by dstlling to reduce the water content of said resulting solvent on a 100 weight percent solvent basis to a range of from about 10 weight percent down to that of an azeotrope mixture of water and tetrahydrofuran and to remove substantially all of said dissolved polymeric materials therefrom.

i 9 6. The process of claim 5 wherein the .solvent charged into said processing equipment is a so recovered solvent.

The following references, cited by the Examiner, are 5 References Cited of record in the patented le of this patent or the original patent.

UNITED STATES PATENTS Rieche etal 252-364 X =Po11y et al. 252-364 X Parry et al 134-38 .X Williams'et al. 134-38 X Grantham 252-364` X l0 3,189,553 6/1965 Lange `134--31 3,321,407 5/1967 Rosenfeld 134-38 3,354,093 11/1967 Early et a1. 252/-364 OTHER REFERENCES JOSEPH lSCOVROlJEK, Primary Examiner U.S. Cl. XR.

134-10, 22, 34, 38; 252-364, Digest 8

Images