STYRENE POLYMERIZATION INHIBITOR

Abstract: Compositions useful for inhibiting polymerization of monomers comprising a solvent miscible in the feedstock for the monomer and miscible in the monomer and a polymerization inhibitor. Processes for producing styrene in which a composition of para-nitrosophenol in n-methyl pyrollidone is used to inhibit polymerization.

Description: STYRENE POLYMERIZATION INHIBITOR BACKGROUND OF THE INVENTION The field of this invention is in the inhibition of the polymerization of styrene during the manufacture of the monomer. Styrene production and chemistry are treated in the Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, New York 1982 (Vol. 21) and numerous other publications. During the production of styrene, inhibitors such as sulfur, dinitrσphenols, and p-tert-butylcatechol have been used in the past as inhibitors. The chemistry of styrene is well-known as one of the earliest of the modern thermoplastics and still one of the largest in volume in production, including many copolymers and polymer blends, such as SBR rubber and acrylonitrile-butadiene-styrene (ABS) . Paranitrosophenol (PNP) and dinitro-orthocresol (DNOC) are currently known polymerization inhibitors, but PNP is a solid and DNOC is highly toxic, which has limited their utility. SUMMARY OF THE INVENTION A solution of PNP in the solvent n-methylpyrollidone (NMP) is a highly effective system for inhibiting the polymerization of styrene and its analogs and for general use in inhibiting polymerization of vinyl and other addition polymerizable monomers during their production from the various feedstocks. PNP in solution in NMP is miscible with both the feedstocks and the monomers produced within certain limits. DESCRIPTION OF THE PREFERRED EMBODIMENT A solution of paranitrosophenol in the solvent n-methylpyrollidone is used as a polymerization inhibitor in the production of styrene monomer. Paranitrosophenol is a well known polymerization inhibitor, but it is a solid not soluble in either ethylbenzene or ethylene, which are the usual feed stocks to the styrene process. However, we have found that, by using sufficient quantities of n-methylpyrollidone as a co-solvent, the solubility of the PNP can be increased to a point where it is a commercially viable inhibitor. Hence, both components of the product, the PNP and the solvent, are vital to commercial success. We have found other solvents which may be used, in general these are aprotic very powerful solvents such as dimethyl formamide, tetrahydrofuran, dimethyl sulfoxide and the like, however, none has been as effective in all ways as NMP because of vapor pressures, corrosiveness, or other associated difficulties. Styrene monomer is made from two basic building blocks, benzene and ethylene. Benzene and ethylene are alkylated using an aluminum based catalyst to form ethylbenzene. Ethylbenzene is then dehydrogenated, removing two hydrogen atoms from the ethyl group attached to the benzene ring, by passing it over a metal-oxide catalyst, usually iron oxide, with steam. The result is a mixture of styrene, ethylbenzene, benzene, and toluene. Polystyrene is also a byproduct because styrene is very reactive and styrene molecules will readily form polystyrene if given a chance, though operating conditions in the process are set to minimize this reaction. The styrene conversion rate is about 60% and the yield is about 90%. An accumulator is then used to take out water, then sequential distillation separates the ethylbenzene, benzene, and toluene from the pure styrene monomer. Divinylbenzene, chlorostyrene, butadiene, isoprene, acrylic acid, methacrylic acid, and vinyl chloride monomers can also be inhibited by this system. Too much polystyrene will cause fouling of the equipment and result in an inefficient process. It is standard practice throughout the styrene industry to add a chemical inhibitor of some kind to help control polymerization. Dinitro-orthocresol works very well, but is extremely toxic. The general trend in the styrene industry is to find alternative inhibitors to DNOC, and that is precisely why our invention was developed. The most widely used polymerization inhibitor in the industry today is DNOC although analogs of this compound are also sometimes used under the same name. The problem with the use of DNOC and its analogs is extreme toxicity. All data are not yet available, however, preliminary studies indicate that PNP is some 20 times less toxic. PNP was developed as a less toxic replacement for DNOC and this has proven to be a strong impetus for change of inhibitors in the marketplace. The general system may be used with a large number of monomers in which polymerization inhibitors, in particularly substituted phenolic compounds, are of low solubility. In this connection it must be pointed out that no materials are completely insoluble in solvents, but only that their solubilities are so low as to be of no utility in that particular application under the normal process parameters involved. In addition to being less toxic than DNOC, PNP has proven to be a more effective polymerization inhibitor on a weight for weight basis. Some results from reflux tests are as follows: INHIBITOR PERCENT POLYMER DNOC at 100 ppm 0.858 PNP at 100 ppm 0.0605 DNOC at 100 ppm + Na 2.452 PNP at 100 ppm + Na 0.0451 PNP at 50 ppm 0.240 PNP at 25 ppm 0.981 A sample of the monomer is washed with NaOH and water to remove any phenolic inhibitors, such as tert-butylcatechol, placed in distillation flask under nitrogen and distilled until polymer starts to form. The polymer is precipitated out of solution with methanol, dried and weighed. The above data shows that PNP is far more effective at equal dosage and almost as effective at one-fourth the dosage of DNOC and field testing has shown even greater effectiveness. A recent trial at the sytrene unit in a commerical plant showed the PNP/NMP solution to be about 10 times more effective than DNOC in polymer reduction at one-fifth the dosage. The table below shows dosage in ppm of the actives in both DNOC and PNP versus resulting polymer levels in ppm. The study was done on two different stops in the styrene process (the Recycle Column and the Continuous Column). Mean polymer levels in the Recycle Column are 4,000 ppm with 500 ppm of DNOC inhibitor actives. Mean polymer levels in the Recycle Column are 455 ppm with only 125 ppm of PNP actives, hence, 10 times more effectiveness with one-fifth the dosage. Fig. 1 illustrates polymer levels in the Recycle Column during the course of the trial. There is a dramatic drop in the polymer level after the start of feeding PNP. RECYCLE COLUMN Inhibitor Rate Dosage Polymer Polymer Range DNOC 465 ml/min. 500 ppm 4, ,000 ppm 3,500-5000 ppm PNP 80 100 725 595-850 PNP 100 125 455 400-510 PNP 130-135 160-170 415 310-515 PNP 170 210 390 240-535 CONTINUOUS COLUMN Inhibitor Rate Polymer Polymer Range DNOC 465 ml/min 46,000 ppm 39,000-55,OOOppm PNP 80 9,700 8,330-11,050 PNP 100 7,550 6,660-8,435 PNP 130- 135 9,730 8,150-11,300 PNP 170 7,750 5,860-9,640

Claims: CLAIMS 1. A polymerization inhibitor system for monomers comprising a solution of a polymerization inhibitor in a solvent which is miscible with the feedstock for the monomer and the monomer. 2. The polymerization inhibitor system of claim 1 wherein the polymeri¬ zation inhibitor comprises a substituted phenolic compound. 3. The polymerization inhibitor system of claim 1 wherein the polymeri¬ zation inhibitor comprises paranitrosophenol. 4. The polymerization inhibitor system of claim 1 in which the solvent is selected from the group of solvents consisting of n-methyl pyrollidone, dimethyl formamide, tetrahydrofuran and dimethyl sulfoxide. 5. The polymerization inhibitor system of claim 1 wherein the solvent is n-methyl pyrollidone. 6. The polymerization inhibitor system of claim 1 wherein from 25 to 210 parts of paranitrosophenol are used to 1 million parts of monomer. 7. The polymerization inhibitor system of claim 1 wherein the monomers are selected from the group consisting of styrene, divinyl benzene, chlorostyrene, butadiene, isoprene, vinyl chloride, acrylic acid, and methacrylic acid. 8. The polymerization inhibitor system of claim 1 wherein the monomer is styrene. 9. The process of producing styrene comprising the use of a solution of paranitrosophenol in n-methyl pyrollidone as a polymerization inhibitor during said process.

Inventors: NICHOLSON GERALD M, PAWLOSKI JOHN N, STAFFORD MICHAEL L, NICHOLSON, GERALD, M., PAWLOSKI, JOHN, N., STAFFORD, MICHAEL, L.
Applicants: PONY IND INC, PONY INDUSTRIES, INC.
Application year: 1989
Main classification: C09K15
IPC codes: C07C7, C07C15, C07C17, C09K15