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Process for Reduction of the Phenylacetylene Contaminants In ~™ mi ii ii ii ii ii mi nil i ii i ii (19) J European Patent Office Office europeen des brevets (11) EP 0 479 753 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publicationation and mention (51) Int. CI.6: C07C 7/167, C07C 15/46 of the grant of the patent: 01.09.1999 Bulletin 1999/35 (21) Application number: 91870151.7 (22) Date of filing : 03.1 0.1 991 (54) Process for reduction of the phenylacetylene contaminants in styrene Verfahren zur Reduktion von Phenylacetylen-Verunreinigungen in Styrol Procede pour la reduction de phenylacetylene presente comme impurete dans le styrene (84) Designated Contracting States: • Kelly, Kevin P. BE DE FR GB IT NL Webster, Texas (US) (30) Priority: 04.10.1990 US 593730 (74) Representative: 04.10.1990 US 593826 Leyder, Francis et al 04.10.1990 US 593705 c/o Fina Research S.A. 04.10.1990 US 593723 Zone Industrielle C 04.10.1990 US 593706 7181 Feluy (BE) 04.10.1990 US 593729 (56) References cited: (43) Date of publication of application: DE-A- 2 155 400 FR-A- 1 513 123 08.04.1992 Bulletin 1992/15 FR-A-2 212 311 US-A- 4 704 492 (73) Proprietor: PATENT ABSTRACTS OF JAPAN vol. 1 1 , no. 268 FINA TECHNOLOGY, INC. (C-444)(2715) 29 August 1987, & JP-A-62 072633 Piano, Texas 75024 (US) (MITSUBISHI PETROCHEM.) 03 April 1987, PATENT ABSTRACTS OF JAPAN vol. 1 1 , no. 297 (72) Inventors: (C-448)(2744) 25 September 1987, & JP-A-62 • Butler, James R. 087534 (MITSUBISHI PETROCHEM.) 22 April Houston, Texas (US) 1987, CO CO LO r»- Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in o a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. Q_ 99(1) European Patent Convention). LU Printed by Xerox (UK) Business Services 2.16.7/3.6 1 EP 0 479 753 B1 2 Description lyst can be costly and causes the concentration of cata- lyst to be difficult to control. This, in turn, causes the [0001] The present invention relates to the field of molecular weight of the polystyrene to be difficult to con- monovinyl aromatic compound purification, and more trol and can result in an increase in the concentration of particularly discloses processes for reduction of the s low molecular weight polymer and even unreacted sty- phenylacteylene contaminants in crude styrene feed- rene in the polystyrene. stock. [0008] During free-redical polymerization, the pres- ence of phenylatetylene can have detrimental effects on BACKGROUND OF THE INVENTION chain length and polymerization rate, because it is a 10 poor chain transfer agent. Consequently, in the manu- [0002] Of all the thermoplastics manufactured today, facture of polystyrene beads, which are used to make probably the most versatile and most widely utilized expanded or "foamed" polystyrene, significant amounts class of materials is polymerized monovinyl aromatic of residual styrene are left in the bead. compounds, such as polystyrene, polymerized alpha- [0009] Styrene is a suspected carcinogen and creates methyl styrene, and polymers of ring-substituted sty- 15 undesirable taste, odor, and health hazards, when renes. present in small amounts in polystyrene. [0003] Some of the most common uses of these com- [0010] Thus, the presence of phenylacetylene in sty- pounds (often referred to collectively as "styrenes" or rene monomer has adverse effects on cost, control of "polystyrenes" are for food and beverage containers, the polymerization process, and purity of the resulting food wrap, and children's toys. One disadvantage asso- 20 polystyrene. The presence of phenylacetylene in poly- ciated with such uses of polystyrene is the residual styrene also results in olefinic bonds in the backbone of monomer and other contaminants in the polymer which the polymer which can increase cross-linking and cause may contribute to off-taste, odor, off-color and other more rapid oxidation of the polymer, both of which adulteration or degradation of the polymer. degrade the polymer. [0004] A particularly offensive contaminant associated 25 [001 1 ] In free-radical polymerization of styrene, as the with such undesirable properties in polystyrene is unre- concentration of styrene goes down during the polymer- acted vinyl aromatic monomer, usually styrene mono- ization process, the relative concentration of pheny- mer. One of the causes of unreacted monomer is lacetylene naturally increases, and, since directly related to the presence of phenylacetylene in phenylacetylene acts as a polymerization inhibitor, the the styrene feedstock going into the polymerization 30 polymerization process is undesirably affected. reactor system. [001 2] Catalytic attempts at reducing the phenylacety- [0005] In the manufacture of monovinyl aromatic pol- lene levels in styrene monomer streams have involved ymer compounds and more particularly in the manufac- the injection of high levels of hydrogen gas into the mon- ture of polystyrene, benzene is reacted with ethylene to omer in an attempt to reduce the phenylacetylene to form ethylbenzene. This molecular compound is then 35 styrene. See for example French patent 2,212,311, US dehydrogenated in an EB Dehydro unit to form styrene patent 4,704,492 and patent abstracts of Japan, vol. 1 1 , monomer. The styrene monomer is then polymerized, n°268 (1987) C-444, 2715. Any hydrogen added into usually in the presence of a polymerization initiator or the stream in stoichiometric excess of the phenylacety- catalyst, to form the final polystyrene raw material. lene present also resulted in a significant conversion of [0006] Unfortunately, phenylacetylene, one of the 40 styrene back to ethylbenzene, causing a lower styrene undesirable side products of the EB Dehydro unit, is concentration and a lower conversion rate. Significant formed when ethylbenzene is dehydrogenated one step reductions in phenylacetylene were achieved only at the too far. Consequently, the product stream from the expense of styrene conversion to EB and resultant loss Dehydro unit contains styrene, ethylbenzene, and of styrene production. traces of phenylacetylene. The ethylbenzene easily is 45 removed by conventional process, such as distillation, SUMMARY OF THE INVENTION leaving styrene monomer and phenylacetylene. The removal of phenylacetylene cannot be accomplished by [0013] The present invention discloses processes distillation and has heretofore been difficult and costly. which achieve a reduction in the phenylacetylene levels [0007] The presence of phenylacetylene in styrene so of monovinyl aromatic monomer feedstock in polymeri- monomer has undesirable consequences regardless of zation systems by the use of either a two-bed reactor to whether the method of polymerization utilized com- replace the single-bed reactor, or a pair of catalyst reac- prises anionic, or free-radical polymerization. During tors, each bed or reactor having injection means for anionic polymerization, phenylacetylene which is injecting a mixture of hydrogen gas and carbon monox- slightly acidic, consumes a stoichiometric amount of 55 ide into the monomer reaction stream to reduce pheny- catalyst, such as butyllithium, wherein one molecule of lacetylene to styrene. butyllithium is removed from the polymerization process by each molecule of phenylacetylene. This loss of cata- 2 3 EP 0 479 753 B1 4 BRIEF DESCRIPTION OF THE DRAWINGS system. In Figure 2, crude styrene flow line F1 leading from the crude styrene tank CST flows through the vent [0014] gas styrene heat exchanger VGS and is controlled by means of flow control valve V1 in line F1 . From there Figure 1 comprises a simplified general schematic s crude styrene flows up through a static mixer SM1 and diagram of a typical monovinyl aromatic polymeri- into the first reactor vessel R1 containing a catalyst bed zation process utilizing the present invention. C1 therein. Catalyst bed C1 may be of any type known Figure 2 is a partial schematic diagram of a two- in the art such as a fixed bed and preferably contains a reactor phenylacetylene reduction (PAR) system. spherical or lobed catalyst made up of about 0.3% pal- Figure 3 is a partial schematic diagram of a two-bed 10 ladium on an alumina carrier. In this embodiment the single reactor PAR system. reactor vessel is preferably a liquid-full, upflow catalyst reactor. DESCRIPTION OF THE PREFERRED EMBODI- [001 9] The flow of crude styrene through the vent gas MENTS styrene heat exchanger serves two purposes, the first 15 being that the styrene crude is brought up to a temper- [0015] Referring first to Figure 1, this is a simplified ature of approximately 65.56°C (150°F) sufficient to ini- schematic flow diagram representing a styrene purifica- tiate the phenylacetylene reduction process. The tion and polymerization process. In Figure 1, styrene second purpose is to provide an optional vent gas sup- monomer which has been created from the dehydro- ply to reactor R1 and reactor R2 to serve as a possible genation of ethylbenzene is provided at valve V1 from 20 hydrogen source for hydrogen injection. where it flows into the crude styrene storage tank CST. [0020] A source of hydrogen designated respectively From the storage tank crude styrene flows from flow line HS1 and HS2 is provided for each reactor R1 and R2 F1 through a vent gas heat exchanger VGHE to raise and is controlled by valves V2 and V3. Alternatively, a the temperature of the styrene, and from there into an single hydrogen source may be utilized such as a hydro- optional preheater PH. From the preheater the crude 25 gen supply pipeline, railroad or truck tank cars of hydro- styrene passes into the phenylacetylene reduction sys- gen or even bottled hydrogen.
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