Polymer grade

Fig. 4. The Amoco purification process for polymer-grade terephthalic acid.

Hydrolysis of Dimethyl Terephthalate. Hoechst Celanese and Eormosa Chemical Eibers Corp. produce a polymer-grade terephthahc acid by hydrolysis of high purity dimethyl terephthalate. Hbls-Troisdorf AG hcenses a process with this step (70). Hydrolysis occurs at 260—280°C and 4500—5500 kPa (45—55 atm) in a hydrolysis reactor without catalysis. The overhead methanol and water vapor is separated and the methanol is returned to the dimethyl terephthalate section for reuse. The reactor hquid is crystallized, cycloned, washed, and further cooled. Einahy, the slurry is centrifuged and dried. The product has less than 25 ppm of 4-formylbenzoic acid and very low levels of other impurities. There may be several hundred parts per million of monomethyl terephthalate, which is incompletely hydrolyzed dimethyl terephthalate. 

Table 20. Specifications for Polymer-Grade Terephthalic Acid...

Polymer-grade terephthahc acid is over 99.9 wt % pure, exclusive of some residual water which has a specification. With this degree of purity. ... 

Small amounts of polymer-grade terephthaHc acid and dimethyl terephthalate are used as polymer raw materials for a variety of appHcations, eg, adhesives and coatings. They are also used to make high performance polymers or engineering resins. Poly(ethylene terephthalate) is itself an engineering resin, although one more widely used is poly (butylene) terephthalate, formed by reaction with 1,4-butanediol as the comonomer. 

Table 5. Product Specifications for Polymer-Grade Propylene ...

Propylene has many commercial and potential uses. The actual utilisation of a particular propylene supply depends not only on the relative economics of the petrochemicals and the value of propylene in various uses, but also on the location of the supply and the form in which the propylene is available. Eor example, economics dictate that recovery of high purity propylene for polymerisation from a smaH-volume, dilute off-gas stream is not feasible, whereas polymer-grade propylene is routinely recovered from large refineries and olefins steam crackers. A synthetic fuels project located in the western United States might use propylene as fuel rather than recover it for petrochemical use a plant on the Gulf Coast would recover it (see Euels, synthetic). 

Butyl alcohol, obtained from hydration of Raffinate 1, can be dehydrated and subsequently refined to high purity, polymer-grade isobutylene (25). Alternatively, the isobutylene from alcohol dehydration can react with methanol in the presence of an acid catalyst to give methyl /-butyl ether (MTBE) gasoHne additive (see Ethers organic). 

Oxirane Process. In Arco s Oxirane process, tert-huty alcohol is a by-product in the production of propylene oxide from a propjiene—isobutane mixture. Polymer-grade isobutylene can be obtained by dehydration of the alcohol. / fZ-Butyl alcohol [75-65-0] competes directly with methyl-/ fZ-butyl ether as a gasoline additive, but its potential is limited by its partial miscibility with gasoline. Current surplus dehydration capacity can be utilized to produce isobutylene as more methyl-/ fZ-butyl ether is diverted as high octane blending component. 

In the Institut Fransais du Petrc le process (62), ethylene is dimerized into polymer-grade 1-butene (99.5% purity) suitable for the manufacture of linear low density polyethylene. It uses a homogeneous catalyst system that eliminates some of the drawbacks of heterogeneous catalysts. It also inhibits the isomerization of 1-butene to 2-butene, thus eliminating the need for superfractionation of the product (63,64). The process also uses low operating temperatures, 50—60°C, and pressures (65). 

In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic 2eohte, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes ahquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of Hquid to various sections of the adsorber (93,94). The cis- and trans-isomers are alkylated and used in the gasoline blending pool. 

The feed streams should be reasonably pure to limit yield losses and protect the purity of the final products. Typically, polymer-grade propylene with 99.5% purity is employed propane impurity can react to undesirable 1-chloropropane (bp 46.6°C), which is very difficult to separate from aHyl chloride (bp 45°C). Both propylene and chlorine should be dry to prevent corrosion in downstream equipment where mixtures with HCl occur. 

The type of chloroprene polymers used is perhaps best illustrated by the variety of special products, designed for adhesive appHcations, that Du Pont has developed. These are described ia Table 8. Standard polymer grades are also often used, especially to modify adhesive properties and to reduce cost. 

The propylene fractionator operates at a pressure of 1.8 to 2.0 MPa with more than 160 trays required for a high purity propylene product. Often a two-tower design is employed when polymer grade (99.5%+) is required. A pasteurization section may also be used when high purity is required. The bottoms product contains mainly propane that can be recycled to the cracking heaters or used as fuel. Typical tower dimensions and internals for a 450,000 t/yr ethylene plant with naphtha feed are summarized in Table 7. 

The propylene-90 bottoms product from the deethanizer may be upgraded to high polymer grade 99.8% purity by superfractionation. Propane bottoms are used elsewhere in the refinery. 

Olefin metatheses are equilibrium reactions among the two-reactant and two-product olefin molecules. If chemists design the reaction so that one product is ethylene, for example, they can shift the equilibrium by removing it from the reaction medium. Because of the statistical nature of the metathesis reaction, the equilibrium is essentially a function of the ratio of the reactants and the temperature. For an equimolar mixture of ethylene and 2-butene at 350°C, the maximum conversion to propylene is 63%. Higher conversions require recycling unreacted butenes after fractionation. This reaction was first used to produce 2-butene and ethylene from propylene (Chapter 8). The reverse reaction is used to prepare polymer-grade propylene form 2-butene and ethylene ... 

Figure 9-3. A flow diagram showing the metathesis process for producing polymer grade propylene from ethylene and 2-butene.

Linear low-density polyethylene (LLDPE) is produced in the gas phase under low pressure. Catalysts used are either Ziegler type or new generation metallocenes. The Union Carbide process used to produce HDPE could be used to produce the two polymer grades. Terminal olefins (C4-C6) are the usual comonomers to effect branching. 

Benzoyl peroxide containing 50% water (see safety section) Polybutadiene (polymer grade)... 

Polymer grade Before After % Mass swell % Dimensional swell... 

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