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PVC / VCM

Polyvinyl Chloride is a chlorinated hydrocarbon polymer. It is produced from vinyl chloride, chemical formula CH²=CHCl, Vinyl Chloride Monomer (VCM) is produced from the raw materials of ethylene and chlorine.

The first step is production of VCM

VCM is produced in three steps (see flow diagram below):

  • Direct chlorination: Ethylene and chlorine are combined in a continuous process to form EDC.

  • EDC cracking: EDC is thermally decomposed into VCM and hydrogen chloride. The hydrogen chloride is recycled as feedstock to a further stage, the oxychlorination. Unconverted EDC is separated and recycled. The VCM is purified for use in PVC production.

  • Oxychlorination: Recycled hydrogen chloride is reacted with further ethylene feedstock in the presence of copper chloride catalyst and oxygen. This produces further quantities of EDC, while excess hydrogen is oxidised to form water.

 

PVC Process

Vinyl Chloride Monomer is polymerized to make poly Vinyl Chloride (PVC). VCM is a gas at ambient temperature and pressure and can be represented chemically as follows:

H   H
I   I
C = C
I   I
H   CL

When an initiator is added to the reaction medium, the VCM becomes more reactive, breaking the double bond. This happens because the attraction between the carbons in the VCM molecule is less than the attraction between the carbons and the initiator. The initiator is carefully selected to insure that this is the case. With the initiator, double bonds are broken and a number of monomers join together to for PVC.

This reaction is exothermic, liberating approximately 1600 kJ/kg VCM. The heat created from the reaction comes from the breaking of the double bond in the vinyl chloride monomer. This heat must be removed in order to maintain temperature control, which in turn, is the key controller of molecular weight. This heat can be removed from the reactor by cooling water, refrigerated water, and ammonia refrigerant or by evaporative cooling.

The normal range of polymerization temperatures is 52-70C. This temperature range produces resins with a molecular weight range of 40,000 to 73,000. The molecular weight is measured by determining the viscosity of a dilute polymer solution.

The molecular weight can be altered by the use of additives to the polymerization that act as chain transfer agents.

PVC is produced by batch polymerization of VCM dispersed in a media generally water in stirred tank reactors (see flow diagram).  The reactor sizes range from 60 to 120 cubic meters. The reactor temperature is controlled by circulating chilled water.  The VCM slurry in water is stripped in a column to remove unreacted VCM. The slurry is then de-watered and VCM granules are dried in a dryer.

Process Economics

 Typical economic parameters are given below. Licensor information should be obtained for accurate estimates.

Item

Consumption per ton of PVC

Raw Matl VCM

1.004

Electricity KWH

170

Steam tons

0.8

DM water, tons

2.2

Cooling Water, KCal

480,000

Additive cost, US Dollars

10-14

Installed Cost

 

 

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