Process analytics in vinyl chloride monomer (VCM) plants – Case Study

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Courtesy of Siemens Industry, Inc. - Process Analytics

Vinyl chloride monomer (VCM), made from ethylene and chlorine by pyrolysis, is the feedstock for the production of the very common plastic material PVC. Modern VCM plants use integrated processes, combining endothermic and exothermic reactions in an almost energy balanced operation. Plants optimize the product quality, plant safety and cost efficiency by using advanced process control equipment including process analyzers.

Siemens, a leader in process analytical instrumentation, has proven over the decades, its capability to plan, engineer, manufacture, implement and service analyzer systems for VCM plants worldwide.

This Case Study provides an overview of the typical processes and describes how Siemens with its analyzer and application know-how meets best the process requirements.

Vinyl chloride monomer (VCM)

Vinyl chloride monomer (CH2=CHCl, monomer, in contrast to polymers, PVC) is a colorless, flammable gas, first obtained in 1912 through catalytic hydrochlorination of acetylene. VCM is heavier than air hence can sink to the ground and may, under specific circumstances, form peroxides, initiating explosive polymerization. Under incineration, VCM decomposes producing toxic and corrosive fumes (hydrogen chloride and phosgene).

VCM is used as feedstock in the production of PVC (polyvinyl chloride), one of the world´s most versatile thermoplastics for a wide variety of industrial applications. As a hard plastic, it is used as vinyl siding, magnetic stripe cards, window profiles, pipe, plumbing and conduit fixtures. It can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, and to make flexible hoses and tubing, flooring, roofing membranes, and electrical cable insulation. The material is often used for pipelines in the water and sewer industries because of its low-cost.

Since 1940, acetylene as feedstock was replaced stepwise by the inexpensive ethylene. Complete changeover to almost exclusive use of ethylene became possible in 1955, when the large-scale oxychlorination of ethylene to 1,2-dichlorethane became possible. Today, more than 90 % of the global VCM production is based on ethylene.

Modern VCM plants mostly use integrated processes which combine both highly exothermic reactions of ethy-lene chlorination and oxychlorination with the endothermic cracking process. This results in an almost energy balanced operation.

VCM process in brief

A modern VCM production can be considered as 'balanced process' where ethylene, chlorine and oxygen are converted into VCM and water. The three process steps direct chlorination, oxychlorination and EDC cracking are summarized in the 'VCM synthesis reaction'. For details see text box below.

Based on the 'VCM synthesis' principle different production technologies are in use. They differ, for instance, in details of the reactor design, type of catalysts, and reaction temperatures applied.

Many hundreds of VCM plants are in use worldwide with capacities from 30 000 up to 600 000 t/year each. All plants are operated with the aim of a high and constant product quality as well as cost efficiency and safety (corrosive, poisonous and explosive substances are handled).

Considerable investments are required in instrumentation and process control to meet these requirements. Process analytics contribute an important part to that.

Process steps

The major steps of a VCM production process (fig. 1) include:

Direct chlorination

Ethylene and chlorine are fed to a reactor where a catalytic reaction (direct chlorination) takes place. 1,2-dichlorethane (EDC) is formed, together with heat, water and HCl-rich waste gas. The EDC is fed to a tank for temporarily storage. The waste gas is fed to the oxychlorination and reused as reacting component.

Oxychlorination

Ethylene, oxygen and hydrogene chloride are fed into a fluidized-bed reactor for the oxychlorination process. Raw EDC is formed, removed by condensation and fed to the EDC distillation unit for purification and heat. Waste gas and effluent are also formed and fed to the HCl recovery and water treatment units.

EDC distillation

To produce pure EDC both the EDC from the oxychlorination and the unconverted EDC from the cracking process (recycle EDC) are purified in the EDC distillation unit. The purified EDC is then fed to the EDC tank for temporarily storage.

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