H2o

By operating both the oxychlorination pathway and the direct chlorination pathway, the waste hydrochloric acid can be used as a raw material and essentially all of the molecular chlorine which originally reacted with ethylene is incorporated into vinyl chloride. As shown in Figure 32.6, the two processes operate synergistically and an efficient design for

Figure 32.6 Direct chlorination and oxychlorination of ethylene in tandem

Figure 32.6 Direct chlorination and oxychlorination of ethylene in tandem the manufacture of vinyl chloride involves both processes. By-product hydrochloric acid from the direct chlorination of ethylene is used as a raw material in the oxychlorination process; by operating the two processes in tandem, chlorine is used efficiently.

Additional efficiencies in the use of chlorine can be obtained by expanding the number of processes included in the network. In the network involving direct chlorination and oxychlorination processes, both processes incorporate chlorine into the final product. Recently, more extensive chlorine networks have emerged, linking several isocyanate producers into vinyl chloride-manufacturing networks (McCoy 1998). In isocyanate manufacturing, molecular chlorine is reacted with carbon monoxide to produce phosgene:

The phosgene is then reacted with an amine to produce an isocyanate and by-product hydrochloric acid:

The isocyanate is subsequently used in urethane production, and the hydrochloric acid is recycled. The key feature of the isocyanate process chemistry is that chlorine does not appear in the final product. Thus chlorine can be processed through the system without being consumed. It may be transformed from molecular chlorine to hydrochloric acid, but the chlorine is still available for incorporation into final products, such as vinyl chloride, that contain chlorine. A chlorine-hydrogen chloride network incorporating both isocyanate and vinyl chloride has developed in the Gulf Coast of the USA. The network is shown in Figure 32.7. Molecular chlorine is manufactured by Pioneer and Vulcan Mitsui. The molecular chlorine is sent to both direct chlorination processes and to isocyanate manufacturing. The by-product hydrochloric acid is sent to oxychlorination processes or calcium chloride manufacturing. The network has redundancy in chlorine flows, such that most processes could rely on either molecular chlorine or hydrogen chloride.

Consider the advantages of this network to the various companies (C.G. Francis, personal communication 2000):

• Vulcan/Mitsui effectively rents chlorine to BASF and Rubicon for their isocyanate manufacturing; the chlorine is then returned in the form of hydrochloric acid for ethylene dichloride/vinyl chloride manufacturing.

• BASF and Rubicon have guaranteed supplies of chlorine and guaranteed markets for their by-product HCl.

Even more complex networks could, in principle, be constructed. As shown in Table 32.3, chlorine is used in manufacturing a number of non-chlorinated products. The table lists, for selected reaction pathways, the pounds of chlorinated intermediates used along the supply chain, per pound of finished product. This ranking provides one indication of the potential for networking these processes with processes for manufacturing chlorinated products (see Rudd et al. 1981; Chang and Allen 1997).

An examination of individual processes, such as those listed in Table 32.3, can be useful in building process networks, but the individual process data do not reveal whether effi-

Figure 32.7 Chlorine flows in combined vinyl chloride and isocyanate manufacturing

cient use of chlorine is a major or a minor issue in chemical manufacturing. To determine the overall importance of these flows, it is useful to consider an overall chlorine balance for the chemical industry. The overall flows of chlorine into products and wastes, as well as the recycling of chlorine in the chemical manufacturing sector, are shown in Figure 32.8. The data indicate that roughly a third of the total chlorine eventually winds up in wastes. By employing the types of networks shown in Figures 32.6 and 32.7, the total consumption of chlorine could be reduced.

Identifying complex material re-use routes, such as those used for chlorine, is difficult and needs to rely on comprehensive, integrated models of material flows in the chemical process industries. Fortunately, such models have been developed. Rudd and co-workers

Table 32.3 Partial listing of non-chlorinated chemical products that utilize chlorine in their manufacturing processes

Product

Synthesis pathway

Pounds of chlorinated intermediates per pound of product

Glycerine

Hydrolysis of epichlorohydrin

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