7 Vinyl Chloride Monomer Process
7.1 Basis of Design 7.1.1 Problem Statement
Vinyl chloride monomer (VCM) is one of the leading chemicals used mainly for manufacturing polyvinyl chloride (PVC). The PVC worldwide production capacity in 2005 was of about 35 million tons per year, with an annual growth of about 3%, placed after polyoleﬁnes but before styrene polymers. In the 1990s the largest plant in the USA had a capacity of about 635 ktons , but today there are several plants over one million tons. At this scale even incremental improvements in technology have a signiﬁcant economic impact. Computer simulation, process optimization and advanced computer-control techniques play a ...view middle of the document...
Alexandre C. Dimian and Costin Sorin Bildea Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31403-4
7 Vinyl Chloride Monomer Process
Table 7.1 Purity speciﬁcations for vinyl chloride .
Impurity Acetylene Acidity, as HCl by wt Acetaldehyde Alkalinity, as NaoH by wt. Butadiene 1-butene, 2-butene Ethylene, propylene Ethylene, dichloride (EDC) Nonvolatiles Water Iron, by wt.
Maximum level (ppm) 2 0.5 0 0.3 6 3, 0.5 4, 8 10 150 200 0.25
7.1.2 Health and Safety
Health and safety regulations (OSHA) require the monitoring of concentrations of harmful species in all facilities where VCM is produced or used. The employees should not be exposed to more than 1 ppm over 8 h, or no more than 5 ppm for periods exceeding 15 min. Chronic exposures to more than 100 ppm may lead to serious diseases. On the other hand, VCM is ﬂammable by heat, ﬂame and oxidizing agents. Large ﬁres are difﬁcult to extinguish. Because of possible peroxide formation, the VCM must be stored or transported under an inert atmosphere. The use of stabilizers prevents polymerization during processing and storage. VCM is generally transported in railroad tank cars and tank trucks.
7.1.3 Economic Indices
Table 7.2 presents some economic indices of a modern VCM technology , in which stoichiometric values are indicated in parentheses. The yield for chlorine may rise over 98%, the yield for ethylene being slightly lower because of losses by combustion.
7.2 Reactions and Thermodynamics 7.2.1 Process Steps
Most of the VCM technologies are based on “balanced” processes. By this is meant that all intermediates and byproducts are recycled in a way that ensures a tight
7.2 Reactions and Thermodynamics
Table 7.2 Economic indices of VCM processes .
Raw material (kg/t VCM) • ethylene • chlorine • oxygen Utility/t VCM • electric power (kWh) • steam (t) • cooling water (m3) • fuel (Mcal) Operators (men/shift)
462 (448) 578–585 (568) 131–139 (128) 105–110 0.05–0.20 150–175 660–700 4
closure of the material balance to only VCM as the ﬁnal product, starting from ethylene, chlorine and oxygen. The main chemical steps are explained brieﬂy below. 1. Direct chlorination of ethylene to 1,2-ethylenedichloride (EDC): C2H4 + Cl2 → C2H4Cl2 + 218 kJ/mol 2. Thermal cracking (pyrolysis) of EDC to VCM: C2H4Cl2 → C2H3Cl + HCl − 71 kJ/mol 3. Recovery of HCl and oxychlorination of ethylene to EDC: C2H4 + 2HCl + 0.5O2 → C2H4Cl2 + H2O + 238 kJ/mol Hence, an ideal balanced process can be described by the overall equation: C2H4 + 0.5Cl2 + 0.25O2 → C2H3Cl + 0.5H2O + 192.5 kJ/mol Therefore, half of the ethylene and the whole of the chlorine are fed to direct chlorination, while the other half of the ethylene goes to oxychlorination. The process block diagram consists of three plants, as shown in Figure 7.1. Note that the puriﬁcation of all EDC streams produced by synthesis, as well as recycled, can be done in a common separation section. In...