6 Unit Operations and Material Balances

Recall the vinyl chloride production process from the last chapter:

\begin{align*}
C_{2}H_{4} + Cl_{2} → C_{2}H_{4}Cl_{2}&\;\;\;\; (1)\\
C_{2}H_{4}Cl_{2} → C_{2}H_{3}Cl + HCl&\;\;\;\; (2)
\end{align*}

We now consider an additional reaction in the vinyl chloride production process. Most vinyl chloride manufacturers use the combination of direct chlorination and oxychlorination, which is called the balanced process .

\begin{align*}
C_{2}H_{4} + Cl_{2} → C_{2}H_{4}Cl_{2}&\;\;\;\; (1)\;\;\;\; \\
C_{2}H_{4} + 2HCl + \frac{1}{2}O_{2} → C_{2}H_{4}Cl_{2} + H_{2}O&\;\;\;\; (2) \\
C_{2}H_{4}Cl_{2} → C_{2}H_{3}Cl + HCl&\;\;\;\; (3)
\end{align*}

1. Direct Chlorination (last lecture)
2. Oxychlorination
3. Ethylene dichloride pyrolysis (last lecture)

What is the overall reaction and what advantage does this have over what we saw last chapter?

To get the overall reaction, we add all the reactions and cancel any compounds that appear in both the reactants and products of the overall reaction. In the separation process, HCl is recycled back to the oxychlorination reactor if it is not depleted to ensure no HCl is released into the environment. Reaction (1) is balanced with the rest of the process in a way to provide the source of [latex]C_{2}H_{4}Cl_{2}[/latex] for reaction (3). This process gives us only 1 by-product: water. Using the balanced process gives us water as a by-product as opposed to hydrogen chloride.

[latex]2C_{2}H_{4}+Cl_{2}+\frac{1}{2}O_{2} → 2C_{2}H_{4}Cl_{2} + H_{2}O[/latex]

This process is carried out in multiple unit operations. To achieve the maximum efficiency and avoid releasing hazardous chemicals, hydrogen chloride produced in reaction 3 is recycled back to the oxychlorination reactor and used reaction (2), as well as unreacted EDC(ethylene dichloride, [latex]C_{2}H_{4}Cl_{2}[/latex]) in reaction 3 is recycled to react again.

• Ethylene enters the process and is split into 2 streams: 1 stream goes to the oxychlorination reactor an the other stream goes to the direct chlorination reactor
• Air and/or oxygen are fed into the oxychlorination reactor and chlorine is fed into the direct chlorination reactor
• EDC produced by both the oxychlorination and direct chlorination processes, and is fed into a purification unit to separate from the other components in the system
• EDC is then fed into the EDC pyrolysis reactor, followed by the VCM(vinyl chloride monomer, [latex]C_{2}H_{3}Cl[/latex]) purification unit, which separates HCl, VCM, and EDC into 3 separate streams

Separation of Components

As depicted above, we have light products (the components that are more volatile than our desired product, EDC), heavy products (the components that are less volatile than our desired product), and a stream of EDC. Let’s look at separating the components after direct chlorination:

For simplicity, say we only have chlorine ([latex]Cl_{2}[/latex]), ethylene dichloride (EDC, [latex]C_{2}H_{4}Cl_{2}[/latex]), and trichloroethane (TCE, [latex]C_{2}H_{3}Cl_{3}[/latex]) produced from a side reaction as products of the direct chlorination step. In reality, there may be other components in the stream, but we will stick to these three as the main components for this analysis. How many streams will come out after the EDC purification processes, and what are the components of each stream? How do we select the temperature for the purification process?

Compound	Boiling Point ([latex]^{\circ}C[/latex])
[latex]Cl_{2}[/latex]	-34.04
[latex]C_{2}H_{4}Cl_{2}[/latex]	83.47
[latex]C_{2}H_{3}Cl_{3}[/latex]	114

From the boiling point temperatures, we can deduce that the light product will be [latex]Cl_{2}[/latex] (boils at a lower temperature) and the heavy product will be [latex]C_{2}H_{3}Cl_{3}[/latex] (TCE) (boils at a higher temperature).

We can consider the separation of EDC from TCE and chlorine using 2 distillation columns. In this course, we’ll assume that distillation columns separate a single feed stream into two exit streams. There are more complex column arrangements with a greater number of entering and exit streams, but we won’t go over these in this course. Distillation columns usually operate with a range of temperatures inside them, and we’ll see why this is later in this course. For now, we’ll just assign them an average temperature.

The first column separates EDC and TCE from chlorine by operating at a temperature that is lower than EDC’s boiling temperature, but higher than [latex]Cl_{2}[/latex]‘s boiling temperature (let’s say on average [latex]60 ^{\circ}C[/latex]):

The second column separates EDC from TCE by operating at a temperature that is lower than TCE’s boiling temperature, but higher than EDC’s boiling temperature (let’s say, [latex]100 ^{\circ}C[/latex]):