Question 1 of 6: Vinyl-Chloride Reaction-Path Selection by Economic Potential

Question 1: Vinyl-Chloride Reaction-Path Selection by Economic Potential (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Molar masses and market values of the materials, on a per-kmol basis:

Find. (a) the economically preferred path on a raw-material/by-product basis; (b) whether the combined ethylene + chlorine process whose only by-product is water is economically attractive (the flowsheet is devised on the way to answering it).

Approach. Screen each path with the economic potential $\mathrm{EP}=\sum(\text{value of products})-\sum(\text{cost of raw materials})$ per kmol of vinyl chloride (the level-2 Douglas screen); then combine the two ethylene paths so that one path’s HCl feeds the other, eliminating the by-product.

1. Define the economic potential (gross margin on materials). On a basis of 1 kmol vinyl chloride (VCM), $\mathrm{EP}=\sum \dot m_i v_i\big|_{\text{products}}-\sum \dot m_i v_i\big|_{\text{feeds}}$, where $\dot m_i$ is kg per kmol VCM and $v_i$ the market value. Oxygen is free and water has no value.
2. Path #1 — acetylene route. $\mathrm{C_2H_2+HCl\rightarrow C_2H_3Cl}$. Feeds cost $26.00+14.04=\$40.04$ and the product is worth $\$28.52$, so $\mathrm{EP_1}=28.52-40.04=\boxed{-\$11.52\ \text{per kmol}}$. Both reactants are purchased and relatively dear (acetylene at $1.00/kg), so this path loses money on materials alone.
3. Path #2 — direct chlorination with an HCl by-product. Overall $\mathrm{C_2H_4+Cl_2\rightarrow C_2H_3Cl+HCl}$. Feeds cost $14.84+16.33=\$31.17$; the products are VCM plus one mole of saleable HCl, worth $28.52+14.04=\$42.56$. With the by-product credit $\mathrm{EP_2}=42.56-31.17=\boxed{+\$11.39\ \text{per kmol}}$ — but if the HCl cannot be sold it becomes $28.52-31.17=-\$2.65$. The path is attractive only if there is a market for the HCl.
4. Path #3 — oxychlorination. The two written steps combine (one HCl released in cracking is re-consumed) to the net $\mathrm{C_2H_4+\tfrac12 O_2+HCl\rightarrow C_2H_3Cl+H_2O}$. Feeds cost $14.84+14.04=\$28.88$ (oxygen free) and the product is worth $\$28.52$, so $\mathrm{EP_3}=28.52-28.88=\boxed{-\$0.36\ \text{per kmol}}$ — essentially break-even. It consumes rather than produces HCl.
5. Answer to (a). On raw-material and by-product economics the ranking is Path #2 $(+\$11.39) >$ Path #3 $(-\$0.36) >$ Path #2 without an HCl market $(-\$2.65) >$ Path #1 $(-\$11.52)$. Path #2 (direct chlorination) makes the most sense — provided the by-product HCl can be sold; the acetylene route is clearly the worst.
6. Answer to (b) — the balanced process. Path #2 produces one HCl per VCM and Path #3 consumes one HCl per VCM, so running them in a 1:1 ratio through a shared EDC pyrolysis furnace lets the cracking HCl feed the oxychlorination reactor. Adding the two overall reactions cancels HCl entirely: $$2\,\mathrm{C_2H_4}+\mathrm{Cl_2}+\tfrac12\mathrm{O_2}\;\rightarrow\;2\,\mathrm{C_2H_3Cl}+\mathrm{H_2O}$$ This uses only ethylene and chlorine as purchased feeds (oxygen is free) and rejects only water. Feeds for 2 kmol VCM cost $2(14.84)+16.33=\$46.01$ and the product is worth $2(28.52)=\$57.04$, giving $\mathrm{EP_{bal}}=(57.04-46.01)/2=\boxed{+\$5.52\ \text{per kmol}}$.
7. Answer to (b) — is it economically attractive? Yes. The balanced process returns a positive material margin of $\$5.52$ per kmol of vinyl chloride (about $\$0.089$ per kg of product, or 19% of the product’s $\$28.52$/kmol market value) while rejecting only water, so it carries no by-product disposal burden and needs no merchant outlet. It is worth less than Path #2 if a full-price HCl buyer exists $(+\$11.39)$, but it beats Path #2 with no HCl market $(-\$2.65)$, Path #3 $(-\$0.36)$ and Path #1 $(-\$11.52)$ — and unlike Path #2 its margin does not move with a thin, volatile merchant-HCl price. A positive margin under every HCl market condition, on a feed slate of ethylene and chlorine alone, is exactly why the balanced process is the commercial route. The reservation is that it buys that robustness with two reaction systems instead of one, so the capital charge it must still cover is roughly double Path #2’s; the material margin is a screen, not a profit.