Question 2 of 5: Tier 1 Economic and Environmental Performance Evaluation – TDI Production Routes

Question 2: Tier 1 Economic and Environmental Performance Evaluation – TDI Production Routes (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. Table 1 stoichiometric factors and costs above; TDI MW = 174.2 g/mol; Eq. 2-1/2-2/2-3.

Find. The Economic Index and TLC Environmental Index for each route, a recommendation, its limitations/assumptions, and why TLV/PEL is an oversimplified exposure metric.

Approach. Compute each route's per-kg-TDI raw-material cost from the negative (consumed) stoichiometric entries, credit each route for its saleable by-product using the same Cost column, then compute the TLC index from the established TLV values for the compounds each route actually handles.

1. Economic Index, Process 1 (amine-phosgene). Raw-material cost sums the three consumed reagents: $$\sum(C_i)_{P1}=1.29(0.810)+0.78(0.876)+0.01(0.550)=1.0449+0.6833+0.0055=1.7337\ \$/\text{kg TDI}$$ The only saleable by-product is HCl: $$\sum(C_j)_{P1}=0.4(0.027)=0.0108\ \$/\text{kg TDI}$$ $$\text{Economic Index}_{P1}=1.7337-0.0108=\boxed{\$1.723/\text{kg TDI}}$$
2. Economic Index, Process 2 (carbonylation). Raw-material cost: $$\sum(C_i)_{P2}=1.03(0.550)+1.0(0.040)+0.01(0.550)=0.5665+0.0400+0.0055=0.6120\ \$/\text{kg TDI}$$ Saleable by-product (CO₂, credited at its Table 1 unit value): $$\sum(C_j)_{P2}=1.0(0.040)=0.0400\ \$/\text{kg TDI}$$ $$\text{Economic Index}_{P2}=0.6120-0.0400=\boxed{\$0.572/\text{kg TDI}}$$ Process 2 is $1.723/0.572=3.0\times$ cheaper on raw-material economics alone.
3. TLC Environmental Index, Process 1. Using established ACGIH TLV-TWA values — phosgene 0.1 ppm, chlorobenzene 10 ppm, HCl 5 ppm (ceiling), TDI 0.005 ppm: $$\sum\left|v_i\right|/TLV_i\big|_{P1}=\frac{1.29}{0.1}+\frac{0.01}{10}+\frac{0.4}{5}+\frac{1.00}{0.005}=12.900+0.001+0.080+200.000=\boxed{212.98}$$ (TDA's contribution is not numerically included — see the callout above — but is carried forward qualitatively as an unquantified carcinogenicity/skin-hazard flag.)
4. TLC Environmental Index, Process 2. Using CO 25 ppm, chlorobenzene 10 ppm, CO₂ 5000 ppm, TDI 0.005 ppm: $$\sum\left|v_i\right|/TLV_i\big|_{P2}=\frac{1.0}{25}+\frac{0.01}{10}+\frac{1.0}{5000}+\frac{1.00}{0.005}=0.040+0.001+0.0002+200.000=\boxed{200.04}$$ (DNT's contribution is likewise carried forward qualitatively.) Both routes' index is overwhelmingly dominated by the shared TDI output term (200.0 of $\approx$213 and $\approx$200 respectively, since both routes make 1.00 kg TDI per kg TDI and TDI's TLV (0.005 ppm) is far smaller than any other listed compound's) — the two routes differ by only $\approx$6%, with Process 1's extra 12.9 coming entirely from phosgene.

[3] Recommendation

Process 2 (carbonylation of dinitrotoluene) is recommended. It is unambiguously superior on the quantified Economic Index (roughly one-third the raw-material cost of Process 1) and matches or slightly betters Process 1 on the TLC Environmental Index. Beyond the indices themselves, Process 1's use of phosgene — an extremely acutely toxic gas historically used as a chemical warfare agent, with a documented history of catastrophic-release incidents in the chemicals industry — carries a low-probability/high-consequence acute-release risk that a steady-state TLV-based index does not capture at all (the TLC index only reflects routine, continuous-exposure conditions; see part [5]). This qualitative acute-hazard consideration reinforces, rather than overrides, the quantitative recommendation: Process 2 is both the cheaper and the lower-acute-hazard route.

[4] Limitations and assumptions

• only Cost values (assumed reliable, 1998 basis, no inflation or regional adjustment applied) support the Economic Index.
• Process 2 is unproven at scale (lab-only, per the question statement): its stoichiometric factors and 70–85% TDI selectivity are estimates, not demonstrated plant performance; a scale-up cost premium (catalyst cost/life, additional purification for lower-selectivity product) is not included in this Tier 1 estimate and could narrow or reverse the economic gap.
• TLV data gap: TDA and DNT, both reactants/intermediates with known carcinogenicity concerns, could not be assigned a numeric TLV and are excluded from the Environmental Index total — the computed indices are therefore a lower bound, not a complete toxicological comparison, for both routes.
• Steady-state, routine-exposure basis only: neither index captures accidental release/upset scenarios, which is exactly the additional acute-hazard consideration raised for phosgene in the recommendation above.
• Solvent and catalyst credits/burdens (chlorobenzene make-up, and any mixed-metal catalyst consumption for Process 2) are assumed negligible beyond the 0.01 kg solvent make-up figure given; catalyst manufacture/disposal burden is not in Table 1 and is not included.

[5] Why TLV/PEL oversimplifies overall human health effects of exposure

A TLV or PEL is a single time-weighted-average airborne concentration set to protect against a defined reference effect (typically the most sensitive well-documented chronic or acute effect) for a healthy adult worker under an assumed 8-hour/40-hour occupational exposure pattern. This oversimplifies real human health risk in several specific ways: (1) it addresses a single exposure route (inhalation) and ignores dermal absorption and ingestion, both directly relevant to TDA/DNT's skin-contact hazard and to the fish-ingestion pathway modelled in Question 3; (2) it is a population-average, not individual, threshold — it does not protect sensitized or hypersensitive individuals (TDI itself is a well-documented respiratory sensitizer, where a previously exposed, sensitized worker can react at concentrations far below the TLV); (3) it assumes a single-substance exposure, ignoring additive or synergistic effects of simultaneous exposure to multiple compounds (both TDI routes involve multi-compound atmospheres); (4) it is built around a time-weighted average and so can mask short, high-concentration excursions unless a separate ceiling or STEL value is also applied; and (5) it reflects an occupational, not general-population, exposure pattern and receptor (healthy working-age adults, not children, elderly, or chronically ill individuals, and not the general public living near a facility). A TLV/PEL is therefore best understood as a regulatory screening threshold for routine workplace exposure, not a complete characterization of human health risk — the same conceptual limitation raised for the single-value LC50 benchmark discussed in Question 1(c) and applied quantitatively in Question 3.