Manufacture of By-Products
In the production of $300 \mathrm{~kg}$ per hour of an active substance for the manufacturing of photographic paper, 20 tons of sewage water accrue per day. The sewage flow is contaminated with an organic dissolver, which is needed for the production of the active substance. The purchase price of the dissolver is $$\$ 1.30$$ per $\mathrm{kg}$.
The current production process has about 6000 operating hours per year and runs on the principle of continuous production. The sewage water needs to be disposed of as waste product. Because of dissolver contamination of approximately $5 \%$ (mass percent), extra costs of $$\$ 5.50$$ per $\mathrm{m}^3$ are caused in comparison to wastewater without organic impurities.
On the basis of thermodynamic calculations and laboratory tests, it was estimated that it would be possible to separate almost all of the dissolver by adding a simple distillation column as a further process step. For the distillation, $80 \mathrm{~kg}$ heating steam (cost: $$\$ 20$$ per ton) is needed per $\mathrm{m}^3$ of sewage water. The regained dissolver can be reintroduced into the production process without any additional effort.
The plant engineer now attempts to estimate how much money can be invested in the distillation device, if management sets a limit of 2 years maximum for payback on this kind of investment. Can you help?