1. Chapter 9 Unit Operations and Pollution Prevention

2. Important Considerations Material selection Waste generation mechanisms Operation conditions Material storage and transfer Energy consumption Process safety

3. Raw Material Selection Raw materials are used as feedstocks, solvents, reactants, mass separating agents, diluents and fuels. Critical questions: –What are the environmental, toxicological, and safety properties of the material? –How do these properties compare to alternative choices? –To what extent does the material contribute to waste generation or emission release in the process? –Are there alternative choices that generate less waste or emit less while maintaining or enhancing the overall yield of the desired product?

5. Solution No. 6 FO: –volume needed = (1000000 BTU/148000 BTU/gal) = 6.76 gal; –mass needed = (6.76 gal)(1 ft^3/7.48 gal)(61.23 lb/ft^3) = 55.18 lb; –SO2 generated = (55.18 lb)(0.0084 lb S/lb)(54.06 lb SO2/32.06 lb S) = 0.928 lb SO2 No. 2 FO: 0.243 lb SO2 Natural Gas: 0.0 lb SO2

6. Waste Reduction Opportunities in Raw-Material Selection 1.The elimination of feedstock impurities. 2.The use of less hazardous raw materials. 3.A reduction in the number of raw materials used. 4.The utilization of waste materials from other processes.

7. Upgrading Raw Materials to Prevent Pollution – A Case Study Figure 9.2-1 shows the major uses of process water at a midsize refinery. When the concentrated calcium salts in the blowdown water from cooling tower met with the alkaline boiler blowdown, salts precipitated in the past. This precipitate plugged sewer lines, degraded wastewater treatment equipment, and resulted in greater sludge generation rates from wastewater treatment processes. It has been shown that every pound of solid precipitate in oily wastewater creates about 10 pounds of oily sludge.

8. RCRA hazardous Waste

9. Upgrading Raw Materials to Prevent Pollution – A case study of the process water in a refinery The quality of water supply is shown in Table 6-1. Reverse osmosis is chosen as the pre- treatment process. The purified water contains 5% of the impurities of the raw water. Rejected water with concentrated impurities is suitable for use in irrigation.

11. Upgrading Raw Materials to Prevent Pollution – A case study of the process water in a refinery The greatest savings are due to the reduction in hazardous wastewater sludge generation. Savings in sludge disposal costs alone are greater than the cost of pretreatment. Consumption of chemicals for treating the boiler and cooling water have been reduced by more than 90%. The resultant savings amounts to 1/3 of the cost of reverse osmosis. Maintenance costs are reduced.

13. Substitute Chemicals Tradeoffs between the environmental impacts of the current material and its potential substitute must be considered. Example: If a VOC solvent is replaced by a less volatile solvent, less emissions result but the heavier solvent will generate more solvent sludge. If it is replaced by an aqueous cleaner, the air emissions are replaced by aqueous wastes.

14. Two Widely Applicable Aspects to be Shown Here 1.The usefulness of normalized indicators of environmental effects. 2.The potential of tradeoffs in environmental impacts that can occur when substitutes are chosen.

15. The Chlorinated Solvents A million tons are used annually in processes ranging from vapor degreasing to the fabrication of electronic components. Useful properties: nonflammable, NBP slightly above room temperature, etc. Cause stratosphere ozone depletion Select substitute based on flammability, volatility, toxicity, environmental fate and impact, and solubility, etc.

16. Ozone Depletion The chlorinated solvents photodissociate if they reach the stratosphere ozone layer. The released chlorine atoms catalyze ozone destruction. The net reaction is

17. The Ozone-Depletion Potential Index For a compound to cause OD, it must have a lifetime in the atmosphere sufficient to reach the stratosphere and it must contain chlorine or bromine. Where is the rate constant for reaction of compound i with atomic oxygen and is the atmospheric lifetime of compound i.

18. Global Warming The overall energy balance of our planet is known to depend strongly on the emission of infrared radiation. The global warning potential of a chemical depends in part on both its ability to absorb infrared radiation and the length of time that it remains in the atmosphere.

19. The Global-Warming Potential Index Where is the infrared absorption band intensity of compound i and is the atmospheric lifetime.

20. Smog Smog is the result of the photochemical oxidation of hydrocarbons such as the organic solvents. The smog-formation potential of a chemical depends in part on the reaction rate for the oxidation of the compound by hydroxyl radical, which is a measure of the tendency of the chemical to participate in photochemical reactions.

21. The Smog-Formation Potential Index Where is the rate constant for the reaction of compound i with the hydroxyl radical.

22. Example: Replacement of CFC-113 with 1,1,1-Trichloroethane (TCA) Production of CFC-113 and TCA in 1989 was 78,000 and 353,000 metric tons. It is assumed that 80% of the produced solvents are eventually released to the atmosphere and, also, the solvent use has been stable since 1989. The molecular weight of TCA is 133 g/mol.

23. Solution The production rate of CFC-113 drops to zero while that of TCA increases to 431000 (=78000+353000) tons per year. Atmospheric emission of TCA is Overall SFP unchanged; overall GWP reduced by 80%; overall ODP reduced by ½.

24. Example: Replacement of Trichloroethylene (TCE) with 1,1,1-Trichloroethane (TCA) Assume that the two solvents are equivalent on a mass basis and that air emission are 80% of production. In 1989, production of TCA and TCE was 353000 and 82000 tons respectively. Molecular weight of TCA is 133 g/mol.

25. Solution The production rate of TCE drops to zero while that of TCA increases to 435000 (=82000+353000) tons per year. Atmospheric emission of TCA in mol is Overall SFP drops to 31%; overall GWP is unchanged; overall ODP increases by 10%.

26. Solution The solvents with high smog-formation potential (methylene chloride, TCE and tetrachloroethylene) have low ODP and GWP. There is a tradeoff between the potential to cause GW and DO and the potential to cause SF.