Use different types or physical forms of catalysts.
Use water-based coatings instead of VOC-based coatings.
Use pure oxygen instead of air for oxidation reactions.
Use pigments, fluxes, solders, and biocides without heavy metals or other hazardous components.
Use terpene or citric-acid-based solvents instead of chlorinated or flammable solvents.
Use supercritical carbon dioxide instead of chlorinated or flammable solvents.
Use plastic blasting media or dry ice pellets instead of sand blasting.
Use dry developers instead of wet developers for nondestructive testing.
Use hot air drying instead of solvent drying for components.
Use no-clean or low-solids fluxes for soldering applications.
Plant Unit Operations
Optimize the relative location of unit operations within a process.
Investigate consolidation of unit operations where feasible.
Optimize existing reactor design based on reaction kinetics, mixing characteristics, and other parameters.
Investigate reactor design alternatives to the continuously stirred tank reactor.
Investigate different ways of adding reactants (e.g., slurries versus solid powders).
Investigate changing the order of adding reaction raw materials.
Investigate chemical synthesis methods based on renewable resources rather than petrochemical feedstocks.
Investigate conversion of batch operations to continuous operations.
Change process conditions and avoid the hydrolysis of raw materials to unwanted by-products.
Use chemical additives to oxidize odorous compounds.
Use chemical emulsion breakers to improve organic-water separation in decanters.
Source: Chadha, 1994.
Market-driven product scheduling and inventory considerations often play an important part in the generation of waste and emissions. A computerized material inventory system and other administrative controls can address these constraints. Another common constraint for pollution prevention projects is conformance with product quality and other customer requirements (Chadha 1994).
An example of reducing emissions through operational modifications is a synthetic organic chemical manufacturing industry (SOCMI) plant that wanted to reduce emissions of a cyclohexane solvent from storage and loading and unloading operations. The tank farms had organic liquid storage tanks with both fixed-roof and floating-roof storage tanks. The major source of cyclohexane emissions was the liquid displacement due to periodic filling of fixed-roof storage tanks. Standard operating procedures were modified so that the fixed-roof storage tanks were always kept full and the cyclohexane liquid volume varied only in the floating-roof tanks. This simple operational modification reduced cyclohexane emissions from the tank farm by more than 20 tn/yr.
Another example is a pharmaceutical manufacturer who wanted to reduce emissions of a methylene chloride solvent from a process consisting of a batch reaction step followed by vacuum distillation to strip off the solvent. The batch distillation involved piping the reactor to a receiver vessel evacuated via a vacuum pump. The following changes were made in the operating procedures to minimize emissions:
The initial methylene chloride charge was added at a reactor temperature of — 10°C rather than at room temperature. Providing cooling on the reactor jacket lowered the methylene chloride vapor pressure and minimized its losses when the reactor hatch was opened for charging solid reactants later in the batch cycle. The nitrogen purge to the reactor was shut off during the vacuum distillation step. The continuous purge had been overloading the downstream vacuum pump system and was unnecessary because methylene chloride is not flammable. This change reduced losses due to the stripping of methylene chloride from the reaction mix. The temperature of the evacuated receiving vessel was lowered during the vacuum distillation step. Providing maximum cooling on the receiving vessel minimized methylene chloride losses due to revaporization at the lower pressure of the receiving vessel.
Table 3.3.5 shows another checklist that can be integrated into an analysis structured like a hazard and oper-ability (HAZOP) study but focuses on pollution prevention.
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