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Waste Minimization

    The concept is simple really.   "Waste" describers material that was not used for its intended purpose or
unwanted material produced as a consequence of another process.  In the chemical industry, waste is either considered inert or contaminated.  Inert waste can be recycled or released into the environment.  Malformed polymer or leaking steam may be considered inert wastes, although they are not chemically inert.  Wastewater is a type of contaminated waste that needs to be treated before the components can be recycled or released to the environment.  No matter what type of waste you have, waste costs money.  Malformed polymer is either sold as scrap for much less than the properly formed parts or it has to be reprocessed.  Either scenario costs a company money.  Treating wastewater is certainly an expensive endeavor.  In short, there is much motivation to minimize waste in the chemical and other industries.

Recovering Materials



Economics Types of Waste Example Applications
Gravity Settling Tanks or ponds provide hold-up time allowing solids to settle Relatively inexpensive; dependent on particle size and settling rate Slurries or streams with suspended solids Industrial wastewater treatment
Filtration Collection devices such as screens, cloth, or other; liquids pass and solids are retained Labor intensive, relatively inexpensive, additional energy needed for pumping Aqueous solutions with finely divided solids Polymer filtration, wastewater, etc.
Floatation Air bubbled liquid to collect fine solids that rise to surface with bubbles Relatively inexpensive Aqueous solutions with finely divided solids Refinery oil/water mixtures, paper wastes; mineral industry
Flocculation Agent added to aggregate solids to together to facilitate separation Relatively inexpensive Aqueous solutions with finely divided solids Refinery oil/water mixtures, paper wastes; mineral industry
Centrifugation Spinning of mixtures and centrifugal force causes separation by differences in densities Competitive with filtration Liquid/liquid or liquid/solid separations Paints
Distillation Boiling off materials by taking advantage of differences in boiling points Energy intensive Organic liquids Solvent separations
Evaporation Solvent recovery by boiling off the solvent Energy intensive Organic/Inorganic aqueous streams Rinse waters from metal plating waste
Ion Exchange Waste streams pass through resin bed where ionic materials are selectively removed Relatively high costs Heavy metal aqueous solutions Metal plating solutions
Ultrafiltration Separation of molecules by size using membrane Relatively high costs Heavy metal aqueous solutions Metal coating applications
Reverse Osmosis Separation of dissolved materials from liquid through a membrane Relatively high costs Heavy metals; organics, inorganic aqueous solutions Seldom used industrially
Electrolysis Separation of positively charged materials by application of electric current Dependent on concentrations Heavy metals; ions from aqueous solutions; copper recovery Metal plating
Carbon absorption Dissolved materials selectively absorbed in carbon Relatively costly thermal regeneration needed; energy intensive Organic/inorganics from aqueous solutions with low concentrations Metal plating
Solvent Extraction Solvent used to selectively dissolve solid or extract liquid from waste Relatively high costs Organic liquids, phenols, acids Recovery of dyes
Precipitation Chemical reaction caused formation of solids which settle Relatively high costs Lime slurries Metal plating wastewater treatment
Electrodialysis Separation based on differential rates of diffusion through membranes Moderately expensive Separation/concentration of ions from aqueous streams Separation of acids and metallic solutions
Chlorinolysis Pyrolysis in atmosphere of excess chlorine Insufficient U.S. market for carbon tetrachloride Chlorocarbon wastes Carbon tetrachloride manufacturing
Reduction Oxidative state of chemical changed through chemical reaction Inexpensive Metals, mercury in dilute streams Chrome plating solutions and tanning operations
Thermal Oxidation Thermal conversion of components Relatively high costs Chlorinated organic liquids; silver Recovery of sulfur, hydrochloric acid

Batch Operations

    When you think of a batch operation, you may not think that these smaller units produce much waste.  To the contrary, batch operations produce much more waste per unit product than do continuous processes.   Traditionally, manufacturers using batch operations could afford this high waste content thanks to the high value of their products.  In recent years, waste has become more and more expensive to deal with while competition has forced product value down.  What has raised the cost of waste "production"?  Increased solvent prices and fees for environmental permitting and monitoring emissions.
    Each run of a batch process differs in many aspects.  Waste generation is no exception.  For example, one run may yield 1.5 lbs. of unwanted by-product while a small pressure variation in the next run causes 1.7 lbs. of waste to be produced.  Waste handling equipment must be designed to handle the worst case scenario waste production conditions.  Careful control of reaction conditions can help minimize waste in such applications.  Reactor loading and unloading are also opportunities to minimize waste.
    During reactor loading, add solids before liquids.  This will minimize the amount of time that a most likely volatile liquid is in contact with the atmosphere.  If possible, use a solvent with a lower vapor pressure to minimize evaporation losses.  Consider using a hopper specific to your solids.  Some hoppers allow locking of the process vessel to minimize vapor losses.  Also, some hoppers are available to open and distribute solids packaged in bags (cut-in hoppers).
    During batch reactor operations, consider using a vapor recycling system if necessary.  The cumulative effects can be well worth the investment.   Install gaskets on all vessel openings.  Use statistical process control (SPC) to regulate reactions rather than using intermediate testing.  When discharging the reactor, try to allow the reactor to cool as much as possible to limit volatile organic compounds from leaking.

Process Modifications

Raw Materials
    Feed quality is very important in waste minimization.  Working with suppliers to improve feed quality reduce waste dramatically.  Even small impurities can lead to giant amounts of waste.  For example, a specific feed impurity may speed up catalysts degradation which in turn produces waste that must be separated from final products.  Even if these impurities are not a compromise of your quality and are left in the product, it may increase the waste in your customer's process.   Raw materials can also be evaluated for reduction or elimination.  Consider a company that uses algae inhibitor in their cooling tower.  By shielding their tower from the sun, they quickly found that they could reduce their inhibitor use by half.

The transition from laboratory to industrial scale can sometimes see a drastic change in product yield if proper mixing is not employed.  By using static mixers before the reactor, by-product yield can be minimized.  Constant searching for better catalyst materials can also help a reactor operate at peak efficiency.  Consider a separate, smaller reactor for recycle streams.  Optimum conditions for recycle streams can vary from those used for fresh feeds.  A separate reactor allows these different, optimum conditions to be used.

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Distillation Columns
Distillation columns contribute to waste by allowing impurities to remain in the product.  The solutions to this problem include ways to better separations.  It is critical that engineers analyze the cost of waste treatment and the additional energy costs required for better separations.  At times, the additional energy may be much more expensive than waste treatment.  Separations can be bettered by the following methods:
1.  Increased reflux ratio
2.  Additional trays
3.  Changing feed location
4.  Insulating column
5.  Pre-heating column feed
6.  Increasing size of vapor lines
    Columns can also produce significant waste due to inadequate condensation.  The vapor can find its way to a vent or flare and add to atmospheric pollution and result in costly fines.  Be sure your condenser is operating properly.

Specific Examples

Acrylonitrile production in the U.S. was 1.45 million metric tons in 1995.  Each metric ton of acrylonitrile manufactured requires 400 metric tons of cooling water for process use.  Typically, about 0.5% of cooling water is discarded to prevent buildup of slime and solids during recirculation.  This discarded water (called blowdown) contains toxins used as bactericides and fungicides and should be considered hazardous.  For every 400 metric tons of cooling water used (per ton of product), 2 metric tons of wastewater are generated.  That's a total of about 2.9 million metric tons of wastewater produced from the acrylonitrile industry in the U.S. alone.
    Assuming a waste treatment cost of $0.08 per gallon, the waste treatment costs per ton of product are:
(2000 kg water) x ($0.08/gallon water) / (3.785 kg/gallon water) = $42 per ton of product
A closed loop refrigerant system could perform the necessary cooling duty for around $17 per ton of product.  For a 100,000 ton/year facility, this could result in a saving of around $2.5 million annually.  Assume a total plant cost of $400 million.  The cooling water system would cost 2% of the total plant cost while the refrigeration system would cost twice as much.  This puts the refrigeration system cost at $16 million.  It's likely that over the life of the plant (especially with tightening regulations), the refrigeration system will easily pay for itself.   Not to mention that is helps keep waste generation to almost nothing.

Paint Stripping
The U.S. Air Force spends much of their time stripping paint from equipment and repainting.  Typically, a methylene chloride solvent is applied, followed by scraping, washing with water (contaminating thousands of gallons), hand sanding, and buffing.
    Hill Air Force Base in Ogden, Utah decided to try a different way.   They use tiny plastic beads in a blasting technique (similar to sandblasting) to remove the paint.  The only waste is pulverized paint, while the plastic beads can be separated and recycled.   Below we'll compare the costs associated with stripping one F-4 aircraft.

  Chemical Stripping Plastic bead stripping
Waste solids generated 9767 lbs sludge 320 lbs dry waste
Waste water generated 200,000 gallons ---
Hazardous solid disposal $967 $32
Waste water treatment $1,485 ---
Investment N/A $647,389
Raw material costs $5,422 $346
Energy costs $231 $127

**Payback occurs after approximately 80 similar sized aircrafts have been stripped

PVC Manufacturing
During the production of polyvinylchloride (PVC), waste streams of HCl are produced.  These streams are traditionally handled in one of two ways.   Chlorinolysis uses the HCl stream to produce carbon tetrachloride.  This process requires high temperature and pressure as well as significant capital investments.   In the past, this investment could be justified through the revenue produced by selling carbon tetrachloride.  As the carbon tetrachloride market has grown weaker, the HCl has become more valuable as a reactant in the PVC process rather than the carbon tetrachloride reaction.  Thus most plants utilizing the oxychlorination method of PVC production are using catalytic fluidized bed reactors to recycle the HCl to the process.   This is a perfect example of how the chemical market can influence waste minimization.

Sulfuric Acid Reduction at Standard Uniform Services
Standard Uniform sought a cost effective replacement for sulfuric acid used to adjust the pH of its wastewater to levels required by discharge permits.  One of Standard Uniform's customers in the industrial gas business suggested using carbon dioxide to alter the pH.  Below the economics over a 10 year period are shown:

  H2SO4 System CO2 System
Installation --- $650
Raw Materials $14,000 $45,000
Permit Filing $29,500 ---
Equipment Replacement $100,000 $20,000
TOTAL $143,500 $65,650

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