Equipment cleaning is one of the most common areas of waste generation. The reduction of solvent wash waste from metal cleaning and degreasing operations as well as various applications in the paint industries is well documented. This section focuses on the options used in the chemical industry's reduction of solvent wash waste (i.e., vessels and associated piping required in the clean-out).
Manual cleaning reduces the amount of solvent used because: manual washing can be more efficient than an automated wash system; and personnel can vary the amount of solvent needed from wash to wash depending on the condition of the equipment (cleanliness).
A variation of this option involves personnel entering the equipment and wiping the product residue off the equipment interior walls with hand-held wipers or spatulas which would minimize or eliminate the need for a subsequent solvent wash. A company should thoroughly review the safety aspect of this option, particularly the nature and extent of personnel exposure, before implementation.
FIG. 3.10.7 Hot stream piped directly into the suction line of the storage tank pump.
Better draining can reduce the amount of product residue on equipment walls and thereby minimizes or eliminates the solvent needed in a subsequent wash. A company can improve its draining simply by lengthening the time between the end of a production batch or cycle and the start of the washout procedure. For a packed distillation column, maintaining a slight positive pressure (with nitrogen) on the column for twenty-four to forty-eight hours facilitates draining. The residue product is thereby swept off the packing and accumulates in the bottom of the column.
Prewashing Equipment with a Detergent and Water Solution
Prewashing contaminated equipment with a soap and water solution minimizes or eliminates the solvent needed in a subsequent wash step.
Flushing the Equipment with the Product and Recycling It Back to the Process
This option applies when more than one product is produced with the same equipment. Prior to processing another product, a company can withhold a small reserve of a product from a previous similar process and then use it to flush the equipment. The contaminated product (used as a flush) can then be reworked or reprocessed to make it acceptable for use.
Instead of using fresh solvent, a company can use the waste solvent from another process in the plant for the equipment flush. This option reduces the plant's total waste load.
Minimizing the Amount of Solvent Used to Wash Equipment
Often, a company can minimize the amount of solvent used for a flush without changing the resulting cleanliness of the equipment.
Increasing Campaign Lengths
With careful scheduling and planning, a company can increase product campaign lengths and thereby reduce the number of equipment washings needed.
Often the specifications for products produced in the same equipment are different. One set of specifications may be more stringent than another. Through careful planning and inventory control, a company can make product changeovers from products with tighter specifications to those with looser specifications.
Often, product residue dries, thickens, and hardens in the equipment between solvent washouts. Immediately washing out vessels between campaigns makes the residue easier to remove when it does not set on the equipment interior walls.
The solvent wash can be replaced with a less hazardous or nonhazardous (i.e., water) flush material. Another variation is to replace the solvent with a less volatile solvent thus reducing fugitive emissions. The solvent can then be recovered and recycled.
A new cleaning system uses a special nozzle and lance assembly which is connected to a high-pressure water source and inserted through a flange at the vessel bottom (see Figure 3.10.8).
As shown in the figure, a chain-drive moves the lance up and down the carriage as needed. A swivel joint at the base of the lance permits free rotation. The nozzle at the tip of the spinning lance has two apertures, which emit
Rotated . Lance
High-Pressure Water Feed r
FIG. 3.10.8 High-pressure water system.
Nozzle cone-shaped sprays of water at 10,000 psi with a combined flow rate of 16 gpm. The operation of the lance is controlled from a panel well removed from the vessel. The process is designed so that no high-pressure spray leaves the interior of the vessel. These precautions assure operator safety during vessel washout.
All solvent waste is eliminated. The product removed from the equipment walls can be separated from the water and recovered for further waste reduction. Even in processes where water cannot be introduced into a vessel, an alternative exists. Vessels can be cleaned with solid carbon dioxide (dry ice) particles suspended in a nitrogen gas carrier. The solid CO2 cleans in a manner similar to that of sand blasting, leaving only the material removed from the equipment (U.S. EPA 1993).
A rotating spray head can be used to clean vessel interiors. This system minimizes solvent use by allowing the solvent to contact all contaminated surfaces in an efficient manner (U.S. EPA 1993).
Pigs are pipe-cleaning mechanisms made of various materials. They are actuated by high-pressure water, product, or air. Pigs remove the residual build-up on pipe walls thereby minimizing or eliminating subsequent washing.
This option uses a system of wipers or brushes that cleans off residual product. (This system is somewhat analogous to a car wash except that it washes the interior vessel walls as opposed to the outside of a car.) This system is appropriate for processes where the product hardens on the vessel walls. The wipers or brushes dislodge the material which subsequently falls to the vessel bottom. This system is not appropriate for a viscous material that would adhere to the brushes or wipers and have to be washed out; this situation would create as much, if not more, waste than the original process.
Using Dedicated Equipment to Make Products
This option eliminates the necessity of washing out equipment between production campaigns thus eliminating the flush solvent stream.
During the design of a new process, a company can minimize flush solvent waste by designing equipment to facilitate draining. This equipment includes vessels with sloping interior bottoms and piping arrangements with valve low points or valves that drain back to the main vessels. After a product campaign, the residue is drained from each equipment section into a movable, insulated collection vessel. The collected material is then reintroduced into the process during the next campaign.
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