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Selecting a Still for On-site Solvent
Recycling
Many solvents can be recycled: reprocessed and then reused. Solvent recycling is preferred to other disposal methods for minimizing adverse environmental effects. Distillation is the most common method of solvent recycling, although filtration can be used to remove solids and nonmixable liquids. Recycling can be done at your facility or off-site.
Off-site solvent recycling is generally economical for solvents generated in truck load quantities over three months. As long as the recycled solvent can be reused, on-site recycling is usually economical at these same volumes and smaller. Economic viability improves as the value of the solvent increases or as the cost of waste disposal increases. Carefully evaluate the feasibility of small-scale, on-site distillation before purchasing distillation equipment. Use this fact sheet as a guide to determine if on-site distillation is right for your company. A regulatory overview related to on-site stills follows.
Questions to
Ask
By answering the questions in the next four sections your company will be better prepared to select the right still for your needs. Distillation equipment suppliers, companies in your industry that already distill solvents for reuse, and trade associations are good sources of information related to economic and safety considerations of specific stills.
Feasibility
Considerations
Are other separation technologies more appropriate?
Distillation works well at separating similar liquids. If solid particles are the main contaminant, consider recovering the solvent by filtration. If the waste consists of liquids with vastly different molecular sizes, like a solvent and polymer or resin, consider using membrane filtration to recover the solvent.
Can the distilled solvent be used
again for the same process at your company? If the solvents in the blend have a wide range of boiling points, they may be unsuitable for the original use because distillation will yield a solvent different from the original blend. Ask your solvent supplier if the solvent can be reformulated to meet your requirements. In other cases, contaminants not sufficiently removed can cause problems. For example, the breakdown of chlorinated degreasing solvents creates acids. These residual acids catalyze the formation of more acid, which can corrode both products and process equipment.
If the distilled solvent cannot
be used in the original process, is there another process
in which the solvent can be used? Uses where requirements
are less stringent than the original use are good candidates
for using distilled solvent.
Do different waste solvents need
to be stored separately? When different solvents are accumulated in a single waste drum simple batch distillation will produce a mixed solvent product which may not be usable for any of the current solvent uses. Make sure that procedures necessary to prevent the cross-mixing of solvents can and will be followed by all employees. Also, if possible, reduce the number of different solvents used.
Technical, Economic
and Operational Considerations
Will the equipment supplier distill a sample of your
waste solvent? You should evaluate the quality and
quantity of both the distilled product and the still
bottoms. Be sure to distill enough waste so that the
recycled solvent can be tested in your facility.
What is the percentage of solids
or nonvolatile liquid contaminants in your waste?
Many stills claim a high solvent recovery percentage.
If only half your waste is solvent, a high recovery
rate may still be inadequate to pay back the investment
after operating expenses.
What is the physical nature of
the still bottoms or residue produced by the still?
Is the still designed to handle this type of residue?
Solids and liquids that gel or polymerize can require
significant amounts of labor to remove from the still.
High solids can also coat and insulate the still’s
heating surfaces. This causes distillation to take more
time and may cause hot spots and chemical decomposition.
Waste of this type may require you to use still liners
for smaller units, or a still with an automated surface
scraper (scraped-surface still) for larger applications.
Should waste
solvent be distilled under vacuum? Since vacuum distillation lowers the temperature at which liquids boil, it has the following advantages: allows the distillation of some materials that are not safe to distill at atmospheric pressure; speeds up most distillations; and protects some materials from breaking down under high temperatures. Vacuum distillation can add significantly to the purchase price and operating costs of the still.
How will the
still bottoms be managed, and what are the disposal
costs? Most still bottoms must be handled as
hazardous waste. The disposal costs for pumpable liquid
wastes will generally be less per pound than for solids
or semi-solids. But, if the volume can be reduced sufficiently,
the disposal of solids may be economically feasible.
In some machining operations oily still-bottoms can
be reused as a lubricant. Regulatory agencies require
evidence that reused still bottoms are providing a valuable
function and are not being disposed of in an unregulated
manner.
What are the
costs of operating the still? Costs include: labor to operate, load, monitor, unload and clean the still; electricity; condenser water, including sewer or treatment charges; and still liners, if needed.
How much space
will the distillation equipment require compared to
the space needed to store waste or original solvent?
At larger volumes floor space should be freed up, while
at smaller volumes extra space will be needed. Consider
the value of this space.
How much labor
will be needed to operate the solvent recycling equipment
and related materials handling? How does this compare
to labor required to manage original and waste solvents?
Generally, recycling will require a little more labor
than materials handling alone. Is it easy to operate
or will it require extra labor or supervision? Is any
surplus time available, or are current employees stretched
now? Do the economics justify adding labor?
Will any more
equipment or special utilities be needed for the still
to operate? At what cost? What is required to
install the still? These indirect costs should be factored
into the total cost of purchasing a still.
How much maintenance
will the still require? Will the solvent eventually
deteriorate any of the still components? Stainless steel
and Teflon fittings and gaskets are required for some
solvents, but they add cost. Check to see if the chemicals
are compatible with the equipment you are considering.
How much operational
support will be available from the manufacturer or distributor?
How much might be needed? Will the manufacturer/distributor
train your operators? Are the written instructions clear?
Will the manufacturer/distributor make an operational
guarantee and back its equipment?
Ask your vendor for references of
companies using the equipment for similar purposes to
verify vendor claims.
Safety Considerations
Is there a safe location for
the still? When choosing a location consider
fire code requirements, sources of ignition, ventilation
needs, and the possibility of spills. Check with your
fire marshal and insurance company to ensure that all
of their requirements are met.
Can the electrical
installation of the distillation equipment be inspected
and approved by the local electrical inspector or the
Minnesota State Board of Electricity? Most industrial
equipment, whether it is a hard-wired installation or
simply plugged into an electrical outlet, must be inspected.
Stills listed under UL 2208 (Underwriter's Laboratories
Standard for Solvent Distillation Units) or equivalent
standards allow for the inspection approval. Using approved
components is the next best situation. Consider making
a purchase agreement contingent on installation approvals
by the electrical inspector and fire marshal.
Are the following
minimum safety features present on the still that you
are considering?
- Automatic shut down for high pot
temperature to prevent the still bottoms from continuing
to cook after all solvent has boiled off
- Automatic shut down for high condenser-water
temperature preventing solvent vapors from being lost
due to a malfunction that causes low or no coolant
flow
- A relief valve to release pressure
buildup
- Controls that are explosion-proof,
intrinsically safe or installed in a remote location
if flammable solvents are processed. Equipment installed
in a hazardous location (as defined by the fire code)
should be approved by an independent testing laboratory
to verify compliance with the National Electrical
Code
- Interlock features to prevent opening
of the still until it has cooled to a safe temperature
How are the
solvent vapors condensed? Water is generally
the simplest and least costly cooling medium. Air cooling
makes sense in some cases for example, when using very
small units. But, electric fan motors are not explosion-proof
and their use should be carefully evaluated. While refrigerated
chillers are an option, they are often too expensive.
Does your waste
solvent contain nitrocellulose? Nitrocellulose
is explosive when dry. Special precautions are required
to distill materials containing nitrocellulose. Automotive
lacquers, and flexographic and gravure inks can contain
nitrocellulose. Contact MnTAP for more information.
Regulatory
Overview
Recycling is a waste treatment process that is exempt
from hazardous waste regulations. Most solvents are
hazardous waste after they are used in industrial applications.
This is because the solvents themselves have toxic or
flammable properties, or they become contaminated during
use making the waste solvents hazardous. From the time
the solvent is no longer used in a process until recycling
occurs, companies need to comply with hazardous waste
storage, labeling, transportation and inspection requirements
for the waste solvents and solvent containers.
Still bottoms are generally hazardous.
If a solvent listed in the Minnesota Rules Chapter 7045.0135
is distilled, then the still bottoms are defined as
a hazardous waste. If the solvent is a characteristic
waste (has hazardous properties but is not specifically
named in the rules) then still bottoms will be hazardous
only if they exhibit significant hazardous characteristics
of their own. This generally needs to be determined
by laboratory analysis.
Recycling can reduce regulatory requirements
for companies if recycling sufficiently lowers their
waste volumes allowing them to be reclassified as smaller
generator-size class. Minnesota hazardous waste rules
state that recycled materials will only be counted as
wastes the first time they are generated. For example,
when a tank is emptied and the solvent is distilled
and then returned to the tank, the annual waste volume
is the solvent capacity of the tank—counted one
time—plus the volume of all still bottoms generated
over one year.
Closed-loop recycling exists when the recycling process is integrated into the manufacturing process and contaminants are removed as they accumulate. In a closed-loop system, wastes (other than still bottoms) are never removed from the manufacturing process and, in the regulatory sense, are never generated. Closed-loop recycling is difficult to accomplish on a small scale, but it is preferred over other recycling methods when possible.
Additional
Resources
The considerations listed above are some of the more
important points to evaluate before purchasing a still.
MnTAP maintains a list of small still manufactures in
its reference list Solvent
Recycling Equipment [#15], available online.
For More Information
MnTAP has a variety of technical assistance services
available to help Minnesota businesses implement industry-tailored solutions that prevent pollution at the source, maximize efficient use of resources, and reduce energy use and cost. Our information resources
are available online. Or, call MnTAP at 612.624.1300
or 800.247.0015 from greater Minnesota
for personal assistance.
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