GAPS Guidelines GAP.9.6.2.2
A Publication of Global Asset Protection Services LLC
SOLVENT RECOVERY WITH ACTIVATED CARBON
INTRODUCTION
Adsorption is the adhesion of an extremely thin layer of molecules to the surface of the solids or
liquids they contact. Activated carbon is the most common material used for such adsorption because
it is versatile and inexpensive. Typically, a carbon-containing material; such as coal, wood or coconut
shells; is reacted with heat and steam. This produces a highly porous substance that preferentially
adsorbs organic compounds from both gaseous and liquid streams. If air containing solvent vapor is
passed through a bed of activated carbon several inches thick, the solvent molecules are adsorbed
by the carbon. Carbon bed adsorption systems operate on this principle.
The carbon bed can be arranged for either batch or continuous operation. In batch type adsorbers,
the carbon bed is usually spread on metal supports in a vertical or horizontal steel tank. The
adsorbed solvent is recovered by blowing steam through the bed of carbon and condensing the
vapors. The solvent is then separated from the condensed steam through decantation or distillation.
In continuous adsorbers, the carbon bed is arranged in a large wheel that rotates in a vertical or
horizontal plane. Air containing solvent vapors is blown through one side of the wheel, which adsorbs
the solvents. The wheel rotates continuously, bringing the solvent saturated carbon bed into a second
stream of hot air that strips the solvents from the activated carbon. The second stream, which has a
lower flow rate than the first stream, has a correspondingly higher vapor concentration. This stream is
typically sent to an incinerator.
The batch type adsorber is older technology and is used where vapor concentrations are high enough
to justify solvent recovery and recycling. The continuous adsorber is used where the solvent
concentrations are low and the goal is to concentrate the vapors to make disposal by incineration
easier.
An entire carbon bed adsorption system consists of:
•Ductwork carrying the vapor-laden air from the point of vapor generation.
•Filtration by water wash, electrostatic precipitators or mechanical filters.
•Carbon bed adsorbers.
•Solvent disposal or recycling (incinerating, distilling, decanting, etc.).
Government regulations are forcing industry to continuously lower the amounts of pollutants emitted
into the atmosphere. As regulations become more stringent, the equipment needed for compliance
becomes larger and more expensive. The chance that a facility would not be permitted to operate
while such equipment is out of service is also increasing. Carbon bed adsorbers have, therefore,
increased in size and importance to point that a unit could cost $20,000,000 and take several months
to replace. Extensive loss prevention and control measures are therefore justified.
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POSITION
Management Programs
Management program administrators should report to top management through the minimum number of steps. They should also institute adequate loss prevention inspection and audit programs to communicate program effectiveness to top management. This management feedback is a key feature of OVERVIEW (GAP.1.0.1). In developing a program, pay particular attention to the following important areas:
Hazardous Materials
Determine the pertinent physical and chemical properties of the materials to be adsorbed. Choose test conditions that represent all possible operating conditions.
Establish routine procedures for testing physical and chemical properties of all materials to be adsorbed to confirm properties required for safe operating conditions.
Process Hazards Evaluation
Determine the safe operating and potential upset conditions of each new or existing adsorption process used by the plant. Include scaling factors in establishing the safety parameters.
Operator Training Program
Educate all operators in the hazards involved and in functions of the safety control equipment. Forbid operators to run the process when any of the safety control equipment is out of service. Train operators in manual emergency shutdown procedures. Forbid deviations from the written procedures.
Schedule re-education and retraining at least annually. Include testing to assure proper performance of all assigned duties with particular emphasis on emergency shutdowns.
Pre-Emergency Planning
Global Asset Protection Services (GAPS) pre-emergency plan section in OVERVIEW (GAP.1.0.1), may be used as a reference to develop a customized plan. This customized plan should include the following features:
•  A fire and disaster alarm system.
•An emergency communications system, including radio where needed.
•An adequately trained, staffed and equipped organization of employees for firefighting and other emergency duties.
•  A planned program of cooperation with neighboring plants and with public firefighting and disaster control organizations.
•  A program to analyze the interruption of business that may result from potential incidents.
•  A program to develop plans for minimizing loss of production during rebuilding after a loss. Inspection, Testing and Preventive Maintenance Programs
Inspect and maintain distillation system equipment, piping, instrumentation, electrical equipment and pressure relief devices according to a schedule established with proper consideration of design and service conditions. Include all appropriate types of modern nondestructive testing, such as IR scanning of switchgear and vibration analysis of rotating machinery, in the inspection techniques. Establish a detailed recordkeeping system that includes equipment retirement forecasts. Management of Change
Apply all management programs to any changes made to the facility’s physical plant or operating procedures. Pay particular attention to the following areas:
•Check all new equipment or parts to determine if it constitutes a replacement in kind or a change.
Manage each appropriately.
•Repeat the process hazards evaluations program for all new adsorption systems or for any modification to an existing system. Determine the need for new or different safety equipment or measures.
•Whenever equipment service is changed, or when the adsorption system is changed, examine the inspection and maintenance program and modify as necessary. Include even daily operating
changes.
•Verify new construction materials and all maintenance parts and supplies as conforming to original (or modified) design specifications.
•Apply the program for handling new construction, especially the control of outside contractors. •Update operations procedure manuals after each modification which results in a change in operating procedure.
•Review and follow through expeditiously on all inspection recommendations from insurance, code enforcement and regulatory agencies.
Other Management Programs
Incorporate these features into the comprehensive management program for loss prevention and control:
•Welding, cutting and other “hot work” permit programs.
•  A program for supervision of impairments of fire protection equipment using GAPS’ “RSVP”
program.
•Smoking regulations.
•Plant security and surveillance.
Duplication of Facilities
Provide duplication with installed spares of adsorption equipment that is highly susceptible to loss or very important for continued operations. If this is not possible, keep spare parts available and maintain them ready for use. Duplicated equipment should be physically separate by at least 50 ft (15 m) or by walls having at least a 3 psi (0.2 bar) overpressure resistance.
For smaller scale or batch type plants, install adsorption systems important to production in the form of multiple small scale units rather than a single unit. Design a single unit to facilitate prompt repair using readily obtainable or prepurchased parts. Where neither is feasible, maintain a backup pollution control method, such as an incinerator, to permit normal production until repair or replacement is complete.
For large-scale plants, provide multiple adsorption systems. Also, maintain spare parts for equipment known to be critical.
Design Considerations
When designing carbon bed adsorption systems, keep the following considerations in mind: •When designing processes, reduce equipment size to limit the amount of flammable liquids or vapors that can be accidentally released.
•Design adsorption systems using proper fail-safe instrumentation and enforce adherence to written operating procedures. Provide means to automatically relieve excess pressure to a safe location.
Provide intermediate alarms to allow operators time to take corrective action.
•Provide redundant power supplies and instrumentation loops for all critical controls. In redundant loop
s, include both separate signal transmitters and signal receivers. Install a comparator to notify operators when control and redundant signals differ. Also duplicate dedicated logic relay panels or PLCs with one acting as an on-line spare.
•To limit the amount of materials released by equipment failure, include the following in shut-down measures: automatic, fire rated block valves; venting to flare stacks or to incinerators; liquid
dumping to blowdown systems; and purging or flooding of equipment with a nonhazardous fluid.
Actuate these shutdown measures with combustible vapor detectors. Interlock equipment to shut down automatically and safely in the event of operator error or equipment failure.
•When designing safety features, assume a minimum of two consecutive errors, one of which may be misinformation because of a faulty instrument or a misunderstanding of instructions. Safety instrumented systems should conform to ISA 84 or IEC 61512. Prove by calculations or testing the response time of sensors and logic circuits.
•Design and specify equipment considering all possible operating conditions, both normal and abnormal. Give particular attention to suitability of the equipment to handle the process materials and to withstand external environmental influences while resisting common mode failure
mechanisms, such as freezing of sensing lines.
Design Features
Equip carbon bed adsorption systems handling flammable vapors with the following features:
•  A CO/CO2 monitor in the exhaust system.
•Temperature monitors within the adsorber bed.
•Sufficient air input to keep the solvent/air mixture at the points of generation (such as ovens) below 25% of the lower flammable limit.
•One or more continuously operating flammable vapor detectors interlocked to audible signals at a constantly attended location; automatic shutdown of the process producing the vapors and
emergency venting of the adsorber to a safe location. Design the system to activate the alarms at 25% of LEL and to initiate shutdown and emergency venting at 50% of LEL. Use a portable
flammable vapor detector to make periodic tests during normal operations and special tests when ther
e are changes in operating temperature, when there are several evaporating units in use, or when there are several kinds of solvents or proportions of solvent mixtures in use. Such detectors must always be listed for safe use in explosive atmospheres.
•Controls to automatically stop or divert vapors to a safe location if air flow through the system stops or falls below a previously determined safe limit. Keep exhaust fans in operation in such cases. Install a diverter valve at the entrance header to the carbon bed adsorber to divert vapor flow to atmosphere at a safe location in the event of low air flow, high bed temperature, high
CO/CO2 ratio, or high levels of volatile organic compounds (VOC) in the system.
•For large systems, separate fans with separate power supplies, connected for emergency operation in case power or mechanical failure shuts down the regular fans. Alternatively, dual drive fans or emergency steam ejectors might be used. Fans located at the inlet are preferred to
exhaust fans to force a continuous flow of air through the adsorber.react with
•Equipment vent lines extending to a point above the roof level of any adjacent building (unless connected back into the recovery system). Equip them with pressure-vacuum vent valves. Equip vent li
nes with listed flame arrestors, unless there is a possibility of vent lines becoming plugged. •Explosion relief disks at the elbows and at intervals of every 30 duct diameters on vapor transfer ducts. Aim the discharge from these explosion vents away from other process vessels. Operations
Batch Type Adsorbers
•Apply steam for a sufficient rate and time to ensure removal of the solvents from the carbon bed.
Continue steaming until the carbon bed reaches a temperature at which all the solvent has been driven off. Steam should reach all areas of the carbon bed.
•Maintain the temperature of the vapor-laden air entering the adsorbers at 100°F (37.8°C) or lower for easily oxidizable solvents to reduce the probability of ignition.
•Perform a specific gravity check of the adequacy of steaming or make an analysis of the condensed steam and solvent vapors leaving the adsorber. These tests should be required before any shut down.
•Test the adsorber and the incoming steam line safety valves frequently for dirt or residues from the air-solvent mixture. If necessary, install filtration at the collection points.
•Maintain the moisture level during the adsorption cycle by precooling, limiting the adsorption time, adding humidity to the air-vapor mixture or by combinations of these methods.
•Thoroughly wet new carbon with water before placing it in service. Keep the first few adsorbing cycles relatively short and thoroughly steam the carbon between adsorptions.
•Before a unit is to be left idle overnight or over a weekend, perform a very thorough steaming. Cool the unit after steaming to decrease the oxidation reaction that could result in spontaneous heating.
Cooling may be accomplished by blowing air through the carbon bed if it is certain that only a few pounds of solvent remain adsorbed. If pressure steaming is not possible or if there is reason to suspect an appreciable amount of solvent is retained, wet the carbon with water.
Continuous Type Adsorbers
•Maintain the temperature of the vapor-laden air entering the adsorbers at 100°F (37.8°C) or lower for easily oxidizable solvents to reduce the probability of ignition.
•Maintain rotation of the carbon bed and apply stripping air at a sufficient rate to ensure removal of the solvents from the carbon bed. The stripping air should reach all areas of the carbon bed. •Make an ana
lysis of the solvent vapors leaving the adsorber. These tests should be required before any shutdown.
•Test the adsorber safety valves frequently for dirt or residues from the air-solvent mixture. If necessary, install filtration at the collection points.
•Maintain the moisture level during the adsorption cycle by precooling, limiting the adsorption time, adding humidity to the air-vapor mixture or by combinations of these methods.
•Thoroughly wet new carbon with water before placing it in service. Keep the first few adsorbing cycles relatively short and thoroughly strip the carbon between adsorptions.
•Before a unit is to be left idle overnight or over a weekend, perform a very thorough stripping. Cool the unit after stripping to decrease the oxidation reaction that could result in spontaneous heating.
Cooling may be accomplished by blowing cool air through the carbon bed if it is certain that only a few pounds of solvent remain adsorbed. If there is reason to suspect an appreciable amount of solvent is retained, wet the carbon with water.
Protection
•Use only metal ducts and protect them according to GAP.2.3.2.
•Protect precipitators according to GAP.9.3.2.1.
•Protect distillation operations according to GAP.9.6.2.1.
•Protect incinerators according to NFPA 82.
•Protect equipment decanting or handling flammable or combustible solvents according to GAP.8.1.0 and GAP.12.2.1.2.
•Apply GAP.2.5.1, GAP.2.5.2, GAP.2.5.3 and GAP.12.2.1.2 to buildings housing carbon bed adsorbers.
•Provide fire protection for the interior of carbon bed absorbers. The following protection methods are presented in descending order of desirability:
°Water Flooding - Provide a manually operated flooding valve fed by the fire protection water system and connected to the bottom of the adsorber. The size of the flooding system piping
and valve should be at least 4 in. (100 mm). Also provide a baffle in front of the flooding inlet to the adsorber to prevent excessive turbulence. Design the adsorber to withstand the weight
and pressure of being filled with water and incorporate means to prevent water from entering
the ducts. Install manually operated drains to remove the water after extinguishment is
complete.

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