Special Report: Volatile Organic Compounds

(Lourtesy. \I ara Internxlonal)

Special Volatile Org

PAUL N. CHEREMISINOFF

1 -__

Solvent recovery systems are used to remove solvent/ organic vapors from air emissions originating from a wide variety of industries and product lines as well as to separate, purify and dehydrate recovered solvent mix- tures. The vapor adsorption process consists of passing solvent laden air through an adsorption or recovery system consisting of a quantity and type of adsorbent spe- cifically designed for the purpose. Without control the emission to the atmosphere would otherwise be pollut- ing, odorous and possibly health-harmful.

There are many types of materials that exhibit adsorptive properties, such as activated clays and Fuller's Earth, aluminum oxide base materials, metal adsorbent chars, silica gels, magnesia base materials, zeolites (molecular sieves) and gas adsorbent carbons.

The most generally used adsorbent for solvent recovery systems for air streams is carbon. Such systems may consist of at least two adsorbers for continuous automatic processing of the solvent-air stream and recovery of the solvent contained in the air. Other systems can be designed with one adsorber for batch recovery service or three or more adsorber units for expanded capaci- ties. Gas streams may additionally be pretreated prior to adsorption by passing through coolers and filters, as required, for particulate removal or pre-conditioning the stream to optimize adsorption conditions.

Carbon Adsorption-The Process The utilization of carbon in some form as an adsor-

bent cannot be dated in history; centuries ago it was used as a purifying agent by the Egyptians, and the Hin- dus used charcoal for water filtration. Purification of su- gar solutions with carbonaceous materials dates back to the thirteenth century and in the latter part of that century it was found that carbon had the ability to adsorb gases. Scheele in 1773 described experiments on gases exposed to carbon. Since then carbon adsorbents have been used, researched and developed to an extent where applications are innumerable.

Adsorption is governea by the chemical nature of the gaseous phase, solid phase adsorptive behavior and in part on the surface area. Carbons having equal weights and total surface areas when prepared by different methods will exhibiir different adsorption characteris- tics. Some adsorptive properties can be explained by differences in relative pore size distributors, but a more important consideration is the difference in surface

Figure I . Major installation of solvent recovery system. (Cour- tesv: Amcec Coro.

Figure 2. Completely automatic solvent recovery system is ca- pable of recovering up to 80% of the totul solvent for reuse in cleaning system. (Courtesy: Baron-Blakeslee.)

properties. The chemical nature of the surface function- al groups depend on activation procedures used in the carbon manufacturing or regeneration process. Effec- tive use of an adsorption system requires basic knowl- edge of the adsorption mechanism and the surface func- tional groups responsible for adsorption of a particular adsorbate.

An activated carbon system can usually effectively operate through thousands of adsorption-desorption cycles often over periods of years. A carbon system usually consists of two distinct operations:

1. Contact (adsorption process), 2. Carbon regeneration cycle or system.

A typical arrangement of equipment is shown in Figures 1 and 2.

Adsorption systems used for solvent recovery or odcr

1. Adequate contact (residence time) between air stream and $orbent bed for adequate sorption;

2. Enough sorption capacity for desired service life; 3. Low pressure drop (resistance) to air flow for effi-

cient and adequate fan operation in moving the gases;

4. Uniform distribution of air flow over the sorbent bed:

5. Air pretreatment where required for removal of materials that might impair the sorbent bed performance or regeneration; and

control require in their design:

6 . A regeneration method after sorbent saturation. Required contact time in an adsorption system can be

obtained by plotting vapor concentration vs bed depth as shown in Figure 3. Such a plot can be obtained ex- perimentally or from a sorbent supplier who has had cs- perience with the particular system. As can be seen in Figure 3, the shape of the vapor concentration curve shifts to the right as a function of time or amount of VOC passing through the unit. When the concentration equals the threshold concentration (C, = C,) this is de- fined as the breakthrough point. The bed depth from the point where VOC concentration equals the inlet concentration (C, = C,) to the breakthrough point is

30 MARCH 1985

CONPLP2 FULLY SATUIUTED VAPOR SATURIITlON

0 DEPTH OF SORBENT BED c

Figure 3. The adsorption wave jront.

termed the minimum transfer zone (MTZ) and is the minimum bed depth permissible. Overall bed depth needed depends on the sorptive capacity of adsorbent, which can be obtained from the adsorption isotherm of the svstem.

Designing the System

quires the following data: Designing an activated carbon adsorption system re-

1. Gas volume to be processed (in actual cubic feet

2. Gas stream temperature ( O F , "C); 3. Materials to be removed (adsorbed) from the gas

4. Concentration of the material to be adsorbed; 5. Presence of other pollntants in the gas stream; 6. Is solvent recovery required or justified; 7 . Cycle time of the system (numberifrequency of re-

The amount of adsorbent required will be deter-

1. Duration of adsorbent service before saturation

per minute);

stream:

generations).

mined by the following factors:

(time); . 2. Sorption efficiency;

3. Air flow rate through the bed (acfm); 4. Character and molecular weight of sorbed vapor

5. Initial vapor (VOC) concentration (ppm by vol-

6. Proportionate saturation of sorbent (fractional). Figures 4 and 5 illustrate typical modern solvent re-

covery plants. In Figure 4, solvent laden air is passed through filters to remove dust and either heaters or coolers to bring the air to approximately 90 F. Where appropriate, heat recovery exchangers are used to reduce energy requirements. The air enters the inlet air manifold via the fan, automatic pneumatically controlled valves allow the air to pass through the adsorber on stream. The adsorber contains a bed of activated carbon usually between 12 and 36 in. deep arranged in either a horizontal or vertical annular bed. As the air passes through the carbon the solvent is adsorbed, so clean air can exit via the exhaust valve and chimney.

A gas analyzer samples the exhaust gas and when solvent is detected, at a predetermined level, the carbon

(VOC) ;

ume);

'OLLUTION ENGINEERING

Ti TI 6

SEAL POT I C

I A - SOLVENT LADEN AIR INLfT E -COOLING WbTER RETURN

F -EFFLUENT TO DRAIN H - CONDENSkTE T O DRAIN

R - fTPlDPr? A,- c2-l.:- i - r r L ~ u v t n t a SOLVENT OUTLET

f Figure 4. Preliminary Flowsheet for Solvent Recovery Plant.

bed has reached maximum capacity. The solvent laden air (SLA) is then diverted to a second adsorber by switching the appropriate valves and atmospheric pressure steam is fed to the saturated adsorber. TRe steam flows countercurrent!y to the SLA. The first few minutes of steaming are used to heat the bed and adsorber. the condensate being fed to the s a l tank w:iic!~ slsa a&; as a pressure relief system.

Once steam ha:. broken through the carbon bed it is passed, together with solvent vapor. to the condenser and cooler exchangers. The condensate is collected in the decanter where water immiscible solvents are separated off for reuse. Water miscible solvents are sent to a distillation or liquid phase adsorber plant for water removal. The adsorbers cycle automatically and the sequence is controlled and monitored by a microprocessor. Figure 6 shows a typical sequence chart for the adsorber valves on a 3-adsorber plant. Heat recovery from the condensing steamlvapor mixture is commonly used to preheat boiler feed water andior provide process hot water.

The quantity of steam required to recover the soi- vents varies with the solvent and the solventlair concen-

Figures. 32,500 cfm ADSOLVsolvent recovery plant. Carbon adsorption with distillation recovers Isopropyl Acetate for a manufacturer of gravure-printed packaging labels. Solvent is reused in printing process directly from recovery system. All equipment operates automatically under computer control. (Courtesy: Rav Solv, Inc. )

I Figure 6. Typical adsorber valve sequence chart for 3 adsorber plant with drylcool mode. (Courtesy: Sutcliffe- Speakman Inc.). I

trations. However, most modern plants use between 2 and 3 lb of s teadlb of solvent recovered.

One type of adsorber is the vertical annular bed design in which the carbon in the adsorber is contained between vertical, cylindrical screens. The solvent laden air passes from the outer annulus through the outer screen, the carbon bed and the inner screen, then exhausts to atmosphere through the inner annulus. The live steam for regeneration of the carbon is contra-flow to the direction of the airstream. In this design a large surface area of carbon is presented to the incoming air. Con- densation of solvent and water on the outer shell of the adsorber during the steaming period joins with the main product flow from the condenser to the decanter. Be- cause of the annular space between the carbon bed and the outer shell. the condensate does not cause wetting of the carbon. The carbon can be easily emptied from the adsorbers, when necessary, through emptying pipes at the bottom of the bed, and filled through the top fill- ing plate.

The flat bed adsorber normally has a cylindrical or horizontal bed arrangement, the carbon being contained between screens. In different designs the airflow passes upwards or downwards through the bed and the lip? steam is contra-flow direction. The adsorbers usual- 1y require insulation oecause cf the contact of 'h e carbon against the side walls of the carbon container in this design G € adsorber. Figure 7 shows a typica! flow diagram for a flat bed adsorber plant. Each of the above types of adsorbers can be fabricated in various materials dictated by the process conditions and the particular solvents to be recovered.

Carbon Life The life of carbon is affected by attritior, rate and re-

duction in the adsorptive capacity of the carbon from adsorption of certain high-boiling materiais such as plasticizers, resins, volatile organosiiicone compounds or other substances which may not easily be removed from the carbon during the desorption cycle. Attrition rates are usually less than 1-3 percent per year, depending on carbon hardness and workload of the system. Containments present in the VOC-airstream can affect the carbon adsorptive properties, reducing system life. When conditions are such that the remaining adsorptive capacity of carbon is below desired performance levels. the carbon may have to be removed from the sysrern and regenerated by reactivation at high temperatures in CI suitab!e furnace.

Depending on types of contaminants in the gas

stream, it may be possible to remove large proportions of these by pretreatment before the carbon bed by liquid scrubbing or including a sacrificial carbon bed for selective contaminant adsorption against the solvent. The sacrificial bed is generally not regenerable in-situ, and for example, would not normally be used for ketones recovery. The oxidation effect when ketones remain on carbon can lead to overheating and possibly carbon ignition.

Materials of Construction The materials of construction must be carefully se-

lected for the type of solvents to be recovered and to protect against any acidity which may be present in the inlet airstream. Some solvents when adsorbed on activated carbon, or in the presence of steam, oxidize or hy- drolyze to a small extent and produce small amounts of corrosive materials. As a general rule, hydrocarbons and alcohols are unaffected and carbon steel can be used. In the case of ketones and esters, stainless steel or copper or other corrosion resistant materials are required for the solvent-wetted parts of the adsorbers, pipework and condensers, etc. For carbon tetrachloride and 111 trichlorethane and other chlorinated solvents, titanium has a high degree of resistance to the traces of acidity formed during recovery.

Pressure Drop

function of Pressure drop or air resistance across :he system is a

i. Sorbent particle size; 2 . Required bed depth; 3. Superficial linear velocity through the bed. Air flows through commercial adsorption units range

from 25 fpm for thin bed units, to 80 fpm for thick bed units. A typical pressure drop vs superficial linear velocity is shown in Figure 8.

Particulates in the air stream may foul or plug the adsorbent bed. This type situation will increase the system pressure drop causing 3 decrease in adsorption efficien- cv. Such cases require a precleaner-dry collector such as filters, Lyclone separarors or electrostatic precipitation. Wet collection methods are generally not desirable be- came they will incresse the relative humiditjj of the car- rier gas. Where scrubbers are employed, use of a chiller and reheater can be used to control moisture content to the adsorption system. Small amounts of moisture do,

Figure 7. Adsorption system.

32

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r

7

I

4 1985

i i f

1

1

I I

5

1

t i

1 i I

E t

I t

~

i I

t f

0.11 I 1 I I I l l

Figure 8. Superficial linear velocity through bed (ft.lmin.).

however, generally enhance the adsorption process, acting as a control for the heat of adsorption (the process is generally exothermic).

Efficient Use of Airflow The quantity of airflow to be handled by a solvent re-

covery plant is a critical factor in the size and cost of the plant. Many processes using solvents have excessively high exhaust airflows, probably because the application of emission control equipment was not taken into consideration in the design of the machines. It is also frequently possible to reduce the airflow on a process machine without affecting the operation of the process, or the effective collection of the solvent vapors, by modifi- cation to the machines to increase the concentration of solvent in air as it is exhausted from the machine.

Most insurance companies will allow processes to operate with solvent in air concentratioas up to 25 percent of the lower explosive limit (LEL) without control de- vices, but above 25 percent concentration it is usually required that meters are used to monitor the concentrations. With these meters it is usually permitted to go to 40 percent of the LEL with an alarm above that concentration and automatic shutdown of the process if the concentration exceeds 50 percent of the lower explosive limit. LEL meters can be used to control exhaust airflows from the processes with suitable monitoring of the operation and minimum settings on exhaust dampers so that a minimum flow of air compatible with safety is al- ways maintainea.

Solvent recovery plants use the least energy when the incoming air is at constant flowrate and high concentration. By recycling process air until the concentration level is at as high a ievel as pracricabie, the heating re- quirement for the air is also reduced.

Reducing airflow as above can mean a considerable saving in the installed cost of a recovery plant. For example, there would be a saving of several hundred thou- sand dollars in reducing from say 100,000 to 50,000 scfm on a typical plant. In addition, there would be considerable savings in both the operation of the process and the recovery plant by power savings and reduction in heat

POLLUTION ENGINEERING

usage in the process by handling less airflow. Less steam would also be required for regeneration of the carbon through working at higher solvent concentrations.

Steam Economy Economy in the use of steam for regeneration of the

carbon and recovery of the solvent is frequently achieved by the use of gas analyzers. This will ensure that the adsorbers are effectively loaded with solvent before being steamed out, so that the best steam to solvent ratio is obtained. The analyzers usually measure the solvent content in the exhaust air from an adsorber. The steaming cycles are delayed until the adsorbers are charged as fully as possible, with timer override in appropriate cases.

Recovery of Water Miscible Solvents The simplest type of solvent recovery plant is one in

which a single solvent is non-miscible with water, e.g., toluene. This solvent will collect in the upper layer in the decanter, and, being virtually insoluble in water can be simply decanted for reuse while the lower (water) layer is passed to drain.

If, however, water miscible solvents are being recovered, then additional equipment is necessary to extract the solvent from water. This usually takes the form of distillation or liquid phase adsorption equipment together with a dehydration plant depending on the solvent(s) involved and the final specification for solvent purity required by the customer.

Ketone Recovery Particular care must be taken in both the design and

operation of plants for the recovery of ketone solvents to protect against the effects of oxidation of the ketone on the carbon which can result in an exothermic condition in the bed. If precautions are taken, most ketones can be safely recovered. Insurance companies normaily require prescribed systems to warn of overheating of

Figtire 9. Skid mounted solvent recovery. system ready to be installed. This system can return 85-95% of the solvent that evaporates at emission points through the plant. With savings in solvent of thb mugnitude. the reiurn in solvent cost savings can amortize the full cost of the unit - inchiding ~ervices expense - in I2ss than a year. Some manufacturers have reported total payback on systems in as little as nine months. ICourtesc.: Hoyt -Manufucturing Corp.)

33

the carbon (which is more likely to occur during shutdown of a plant than when the plant is in normal operation). Other warning systems include carbon bed and exhaust air thermometers (the latter with high temperature alarms) and monitoring of the carbon monoxide content of the air in the adsorbers during shutdown.

Economics Recovery of solvents by the activated carbon system

can be economically attractive, particularly in cases where utilization of the equipment is at a high level. The value of the solvent recovered obviously depends on solvent quantity, so the greater the plant utilization the more attractive the payback. Many plants amortize in less than 12 months. (See Figure 9).

The following is a typical schedule of economics for a plant recovering esters and toluene solvents as used in many coating and gravure printing processes. The main solvent wetted parts of the plant are fabricated in stainless steel. The system includes equipment to extract the water soluble fractions of the solvents from the water phase and dry the solvents. The solvent recovery operation is controlled by gas analyzer and microprocessor to minimize the steam consumed.

Aimow Solvent Load Solvent Cost Steam Cost Water Electrical

30,000 scfm 600 lblhr

$ 0.33llb $ 0.005ilb $ 0.35!1000 gal $ 0.045IKwH

Capitalcost Equipment Installation

$575,000 225,000

$800,000

Operating cost (6,000 hriyr. operating at an average of 68 percent of designed capacity)

Steam $ 48,000 Water $ 2,000 Electricity $ 27,000 Operating & Maintenance $ 15,000

$ 92,000

Depreciation (2 yr> $160,000 Interest on Capital $ 75,000 Yearly Operating Cost $327.000 Value Recovered Solvent $712.000 Value Net Operating Profit $385,000 Simple Pavback 2 Years

The economics can usually be improved by incorporat- ing a heat recovery system. This i s particularly true when the inlet air from the process is at high temperature, or the solvent load is at a consistently high level.

Case History-Canister Type Carbon Adsorption System

In the process of coating bulbs, a very small amount of aromatic solvent, 1 to 1.5 lbimin, escapes into a 9000 cfm process exhaust. The challenge by the company was to find a system that would economically and efficiently handle their particular combination of high airflow and

Figure 10. In a coating bulb process, a very small amount of aromatic solvent, 1-1.5 Ibsimin escapes into the exhaust. The challenge was to find a system that would economically and efficiently handle high airflow and low solvent concentration. (Courtesy: VIC Manu- facturing Co.)

low solvent macentration. (See Figure 10). In their quest for an efficient, economical solution to

the problem, the company’s pollution engineers investi- gated incineration as well as conventionai solvent recovery techniques employing deep bed carbon adsorption. They explored the advantages of a new form of carbon adsorption system-a canister design that relies on the injection of low pressure steam. After a careful analysis and comparison of systems, they chose a canister design. The system employs carbon canisters designed to filter large quantities of air through relatively small amounts of carbon,

The customized system required a canister depth of only 4 inches, completely adequate €or the amount of carbon needed for their application. The system consists of two 6 ft diameter vertical stainless steel vessels. Each vessel contains 12 carbon canisters for high airflow. One carhon vessel adsnrhs the vnmatir m!vrlpf

vapors from the process airstream while the second vessel is desorbed for reuse. During desorption, low pressure steam is injected into the canisters’ counterflow to the adsorption process. The steam and solvent within the gas vapors are condensed on a cool water coii. Liq- uids are gravity-separated providing reusable solvent and steam condensate.

Steam regeneration has provided the system with the kev to energy-saving pollution controi. Although the carbon has been in constant use since the system was installed. the companv expects it to operate another four or five years before carbon replacement becomes necessary. The relatively small energy requirements of the

I

MARCH 1985 34

.process and the reusability of the recovered solvent ex- plain why thermal incineration was not employed.

Flxd Bed Adsorption wlth Incineration Direct or catalytic incineration of VOC laden air be-

comes costly when solvent concentratlons are low. De- pending on geographical location, energy costs will vary somewhat but the cost of heating thousands of cfm of air to incineration temperatures can be prohibitive when solvent impurities are present in the 0-100 ppm range.

In this application, activated carbon acts as an accu- mulator, actually adsorbing solvents from the solvent laden air (SLA) and storing them until the carbon becomes saturated. The solvents are desorbed from the carbon (regeneration) by passing hot flue gases through the bed. These solvent-laden flue gases are then passed through the incinerator where the solvents are inciner- ated to combustion prooucts. Since this regeneration process is, by design, performed infrequently, incineration costs can be reduced by as much as 95 percent when compared with catalytic incineration.

SLA is directed into one of two adsorbers (See Figure l l ) , D-1 or D-2 by blower C-1. As the SLA passes down through the horizontal bed of carbon in D-1, the solvent is adsorbed and the clean air is discharged to the atmosphere or returned to the work-area for additional energy savings. The discharge air from D-1 is monitored by a hydrocarbon analyzer which measures the hydrocarbon concentration leaving the carbon bed. As soon as solvent breakthrough is established, the system automatically shifts the SLA to the alternate adsorber D-2 and isolates D-1 from the adsorption process.

Regeneration of the carbon in D-1 is automatic and the following steps take place:

1. The incinerator is lit using gas or fuel oil and al- lowed to come up to operating temperature (1200 F). During this period (15-20 minutes) the flue gas from the incinerator is discharged to the atmosphere.

2. When the incinerator conditions are stable, blow- e r C-2 is started and a slip stream of hot flue gas is pumped through the air cooled exchanger (cooling the gas to 300 F) into the adsorber and downward through the carbon bed. The hot gas heats the vessel internals and vaporizes the solvents.

3. The solvent-flue gas mixture, exiting the adsorber vessel, passes through a gas to gas heat exchanger and is preheated prior to incineration. The 1200 F slip stream flue gas from the incinerator stack is the source of heat.

4. The preheated solvent vapor flue gas stream is mixed with combwion air and burned in the incinerator forming additional flue gases. The concentration of solvent vapors in the mix is less than 20 percent of the LEL but is high enough to provide for self sustaining combustion (at which time the fuel gas is automatically cut off). The combustion air flow is controlled by mea- suring the concentrations of oxygen in the flue gas.

5. At the end of regeneration, as indicated by a drop in the fuel gas temperature, the fuel (gasloil) is reignit- ed. The system, including the carbon bed, is purged and cooled with air, then the fuel gas is cut off and system is shutdown.

Regeneratlon of Spent Carbon Activated carbon placed in solvent recovery service

will likely have a useful service life of 1 to 10 years depending on the application, solvent type or mix. During the course of its service, there will be a gradual solvent buildup which is not recoverable in the normal stearmng cycle. This is due in part to solvents gradually diffusing deep into the carbon internal pore structure and becom- ing inaccessible to the usual stripping action of steam. This gradual loading will decrease the carbon’s activity.

While carbon can remain in service in a reduced capacity state, it represents a non-optimal operation of the system that results in increased costs. Symptoms that typically appear include reduced pounds of solvent recovered per cycle and, as a result, increased steam and overall costs per pound of solvent recovered. Sys- tems, whex cycles zrc regulate& bj preset timer, may also experience higher emissions as well as increased replacement solvent costs. Systems regulated by a gas analvzer will experience shorter adsorption cycles afid may not be able to cycle fast enough to meet production demand. Eventually. recovering capacity will diminish; it i b more economczl to replace carbon than continue its use in a deteriorated state.

In replacing spent carbon there are two options: (1) Replacing it with virgin carbon, which can be very costly and does not address the solid waste problem posed by spent carbon; and (2) reactivating the spent carbon to its virgin adsorptive capacity for a fracilor, of the CUSK. There are services which provide comprehensive programs to customers, including carbon testing to track decline in carbon’s activity, custom offsite reactivation service and replacement carbon.

Reactivation is very similar to the Frocess used to ac- tivate carbon originally. It is carried out at very high temperatures in the presence of a controlled atmosphere of activating gases. A custom, segregated ap- proach is used in reactivating the spent carbon. The carbon is processed separately from all other users’ carbon thus guaranteeing return of the same carbon to the same application. Segregated processing also allows reactivation conditions to be optimized to suit a specific carbon type, adsorbed contaminants, and the carbon’s general service life. It typically restores spent carbon to a full 100 percent of its virgin carbon activity specification for between one-half to one-third the cost of pur- chasing new carbon. Furthermore, since reactivation is

U J FROM

PROCESS

Figure 11. Activated carbon adsorption with incineration. (Courtesy: Nuclear Consulting Services Znc.)

POLLUTION ENGINEERING 35

Figure 12. Linde’s new vent gas condensation system. (Courte- sy: Linde Div., Union Carbide Corv.)

a recycling process it eliminates spent carbon disposal costs.

Such service also includes screening carbon to its original mesh size. This eliminates fines that are gener- ated in handling that may cause increased pressure drop or that are too small for system retaining screens. Charges for these services are usually based only on the dry on-size reactivated carbon returned.

Reactivated carbon quality can be confirmed in ad- vance by lab evaluation of a representative quart sam- pie of spent carbon supplied by the customer. Rapid turnaround is usually afforded.

Vapor Condenser Vapor condensers can also be used to recover vapors

from trucks, storage tanks, reactor barges and rail tank cars. A new vent gas condensation system designed to recover chemical vapors as they are vented from tanks or other vessels, has been developed by the Linde Divi- sion of Union Carbide Corp. The new system-using cryogenic nitrogen as the refrigerant-recovers vapors which should not be vented into the atmosphere for either environmental or economic reasons. The first installation of this system was recently completed at the El Dorado Terminals Co. facility in Bayonne, NJ. El Dorado is a joint venture of Dow Chemical U.S.A. and Powell Duffryn Terminals, Inc. At El Dorado, the system is used to recover dichloromethane and trichloroethane vapors which emerge from trucks as these solvents are being loaded in liquid form. The apparatus can be used to recover many other chemical vapors by adjusting its operating temperature.

A shell-and-tube heat exchanger is used as the vent condenser. Solvent laden air is ducted from the truck body and passed over the fin side of the heat exchanger. On the tube side of the condenser cold nitrogen gas is passed by blending liquid nitrogen with warm nitrogen gas. This particular application uses warm nitrogen gdS

for blanketing, so some of that is blended with liquid to produce cold gas. As the solvent vapors cool and con- dense, the resuiting droplets fall into a 300 gal holding tank from which the liquid is later retrieved. The remaining relatively ciean air, containing iittie residual vapor, can be vented in an environmentally safe man- ner to a proper location.

The Linde gas recovery device built for El Dorado actually contains two separate systems mounted on a single skid. One is to recover the dichloromethane and the other the trichloroethane. The former system utilizes cold gas at -225 F to cool the vent stream to -10 F. while the latter uses cold gas at -100 F to bnng the vent stream to -7 F.

The beauty cf this system is :hat .rirtuall\i no nitrogen is consumed in the process. Once the nitrogen uasses through the tin coils to chill the vapor on the outside of the c d s , an ambient sir heater warms the nitrogen f a -

LIQUID GASOLl N E

Figure 13. Hydrocarbon vapor recovery unit for gasoline bulk starions, condensation process. (Courtesy: Edwarh Enginerrinw

36 MARCH lf3

Figure 14. An indirect steam-heated unit, processes both aqueous sludges and contaminated solvents. In 8-10 hours, 55 gallons of waste water or 110 gallons of solvent are processed, with the contamination left behind in an easy-to-clean Teflon-coated liftpan. The unit is available with an in-plant steam hookup or remote 15 or 30 kw boiler package. (Courtesv: Finish Enfineerinp Co. )

ther for use in the blanketing system. And, since El Dorado already had a Linde liquid nitrogen storage system in place for its blanketing system, the kapor condenser merely does some of the work of the conventional vaporizer normally used to convert liquid nitrogen to blanketing gas.

While other types of vapor condensation systems are available, such as those that use mechanical refrigeration to promote condensation, the Linde system is espe- cially economical for those who already have liquid nitrogen storage onsite. Where liquid nitrogen is available, the initial cost of this condenser system is less than a mechanical system of equivalent size. In addition to condensing vapor from trucks, the system can be used when venting chemical storage tanks, reactors, barges, and railroad tank cars. A schematic of this recovery system is shown in Figure 12.

Figure 13 shows hydrocarbon vapor recovery unit for gasoline bulk stations by the condensation process. A conventional refrigeration chiller cools glycol and water to 34 F for precooling the vapors to remove as much water vapor as possible without the formation of hydrates. The effluent vapors leave the precooler at a standard- ized water vapor dewpoint condition of approximately 34 F wet bulb and 34 F dry bulb. The vapors, after leaving the precooler, enter the top section of the condensing column where moisture and hydrates are removed. In the next section, heavier molecular weight ends are condensed in the column. The design and use of a direct

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T O T E Y P E R A T U I E INDICATOR

.CONTAINS T W O 1 SEALED i m p . CONTROLLERS

-*.-.-,..*.-.-,-.\

Figure 15. Schematic flow diagram.

expansion refrtgeratioii condenser coil heat exchanger permits raising the refrigeration compressor suction pressure and so increasing the capacity of the unit.

At periodic intervals. defrosting of the finned sur- fcces is axajmplished by circulation of war= brine stored in a separate reservoir. The temperature of the warm defrost brine is maintained by heat reclamation from the refrigeration equipment.

Minimal shutdown time is rcquired to accomplish defrosting in the unit, since the unit is equipped with a precooler. The precooler acts to remove most of the water vapor in the entering hydrocarbon vapor-air mixture, thereby reducing the time required for defrost. Defrosting is completed in 30 to 60 minutes, depending upon the amount of frost collected on the finned coil. Dual condenser units with no time lost for defrost are also available.

~- ~

Figure 16. The problem of recycling and disposal of waste solvents can be eliminated entirely by using a commercial source which takes complete responsibility for continually supplying vou with clean solvent. (Courtesv: Safetv-Kleen CorD. j.

37

Selected Equipment & Systems Suppliers of Solvent Recovery Systems and Carbon

Reader Reader Service Service Number Company Name Number Company Name

400 APV Equipment Inc. 425 Glitsch Inc.

401 Advanced Ind. Technology Corp. 426 Gold Shields Solvent Div., Detrex 402 Air Plastics Inc. 427 W. A. Hammond-Drierite Co.

403 Allied Industries 428 Heil Process Equipment 404 Amcec Corp. 429 Hoyt Corp.

405 American Vicarb Corp. 430 IC1 Americas 406 Ametek Havee Div. 43 1 Koch Eneineerine Co.

407 Andersen 2000 Inc. I 432 Mateson Chemical

408 Anspec Co. Inc. 433 Multi-Metal Wire Cloth Inc.

409 Artisan Industries Inc. 434 Norton Co. 410 Baron Blakeslee 435 RaySolv Inc.

411 Beco Engineering Co. 436 Reco Industries Inc.

412 C-E Natco

413 Calgon Carbon Co. 438 Safety-Kleen Corp.

414 Carbone USA Corp. 439 Serfilco Ltd. i

415 CECA Inc. 440 Stebbins Eng. Mfg. Co.

416 Ceilcote Co. 441 Sutcliffe Speakman Inc.

417 Chem-Pro Com. 442 Texcel Intemational

451 Control Instruments Inc. 443 Tigg Corp.

418 Coming Glass 444 Union Carbide Corp.

419 Croll Reynolds 445 Vara International Inc.

452 DR Technoloev Inc. 446 VIC Manufacturing Co.

420 Edwards Enpineering Co. I 447 Westvaco Com.

42 1 Envirotrol Inc. I 448 Whitco Chemical Co. 422 ES Industries 449 York Process Equip. Co.

423 Ferguson Industries 450 John Zink Co. 424 Finish Engineering Co.

Recovery of Collected Solvents The distillation of contaminated solvents after ad-

sorption and regeneration has become very imponant due to solvent cost and their problem in hazardous waste disposal. Distillation technology has now even been packaged for the small user.

A package distillation umt is shown in Figure 14. Ba- sically, the unit boils contaminated solvent in a special vessel. Solvent vapors pass through a water-cooled heat exchanger, are condensed to a liquid, and fall into a re- ceiving drum.

Referring to Figure 15, the process is as follows: Tef- lon-coated boiling vessel (A) contains a cast-in aluminum heater and thermowells (E). In use, a Teflon-coated Stilpan (B) is inserted into the vessel. Contaminated solvent (C) is pumped or poured in from a properly grounded vessel. Lid (D) covers the vessel. After a heatup period, the vapors pass through the shell and tube heat exchanger (G), are condensed into a liquid

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ana coiiecred in a standard drum rd). This drtm and a:: external vessels must be properly grounded. The process boils and seDarates the solvent dunng a 6-8 hr ruc.

Because of the excellent economics, larger production rates are often satisfied by using two or more of these small umts. Several units allow flexibility in operation. For exampie, if a user iias two solvent mixes that he wants to keep totally separate, one still can be dedi- cated to each solvent. Another user may have a case in which solvent separation results in a toxic residue. He can dedicate one unit to this use to cook it down as far as possible to minimize volume to reduce final disposal fees and to render it “bone dry” to insure its rating as a solid non-flowable, non-volatile material. PE

Paul N. Cheremisinoff, P. E . is Director of Physical Treatment Div., Industry1 University Cooperatwe Center For Research in Hazardous & Toxic Substances, New Jersey Institute of Technology, Newark, N . J .

MARCH 198:

Special Report: Volatile Organic Compounds

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