6

' SPECIAL SECTION 7

Kenneth E. Kogut is owner of Kogut Engineering, a consulting engineering firm in Concord, Cal$ Licensed in chemical engineering, he is an expert in hazardous waste treatment processes. Kogut also writes problems for the Principles and Practice of Engineering examinations in chemical engineering.

Solvent Extraction Processes Compared

Deciding which method will work at a particular site will be less confusing with an understanding of all the possible applications.

By Kenneth E. Kogut, PE

ith similar limi- tations and applications, sol- W vent-extraction

processes may be difficult to com- pare. But by understanding how they work, waste processers can determine the best choice for their clean-up purposes.

The technologies used by the Carver-Greenfield Process, B.E.S.T and NuKEMs method are described below.

The Carver-Greenfield Proc- ess, a commercial-scale method, applies solvent-extraction tech- niques to hazardous sludges, con- taminated solids and other industrial wastes. These can con- tain polychlorinated biphenyls (PCBs), polynuclear aromatics, dioxins and other chemicals. The process produces a clean, dry powder, nearly clean water and a concentrated liquid mixture of hydrocarbon-solvable contaminants.

In this process, waste is added to a solvent that will extract the hazardous oily material from contaminated solid particles and concentrate the hazardous material in the solvent phase. The resulting slurry is transferred to an evaporator system to vaporize the water that had been in the feed. If the water content of the feed is high, an energy-saving multiple-effect evaporator may be used.

The slurry of dried solids is fed to a device - centrifuge, hydroclone or other - that separates the bulk of the solvent from the solids. Solids are

reslurried with clean, recycled solvent in subse- quent stages and separated until the desired degree of non-contamination is achieved. Centrifuging separates the remaining solids from the solvent.

This is followed by removing the residual solvent by hydroextraction, a desolventizing process that uses hot, recycled nitrogen to vaporize the solvent from the solids. For test results, see Table 2, page 26.

During the last 30 years, more than 80 commer- cial Carver-Greenfield plants have been licensed. They concentrate wastes from slaughterhouses (for rendering), municipal waste, industrial sewage,

24 \

ENVIRONMENTAL PROTECTION

I , Tank Site Assessments

continued from page 21

ing to the author, Chad Van Sciver, the guide is intended to standardize quality assurance/quality control procedures for various groundwater sampling tech- niques so that regulators in the state can approve their proper use on a site-by-site basis.

According to Van Sciver, the depart- ment recognizes the need to eliminate large numbers of monitoring wells from sites. “Two or three wells might be enough to monitor a site instead of the 10 or 15 commonly found,” he said. “If alternate sampling methods are used to define the extent of groundwater contamination first, the number of per- manent monitoring wells can be reduced significantly. The department recognizes the need to reduce costs in conducting these assessments. This guide will help provide a way to do that.”

Lower Lab Costs The potential cost savings of using

field-screening techniques includes the reduced cost of lab analyses, a reduced number of permanent monitoring wells

and the savings in eliminating sampling remobilization costs.

In a typical accelerated site charac- terization, as many as 60 to 80 samples might be taken from 30 to 40 temporary points. They might be analyzed for several different parameters including total organic volatiles, CO,, 0, and constituent-specific compounds. The cost for this level of sampling using tradi- tional methods can triple the cost of using field analytical and narrow-point sampling tools.

There are a number of reasons for this. Traditional drilling methods usu- ally create large-diameter holes that must be filled and cuttings that must be properly disposed. Analysis by a fixed laboratory can cost double or triple that of field-generated analysis. And the savings in the reduced number of moni- toring points can be astronomical.

A recent study by a major oil company added all the costs associated with monitoring wells that included installa- tion, sampling, maintenance and closure and determined that each well in its inventory cost more than $10,000 over

its lifetime. The real savings does not come from

simply employing these tools as they have been used in the past but in working with the client to establish objectives up front and then sending a senior project manager into the field with the right tools to meet them. This is what ensures a comprehensive investi- gation that will be acceptable to the regulators and allow for the devel- opment of a strategy for closure. Only a person with experience in all facets of the corrective action process can stay focused on the goal of the project and collect the field data necessary to help him get there. (D

References 1. Taken from LTR presentation on “ESCA: the risk-based approach to site closure,” developed by M. Taylor and N. Thompson, November 1993. ESCA is a registered trademark of Land Tech Remedial Inc. and stands for Expedited Site Closure Approach.

Michael Taylor is director of corporate development for Land Tech Remedial inc., a Monroe, Conn., assessment and remediation company. He has worked with researchers, regulators and responsible parties to increase the acceptability of site assessments using field measurements.

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Bench-scale testing involves deter- mining the suspendibility of the raw feed in suitable solvents followed by water evaporation and feed material extraction. After successive extractions the solids are centrifuged, and the cake is desolventized in a vacuum oven. Then the separated oil, water and solids streams are analyzed.

Another commercial-scale process, B.E.S.T., uses triethylamine, a biode- gradable solvent that occurs in nature. The key to success of the B.E.S.T. process is triethylamine's property of inverse miscibility; below 65 degrees F, triethylamine is soluble in water and above 65 degrees F, it is insoluble in water. Therefore, cold triethylamine can extract PCBs, pesticides, semi- volatile organics and volatile organic compounds (VOCs).

When treating sludges and sedi- ments, extraction is continuous. When treating soils, however, the process is configured as a batch extraction sys- tem.

For batch operations, waste is screened to less than 1-inch diameter and then combined with 50 percent sodium hydroxide. By batch operations, cold triethylamine is added, the mix- ture is agitated, then allowed to settle. The resulting solution from this extrac- tion is a mixture of water, solvent and solvated oil. The mixture is decanted from the solids and the solids are removed with a solid-bowl centrifuge. The solvent is recovered from the organic phase via flash evaporation and from the water phase via steam stripping. To accumulate enough solids to per-

form subsequent extraction cycles, the cold extractions are repeated while feed is added to the tank. Solids with high moisture content may require more than one cold extraction.

Once a sufficient volume of moisture- free solids is accumulated, it is trans- ferred to the steam-jacketed extractor1 dryer. Warm triethylamine is then added to the solids. The mixture is heated, agitated, settled and decanted. Subsequent extractions separate the organics not removed during the initial cold extractions. The process produces- solids, water and concentrated oil con- taining organic contaminants.

The B.E.S.T. process has been suc- cessfully demonstrated at bench-, pilot-

and commercial-scale levels. Three pilot-scale units have treated soils, sludges and sediments contaminated with PCBs, PAHs, VOCs and pesti- cides.

The B.E.S.T. bench-scale treatability test protocol requires 1 kilogram to 5 kilograms of waste. The waste is screened and the pH adjusted to 10-11. A solvent compatibility test is con- ducted. Solvent extraction is conducted with the waste. Solvent ratios and mixing rateshimes simulate commer- cial-scale operation. SolidsAiquid sepa- ration is conducted and the solvent is

recovered from the oil product via evaporation. Solvent is recovered from the water product through steam strip- ping. The treated products are dried and analyzed to determine contami- nant removal efficiency. See Table 3, page 27, for test results.

NuKEM, the company that devel- oped the third method described here, does not have a trade name for its solvent-extraction process. The bench- scale process is designed to remove hydrocarbons from fine-sized soil parti- cles. The system consists of aqueous

continued on page 26

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1 Solvent Extraction

continued from page 25 solid washing followed by solvent extraction.

Aqueous solid washing cleans the coarse particles and significantly reduces the material that must be treated by the solvent extraction circuit. A chemical additive enhances the sol- vent extraction of hydrocarbons from soil particles in the aqueous slurry.

The soil washing system is used to classify the particles into coarse and fine fractions. The coarse fraction is cleaned by the use of temperature, chemicals and energy. This material can be returned to the site as clean backfill. The fine fraction is thickened and sent to the solvent-extraction cir- cuit.

In this circuit, the slurry is counter- currently mixed with solvent in a series of tanks. The solvent/slurry mixture is separated by a centrifuge and sent to the next tank. The solvent from the last stage is sent to a distillation circuit. The cleaned solvent is recycled back to the process, and the concentrated stream of

continued on page 28

I Table 2. Selected Test Results Carver-Greenfield Process Pilot-scale test with total petroleum hydrocarbons on solids:

Waste description Oily drilling mud Untreated concentration* 89,000-1 47,000 Treated concentration 7,300-8,400 Matrix Sludge Soil classification Clay, silt

Bench-scale test with BTEX on solids: Waste description Untreated concentration Treated concentration Matrix Soil classification

Waste description Untreated concentration Treated concentration Matrix Soil classification

Waste description Untreated concentration Treated concentration Matrix Soil classification

Bench-scale test with TPH on solids:

Bench-scale test with PCBs:

Oily drilling mud 0-20.85 0-.34 Sludge Clay, silt

Hydrocarbon-contaminated soil 0-1 0,000 0-400 Saturated soil Clay, fine sand, coarse sand

Petroleum hydrocarbon/PCB contaminated soil 5 ND-.00011 Saturated soil Fine sand

Pilot-scale test with oil and grease: Waste description Refinery slop oil Untreated concentration 10,000-571,000 Treated concentration 2,000-20,000 Matrix Sludge Soil classification Not applicable

*Concentrations are in mg/kg units. For more information on this process, contact Theodore D. Trowbridge at Dehydro-Tech Corp. in East Hanover, N.J., (201) 887-2182.

DEWATER & SEPARATE SLUDGE & MEET MOISTURE CONTENT REGULATIONS FOR LAND FILL DISPOSAL OF SOLIDS WASTE.

THE CONTAINER FILTER (ROLL-OFF STYLE)

The liquid suspension is pumped or conveyed into the Container Filter for dewatering. The solids are retained by the filter medium in the container while the cleaned liquid leaves the container through ports below the filter support basket. The dewatering process may be accelerated with a self priming pump. The Container Filter is available in hoppers, luggers, roll-offs, and sealed vessels of various sizes. Standard units are constructed of carbon and stainless steel. We offer a wide range of filter fabrics for selected micron size and product compatability.

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26 Circle 30 on card. Circle 31 on card.

Table 3. Selected Test Results B.E.S.T. Process Pilot-scale test with PCBs:

Waste description

Treated concentration less than 2 Matrix Soil classification

Waste description Clay, silt

Treated concentration less than 1-10 Matrix Soil classification Clay, silt

Waste description Oily sludges, clay

Treated concentration Matrix Sludge Soil classification Clay

Waste description Clay soil Untreated concentration 1,000-1 0,000 Treated concentration less than .l-5 Matrix Unsaturated soil Soil classification Clay

Soils, sludges and sediments Untreated concentration* 50-2,500

Unsat. soil, sat. soil, sediment, sludge Clay, silt, sand, gravel

Pilot-scale test with PCBs:

Untreated concentration 19-1,500

Unsat. soil, sat. soil, sediment, sludge

Full-scale test with PCBs:

Untreated concentration 10-15 less than .l-less than .1

Pilot-scale test with naphthalene:

Bench-scale test with DDT, aldrin, eieldrrin, endrin and lindane: Waste description Clay soil

Treated concentration less than .02-1.9 Matrix Unsaturated soil Soil classification Clay

Waste description Clay soil Untreated concentration 490 Treated concentration .9 Matrix Unsaturated soil Soil classification Clay

Waste description Clay soil Untreated concentration less than 45 Treated concentration less than .9 Matrix Unsaturated soil Soil classification Clay

Untreated concentration 37-2,600

Pilot-scale test with dibenzofuran:

Pilot-scale test with benzo(a)pyrene:

*Concentrations are calculated in mg/kg. For information on B.E.S. T , contact Lanny D. Weimer at Resources Conservation Co. in Nlicotf City, Md., (301) 596-6066.

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continued from page 26 liquid contaminants are sent off-site for disposal. The slurry from the last extrac- tion contains some residual solvent that is removed in a steam stripping column. The solvent vapors go to the distillation circuit, and the cleaned slurry goes to the filter for dewatering. The cleaned soil can be returned as backfill.

Hydrocarbon removal efficiencies are generally 95 percent to 99 percent.

Particle-size distribution and the rela- tionship between particle size and con- taminant concentration are determined. Unit operations such as screening, attritioning, flotation, humus removal and thickening are tested. The effect of surfactant addition, pH and water tem- perature also are evaluated.

Extraction efficiency under various conditions is evaluated for fine-sized particles. Then solvent/aqueous slurry phase disengagement is evaluated, removal of residual solvent from the aqueous slurry is tested, and the pre- liminary filtering rate on cleaned par- ticles is measured. For test results, see

(D Table 4 on this page.

Table 4. Selected Test Results NuKEM Solvent Extraction Process Bench-scale test with TPH:

Waste description

Treated concentration ND to 100 Matrix Unsaturated soil Soil classification

Waste description

Treated concentration 60 to 100 Matrix Unsaturated soil Soil classification

Waste description

Oily refinery soil contaminated with cat cracker feed Untreated concentration* 7,000-8,000

Clay, silt, fine and coarse sand

Unknown mixture of hydrocarbons Bench-scale test with TPH:

Untreated concentration 6,000-1 0,000

Clay, silt, fine and coarse sand

Mix of volatile and semi-volatile hydrocarbons Bench-scale test with TPH:

Untreated concentration 6,000-8,000 Treated concentration 300-600 Matrix Unsaturated soil Soil classification

Waste description Jet fuel JP-6 Untreated concentration Treated concentration Matrix Unsaturated soil Soil Classification

Waste description

Clay, silt, fine sand Bench-scale test with Jet fuel JP-6 (TPH):

6,000-8,000 60-80

Clay, silt, fine and coarse sand

Mix of hydrocarbons and PAHs Bench-scale test with TPH:

Untreated concentration 1,000-1,500 Treated concentration 200-300 Matrix Unsaturated soil Soil classification Clay, silt, fine sand

Toncentrations are calculated in mg/kg. For information on this process, contact John R. Weber at NuKEM Development Co. in Houston, Texas, (713) 520-9494.

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28 Circle 34 on card. ENVIRONMENTAL PROTECTION