The U.S. Department of Energy Brayton Cycle Solvent Recovery Heat Pump Project

Status Report

VOClCFC / WASTE STREAM

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US. Department of Energy Office of Industrial Programs

February 1990

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Abstract

A project researching the capture and recycling of volatile organic compounds (VOCs) and chlo- rofluorocarbons (CFCs) by the use of a reverse Brayton Cycle is described. This project, called the Brayton Cycle Solvent Recovery Heat Pump (BCSRHP), is sponsored by the U.S. Department of Energy’s Off ice of Industrial Programs in cooperation with the 3M Company. Two units handling approxi- mately 8,000 CFM solvent air streams are presently operational at the 3M Company. Both of these systems employ a reverse Brayton cycle as a means of condensation. In one unit an activated carbon bed, which concentrates solvent, is regenerated with hot inert gas as provided by the BCSRHP. The statu! of the two operating plants, a mobile solvent recovery system (soon to be demonstrated in the Southern California area), and new development efforts to further improve the performance and lower the capital cost of this VOC control device is detailed.

Introduction

The use of solvents in industry is widespread. At present, solvent use in most industrial applications results in volatile organic compound (VOC) emis- sions to the atmosphere. Some examples of proc- esses in which VOC emissions are prevalent in industry are: coating processes such as painting, tape and film manufacture, and textile manufacture; printing processes; hydrocarbon production and distribution; and manufacture of materials such as wood composites and plastic foams. Although the actual amount of VOCs released is difficult to esti- mate precisely, the EPA has estimated the 1986 VOCs emissions within U.S. industry at 2.7 billion pounds.(’) Mechanical Technology, Inc., as a con- tractor to DOE, surveyed over 2,300 VOC sources in New York state and found that only 10 percent of the sources were controlled.(2) VOC emissions repre- sents a large economic loss to industry (easily over 1 billion dollars annually since solvents cost from 10 cents to 1 dollar per pound) and contribute signifi- cantly to air pollution.

Recently, environmental concerns have prompted stricter legislation to control VOC emis- sions. Some industries are in a critical situation, especially those industries that use and manufacture

halogenated solvents such as chlorinated and fluorinated compounds (many of which are chlo- rofluorocarbons-CFCs). CFCs are being phased out of production. For instance, the blowing agents that the foam industry uses such as R-1 1 are not only being phased out, but have recently seen a rapid increase in purchase price (e.g., above $1 .OO per pound for R-1 1). Increasing excise taxes will be imposed on the sale of CFCs throughout the 199O’s, and in some regions of the country, fees on VOC emissions of over $1 .OO per pound are being consid- ered by local environmental regulatory agencies. To be able to address the VOC problem and yet remain profitable and competitive in the world market will be a challenge to those industries depending heavily on solvents to manufacture their products.

choices: Industry is thus presented with the following

1) eliminate the need for solvents in the manufacture of their product.

2) reduce solvents emissions by making the process “tighter” (i.e., prevent fugitive plant emissions).

3) control the VOC emission by incineration or destruction.

4) control the VOC emission by capturing and recy- cling it back to liquid solvent.

The BCSRHP offers a new approach to captur- ing and recycling solvents. The basic goals of the DOE program are to develop equipment so that; 1) the recycled solvent value returned to the large industrial solvent user (e.g., over 200,000 pounds per year), will pay for the capital investment in a 1 to 2 year time frame and; 2) for the small solvent user (e.g., less than 200,000 pounds per year), the equipment cost will be affordable (e.g., less than $50,000 annualized cost). That is, for the small user who will need to comply to new air quality standards, the BCSRHP may lessen the economic impact by providing lower capital cost equipment. By achieving these goals, nationwide energy conservation of 0.2 quadrillion BTUs per year can be realized.

Background of the DOE BCSRHP Project

The Department of Energy (DOE) identified solvent recovery and recycling in the 1970s as a means for significant nationwide energy conserva-

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tion. Even today existing technologies to recover or control VOC emissions are energy intensive. Incin- eration of VOCs in most cases requires additional fuel, such as natural gas, to provide combustion flame temperatures high enough to properly burn the VOCs (Le., above 1,400 F). Many solvent adsorption systems require large amounts of steam (3 to 12 pounds of steam per pound of recovered solvent) to regenerate activated carbon beds. In addition to the energy to control VOCs, an even larger energy loss nationwide is the "embodied energy" of the solvent. Solvents can require up to 30,000 BTUs per pound to be manufactured when considering the chemical energy of the feedstock and the process energy requirement. Loss of solvents to the environment therefore results in a loss in the 'embodied energy" since additional solvents must be manufactured.

To address these problems, DOE entered a cooperative agreement with the AiResearch Corpora- tion in 1979 to design a BCSRHP. The 3M Com- pany, which uses a large amount of solvents for the manufacture of their products (e.g., tape), was selected as a host for the BCSRHP and agreed to use one of their facilities to demonstrate this new technology. This project culminated in the installation of an 8,000 CFM BCSRHP on a drying oven located at, the 3M tlutchinson, Minnesota Magnetic Media

Plantc3). This drying oven produces an 8,000 CFM solvent laden air stream with MEK, toluene and cyclohexanone as the solvent mix. The solvent use on this oven is above 3 million pounds a year.

Description of the Brayton Cycle

The BCSRHP is an alternative means of condensing solvents from a gas stream by use of a reverse Brayton cycle. Most solvent streams must be refrigerated to temperatures below -50 F (-45 C) to condense greater than 90 percent of the solvent in a single pass through the condensing system. Other refrigeration methods, such as a reverse Rankine cycle using chlorofluorocarbon working fluids can and have been used to condense solvents to these low temperatures. Reverse Rankine cycle systems typically take two stages of compression and expan- sion to reach temperatures below -50 F (-45 C) when the ambient environment is used as a heat sink. The advantage of the Brayton cycle is that only one stage of compression is typically required. In addition, the time to reach steady state operation with the Brayton cycle has proven to be as low as 3 minutes, whereas Rankine cycle designs used at 3M for solvent recov-

5 Regenerator 4

Compressor Turbine

W

I' T

2

Salients Entropy

Figure 1 - Ideal Regenerative Reverse Brayton Cycle

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ery have taken up to 2 hours.

with the reverse Brayton cycle, Figure 1 shows an ideal regenerative reverse Brayton cycle on a temperature/entropy diagram. At station 1 the solvent laden gas stream is compressed isentropi- cally to station 2. Heat is extracted in the regenerator from station 2 to 3 at constant pressure. Because heat is extracted at station 2 to 3 the cycle is termed a reverse Brayton cycle. If heat was added by heat exchange or combustion (such as in a gas turbine power plant or jet engine), then the cycle would be a forward Brayton cycle. At station 3 the gas stream temperature will be cold enough to condense a majority of the solvents. From station 3 to 4 the gas stream has work extracted isentropically in an expander to lower the gas stream temperature even further. Essentially all of the remaining solvents are condensed at station 4. The work produced by the expander partially drives the compressor (the remain- der of the compressor work requirement comes from

To illustrate how refrigeration is accomplished mechanical shaft work input). The cold gases at station 4 are then passed through the other side of the regenerator to recoup heat at constant pressure to complete the cycle.

Although the Brayton cycle requires high speed turbomachinery, this machinery is commonly used today. All air conditioning systems used on commer- cial and military jet aircraft use a reverse Brayton cycle system. For aircraft applications, Brayton cycle equipment has been selected for reasons of low weight and high reliability. Another application of the reverse Brayton cycle is in cryogenic air separation plants. Here too, the high speed equipment required of the Brayton cycle has proven reliable for over fifty years. At present, the most common application of the Brayton Cycle is the automotive turbocharger.

CONDENSER,

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SOLVENT LAOEN

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COOLED ov EXPANSION - 1 2 0 ' F TO & SOLVENT LEAN A I R

RETURN TO OVEN IIII IIII .120*F

Figure 2 - Simplified 8000 SCFM 3M Hutchinson Brayton Condensation System

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Progress to date on the BCSRHP

Development of the BCSRHP from 1975 to present has resulted in two operating solvent recov- ery units, both at 3M manufacturing plants. A description of the two plants is as follows:

3M Hutchinson, Minnesota Magnetic Media Plant (8,000 SCFM Brayton Cycle Solvent Laden Air Condensation System)

This system, shown in Figure 2, processes the solvent laden air from a drying oven that produces magnetic tapes. The solvents used in this applica- tion are methyl ethyl ketone, toluene and cyclohex- anone. Simply described, solvent laden air (SLA) is drawn intp the compressor of the BCSRHP. The air is compressed to two atmospheres. The heat of compression is removed with a condensing heat ex- changer. This heat exchanger lowers the SLA temperature to a point where a majority of the solvents condense. The heat removed is regener- ated with a liquid run-around loop coupled to down- stream of the expander. Not shown in Figure 2 for simplicity is a redundant condensing heat exchanger which operates in parallel. While one condenser is on-line condensing solvents, the other condenser is defrosting the ice that collects over the condensing cycle. The air is cooled further by the work of ex- pansion through the expander to -120 F. After ex-

pansion, essentially all of the remaining solvents are condensed. The work from the expander (about 250 HP) is transmitted by a rotating shaft to partially supply the horsepower required of the compressor (about 750 HP). The remainder of the compressor horsepower is provided by an electric motor and gear box package (about 500 HP). Figures 3 and 4 show the BCSRHP installation at the Hutchinson site.

The 8,000 SCFM system removes 500 pounds per hour of solvent. With nearly full-time operation the system can save as much as 3,000,000 pounds of solvent per year. 3M is in the process of qualifying a number of products to insure that the tape manufac- tured during the BCSRHP operation meets 3M’s quality standards. More operation time is needed to properly evaluate the reliability of this system in excess of the approximate 200 hours that the BCSRHP has already operated. For more detailed information reference (3) by Fox and Jain of the 3M Company is recommended.

3M Weatherford, Oklahoma Magnetic Media Plant (70,000 SCFM Solvent Laden Air Adsorption System/3,600 SCFM BCSRHP)

This system recovers toluene, MEK and cyclo- hexanone also from a drying oven which processes magnetic discs. In the mid 1980’s 3M decided to further pursue and evaluate the Brayton Cycle tech- nology by installing a unit to control a 10000 SCFM stream at the Weatherford plant. The Weatherford

Figure 3 - 3M Hutchinson Turbocompressor Assembly

Figure 4 - 3M Hutchinson Overall System View

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plant was jointly designed between 3M and NUCON of Columbus, Ohio. As an extension of the 3M Hutchinson BCSRHP development effort, the follow- ing basic design changes to the Weatherford plant were implemented:

1) Activated carbon beds are used to adsorb and concentrate solvent from the VOC stream.

2) The Brayton Cycle heat pump is used to regener- ate the carbon bed with inert nitrogen gas

3) A molecular sieve is used as an intermediate step between the adsorption and regeneration of the carbon bed to eliminate water (and thus ice formation) from the BCSRHP solvent laden nitrogen stream.

4) A free spindle compressor-expander is used in combination with a rotary blower to provide the compression/expansion package. That is, the shaft power input to the BCSRHP system was de- coupled from the compressor-expander to elimi-

nate the high speed gear box used in the Hutchin- son design.

Figure 5 shows the overall solvent recovery process in which the two carbon beds are cycled between step 1 and steps 2 to 4. That is, while one bed is adsorbing solvents at step 1, the other bed is being regenerated by step 2 (carbon dehydration), step 3 (carbon solvent desorption with the reverse Brayton cycle) and step 4 (molecular sieve regenera- tion). Figure 6 shows the free spindle compressor- expander used at the 3M Weatherford plant, which is a modified turbocharger that is mass produced for use in diesel truck engines.

over one year with over 95 percent up time. A more detailed description of the Weatherford process is presented in reference(4) by Kovach of NUCON.

The Weatherford system has been operating for

Exhaust t

Process (2 Beds)

Step 1 So Ive n t Ads or p t i on

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Heat

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Step 2 Carbon Dehydration

I Molecular } ,@cool, Sieve

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Step 3 - Carbon Regeneration Step 4 Molecular Sieve Regeneration (Reverse Brayton Cycle)

Figure 5 - 3M Weatherford Brayton/Concentrator System - Overall Process

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a

Figure 6 - 3M Weatherford Free Spindle Turbocompressor

Small Mobile BCSRHP Demon st rat ion

The first demonstration of this technology to industry (other than 3M) will occur in the Spring of 1990 with a mobile system rated to handle a 250 SCFM SLA stream. The development and demon- stration of the mobile unit is a cosponsored project between DOE, Southern California Edison, the Electric Power Research Institute, NUCON and 3M. Four host sites have been selected to encompass a wide range of solvent type (i.e., perchloroethylene, methylene chloride, pentane, R-22, ethyl acetate, toluene, heptane). The data from these sites will feed critical design data to future development efforts.

Future DOE Plans and Objectives

Future development efforts planned by the DOE will be to improve the economics of recycling VOCs with the Brayton Cycle heat pump technology and yet demonstrate the energy conservation benefits of the system. The reduction of the system capital cost and the quality of the recovered solvents are the main drivers to the economics of any solvent recovery device. To this end, the DOE development effort will be for both a large and small system. 3M is propos- ing to install an 8,000 SCFM unit at the 3M Green-

ville, South Carolina plant. A small system rated at 2,000 SCFM that can be decoupled from the process will also be developed. The development of both large and small systems will concentrate on the following:

Capital Cost Reduction

Kova~h(~) gives a comparison of a steam and BCSRHP system servicing a concentrator for a 15,000 SCFM SLA stream. For the steam system he quotes $72 per installed SCFM and the Brayton system $50 per installed SCFM. He assumes distilla- tion of the condensed solvent is required for the steam system and not required for the Brayton. The cost advantage of the Brayton system by not requir- ing distillation remains to be demonstrated in future development efforts. However, one can safely con- clude that the Brayton system cost presently appears competitive with conventional solvent recovery system s .

Further reduction of the system capital cost will require lower cost of all components. To illustrate this point, Table 1 below shows the cost distribution of one quotation of a concentrator/BCSRHP design.

I Table 1 - Equipment cost breakdown for a

BCSRHP system quoted in 1989

Heat exchangers 20% Adsorbent bed structure 28% Adsorbent bed material (carbon) 12% Rotary blowers 1 0%

, Com pressor-expander 8%

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Instrumentation 9% Piping, valves, separators 13%

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The solvent concentrator for this application accounts for forty-eight percent of the cost (i.e., the adsorbent bed material and structure cost combined). Therefore, new adsorbents with increased "working capacity", lower cost adsorbent bed structure ma- treials (e.g., carbon steel and non-metallics) and improved process operation are areas of development where signifcant cost reduction can be gained. Conversely, for some applications, it may be more beneficial to couple the BCSRHP to the proc- ess directly and not use a concentrator. The scenar- ios where this may be the case are: high solvent

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concentrations in the air stream, high operation times, incompatibilities of the solvent with adsorbents or the flow rate of the stream. Each case is site and solvent stream specific, and must be treated accordingly.

Energy Efficiency

The energy efficiency of the BCSRHP is impor- tant but will probably play a minor role in determining the economic viability in most solvent recovery processes. For instance, the process supervisory manpower costs may outweigh the operating energy cost of the process. Table 2 gives a rough estimate of the energy use and cost for state-of-the-art solvent recovery technologies, the existing BCSRHP installa- tions, and the projected advanced BCSRHP systems.

As one can see, the advanced BCSRHP designs will offir some very low energy operating costs compared to some of the competing technologies but since most solvents cost over 20 cents per pound the controlling factors to the economics of most solvent recovery system will be the recyclability of the sol-

vents and thus the avoided costs of solvent pur- chases and possible emissions fees. That is, in most solvent recovery situations, energy cost is less than 10 percent of the recycled solvent value.

Solvent Recyclability

As mentioned above, capital cost and solvent recyclability will dominate the economics of any solvent recovery installation. If solvents cannot be reused in the process then further refining is the next alternative. Selling solvents back to a solvent reclaimer usually nets only one-half of the solvent market price. Direct solvent reuse will therefore be a key driver in the marketability of any solvent recovery system. Trade-offs of solvent recovery systems are explored in more detail in reference(5) by Priebe.

Table 2 - Energy use comparison of solvent recovery systems per pound of solvent

Energy Use Cost of Energy(")

Carbon bed/stream desorption (with distillation) 7,000 to 12,000 B T W $0.010 to $0.050

Carbon bedstream desorption (without distillation) 3,000 to 5,000 BTUb) $0.006 to $0.020

Incineration LO0 to 60,000 BTUb) $0.001 to $0.30

Hutchinson BCSRHP

Weatherford BCSRHP

Advanced BCSRHP Designs

2,400 BTU(") 0.068 Kw-Hr

2,000 B T W 0.59 Kw-Hr

$0.027

$0.023

500 to 1,000 BTU(@ 0.1 5 to 0.30 Kw-Hr

$0.005 to $0.012

(a) Cost of thermal energy of $2 and $5 per million BTU assumed. Electrical energy of $0.04 per Kw-Hr assumed. (b) Thermal energy used in process. (c) Mechanical shaft power input to process with electric motor drive.

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Conclusions

A development program sponsored by the U.S. Department of Energy Office of Industrial Programs to capture VOC streams and recycle them back to solvents with a reverse Brayton cycle heat pump has shown promising capital cost and efficiency perform- ance at two 3M Company manufacturing plants. Further efforts by the Department of Energy will be to develop equipment affordable to the large and small solvent user. Increased solvent recycling in U.S. industry will result in large nationwide energy savings as a result of reducing the energy requirement to manufacture replacement solvents of those emitted to the environment. In addition, the economic re- cyclability of solvent offers U.S. industry a competi- tive advantage in conjunction will the obvious envi- ronmental improvements to be gained.

Ref e re n ces

(1) "The Toxics-Release Inventory, A National Per- spective", U.S. EPA Report 560/4-89-005, June, 1 989.

(2) "Assessment and Development of an Advanced Heat Pump for Recovery of VOCs", Phase I report by Mechanical Technology, Inc., for DOE contract DE-FC07-881D12788, February, 1990.

(3) Fox, B. J. and Jain N. K.; "Brayton Cycle Heat Pump Solvent Recovery System", ASME Sympo- sium volume AES-Vol. 8 presented at ASME Winter Annual Meeting December, 1989.

(4) Kovach, J. L.; "Full Size Industrial Application of the Brayton Cycle Heat Pump in Adsorption Concentrator Solvent Recover System", ASME Symposium volume AES-Vol. 8 presented at ASME Winter Annual Meeting December, 1989.

(5) Priebe, Stephen J., "The Recovery of Volatile Organic Compounds", Idaho National Engineering Laboratory Report EGG-EP-8486, April, 1989.

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Further information can be acquired by writing or telephoning:

Mr. Paul E. Scheihing Program Manager Office of Industrial Programs, CE-141 U.S. Department of Energy 1000 Independence Ave., S.W. Washington, DC 20585 (202) 586-7234