Nuclear Consulting Services, Inc.

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1 Nuclear Consulting Services, Inc. Utilization of the Brayton Cycle Heat Pump for Solvent Recovery J. L. Kovach NUCON International, Inc. The Brayton Cycle Heat Pump (BCHP) is a thermodynamic cycle consisting of two reversible constant pressure processes meshed with two constant entropy processes. An engine operating on this principle was patented by G. B. Brayton in Actually, the reverse Brayton cycle is used for solvent recovery because the main purpose is to achieve as low a condensing temperature as needed for the condensation of the particular solvent (1)(2). The actual application of the BCHP turbo machinery used in solvent recovery occurred approximately 100 years after the invention of the Brayton cycle engine. The BCHP process was applied for solvent recovery application by Bryce Fox of 3M (3)(4). The process can be typically used in four different modes. These modes are: a) Indirect Condensation b) Direct Condensation c) d) Concentrator - Direct Condensation Regeneration Coupled Concentrator - Direct Condensation Regeneration Decoupled The processes are selected in sequence as the solvent concentration decreases. The process of Indirect Condensation is used when the solvent concentration is very high and there is insufficient noncondensible gas to Ildrivell the turbo expander. This type of application can condense a pure (gas free) solvent stream. In the range of 1-20v% solvent concentrations the Direct Condensation method is suitable for most solvents. Below l.ov% solvent concentrations, the use of a coupled concentration step may be necessary. At very low solvent concentrations, the concentrators can be decoupled from the regenerator/condenser. The entire concentrator or the concentrator media can be moved to the location of the Direct Condensation-Regenerator or a mobile Direct Condensation-Regenerator can be moved to the multiple sites of the concentrators. Presented at the USDOE Industrial Solvent Recyclixig Conference, Charlotte, N.C., 15 October,

2 General Advantaaes of the BCHP Process The BCHP process permits very high (- 99.9%) single pass condensation of volatiles and, even in a single step, can achieve condensation temperatures as low as -150'F. While the process can use a motor gear driven device, it is simpler to use a separate compressor or vacuum pump as the energy input source and use a free-spindle turbo compressor/expander pair. The turbo machinery used is small in size. Typically, an 8 inch diameter wheel can handle 1500 CFM. Multiple parallel free-spindle turbo compressor/expanders have been used successfully (5). The process is capable of returning clean, hot gas for front end process utilization or as regeneration energy for the solvent desorption step. Due to the IIdryIl regeneration gas used, the adsorber bed is always exhausting high purity, dry air which is suitable also for recycle. There are no waste water streams generated and the recovered solvent is typically higher purity than that from conventional systems. Because the actual carrier gas stream is the refrigerating fluid, the cooldown/heat-up steps are nearly instantaneous and only the cooldown of the heat recovery gas-to-gas heat exchanger is not instantaneous. However, compared to conventional Rankin cycle refrigeration, the BCHP system cooldown rate is more than 10 times faster. Indirect Condensation - Use The BRAYCYCLER process uses the expansion of noncondensible gas to achieve the low solvent condensation temperatures needed. When the concentration of the solvent is very high, there is insufficient "carrier gas" in the stream to supply the expansion cooling. An example can be the recovery of gasoline vapors displaced from storage tanks or tanker truck terminals. In such cases, as much as 50% of the stream is condensible organic material. Figure 1 shows a recovery schematic diagram for this version of the BRAYCYCLE process where the BCHP is used as an indirect refrigerator, The BCHP loop consists of a mechanical compressor energy source, the free-spindle turbo compressor/expander, where the pressure of the turbo compressor is also utilized to create additional pressure differential for expansion, a cooler to remove heat of compression and the heat recovery heat exchanger all in a closed loop. This BCHP refrigeration loop is not in direct contact with the solvent (gasoline) air stream. Additional heat recovery is obtainable by the utilization of the heat capacity of low temperature liquid and gas streams for both condensation at 32'F to remove water and to pre-cool the inlet stream. 2

3 The operation of the closed cycle BCHP permits the use of any gas with optimized heat capacity and only a small "gettert1 package is needed to keep the stream dry. The operation of such systems at remote locations creates a potential for diesel engine driven compressors or turbo packages also. An indirect condensation unit is being designed by NUCON for gasoline vapor recovery. Direct Condensation Use The utilization of the BCHP in a direct condensation mode is one of the examples of the BRAYCYCLE process. A typical flow diagram is shown in Figure 2. Depending on solvent type and stream humidity, the first step is dehumidification. This step is needed if moisture is present and if the solvents being condensed do not depress the water freezing point to below the condensation temperature. The next component is the gas-to-gas heat exchanger where the inlet stream is pre-cooled by the gas stream exiting form the solvent separator of the turbo expander. A typical solvent laden air (SLA) exit temperature from the gas-to-gas heat exchanger is 0 F. Solvents condensed at this temperature are removed in a gas/liquid separator. The stream exiting this first separator enters the turbo expander where the temperature is lowered to typically -100 F. (The exact temperature ranges vary depending on total system differential pressures.) The clean, solvent free gas is pulled through the system - in this example - by the vacuum pump and after absorbing the heat generated by the vacuum pump and the turbo compressor, it is returned to the process as hot, clean gas. If higher process pressure differentials are needed to achieve lower condensing temperatures, the SLA can be passed through the turbo compressor, cooled in a water cooled heat exchanger and then enter the low temperature gas-to-gas heat exchanger. Such an example is shown in Figure 3. This type of process arrangement can result in as low as -150'F final solvent condensing temperature. Currently, two such systems are being constructed for methylene chloride recovery. Concentrator-Direct Condensation Resenerator Coupled In the typical solvent concentration ranges of 20 vppm to 10,000 vppm, it is generally not energy economical to directly condense solvents. In these cases, a "concentrator" is used to accumulate solvent which is removed from the concentrator for condensation in a coupled BCHP system, if the solvent concentration is in the 500 vppm to 10,000 vppm range. 3

4 The concentrator is typically adsorption based, however, absorption or membrane separator concentrators can also be used. This concentrator is also used to separate water from the solvent. The l~dehumidificationvl step can be achieved by the use of hydrophobic adsorbents or in case of conventional adsorbents by special switching of the adsorber units to displace the adsorbed water with solvent. The regeneration of the adsorbent beds in the BRAYSORBR process takes place first by displacing the air from the adsorbent. The degree of I1inertinglt needed is different for various solvents. Ketones need a complete oxygen displacement, other 'Inondecomposingll solvents need to be inerted below about 5% oxygen content. After inerting, the system is regenerated by utilizing a directly coupled BCHP system with it's heat recovery and gas/liquid separation steps. The process is shown on Figure 4. Typically, no external heat source is needed: the compression heat is satisfactory for the stripping of the solvents from the adsorbent. Condensation temperatures of -80'F are generally adequate for complete condensation of typically used solvents. A 16,000 SCFM system built by NUCON has been in commercial operation for 3 years at a 3M site recovering MEK, toluene and cyclohexanone (5)(6). There is a portable unit generating application data at various sites in the Southern California Edison service area. The pilot unit and the site work is a joint funded program of the US DOE, Southern California Edison, EPRI, 3M and NUCON. An 8,000 CFM unit is in the design stage for another 3M site. The latter is a DOE, 3M, NUCON project. Concentrator Direct Condensation Reaenerator z DecouDled The application of the BCHP in this mode permits economical pollution control/solvent recovery at locations releasing small volumes of solvent (in the concentration ranges of vppm). These are the sites where conventional solvent recovery/pollution control devices are not economical. In the decoupled arrangement, only the concentrators are located on-site and either the adsorbent only or the adsorber unit is moved to a conveniently located central regenerator. Another version of the decoupled approach is the use of a mobile regenerator moved to the site for regeneration of the concentrator and the recovery of the solvent. Typical regeneration frequencies can be from one/day to one/month. This approach is also suitable for single plants with numerous small sources dispersed within the facility, large tank farms, etc. 4

5 Two approaches of this methodology are currently in the design/ construction stage. One in the San Bernadino area which is a DOE, Southern California Edison, Dow Chemical and NUCON joint project, the other in the Connecticut area which is a Dow-NUCON project. Summary and Conclusions The application of the BCHP for pollution control and solvent recovery has been successfully initiated. Current activity using improved process technology is in the refinement and demonstration of the BCHP process application in a wide area of industry. A particular emphasis is placed on the application of the process for small sources where currently there are no economical solvent recovery method. Both the capital cost and energy cost of the BCHP based solvent recovery methods has been improved (6) and the process design has been greatly simplified. References 1. Priebe, S. J., "The Recovery of Volatile Organic Compoundst1 USDOE Report EGG-ED-8486 (Apr. 1989) 2. Enneking, J. C. & Kovach, J. L., "Control of Toxic/VOC Emissions Using Brayton Cycle Technology" Air & Waste Management Association Annual Meeting (June 1990) 3. FOX, Be J. 1 "Heat and Liquid Recovery Using Open Cycle Heat Pump Systems" US Patent 4,298,282 (OCt. 1981) Flink, L. R., et.al., Vapor Recovery Method and Apparatus" US Patent 4,480,393 (Nov. 1989) Kovach, J. L., I'Full Size Industrial Application of the BRAYTON Cycle Heat Pump in Adsorption Concentrator Solvent Recovery System" ASME Symposium Volume AES - Vol. 8. Presented at ASME Winter Annual Meeting (Dec. 1989) Jain, N. & Scheihing, P. E., "Solvent Recovery Using the Brayton Cycle Heat Pump1@ AIChE Symposium (Aug. 1990) BRAYCYCLE and BRAYSORB are Trademarks of NUCON International, Inc. 5

6 FIGURE 1 BRAYCYCLE SOLVENT RECOVERY SCHEMATIC DIAGRAM INDIRECT NUCON INTERNAIONAL \ACAO\OwC\l ONU883\

7 FIGURE 2 BRAYCYCLE SOLVENT RECOVERY SCHEMATIC DIAGRAM

8 FIGURE 3 BRAYCYCLE SOLVENT RECOVERY SCHEMATIC DIAGRAM VACUM PUMP NUCON INTERNATIONAL \ACAO\DWC\l ONU883\@01-08

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