Dynamically Optimized Recirculation Coupled with Fluidized Bed Adsorption to Cost Effectively Control Emissions from Industrial Coating and Solvent Operations

This page updated November 18, 2005.

Air Quality Specialists, Inc.

Dynamically Optimized Recirculation Coupled with Fluidized Bed
Adsorption to Cost Effectively Control Emissions from
Industrial Coating and Solvent Operations

CARB Grant Number 95-347

The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Executive Summary
The implementation of energy-efficient, low-cost strategies for controlling emissions of volatile organic compounds (VOCs) from industrial coating and solvent operations is a key objective of the California Air Resources Board (ARB). According to the 1995 California Statewide Estimated Emission Report, coating and related industrial processes comprise the fourth largest stationary source category, and release 230 tons of organic compound emissions per day. However, add-on pollution controls for these sources have not historically been required due to the excessive costs that are incurred. Correspondingly, significant economic and environmental benefits can be derived by developing more cost effective pollution control strategies that are applicable to this source category.
In the summer of 1996, Air Quality Specialists, Inc. (AQS) joined with Southern California Edison (Edison) and Steelcase North America (Steelcase) to demonstrate two innovative strategies for reducing VOC emission control costs. This program was conducted under the auspice of the South Coast Air Quality Management District (SCAQMD) and ARB as part of the Innovative Clean Air Technology (ICAT) Program. The two separate and distinct technologies that were evaluated under this project were:


Dynamic Recirculation - A ventilation system which enables a facility to reduce the size and cost of an add-on pollution control device and


Fluidized Bed Solvent Concentrator - A technology which highly concentrates VOC levels in process exhaust streams and therefore reduces equipment and energy requirements of the associated solvent recovery (or destruction) process.
Both of these technologies were installed at a Steelcase office furniture production plant located in Tustin, California. The Steelcase facility served as the host site for this program continues to maintain and operate the equipment in the same manner as was used throughout the demonstration program described herein. These strategies will enable ARB to meet regulatory needs pertaining to state and federal ozone attainment standards as well as air toxic exposure risk reduction provisions.
Technology Details
Dynamic Recirculation: Industrial coating operations are typically enclosed and ventilated by the introduction of clean air through an intake face; the ventilation air passes through the enclosure (removing solvent vapors and overspray particulate) and is then vented to atmosphere. This "single pass" ventilation mode generates high process exhaust flow rates as well as excessive heating, ventilation, and air conditioning (HVAC) costs. In addition, add-on pollution control systems are sized based on the process exhaust volume flow rate, thus single pass operation also results in high emission control equipment installation and operating costs. Thus, development of an exhaust flow reduction strategy provides considerable economic / environmental benefits.
Dynamic recirculation provides a safe and efficient means of reducing process exhaust flow rates. As indicated in the schematic diagram provided in Figure 1, dynamic recirculation employs a return air system to recirculate a portion of the exhaust air back into the booth; the remainder of the exhaust is vented to an air pollution control system. Prior to re-entering the booth, the recirculated air is mixed with fresh make-up air which is provided to replace the exhaust air vented to the control device. The recirculation rate that may be employed is limited by applicable health and safety standards requiring that hazardous compound concentrations in the respirable air of the enclosure do not exceed established safety limits.

Figure 1. Schematic Diagram of a Dynamic Recirculation System

To ensure compliance with these limits, dynamic recirculation employs a continuous monitor to evaluate the quality of the air that is recirculated. Based on the monitor output, the dynamic recirculation central control system continually adjusts the exhaust and recirculation flow rates to optimize ventilation system operation. This dynamic mode of operation allows the facility to reduce the process exhaust flow rates to the lowest possible level and, correspondingly, reduce emission control and HVAC operating costs to the lowest possible level. The key to successful operation of dynamic recirculation is a continuous monitor that provides accurate, real-time constituent concentrations data. Fourier Transform Infrared (FTIR) monitoring is well suited to this application; FTIR systems can speciate quantify individual organic components in a gas mixture, and work quite well on paint solvent compounds. The FTIR operates based on the fact that each organic compound responds to infra-red light differently. As such, each compound can be identified and quantified based on its unique intra-red absorbance signature.
Fluidized Bed Solvent Recovery: One approach for reducing emission control system costs is the use of a VOC concentrator device that reduces process exhaust flow rates and correspondingly reduces the emission control system requirements. Concentrator devices typically employ adsorbing media to collect solvent vapor molecules when the adsorbing media is saturated, the media is regenerated using a low-flow hot gas stream to thermally desorb the solvent molecules from the media. The low flow, high concentration regeneration stream is directed to an emission control device, where the solvent vapors are either recovered or destroyed via oxidation.
As indicated in the fluidized bed schematic diagram (Figure 2) the system is comprised of two separate components; one is designated as the adsorber module (which receives the process exhaust air stream) and the second component is designated as the desorber module (where the media is regenerated via hot gas desorption). The adsorber media is continuously transferred between the two modules to achieve the high flow reduction levels achieved by this technology. Process exhaust air enters the bottom of the adsorber module and flows upward through sieve trays containing the adsorber media (flowing in a downward, counter-current direction). The process exhaust air passes vertically through the adsorber module, where it contacts progressively cleaner media. Purified air exits the top of the adsorber vessel, and spent (saturated) media exits the bottom of the adsorber module. The spent media is then transferred to the top of the desorber module, where it flows downward and is stripped of solvent vapors by a low-flow, hot gas stream flowing vertically in an upward (counter current) direction. As the media progresses down the desorber module, it contacts progressively cleaner desorption gas, and is fully regenerated before it is transferred back into the adsorber module. The low flow rate of the desorbed stream, coupled with high solvent vapor concentrations, maximizes the emission control device operating efficiency. The Steelcase system operates in conjunction with a solvent condenser, which enables recovery of purified solvent for re-use in the coating process.

Figure 2. Schematic Diagram of a Fluidized Bed Concentrator System

Program Implementation
The objectives of the ICAT program were to demonstrate that dynamic recirculation is a safe and effective means of reducing process exhaust flow rates to the lowest achievable level on a real time basis; and explore the long term applicability and cost-effectiveness of the fluidized bed concentrator system. To achieve these objectives, AQS implemented a three-phase approach: 1) Configure the dynamic recirculation ventilation system and integrate operation with the fluidized bed concentrator system, 2) Conduct a long-term performance evaluation of the fluidized bed concentrator and dynamic recirculation systems and 3) Confirm and / or modify technology performance predictions, assess economic benefits of the technologies evaluated, and coordinate technology transfer activities.
Program Results
Dynamic Recirculation Performance Evaluation: The performance of the dynamic recirculation system was evaluated in terms of the work environment that it provided, as well as the ventilation system optimization level achieved. To evaluate the work environment, AQS performed extensive organic compound and particulate sampling in each of the recirculating spray booths. The data demonstrate that the air quality / work environment created by dynamic recirculation ventilation is well within regulatory limits. To evaluate the energy efficiency advantages of dynamic recirculation, AQS continually monitored the ventilation system exhaust and recirculation flow rates on a minute-by-minute basis, and generated more than 100,000 data points. These data provide the basis for determining the frequency in which the ventilation system operated in maximum recirculation mode vs. minimum recirculation mode. The results of this evaluation indicate that the system operated in maximum mode 91 percent of the time, and operated in minimum mode 1 percent of the time; for the remaining 8 percent, the system operated in mid-recirculation mode. The air quality assessment and the ventilation system profile evaluation data clearly demonstrate the technical viability, energy efficiency, and cost-effectiveness of dynamic recirculation.
Dynamic Recirculation Economic Assessment: The advantage of dynamic recirculation over other ventilation system strategies is that it optimizes HVAC and ventilation system operation and reduces process exhaust flows on a continual basis. For this study, the economic benefits of dynamic recirculation were assessed through a cost comparison analysis in which the following ventilation system / emission control scenarios were projected for a typical operation: 1) The facility has no recirculation, and installs a regenerative thermal oxidizer (RTO) pollution control device, 2) The facility installs a simple recirculation system and an RTO and 3) The facility installs a dynamic recirculation system and an RTO. The results of this evaluation reveal that implementing dynamic recirculation reduces annualized system costs by 30 percent. Moreover, dynamic recirculation achieves a further annualized cost reduction of 10 percent compared to simple recirculation. The results of this economic analysis clearly demonstrate the cost effectiveness of recirculation in general, and dynamic recirculation in particular.
Fluidized Bed Concentrator Performance Evaluation: The long-term performance evaluation of the fluidized bed concentrator focussed on two areas: 1) The durability and effectiveness of the BAC adsorption media and 2) The emission control capability of the equipment. Adsorber media samples were analyzed to monitor changes in media surface area, solvent retention, and specific gravity; these are indicators for determining the media reactivation frequency (which varies from application to application). The media reactivation frequency is important not only in establishing system performance characteristics, but also in evaluating the overall fluidized bed system economics. For the Steelcase application, these results (summarized in Figure 3) indicate a media reactivation frequency of 22 weeks. The results also indicate that the media is not particularly susceptible to attrition, erosion, or decreased adsorption capacity, even after reactivation. Additionally, Steelcase performed several third-party emission control tests; these efforts consistently demonstrated a solvent emission concentration of less than 2 parts per million (ppm) and, depending on the test method that is used, an overall efficiency of 89 percent to 96 percent.
Fluidized Bed Concentrator Economic Assessment: A detailed economic evaluation of the fluidized bed concentrator system was performed to establish the cost competitiveness of this technology. As part of this evaluation, a cost profile of the fluidized bed system was developed based on the process data collected from the Steelcase facility. Additional cost profiles were developed for three competing technologies based on installation and operating cost information provided by different manufacturers. The results of this analysis, summarized in Table 1, demonstrate the cost-competitiveness of the fluidized bed concentrator technology.

Figure 3. BAC Media Characterization Analysis Results

Table 1. Cost Comparison of Competing Control Technologies

Cost Item



Fixed Bed


Capital and Installation Cost





Annualized Capital Cost





Electrical and Natural Gas Cost





Other Operating Cost





Solvent Savings





Total Annual Operating Cost





Total Annualized Cost





Performance and economic evaluations of two separate technologies were performed under this ICAT Program. These technologies, which can be implemented separately or in tandem, provide alternatives for achieving more-cost effective pollution control. The program results indicate that dynamic recirculation provides a safe and effective means of reducing pollution control costs and increasing ventilation system energy efficiency. The results also support the conclusion that the fluidized bed concentrator is a viable control option, and presents a cost-competitive alternative.

Funding Source

Funding Amount





Southern California Edison and
Steelcase North America


Click here for the entire final report.

ICAT Funded Projects