Regenerative thermal oxidizers (RTOs) are commonly used for abatement of volatile organic compounds (VOCs), but in some cases, solvent-recovery systems can realize significant economic and environmental benefits
In the field of industrial air-pollution control, the regenerative thermal oxidizer (RTO) is widely regarded as the default solution for abatement of volatile organic compounds (VOCs) due to its thermal efficiency. However, for manufacturing processes utilizing high-value solvents, oxidation represents destruction of a valuable resource.
This article evaluates the technical and economic feasibility of solvent-recovery systems (SRS) as an alternative. A comparative case study of an adhesive manufacturing facility demonstrates that despite a higher initial capital expenditure (capex), SRS technology can yield a sub-12-month return on investment (ROI) and can reduce carbon dioxide emissions (CO2e) by over 90% compared to oxidation.
Technology landscape
When a manufacturing facility must control VOC emissions to comply with U.S. Environmental Protection Agency (EPA; www.epa.gov) or other local environmental regulations, the choice typically comes down to one between destruction of the VOCs (oxidation) and recovery of the materials.
The RTO is considered to be the industry standard for destruction. It utilizes ceramic media beds to capture heat, allowing the system to oxidize VOCs at temperatures around 1,500°F. Very little auxiliary fuel is required when VOC concentrations are sufficiently high. RTO is robust, reliable and generally is the option associated with the lowest capex.
Recovery involves separating the VOCs from the airstream and purifying them for reuse. Three primary technologies exist for this purpose:
Condensation. This method uses refrigeration to reduce the air temperature below the solvent’s dewpoint (Figure 1). It is generally limited to solvents with high boiling points or at high concentrations.
FIGURE 1. Condensation, where refrigeration is used to lower air temperature below the solvent’s dewpoint, is one approach to separating solvents for reuse
Scrubbing. This approach uses a liquid absorbent to strip VOCs from the gas phase. Scrubbing is effective for water-soluble solvents (like alcohols), but typically requires multiple stages and is susceptible to biological growth issues in the scrubber media.
Regenerable carbon adsorption. This is the most versatile solution. Activated carbon acts as a sponge, adsorbing solvents from the air. The carbon is then regenerated with steam to recover the solvent. This article focuses on this method compared to RTO.
Selection criteria for recovery
Solvent recovery is not a universal solution. It requires specific process conditions to be economically superior to oxidation. The four critical feasibility factors are the following:
Solvent value. The economics of SRS are driven by the replacement cost of the solvent. Commodity solvents (such as toluene or methyl ethyl ketone) often justify solvent recovery while low-value solvents or fuels may not.
Solvent purity and chemistry. The purity of the solvent, and its interaction with water are important for determining whether recovery of the solvent will be economically and technically viable. For example, single-solvent systems are the easiest to recover. Water-immiscible solvents (those that readily separate from water, such as toluene or hexane) allow for simple gravity separation by decanting, while solvents that mix with water (ethanol, for example) require distillation. Higher levels of purity typically equate to higher recovery cost, so understanding the purity required for reuse of the solvent is important.
VOC concentration. Recovery systems are sized by airflow (ft3/min). Low solvent concentrations result in large equipment recovering very little solvent, diminishing the ROI.
Corrosivity. Halogenated solvents create acid gases when oxidized, requiring RTOs to be built with expensive exotic alloys, custom ceramic materials and downstream acid-gas treatment. Recovery avoids combustion, often allowing for the use of less expensive materials of construction.
Case study: Basis of design
To quantify the difference between RTO and SRS, consider the example of a real-world application for a manufacturer of adhesives using the solvent toluene. The following process conditions were in place.
• Airflow: 77,000 std ft3/min
• Solvent: Toluene (C7H8)
• Loading: 2,400 lbs/h
• Operating schedule: 8,000 h/yr (24/7 operation with downtime)
The case study uses the following utility costs and financial assumptions for the purpose of the economic analysis:
• Electricity cost: $0.08 per kWh
• Natural gas: $7.00 per million Btu
• Steam cost: $11.00 per 1,000 lb generated
• Cooling water cost: $0.16 per 1,000 gal
• Toluene value: $0.635 per lb (market replacement cost)
Technical scope comparison
The primary barrier to SRS adoption is the initial capital cost. The technical scope of supply for an SRS is significantly more complex than that for an oxidizer.
Option A: RTO. The total installed cost for the RTO amounts to $3,900,000. The required equipment includes a two-canister RTO, a combustion blower, the main fan, a burner/gas train and an exhaust stack. The complexity of the system is considered to be low because the system is largely self-contained.
Option B: Solvent recovery system (SRS). The total installed cost for the SRS is $7,700,000. The SRS functions as a chemical processing plant attached to the exhaust. It includes the following equipment:
• Adsorber: This would be a three-vessel carbon adsorber (made from 304 stainless steel) with automated sequencing valves (Figure 2)
FIGURE 2. A solvent-recovery system consists of a three-vessel carbon adsorber, air-handling equipment and a regeneration system
• Air-handling system: This includes the main fan, inlet filter house, cooling coil, dampers and ductwork
• Regeneration system: This system consists of a steam system, condenser, decanter (for gravity separation), drying and cooling system (fan, filter, heaters), cooling tower and air stripper and multiple system pumps
The SRS requires nearly double the capital investment ($7.7 million versus $3.9 million).
Financial analysis (opex & ROI)
The annual operating and maintenance (O&M) costs for the SRS are driven by the steam required to regenerate the carbon and by the cooling water required to condense the solvent. Table 1 shows the O&M costs for the case study.
FIGURE 3. The ROI for an SRS depends on the value of the recovered solvent
The economic viability of the SRS hinges entirely on the value of toluene (Figure 3). The following values are used to calculate the value of the recovered solvent:
• Recovery efficiency: 96%
• Annual recovery amount: 2,400 lb/h × 8,000 h × 0.96 efficiency = 18,432,000 lb of toluene recovered per year
• Value: 18,432,000 lb × $0.635/lb = $11,704,320 per year
Given the value of the recovered solvent, the payback for the solvent recovery system is evident, as shown here:
• SRS initial capital investment: $7,700,000
• SRS annual operations and maintenance cost: $1,520,000
• SRS annual revenue from toluene recovery: $11,700,000
• Simple economic payback: $7,700,000 / ($11,700,000 – $1,520,000) = 0.756 years, or just over 9 months
To compare the full financial picture, the capital costs were annualized over a 20-year period at 8% interest (Table 2).
With a payback of less than one year and potential annual savings of over $10,000,000/yr at design capacity, the advantage of an SRS is obvious in this application.
Risk mitigation
A common objection to large-capex projects is the risk of production downturns. For example, does the system still make sense if the facility operates at only 25% capacity (2,000 hours/year)? This scenario is outlined in Table 3.
Even at drastically reduced production rates, the SRS provides an annual advantage of over $2 million compared to the RTO. The system remains financially sound, even in severe economic downturns.
Environmental impact
Environmental, social and governance goals (ESGs) are increasingly driving technology selection. To understand the true impact, the analysis needs to include Scope-3 emissions (lifecycle), not just direct combustion. To evaluate this, the following calculations are made:
• Production: Petrochemical data indicate that producing 1 lb of toluene generates 4.94 lb of CO2 emissions.
• Transportation: Shipping 461 truckloads of solvent per year generates 779 tons of CO2 emissions.
• Utilities: Electricity and steam CO2 emissions are based on the use of natural gas as the energy source:
o Natural gas: 116 lb CO2 emissions per million Btu
o Electricity: 0.96 lb CO2 emissions per kWh
o Steam: 159.5 lb CO2 emissions per 1,000 lb steam
• Combustion: Stoichiometric analysis (C7H8) shows that oxidizing 1 lb of toluene generates 3.34 lb of CO2 emissions.
Using these data, the total annual CO2 emissions can be calculated, as shown in Table 4.
The result indicates that using an SRS reduces the facility’s global carbon footprint by 90%. The RTO, by necessitating the production of new virgin solvent, is responsible for ten times the environmental impact.
While the RTO remains a reliable workhorse for general VOC control, it is financially and environmentally inefficient for high-value-solvent applications. For facilities utilizing non-water-soluble solvents like toluene, the SRS offers a compelling business case. Despite a 100% premium on initial capital equipment, the SRS transforms a regulatory compliance cost into a significant profit center, paying for itself in under a year, while simultaneously aligning with aggressive corporate sustainability goals.
Edited by Scott Jenkins
Acknowledgement
The images that appear in this article were provided by Dürr CTS.
Authors
Jeff Rudolph is the process engineering manager at Dürr CTS Inc. (830 Prosper Street, De Pere, WI 54115; Phone: +1 920 337-1497; Email: Jeffrey.Rudolph@cts-durr.com; Website: www.cts-durr.com). He has over 30 years of experience in air-pollution control and solvent recovery. He holds a B.S.Ch.E. degree from Rensselaer Polytechnic Institute (RPI).
Brian McCarthy is a process engineer at Dürr CTS (same address as above; Phone: +1 920 337-1403; Email: Brian.McCarthy@cts-durr.com). McCarthy holds a bachelor’s degree in chemical and biological engineering from Colorado State University.