POLLUTION CONTROL THAT PAYS ITS WAY

by Martin Decker, Prakash Naik & Mike Worrall

Reprinted from INDUSTRIAL WASTES

An activated carbon technology recovery system solves an environmental problem and represents an economical return on the investment.

Through the installation of an activated carbon technology recovery system, the Armak plant in Marysville, Michigan, is able to consistently meet air emissions standards as well as recover a significant amount of solvent used in its tape manufacturing operations.

Armak's ability to continue its tape operations at Marysville, was contingent upon its air emissions being in compliance with the Michigan State EPA requirements. The State Implementation Plan mandates installation of "Reasonable Available Control Technology (RACT I)" on existing pollution sources. For tape manufacture and other similar operations, RACT limits emissions to 2.9 lbs of volatile organic compounds (VOC) per gallon of coating mix applied.

Armak's Central Engineering evaluated the technologies available to solve the problem and decided to purchase an AMCEC activated carbon technology recovery system. This article summarizes the background of the problem and how an economic solution was implemented.

In the tape manufacturing operation, solvent based coatings and adhesives are applied to a continuous web of material, after which the solvents are evaporated leaving the pressure sensitive tape. Solvents are evaporated from the tape in the initial stages of long tunnel ovens and these solvents are exhausted to the atmosphere as dilute vapors at concentrations of about one-quarter percent by volume in air, which is safely less than the lower explosive limit.

It was estimated that the solvent laden air volume exhausted from the solvent drying stages of the various ovens was 125,000 standard cu. ft. per min. containing a mixture of hydrocarbon solvents at a max. rate of 3,750 lbs. per hour.

In addition to the environmental aspects involved, Armak was concerned with the rapid increases in solvent prices experienced in recent years, as well as with the future availability of solvents due to the projected worldwide shortage of petroleum products in the future.

Solvent recovery

Corrective Processes Evaluated

Various processes were conceptually evaluated with the above objectives in mind. This survey took two months. The processes studies included:

  • Condensation: This involves cooling of solvent laden air (SLA) to below the condensation point. The relatively low concentration of solvents in air requires cryogenic temperatures resulting in high energy costs (electrical power) and, hence, poor economics. Furthermore, recovery by cryogenic condensation will condense and freeze water vapor from the air stream requiring complex exchanger thawing techniques.

  • In Absorption, the solvent vapors are absorbed by a scrubbing liquor, such as water, for miscible solvents; and paraffin oil, mineral oil, etc., for immiscible solvents - the later being necessary for the Armak solvents. The solvents are subsequently recovered by distillation. Although the system is generally simple and easy to operate, the low concentration of solvents resulted in high capital costs, poor recovery efficiency and was not economically attractive.

  • Recovery of solvents from Inert Gas generally requires replacing the once through air in the ovens with recirculating inert gas and taking a small slip stream of solvent laden inert gas through a series of condensers to recover the solvent. Inert gas permits safe operation with much higher concentrations of solvent in the inert gas compared to air systems, and substantially reduces the gas volumes exhausted from the ovens resulting in considerable energy savings. Advantages are low operating costs and improved safety. Disadvantages are high capital requirements for major modifications to existing ovens, loss of production time while modifying the ovens, changes in oven operating procedures and possible contamination of recovered solvent due to build-up of high boiling resins.

  • Incineration requires preheating of solvent laden air to ignition temperatures typically 500oF to 900oF for catalytic oxidation or 1200oF to 1500oF for thermal incineration, and burning the solvent vapors to carbon dioxide and water. Heat is recovered from the exhaust gases by exchange with the entering air, and as preheat to the air required to evaporate and sweep solvents from the ovens. The heat available from the combustion of solvents exceeded that required in the process. Thus, it would be lost and this, combined with lower fuel value of the solvents compared to their purchased cost, made this alternative unattractive.

  • Carbon Adsorption involves adsorption of solvents onto activated carbon followed by recovery of solvents by steam desorption condensation and decantation. Fixed bed carbon adsorption offered high recovery efficiency, relatively attractive economics and proven operation of such systems in a variety of industries, including pressure sensitive tape.
    After visiting a number of systems built by various carbon adsorption vendors for the printing and tape industries, the AMCEC fixed bed carbon adsorption technology was chosen. It was decided that the SLA volume of 125,000 SCFM should be processed by a single solvent recovery unit able to recover a mixture of hydrocarbon solvents at a maximum rate of 3,750 lbs. per hour.
    In designing this recovery unit, Armak (now Intertape Polymer) asked that consideration be given to two of their operating concerns, namely reasonable carbon life and suitable materials of construction for equipment.
    Carbon life is defined as a period between carbon changes during which the unit can be run "optimally." The fouling of the activated carbon can occur due to trace contaminations in the SLA stream. The unit has been designed to give five years estimated carbon life through later additions of approximately 30% excess carbon over the initial charge.
    In anticipation of possible corrosion due to chlorides, sulfur compounds, etc. in the SLA or steam, a number of steps were taken. The carbon pellet support screens are constructed of Incoloy 825 material. The adsorber vessels are of carbon steel fabrication, but with 1/8" corrosion allowance. Steam solvent vapor and plume recycle ducts are 316 stainless steel. Heat exchangers have 316 stainless tubes. Decanter and process condensate tanks are fabricated from 304L stainless steel.

The System Being Used

The solvent recovery system is designed to continuously process up to 125,000 SCFM of solvent laden air exhausted from the solvent drying zones of various coating lines. Four adsorbers, each containing a bed of activated carbon pellets, process the SLA such that the adsorber exhaust solvent vapor content is reduced to less than 50 ppm - more than 97% reduction.

The SLA has to be cooled to the optimum adsorption temperature of 100oF before entering the carbon beds. Cooling coils fed with cooling tower water air mounted in two air-tight houses. Ahead of the cooling coils are filter elements which remove particulates such as paper dust and viscous oil like droplets (high boilers) found in the SLA. These filters protect both the coils and the carbon beds from the particulates.

Each filter/coil house feeds to a centrifugal fan which blows the SLA through the adsorbers. The fans are each designed to handle half the total SLA capacity. Each fan is directly driven by a large explosion proof electric motor. Both fans have variable inlet vanes, with automatic actuators, to adjust the fan performance to that required by the suction controls. The variable inlet vanes reduce fan power at lower than design air flow, thus conserving electrical demand.

There are four horizontal cylindrical adsorbers. Each of the 12 ft diameter adsorbers is 47 ft long and contain a deep bed of 43,000 lbs. of CECA-AC35 activated carbon pellets. Horizontal static beds of carbon pellets are used in all AMCEC adsorbers, since the concept has been proven to protect the carbon pellets from attrition. Also, the adsorber design ensures even distribution of the SLA, thereby providing high adsorption performance with low regeneration energy needs. In the adsorbers, the carbon pellets are supported by an expanded Incolloy 825 mesh. Incolloy was selected because of its known resistance to sulfuric vapors, chlorides and the galvanic cell that may form between carbon pellets and the vessel wall.

A simplified flow diagram shows the adsorption and desorption phases (see Fig. 1). This depicts four adsorbers with three in parallel adsorption phase while the fourth is being steam regenerated.

Fig 1- A simplified flow sheet : two vessels adsorbing, the right hand unit is being steam desorbed

The SLA is processed by three adsorbers operating in parallel adsorption service in a staggered cycle. When an adsorber is fully charged with solvent, it is regenerated (desorbed) by the counterflow passage of live steam. The steam heats the carbon to about 220oF causing release of the solvent vapors from the carbon pores, and these are carried as a steam/solvent mixture to the condensing station. There are two heat recovery condensers which recover much of the heat in the steam solvent vapor for use as hot water for process heating and boiler feed. The final exchanger uses cooling tower water to remove that heat which cannot be used by the Armak plant. The process condensate separates in a gravity decanter into solvent and water phases. The solvent, after quality control tests, is pumped to the storage tanks for reuse in the process. After neutralization, the aqueous phase condensate is added to the evaporative cooling tower as make-up water.

A special feature of this plant is the use of the unique AMCEC recycle loop, which cools the carbon bed after desorption steaming. A small flow of SLA passes through the hot adsorber taking heat and steam vapor to the condenser, and then discharges it to atmosphere via another adsorber. This loop improves the plant's adsorption performance by ensuring that the carbon bed is properly cooled prior to reentering adsorption service. Also, the recycle loop substantially reduces the visible water vapor plume which is normally emitted from a carbon adsorption system after each steaming, thus avoiding possible complaints from neighbors.

To minimize operator attention and to maximize performance, the solvent recovery system is furnished with modern controls including a programmable controller (PC). Among the PC functions is sequencing of the adsorbers through the various process stages. All automatic valves are continuously monitored and, in the event of valve malfunction, the PC identifies the valve. An analyzer monitors the SLA exhausted from each adsorber and, when a carbon bed is fully charged, its steam regeneration is initiated. The analyzer loop avoids premature regeneration of the adsorbers, ensuring effective use of desorption steam. Another function of the PC is to monitor operation of the coating lines and only connect coater exhausts to the solvent recovery system when a web is being coated. This avoids unnecessary processing of SLA, thereby saving fan power and dilution of the solvent air which would reduce adsorption performance. At reduced coating loads, the PC stops one SLA fan and takes an adsorber out of service.

Automatic controls mean that the whole system - collective ductwork, solvent recovery, boiler unit, cooling tower, solvent pumping, etc. - requires less than four hours attention per shift, most of which is monitoring, routine inspection and quality control.

The battery limit solvent recovery system was conceived, engineered and installed in about 18 months at a cost of $2.7 million. This price included process and equipment design, equipment supply, foundations, erection, electric wiring, controls and thermal insulation. The system was commissioned in the summer of 1982 and performance testing was completed within a few weeks.

After nearly a year's operation, working 24 hours a day, 5 days a week, the system is consistently achieving or exceeding its design specification. The emissions from the whole site has averaged 0.9 lb. VOC per gallon of coating which is well within the applicable EPA requirement of 2.9 lbs. VOC per gallon of coating. The overall performance, in terms of recovery and utilities consumption, is summarized in Table 1.

The main operating costs of the unit - natural gas (for steam generation), electricity, water and chemicals for treatment of boiler feed and cooling tower water are summarized in Table 2.

TABLE 1
Recovery and Utilities Information on the Marysville Tape Plant
Design
Actual
Adsorption efficiency, recovery unit (%)
97
97.7
Overall site recovery of solvent (%)
75
80
Steam consumption (lbs/lb recovered solvent)
3.2 - 3.5
3.3
Power consumption (kw/1000 SCFM)
3.5
3.3

TABLE 2
Operating Costs of the Solvent Recovery Facilities (1982 basis)
Natural Gas $0.18/gal recovered solvent
Electricity $0.12/gal recovered solvent
Water $0.01/gal recovered solvent
Chemicals $0.01/gal recovered solvent

Conclusions

The recovered solvent is of acceptable quality and is recycled back to the process without further treatment.

There is no evidence of carbon pellet contamination. Some high boilers are being condensed at the SLA cooling stage.

To date, no corrosion has been experienced. The pH of process condensate is 4.8 to 5.0, which is higher than expected and therefore, anticipated corrosion of the equipment is unlikely.

No more than normal maintenance has been experienced, and the unit has been operational more than 99% of the time that the coaters have been working.

The system has more than solved an environmental problem and represents an economical return on the investment. At design conditions, some 15,000 tons per year of organic solvents could be recovered for reuse.