CALGON CARBON CORPORATION

Thermal reactivation is becoming an increasingly important technique for recovering spent carbon used in waste-treatment processes. Two types of furnaces are commercially available - here are the advantages and limitations of each.

Roger H. Zanitsch, Calgon Environmental Systems Div.
Richard T. Lynch, Calgon Corp.

The thermal-reactivation system

Spent carbon from the adsorbers is transferred as a slurry, first to a storage tank (Fig.1) and then to an elevated furnace-feed tank, from which it is metered to a dewatering screw - an inclined screw-conveyor that serves the dual purposes of draining slurry-water from the carbon and providing a water seal for the top of the furnace.

Drained but still wet, with 40% to 50% water, depending on carbon granule size and temperature, the spent carbon flows by gravity into the furnace. Reactivated carbon also leaves the furnace by gravity, flowing into a quench tank that both wets the reactivated carbon and provides a water seal for the bottom of the furnace.

From the quench tank, a slurry of reactivated carbon is transferred via a storage tank to the adsorbers. In most plants, an afterburner and scrubber are provided for destroying organics and removing residual particulates from the furnace off-gases. Makeup virgin carbon is added at the reactivated-carbon-storage tank, in amounts required.

Both of the storage tanks, which are usually lined-steel tanks with conical bottoms, are designed for 5 to 10 days holdup, in order to allow for routine furnace maintenance and unscheduled shutdowns. The furnace feed-tank, which is also made of lined steel and has a conical bottom, is sized for a holdup equivalent to one shift of operation.

The carbon slurries are transferred with either eductors, slurry pumps, or blowcases operating under air or water pressure. If eductors or pumps are used, the slurry must be diluted to less than one pound of carbon per gallon of water, in order to reduce carbon abrasion or pipeline erosion. If a blowcase is used, the slurry can be denser, on the order of 3 lb/gal.

As long as slurry lines are flushed free of carbon after each transfer, galvanic corrosion of carbon steel will not result, and carbon steel lines will be satisfactory. If the wastewater is corrosive, however, materials of construction resistant to the wastewater must be used. All carbon-slurry lines in the system should be equipped with flush connections.

 Fig. 1 - Furnace is key element in reactivation system for spent granular carbon

Thermal-reactivation furnaces

The heart of the above system is the furnace, which takes the carbon through three stages: drying, baking and activation.

During the drying stage, temperatures are increased to about 212^oF, simultaneously evaporating moisture and steam-distilling the more-volatile organics from the granular carbon. The carbon temperatures are then raised to 1,200 to 1,400^oF, at which point some organic molecules are thermally cracked to volatile hydrocarbons, and other organics are distilled. At the same time, a carbon char is deposited in the pore structure of the carbon granules.

This char is burned, along with a small amount of the original carbon, in the final reactivation stage, where temperatures range from 1,600 to 1,800^oF. Since the char and granules are both carbon, the critical feature of the entire process is this reactivation stage, during which the char is selectively burned. Steam is added to the furnace, and the O₂ concentration is controlled to obtain optimum char gasification with minimum loss of carbon granules.

Either of two types of furnace is used: the multiple-hearth (Fig. 2) or the rotary-kiln (Fig. 3). Both are being employed in commercial installations, and both can achieve an equivalent suitable quality of reactivated carbon. The choice between the two thus rests on a balance among many factors, such as: cost, area requirements, fuel consumption, turndown ratio, control, corrosion, maintenance, ease of shutdown, and overall operating factors.

Furthermore, the choice of reactor type also bears on the more common problems that have been experienced in reactivation systems, for example: corrosion, slagging, poor quality of reactivated carbon, high carbon losses, feed interruptions, hearth failures, and erosion and corrosion in the slurry lines. The role of the reactor in all these considerations is seen via an understanding of the furnace or kiln.

The multiple-hearth furnace

From four to eight hearths are used in carbon-reactivation furnaces, which consist of a cylindrical refractory-lined steel shell containing the hearths, one above the other, and a central rotating shaft that drives rabble arms across the hearths (Fig. 2). The central shaft and rabble-arms are hollow, so that they can be cooled by air from a centrifugal blower discharging into the bottom of the shaft. A sand-seal at the top of the furnace, and a sand- or water-seal at its bottom, prevent extraneous air from entering.

Carbon entering near the rim of the top hearth is raked toward the center and down a central hole to the center of the next hearth, from where the rabble teeth move it outward again in a spiral path toward the rim. This flow pattern is repeated through subsequent pairs of hearth, until the reactivated carbon is discharge from the rim of the bottom hearth (Fig.2).

Nozzle-mix-type burners using either fuel oil or natural gas are mounted on the shell so as to direct their flames tangentially into the space above the hearths. Generally, these burners are mounted at the bottom two or three hearths, with two burners at each hearth of a small furnace and three burners similarly located in larger ones. Also, the afterburner for destroying organics and removing residual particles may be mounted above the top hearth.

Steam for controlling the reactivation stage is added through ports above the bottom three hearths.

Frequent feed interruptions, resulting in temperature excursions on the upper hearths of this type of furnace, can weaken the hearths and ultimately lead to failure. Minimizing the number of feed interruptions  and maintaining continuous operation can usually afford a 3- to 5-year life for the upper hearths.

Sodium compounds can also lead to hearth failure by attacking the brick, in addition to forming slag and clinkers. The slag is formed when sodium and/or organic phosphate from the pores of the carbon react with silica and alumina in the furnace refractory. Generally, slag can be kept to a minimum by pre-treating the carbon and carefully controlling furnace conditions.

Also, improper dewatering, resulting in excessive water and thermal shock, can lead to failure of the upper hearth.

In general, hearth life is a function of the manner in which the furnace is operated. If frequent interruptions due to improper carbon feed or cyclic operations are encountered, poor hearth life can be expected.

Fig 2 - Multiple-hearth reactivation furnace:
            lower fuel consumption, better control

The direct-fired rotary kiln

The rotary kiln (a refractory-lined steel cylinder closed at the ends by stationary hoods and mounted on two or three sets of trunnions, so as to slope downward from inlet to outlet - Fig.3) passes the carbon granules either cocurrent with or countercurrent to combustion gases and steam.

A variable-speed drive , coupled to a speed-reducer and pinion gear meshing with a girth gear on the shell, rotates the kiln. Wet spent carbon enters onto flights through a feed-screw or chute, and reactivated carbon at 1,600 to 1,800^oF discharges from the lower end to fall down a chute into a quench tank, which acts as a seal over the lower end of the chute.

Control of the atmosphere inside the kiln is accomplished by means of the air-to-fuel ratio at a burner, plus steam addition through a port, which with the burner is mounted on the discharge hood. The steam and combustion gases are contained by a rotary seal between the kiln and the refractory-lined hood; they exhaust, at 500 to 800^oF, through a duct in the feed hood.

In most installations, the exhaust gases are passed through an afterburner for complete combustion of organics and the carbon fines that are swept out of the kiln. New installations will probably be required to include a wet scrubber to meet air-pollution codes in most parts of the country.

Fig.3 - Rotary kiln reactivator: fewer corrosion problems, lower maintenance costs

Selection criteria

Cost: Total installed costs for either a multiple-hearth furnace or a rotary kiln are about the same. The purchase price for the multiple hearth is generally higher, but its installation costs are lower, so that total installed costs are about the same.

Area: Site preparation, foundations, and structural costs are higher for a rotary kiln because of its much greater area requirements.

Fuel consumption: Insulation used behind the brick lining of a multiple-hearth furnace reduces its heat losses, whereas similar insulation is not feasible with the rotary kiln. Also, the rotary kiln has larger surface area. Consequently, fuel consumption is higher with a rotary kiln than with the multiple hearth in the following ranges:

Capacity turndown: Because of the greater control possible in the various zones of the multiple-hearth furnace, the ratio of minimum to design capacity at equivalent reactivated carbon quality is about 35%. With only one burner and one point of steam addition, rotational speed is the major element of control for the kiln, which has a turndown ratio of about 50%.

Control: Because of its distinct zones, each with possibility for regulating burning, steam concentration and air concentration, the multiple-hearth furnace offers better control during reactivation. However, when the kiln is properly sized, this is not a distinct advantage, since the kiln will perform satisfactorily.

Corrosion and slag: Many industrial waste-streams contain inorganic impurities - mostly chloride and sulfur salts of calcium and sodium - that cause corrosion and slag formation in the reactor. Since the multiple-hearth furnace has more exposed parts, it is more susceptible to corrosion from such materials. Rabble teeth and arms are expensive, long-delivery castings, compared with the alloy flights in a kiln, which are fabricated from readily available plates. Also, considerable corrosion of the flights can occur before replacement is required.

Slag buildup in a multiple-hearth furnace will require periodic shutdowns to remove accumulated material, whereas slag is discharged, along with the carbon, into the kiln's quench tank, from which the slag can be removed without shutting down the kiln.

Maintenance: Experience indicates that multiple-hearth furnaces have higher maintenance costs, for these reasons:

• Corrosion and slag cause shutdowns for repairs.
• Rabble teeth and arms are more expensive to replace than kiln flights.
• Multiple-hearth furnaces are difficult to work on, so that it takes more manhours to rebuild a hearth than to replace brick in a kiln.
• More instrument components are required with a multiple-hearth furnace.

Feed outages: Although periodic planned shutdowns can be carried out without damaging the multiple-hearth furnace, the upper hearths can be damaged by temperature cycling caused by interruptions in the feed. In a rotary kiln, by contrast, feed outages are not much of a problem, since the refractory is less affected by temperature cycling.

Operating factors: Because of the foregoing characteristics, the operating factor for the rotary kiln is generally 85 to 95%, compared to 75 to 90% for the multiple-hearth furnaces in industrial-wastewater applications.

System problems

In addition to the problems associated with a kiln or multiple-hearth furnace, the carbon handling system exhibits some characteristic problems, such as corrosion, erosion, and carbon loss.

Corrosion: Thorough testing with corrosion coupons should be made before selecting a lining for the storage tanks, etc. Also, erosion of lining material at carbon outlet nozzles has been a problem. Sacrificial wear plates or stainless steel cones minimize this problem. Similarly, dewatering screws and quench tanks are generally constructed of stainless steel type 304 or 316L. Since the dewatering screw is exposed to the spent-carbon slurry, it must be compatible with the wastewater.

Slurry-line erosion: We recommend that the slurry velocity be 3 to 5 ft/s, in order to prevent settling on the one hand and abrasion on the other. The lines should be as direct as possible, with a minimum number of bends; and where bends are necessary, they should have long radii. Also, all bends should be accessible for periodic inspection and replacement. Flush connections should be provided at frequent intervals.

Carbon losses: Makeup carbon is the single most important cost element for a reactivation facility. Through proper design, losses can be held to about 5 to 7% in the adsorbers, the carbon transfer and handling systems, and the carbon storage and reactivation systems. Losses within the reactivation furnace should not exceed 1 to 3%; and the carbon lost due to oxidation during destruction of the organics is generally 0 to 2%.

Most losses of granular carbon occur during backwashing of the adsorbers, because of abrasion in the slurry lines, and due to spillage and carryover in overflow lines. These can all be controlled through proper design and operating techniques.

Capital and operating cost estimates

Capital costs and total installed costs for reactivation systems using multiple-hearth furnaces and rotary kilns are shown in Table I and Fig. 4, assuming that utilities and offsite facilities are available at the battery limits of the system. The time required to design, procure, install and start up a reactivation is estimated to be two years, assuming a 12-mo delivery time for the furnace and associated equipment.

Operating costs are shown in Table II and Fig. 5.

These costs are based on the following assumptions:

• One operator per shift at $10/h, plus 25% for supervision.
• 8,000 Btu/lb of carbon reactivated, at $ 3/million Btu. (Of the 8,000 Btu, approximately half is allowed for afterburner consumption and idling.)
• Minimal power costs at $0.03/kWh.
• One lb of steam per lb of carbon reactivated, at $4/1,000 lb of steam.
• Maintenance costs equivalent to 8% of the installed costs per year. (These costs can vary from 8% to 15% of the installed cost per year.)
• Carbon loss equivalent to 7% of the reactivation rate, with makeup carbon at 57¢/lb, delivered. (Although carbon losses can range from 3% to over 10%, most plants seem to operate in the range of 5% to 7%).
• A general plant overhead of 10%, for insurance, taxes, monitoring, accounting and administration.

Depending on capacity, the direct operating cost, excluding amortization of the investment, ranges from 11¢/lb to 19¢/lb, plus or minus 20% (Fig.4). The total installed cost for a unit of 10,000 lb/d capacity is $1.25 million, plus or minus 20% (Fig.3)

Fig. 4 - Installed cost estimate for reactivation systems	Fig. 5 - Direct operating cost for reactivation systems

Conclusions

Granular activated carbon has been demonstrated effective in treating a wide variety of industrial wastewaters. Both multiple-hearth furnaces and rotary kilns can be used to reactivate spent carbons, if adequate consideration is given to selecting materials, sizing the equipment, and plant operation.

Experience gained over the past ten years indicates that corrosion, slagging, poor reactivation quality, carbon losses and line erosion can all be kept to a minimum through good design.

Although the problems experienced by adsorption systems treating industrial wastewaters are the same as for the systems treating municipal waters, the problems of industrial systems are magnified where wastewater quality, and thus carbon exhaustion rates, are more variable, and the waters substantially more corrosive. However, experience with over 100 million lb of spent carbon from more than 75 different industrial applications indicates that a high-quality product can be made in several reactivation systems on a reliable and economical basis.

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Roger H. Zanitsch is Director, Air Purification Venture, Activated Carbon Div., Calgon Corp., Calgon Center, Box 1346, Pittsburgh, PA 15230, where he was previously Director of Engineering for the Environmental Systems Div., and where he has advanced through several engineering assignments since joining Calgon in 1969. He holds a B.S. in chemical engineering and an M.S. in environmental engineering from the University of Cincinnati.

Richard T.Lynch is a senior engineer in the Process Engineering Group of Calgon Corp.'s Engineering Department. He has been a project manager for the design of several carbon adsorption reaction systems treating industrial waste streams. A member of the AIChE and a registered professional engineer in Florida, he holds a B.S in chemical engineering from the University of Florida.

Reprinted by special permission from CHEMICAL ENGINEERING, January 2, 1978.
Copyright 1978 by McGraw-Hill, Inc., New York, N.Y. 10020