In foul condensate applications, spiral units tend to have lower maintenance costs and less downtime than other designs

Steam Stripping Foul Condensate More Efficient With Spiral Heat Exchanger Use

Steam stripping will be a common treatment technology due to the relatively low installation and operating costs of this type of system. In a steam stripper system, the foul condensate is filtered and fed to a stripping column (Figure 1). A heat exchanger is used for energy recovery to preheat the column feed and cool the effluent from the column.

FIGURE 1: In a typical steam stripping system, foul condensate is filtered and then fed to a stripping column.

In the column, the condensate is heated with steam to remove vapors, including methanol and total reduced sulfur (TRS) compounds, which includes hydrogen sulfide and trace amounts of dimethyl disulfide, methyl disulfide, and methyl mercaptan. Water is removed from the column overhead vapor in a condenser and then the vapors are recovered and processed. The stripped condensate leaves the bottom of the column and can be reused in the pulping process or sent to the waste treatment system.

The proper design of the column preheater is critical to ensure maximum efficiency and reliability of the system. The stripped condensate leaves the stripping column at high temperatures, which can cause problems in the process or in the waste treatment area. The foul condensate is pumped to the stripping column at low temperatures and must be heated to bring the foul condensate up to temperature before any stripping can occur.

In many mills it is important to the overall efficiency of the system to recover as much heat as possible in the heat exchanger to minimize steam costs. Deep temperature crosses are often required to meet the desired efficiency (Figure 2).

FIGURE 2: A temperature cross, where the hot outlet temperature is cooler than the cold outlet temperature, is difficult to achieve in a single shell and tube heat exchanger.

When foul condensate stripper systems were first introduced more than a decade ago, most were equipped with shell and tube heat exchangers since they are the most common type used in the pulp and paper industry (Figure 3). In most cases the foul condensate flows on the tube side and the stripped condensate flows on the shell side.

FIGURE 3: Schematic of a standard shell and tube heat exchanger design.

Most of these shell and tube heat ex-changers are equipped with a removable tube bundle to allow access to the shell side for cleaning, since the stripped condensate can still contain chemicals that precipitate out of solution when in contact with the cold heat transfer surfaces. The duty usually requires multiple shells in series to meet the thermal requirements, resulting in significant installation costs. Having the units installed in series also means that if one unit needs to be shut down for maintenance, the entire heat exchanger train is normally shut down.

NEW NEEDS, NEW DESIGNS. Due to the inefficiencies of the shell and tube design, other more efficient heat transfer technologies have begun to be installed in the stripper systems. One commonly used piece of equipment is the plate heat exchanger (PHE). The PHE consists of a series of thin, gasketed metal plates compressed between two thick heavy frame plates (Figure 4).

FIGURE 4: An exploded view of a plate heat exchanger shows that the unit consists of a series of thin, gasketed metal plates compressed between two thick heavy frame plates.

The fluids flow in a countercurrent arrangement, and turbulence is induced by the corrugated pattern on the plate, resulting in high heat transfer efficiency. Temperature approaches of 5ºF to 10ºF can be achieved in one compact unit.

The gasketed PHE, though, has had problems of its own with fouling and gasket compatibility. The plate has difficulty handling a large amount of solids, so if fibers are present in the foul condensate, plugging will occur in the PHE. As with the shell and tube design, organics precipitate out of solution onto the heat transfer surfaces. Also, the high temperature of the stripped condensate (230ºF and higher) and the high methanol content of the foul condensate stream cause the EPDM gasket used in the plate heat exchanger to swell. Several other gasket formulations have been considered to handle the problem, but these add significantly to the cost of this inexpensive solution and have not worked well in service.

In recent years, several plate heat exchangers and a number of shell and tube heat exchangers have been replaced with spiral heat exchangers. The spiral heat exchanger is composed of two long, flat plates that are wrapped around each other, creating two concentric channels (Figure 5).

FIGURE 5: An exploded view of a spiral heat exchanger shows two long, flat plates that are wrapped around each other, creating two concentric channels.

The channels are seal welded on alternate sides to prevent mixing of the fluids. Covers are fitted on either side of the spiral with a full-faced gasket to prevent bypassing of the fluid and leakage to the atmosphere. Access to the hot or cold heat transfer surfaces is obtained by removing the respective cover. The covers are frequently fitted with hinges to facilitate the opening and closing of the heat exchanger.

The hot fluid flows into the center of the unit and spirals outward in the long, flat, rectangular channel toward the periphery. The cold fluid enters at the periphery and spirals inward, exiting at the center.

Due to the long thermal length of the spiral channel, the spiral heat exchanger can obtain a deep temperature cross in a single shell. The spiral heat exchanger eliminates the gasket problem while still being compact and easy to clean. The single channel design of the spiral heat exchanger reduces the fouling rates associated with the plate heat exchanger and the shell and tube design.

If solids begin to accumulate in the channel, the cross sectional area of the channel is decreased. Since there are no other channels for the fluid to be diverted to, the velocity in the channel increases, creating a scrubbing effect that works to remove any solids that have deposited on the wall (Figure 6).

FIGURE 6: If solids begin to settle on the wall of the spiral in a spiral heat exchanger, the cross sectional area decreases. Since the spiral is built with only a single channel, this causes the velocity of the fluid to increase and creates a self-cleaning effect in the spiral.

In many cases field data has shown that the spiral heat exchanger does not need to be cleaned at all in foul condensate service. The spiral heat exchanger requires higher capital costs and is not as efficient as the plate heat exchanger, but in most cases the life cycle costs of a spiral heat exchanger are less than the plate heat exchanger due to lower maintenance costs and downtime. In most cases, the spiral heat exchanger is similar in cost to a shell and tube unit, and operating costs are lower due to the lower fouling tendencies and more efficient heat recovery.

SPIRAL UNIT APPLICATION. Willamette Industries in Johnsonburg, Pa., underwent an extensive renovation in 1993 to upgrade its facilities and improve quality. Federal regulations at the time required foul condensate treatment, so a steam stripper was installed. A spiral heat exchanger with 1,715 ft² of heat transfer area was included with the system to preheat 270 gpm of foul condensate.

The spiral heat exchanger is made of 304 stainless steel and is approximately 5 ft wide and 5 ft in dia. It recovers approximately 85% of the available heat from the stripped condensate. The stripper system treats foul condensate from the black liquor evaporator system and the digester vent condenser.

The foul condensate is filtered prior to entering the heat exchanger with an automatic backflushing strainer. The strainer is required to remove fibers from the fouled condensate in order to eliminate fiber buildup in the stripper column. The stripped condensate must be cooled to prevent flashing before it is reused in the mill’s pulp washers.

The spiral heat exchanger has performed at design conditions for five years without need for cleaning. According to Bill Shuey, production and maintenance coordinator at Willamette Industries, “We’ve never had a problem with the spiral heat exchanger. Fouling is not a concern. We are now installing another spiral unit to heat our green liquor feed to our slaker.”

MARK CRUTCHER is product manager, spiral heat exchangers, Alfa Laval Thermal Inc., Richmond, VA.

CHRISTOPHER S. BULLOCK is a registered professional engineer who has served in the paper industry in various capacities with Buckeye Cellulose, MacMillan Bloedel, Weyerhaeuser, and Willamette Industries.