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Comment PDF Separation Processes

Evaporators: Energy Conservation Strategies and Process Control

By Alan Gabelman, Gabelman Process Solutions |

Several strategies for improving energy consumption and process control in evaporation processes are presented here

In industrial evaporation, a heat source (usually condensed steam) is used to concentrate a material through the removal of a liquid solvent. There are several types of evaporators and a number of important design principles. This article describes techniques to reduce energy usage in industrial evaporation, and also control methods for evaporators. For more information on evaporation fundamentals and equipment selection, please see Ref. [1], Evaporators: Design Concepts and Equipment Selection.

Multiple-effect evaporators

To a first approximation, the heat obtained upon condensation of 1 pound (lb) of steam is enough to evaporate 1 lb of water, leading to an expected steam economy of about 1 lb of water evaporated per 1 lb of steam for an ordinary evaporator. However, the evaporated vapor also contains latent heat of vaporization. Rather than releasing this heat to cooling water flowing through the condenser, why not use it to evaporate more water? Such an arrangement is shown in Figure 1 for a double-effect evaporator. Note that in Figure 1, the temperature in the first effect (T1) is greater than that in the second effect (T2), as required to provide the driving force for heat transfer in the second effect, and the effect pressures also correspond so that p1 > p2.

FIGURE 1. This double-effect rising-film evaporator uses the latent heat contained in vapor from the first effect to evaporate additional water in the second effect. More energy is saved by flashing the first-effect steam condensate into the second-effect heat exchanger

The feed is partially concentrated in the first effect, then the first-effect product is directed to the second effect for final concentration. The heat source for the first effect is steam from the boiler (or a waste-heat source), while the second-effect heat source is the vapor produced in the first effect. In this double-effect evaporator, 1 lb of steam results in approximately 2 lb of evaporated water, one from each effect.

One can extend the concept and build an evaporator with N effects, delivering N lb of evaporated water per 1 lb of steam. However, there is an upper practical limit, dictated by economics. The total available temperature driving force (ΔT), the difference between the temperature of the steam to the first effect and the evaporation temperature of the final effect, is distributed over all effects. With a greater number of effects, each effect receives a smaller portion of the overall ΔT, and for a given heat-transfer area per effect, the evaporation rate at each effect decreases. Consequently, more effects are needed to achieve a targeted evaporation rate, driving up the required investment. The optimum number of effects is reached when the energy savings no longer supports the increased cost of capital. In most cases, more than five or six effects is difficult to justify. Note that multiple effects do not provide increased capacity over a single-effect evaporator, only reduced energy usage.

By convention, effects are numbered according to the direction of the flow of vapor. The first effect is heated by boiler steam, and effects 2 through N are heated by vapor from effects 1 through N– 1, respectively. However, the process liquid flow is not necessarily sequential in this manner, as shown in Figure 2 for a four-effect evaporator. In forward flow (Figure 2a), feed enters the first effect, and like the generated vapor streams, the intermediate product from each effect flows to the next one in line. The main advantage is the successive decrease in pressure (corresponding to the decrease in temperature), which either reduces the size of the required interstage pumps, or eliminates the need for them entirely.

FIGURE 2. By convention, effects of a multiple-effect evaporator are numbered to follow the steam and vapor flow. Options for process material are as follows: (a) forward flow (in the same direction as vapor flow); (b) reverse flow (in the opposite direction of vapor flow); and (c) mixed flow (a combination of forward and reverse flow)

In reverse flow (Figure 2b), feed enters the final effect, and each intermediate product flows to the preceding effect, with concentrate removed at the first effect. The advantage is that product streams of increasing concentration are handled at higher temperatures. This counteracts the high viscosity often encountered at higher concentrations, resulting in improved heat transfer and ease of handling. The disadvantage is the need for interstage pumps to overcome the increase in pressure between a given effect and the previous one.

Mixed flow (Figure 2c) may be a reasonable compromise. As in forward flow, feed enters the first effect, and intermediate product streams flow sequentially until effect k is reached. Product emanating from effect k is directed to the last effect N, then product from there flows in reverse, leaving effect (k + 1) as final concentrate. Mixed flow allows some interstage pumps to be reduced in size or eliminated, while still allowing the viscous concentrate to be handled at a higher temperature than with forward flow.

 

Vapor recompression

Like the use of multiple effects, vapor recompression is a method of capturing the energy contained in the process vapor. This technique comprises compressing the vapor, either mechanically or thermally, then reusing it as heating medium.

Unlike some other evaporator operations, mechanical vapor recompression (MVR) does not employ boiler steam as the heat source (except for a small amount used at startup and as makeup). Instead, the required energy is provided by a compressor, which raises the pressure and temperature of the process vapor high enough for it to serve as the heat source for additional evaporation. A schematic diagram is shown in Figure 3. Note that a condenser is not needed, because all of the vapor is recompressed and reused. Once reused, the vapor is removed as condensate, to be replaced by freshly produced and recompressed vapor. Compressors can be driven by an electric motor, steam turbine, gas turbine or internal combustion engine [2, 3].

FIGURE 3. This rising-film evaporator uses mechanical vapor recompression, which typically provides steam economy equivalent to between 10 and 15 evaporation effects

Typically, the energy economy of an evaporator with MVR is equivalent to 10 to 15 conventional evaporation effects, although equivalence to 30 effects or more is possible. The economy is a function of the required compression ratio, which in turn depends on the evaporation pressure, the boiling point elevation (BPE; further discussed in Ref. [1]), and the required ∆T. Savings are higher when the cost of electrical or mechanical power is low, when low-pressure steam is not available or when the cost of providing cooling water is high. For a reasonable compressor size and power requirement, usually the ∆T is kept in the range of 10 to 18°F. This limitation may lead to the need for a high heat-transfer area, which in turn may increase the cost of an MVR evaporator when compared to a multiple-effect unit with similar capacity. However, this is at least partially mitigated by the omission of steam boiler and cooling tower (or other cooling water source) capacity, which are not needed with MVR [2, 4].

MVR is particularly well suited for film evaporators, because the high heat-transfer coefficients reduce the required ∆T, and in turn, the compression ratio. The technique is not used for high-fouling applications or if the BPE is high, because the higher temperature needed to obtain the necessary ∆T  leads to compression ratios that are not economical. These cases may be handled by using MVR in conjunction with a conventional evaporator. For example, the initial concentration may be done with a film evaporator with MVR, then a forced-circulation unit may be used for the final concentration.

Rather than a mechanical compressor, thermal vapor recompression (TVR) uses a steam jet ejector to recompress the process vapor (Figure 4). Unlike MVR, only part of the vapor is recompressed, while the rest is directed to the condenser in single-effect units, or the second effect in multiple-effect evaporators. Because TVR is better suited to vacuum operation than MVR, the former is often preferred for heat-sensitive products that require low temperatures. Similarly, TVR can add some steam economy in applications that cannot use multiple effects because the temperature in the earlier effects would be too high. The ratio of motive steam to entrained vapor depends on the steam supply and the evaporation pressures. As a rule of thumb, a TVR adds the equivalent of one evaporator effect.

FIGURE 4. Thermal vapor recompression usually increases steam economy by the equivalent of one evaporation effect

TVR offers simplicity of design and construction, low cost, low maintenance, ability to handle large volumes of vapor and the ability to handle corrosive gases. The main disadvantage is lack of operating flexibility, with a rapid decrease in efficiency upon deviation from design conditions. As with MVR, this precludes high-fouling applications, because conditions would change with the buildup of deposits on the heat-transfer surface. Installations that require more flexibility may use multiple jets in parallel, with valves to match the number of online jets to the targeted flowrate.

 

Control of evaporators

As with most types of process equipment, there are numerous ways to control an evaporator, ranging from simple to sophisticated. An example of a control strategy that is intermediate in complexity, applied to a falling-film evaporator with recirculation, is shown in Figure 5. The desired steam flowrate is set by the operator, and maintained by virtue of a flow sensor and control valve. Alternatively, steam pressure can be controlled instead of flowrate, using either a pressure sensor and a control valve, or a simple pressure regulator with a spring-loaded diaphragm. The latter is the least expensive, and provides adequate control in many applications. One disadvantage of pressure control, regardless of the method, is the need to increase the setpoint over time as the heat exchanger fouls. This is not necessary when the steam flow, rather than the pressure, is controlled, because the pressure is self-adjusting. That is, the steam pressure increases until the ∆T is sufficient to obtain a heat-transfer rate high enough to condense the steam entering at the controlled flowrate.

FIGURE 5. An evaporator can be controlled in a variety of ways. Here, the steam flowrate is set manually, the feedrate is regulated to maintain the desired level in the vapor-liquid separator, and concentrate flowrate is varied to maintain the desired concentrate solids concentration

Feed flowrate is regulated to keep the vapor-liquid separator level at setpoint, using a cascade loop. Concentrate solids content is maintained at setpoint by controlling the rate at which concentrate is removed, employing a second cascade loop. Solids content can be inferred from sensors that measure, for example, refractive index or density. The former is obtained from an inline refractometer, a relatively inexpensive device. Density can be measured using a gamma-radiation density meter, which relates the attenuation of gamma radiation by the process fluid to its density, and in turn, solids content. While considerably more expensive than other methods, this technique offers high accuracy and no contact with the process fluid.

For the vacuum evaporator shown in Figure 5, evaporation pressure is regulated by a controlled bleed of air (or inert gas). Also, there is no control on the condenser water, only temperature indication. While not shown in Figure 5, some designs include a sensor in the vapor-liquid separator to detect the presence of foam at a pre-determined level, followed by automatic addition of a chemical antifoamer.

While a single-effect evaporator is shown in Figure 5, the same control strategy is applicable to multiple-effect units. Pressures, temperatures and solids contents of the intermediate streams are not controlled, because the operation is self-regulating, but a level-control loop at each vapor-liquid separator may be desirable. Alternatively, separator levels can be controlled by simple overflow through a side port, with the connecting pipe sized to provide sufficient pressure drop so that the downstream pump (if used) suction is not starved. ■

Edited by Mary Page Bailey

 

References

1. Gabelman, A., Evaporators: Design Concepts and Equipment Selection, Chem. Eng., Jan. 2020, pp. 27–38.

2. Perry, R.H., Green, D.W., Maloney, J.O., eds., “Perry’s Chemical Engineers’ Handbook,” 7 th ed., McGraw-Hill, New York, 1997.

3. McCabe, W.L., Smith, J.C., Harriott, P., “Unit Operations of Chemical Engineering,” 7 th ed., McGraw-Hill, New York, 2005.

4. APV Evaporator Handbook, SPX Flow, Inc., 2009.

Author

Alan Gabelman is president of Gabelman Process Solutions, LLC (6548 Meadowbrook Court, West Chester, OH 45069; Phone: 513-919-6797; Email: alan.gabelman@gabelmanps.com; Website: www.gabelmanps.com), offering consulting services in process engineering. Gabelman’s over 40 years of experience include numerous separation processes and other engineering unit operations, equipment selection, sizing and design, process simulation, P&ID development, and process economics. He holds B.S., M.Ch.E. and Ph.D. degrees in chemical engineering from Cornell University, the University of Delaware and the University of Cincinnati, respectively. He is a licensed professional engineer and has served as an adjunct instructor in chemical engineering at the University of Cincinnati. Gabelman has edited a book on bioprocess flavor production, and he has authored several technical articles and a book chapter.

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