Energy Management / Energy Optimization

The operating costs of an evaporation plant are largely determined by the energy that is required to achieve a certain evaporation, distillation or crystallization rate.

Under steady state conditions there must be a balance between the energy entering and leaving the system and a simple relationship results: the sum of all energy and enthalpy inputs = the sum of all energy and enthalpy outputs.

The arrangement of the evaporation plant, the number of effects, the use of vapor recompression, and the number of preheaters are factors which are evaluated during the design phase of a custom tailor-made system in order to optimize the energy usage in the system.

In general there are different ways to heat a system

• Direct Heat (single or multiple-effects)
  • Live Steam
  • Waste Heat (e.g. drier vapor, rectification vapor.)
• Thermal vapor recompression (TVR)
• Mechanical vapor recompression (MVR)

Energy can be saved by re-using vapor formed from the boiling product. There are a few ways that this can be accomplished by either multiple-effect evaporation, thermal or mechanical vapor recompression.

Application of one of these techniques will considerably decrease the energy consumption. Often, it is feasible to combine two of these possibilities to minimize capital and operating costs. All three techniques may be utilized in advanced evaporation plants.

Direct Heat (Single or Multiple-Effects)

With direct heating different options are possible. You can use either live steam or waste heat from other parts of the plant like drier vapor for example. In considering the heat balance for a single-effect evaporator, the heat content (enthalpy) of the evaporated vapor is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1 lb/hr (0,5kg/h) of vapor will be produced by 1 lb/hr (0,5kg/h) of live steam, since the values for the specific heat of evaporation on the heating and product side are about the same. In case of a 1-effect evaporation plant as shown in Figure 1, you would need the same amount of live steam as you need to do the necessary evaporation ( see Figure 2).

Figure 1: 1-Effect Evaporation Plant Layout

Figure 2: Specific Steam Consumption of Number of Effects

The steam consumption of evaporation plants can be considerably reduced by using the enthalpy (heat content) of the vapor to heat another effect. The vapor produced in this effect can be further used for heating of a third effect (at a lower temperature) as illustrate in figure 3.

Figure 3: 3-Effect Evaporation Plant Layout

As shown in the figure 2, with the use of only 33% of live steam in the 3-effect evaporation plant compared to the 1-effect evaporation plant you are still able to do the same evaporation. With the number of effects the capital costs increase.

The total temperature difference is that between the maximum heating temperature in effect 1 and the lowest boiling temperature in the last effect. This is distributed between the individual effects and therefore the larger the number of effects, the smaller the temperature difference for each effect. This in turn increases the heating surface required to achieve a given evaporation rate. Increasing the number of effects increases the complexity of the plant arrangement and renders the operation and control more difficult. The product residence time will also increase.

When accomplish calculations for our customers we consider all these and many more facts to optimize each plant to the client's specific requirements. With the cost analysis; only briefly introduced here; we support our customer to meet their budgets and the plant performance.

Thermal Vapor Recompression (TVR)

Multiple effect evaporation plants save heating steam by repeatedly using the same quantity of heat from effect to effect. The heat of condensation can also be recovered if the vapors of a boiling chamber are compressed to the higher pressure of the heating chamber according to the heat pump principle. The saturated steam temperature corresponding to this pressure is also higher and the vapor can be reused for heating several times. Steam jet vapor recompressors are frequently used for this purpose.

		A Product B Vapors B1 Residual Vapors C Concentrate D Motive Steam E Heating steam condensate 1 Motive nozzle 2 Diffuser
Figure 4: Falling Film Evaporator with Thermal Vapor Recompression

Jet compressors operate at very high flow velocities and have no moving parts. Construction is simple and operation reliable.

A certain quantity of steam is required to operate a jet compressor. This represents the heat input to the plant and can be calculated from the available motive steam pressure and the required compression ratio. Due to the presence of this motive steam in the mixed flow, more vapors will be evaporated than the compressor can recompress.

If 1 lb/hr (~0.5kg/h) of motive steam is required to compress 1 lb/hr (~0.5kg/h) of vapors, 2 lb/hr (~1kg/h) of mixed steam are produced on the pressure side of the jet compressor, and this will in turn evaporate approximately 2 lb/hr (~1kg/h) of vapor. Excess vapor will be conveyed to the next effect at the branch downstream of effect 1. In this example, thermal vapor recompressors produce the same result as an additional effect for a directly heated plant. Depending upon the operating conditions, the jet compressor can act as several additional effects. The heat of condensation of the last effect is removed via the cooling water.

Mechanical Vapor Recompression (MVR)

Evaporation plants equipped with mechanical-vapor-recompression-type heat pumps require low live steam input during normal operation. Jet compressors can compress only a part of the vapor and the energy of the motive steam is discharged as residual heat via the cooling water. In mechanical vapor recompression systems, however, all vapors are compressed to a higher condensation pressure.

The major advantage of a MVR type of arrangement is that you can lower the live steam consumption during normal operation and shift the necessary energy to electric energy.

In Figure 5 you can see such kind of configuration. The vapor from the first effect is compressed in the MVR to a high pressure. At this higher pressure it can be utilized again for heating the first effect evaporator. In this case it is illustrated in a 2-effect arrangement.

Figure 5: 2-Effect Evaporation Plant Layout with mechanical vapor recompression for the first effect

Some additional steam or condensation may be required to maintain the evaporator heat balance and give stable operating conditions.

To keep the evaporation plant as simple as possible and for easy operation, single-effect centrifugal recompressors are frequently used. This can be high-pressure fans or centrifugal-compressors. For high pressure increases, multiple-stage compressors can be used.

The specific energy consumption corresponds to the compressor energy input/evaporation rate ratio. This is determined by the compression ratio, which represents the temperature difference between the heating steam and boiling liquid. Furthermore, it depends on the boiling point elevation and the pressure loss in the system. Under favorable conditions this value can be as low as 4 kWh per 1,000 lb (~450kg) of water.

Electric motors can be used to supply the motive power for the compressor. They are reasonable priced and easy to operate and maintain. Combustion engines have been used in cases where there is heat available from the cooling water and exhaust gas. If high-pressure steam is available, it is reasonable to install a steam turbine. Utilizing the turbine exhaust steam may provide very high total energy efficiency.