Evaporation Process Principles

Evaporation is a thermal separation process, widely used for concentration of liquids in the form of solutions, suspensions, and emulsions. Concentration is accomplished by boiling out a solvent, normally water, from the liquid. In most cases, concentrate resulting from the evaporation process is the final product. Sometimes, however, the evaporated, volatile component is also a main product, as, for example, during solvent recovery.

Evaporation processes fall into two general categories:

• Film type evaporation.
• Suppressed boiling type evaporation.

Film type evaporation involves the generation of a thin product film over a heat transfer surface to promote minimal resistance to heat transfer. Parameters are set such that optimum film boiling occurs within the evaporator body. Caution must be used to ensure a continuous film hydraulic condition (wetting rate) and to prevent nucleate boiling; otherwise, the rate of heat transfer will fall off dramatically, while the rate of scaling on the heat transfer surface will dramatically increase.

Suppressed boiling type evaporation involves superheating a product above its boiling point, while maintaining a backpressure within the system to prevent boiling in the evaporator body. This is commonly referred to as a "heat and flash" principle.

A major requirement of the evaporation process is to maintain the quality of the liquid during evaporation and to avoid heat damage to the concentrate. This normally necessitates exposing the liquid to the lowest possible boiling temperature for the shortest period of time. Protecting concentrate quality and other physical requirements of the finished product have resulted in development of many different evaporator types. Additional demands for energy efficiency and minimal impact on the environment have driven development toward innovative evaporation process designs and plant configurations. 

GEA Evaporation Technologies companies have made major contributions to these developments, making the company a world leader in evaporation, distillation, and crystallization technologies. The company is supported by its test and development facilities, where its core technologies are continually improved and applied to new products.

Evaporation Plant Selection

Design of evaporation plants requires consideration of numerous, and sometimes contradictory, requirements. The most important requirements are as follows:

• Product characteristics, including heat sensitivity, viscosity, corrosiveness, foaming tendency, fouling and precipitation, boiling behavior
• Capacity and operation data, including quantities, concentrations, temperatures, annual operating hours, change of product, controls automationRequired operating media, such as steam, cooling water, electric power, cleaning agents, spare parts
• Capital and other financial costs
• Personnel costs for operation and maintenance
• Standards and conditions for manufacture, delivery, acceptance
• Choice of materials of construction and surface finishes
• Site conditions, such as space, climate (for outdoor sites), connections for energy and product, service platforms
• Legal regulations covering safety, accident prevention, sound emissions, environmental requirements

At GEA Evaporation Technologies careful attention is paid to all of the above criteria, taking into account individual requirements, during evaporation process selection, and plant design.

Energy Efficiency of Evaporation Plants

Operating costs of an evaporation plant are largely determined by the energy required to achieve the desired evaporation rate and finished product capacity.

During plant operation 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 equals the sum of all energy and enthalpy outputs.

However, energy can be saved by re-using vapor formed from the boiling product. There are a few ways that this can be accomplished:

• Multiple-effect operation
• Thermal vapor recompression
• Mechanical vapor recompression

Application of one of these techniques can considerably decrease energy consumption. Often it is feasible to combine one or even all of these possibilities to minimize capital and operating costs.

Multiple-Effect Operation

Evaporation duty is separated into stages operating at different absolute pressures (temperatures). External heating media drive the first effect of the evaporator, with subsequent effects being driven by vapor generated from the previous (higher temperature) effect. Product may be passed through the evaporator in a forward flow, back flow, or mixed flow configuration.

Additional efficiency is achieved with the use of regenerative heaters, condensate heaters, and vapor heaters.

Thermal Vapor Recompression

A portion of evaporated vapor from either a single- or multiple-effect evaporator stage is mixed with higher pressure live steam to yield an intermediate temperature (pressure) heating media. By means of a steam thermal compressor, evaporator vapors are typically "boosted" between 8-17°C.

This is a relatively inexpensive means of imparting thermal efficiency to an evaporator.

Mechanical Vapor Recompression

Evaporated vapors are compressed to a higher pressure (temperature) in either a low-speed blower or turbofan, or a high-speed turbo compressor. The compressor is typically driven by electricity or a steam turbine. Equivalent compression ranges of 4 - 7°C for turbofans, and 10 - 18°C for turbo compressors, are not uncommon.

MVR technology yields the greatest energy efficiency for an evaporator, and results in the lowest cooling water requirement for the waste heat condenser. An MVR evaporator may also be coupled with TVR and/or multiple-effect evaporation stages.

The high initial capital cost of an MVR unit must be weighed against operating cost savings. Generally, when considering medium to high evaporative duties in the range of 20,000 - 100,000 kg/hr of water removal, the saving associated with operating costs of an MVR evaporator (over those of a multiple-effect or TVR plant) will offset the added capital expenditure within 3 months to two years of operation.