Best practice for evaporation processes

Evaporation is a process where a material moves from its liquid state to a vapor (or gas). The most common example of evaporation seen in everyday life is the transfer of water from the earth’s surface to the atmosphere as part of the water cycle. When liquids evaporate, and materials which they contain in solution or suspension are left behind, this becomes a useful process to concentrate solutions or to help separate materials.

Evaporation is different from dehydration or drying because the product is a concentrated liquid, rather than a dry solid. However, evaporation and drying processes can be combined: an evaporation process first removes the bulk of the water, and a final dryer eliminates the last bit, to obtain the final dried product. As evaporation plants are usually more energy efficient than dryers, it makes sense to combine the two technologies. Evaporation is different from distillation: a distillation process separates two or more liquids that have different boiling points (and which normally contain no solids). In contrast, evaporation eliminates water from a solution that contains dissolved and/or suspended solids, thus obtaining a more concentrated solution.

In general terms, the rate of evaporation depends on the temperature (the warmer it is, the faster the rate of evaporation), but the boiling point of a material varies with pressure (water boils at a lower temperature as pressure reduces), and so industrial processes often use a reduction in pressure to speed up the evaporation process.

Evaporation is used for different purposes in multiple industries and sectors. In the food industry, some products are concentrated to increase shelf life, reduce volume or weight, or reduce storage and transport costs. In contrast, in the pharmaceutical sector, evaporation is often used to create concentrated solutions that can then be dried to create powdered products. In each sector, the basic principles of evaporation remain the same — the removal of water (or another solvent) from a solution by converting that water or solvent into its vapor phase.

Different techniques, using different temperature and pressure conditions, can be applied in different situations. The type of evaporation that is most suitable for a particular purpose depends on many factors, including the nature of the solvent and the solution, the required end products, and the energy available for the process.

Types of evaporation equipment

Evaporation may be carried out in batches or as a continuous process. Evaporation consists of two elements: a heating phase (bringing the product to a boil) followed by an evaporation phase (where molecules leave the liquid phase and become gaseous). There are several types of equipment for performing these processes, including:

Jacketed tank evaporators (JTE)

These are among the simplest evaporators and are ideally suited to applications of small capacity and where capex is limited. The product is fed into a tank which has an external heating jacket in which the heating media flows. The product is raised to its boiling point and steam evaporates and exits from the top. For good heat transfer or if product fouling is likely, an agitator or scraper is installed to increase product turbulence and heat transfer rates. Jacketed tanks have less heat transfer area per unit of product volume compared to tubular or plate evaporators making them only suitable for relatively small evaporation tasks. However, there is no need for a recirculation pump, making them simple and easy to install.

Forced recirculation evaporators (FRE)

This type of evaporator was one of the first systems developed industrially and refined by HRS using our proven corrugated tube heat exchanger technology. In a FRE system the product is super-heated to a temperature above its boiling point. Upon exiting the evaporator, the product is introduced in a flash separation vessel where the pressure is lowered. Due to the reduction in pressure part of the product will flash off, and the concentration of the fluid in recirculation is increased. The flashed steam is then recovered by condensing it back to water in a condenser. Depending on the product, FRE can use plate, corrugated tubes or scraped surface heat exchangers.

The evaporated steam is condensed and collected in a condensate tank. A vacuum pump can be connected to the condensate tank to control the evaporation pressure. The evaporated-condensed steam can be used to preheat the incoming product, allowing for significantly increased levels of thermal efficiency.

Falling film evaporators (FFE)

In falling film evaporators, the product is introduced at the top of a vertical tube bundle, where it is evenly distributed and falls downwards as a thin film against the tube walls. On the outside of the tube a heating medium, normally steam, is applied to raise the temperature of the product and evaporation takes place at the liquid film surface. The vapor generated as the product is evaporated, travels down with the liquid film and the steam velocity helps to move the film along the surface of the tube wall.

This method offers several advantages: Falling film evaporation generally leads to very high levels of heat transfer, while the product recirculation flow rate required is far less than for FRE evaporators, resulting in lower power consumption for pumps. Finally, as evaporation takes place inside the evaporator tubes themselves, no temperature gradient is applied to the recirculating product.

The characteristics of falling film evaporators make them particularly suited for applications where the service fluid temperature is close to the evaporation temperature, such as with thermal vapor recompression (TVR) or mechanical vapor recompression (MVR) evaporators (see below for more details). Furthermore, residence times in FFEs are short, making them particularly suitable for heat-sensitive foods such as fruit juices and milk.