Evaporation is essential to many engineers in the process industry. Here we explore some of the key techniques used.
Evaporation is a process used to concentrate liquids that multiple industries and sectors employ. In the food industry, many products are concentrated to increase shelf life, reduce volume or weight, or reduce storage and transport costs. While in the pharmaceutical sector, evaporation is often used to create concentrated solutions which can then be dried to create powdered products. The basic principles of evaporation remain the same regardless of the sector.
Evaporation may be carried out in batches or as a continuous process and will consist of a heating phase followed by an evaporation phase. There are several types of equipment for performing these processes. Arnold Kleijn, from HRS Heat Exchangers here explores them in some detail.
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.
Forced recirculation evaporators (FRE)
In a FRE system the product is super-heated to a temperature above its boiling point, and upon exiting the evaporator, the product is introduced in a flash separation vessel where the pressure is lowered.
Owing 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. 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. This method 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.
Evaporation processes can be optimised in terms of energy use, by reusing the energy (latent heat) contained in the water that is evaporated from the product, or the use of steam compression devices. There are three possible ways of doing this.
Multiple effect evaporators (MEE)
In a multi-effect evaporator, the evaporated steam generated in the first stage of evaporation is used as the thermal energy source for the next stage. This can be repeated several times over where the same quantity of steam is reused to evaporate multiple volumes of water. In such systems, the pressure of each consecutive stage is lower than the previous one, which also lowers the boiling point. Multi effect evaporation is used to significantly increase the thermal efficiency of the process. For example, going from one stage to two stages reduces the thermal energy consumption by half. Adding a third stage reduces energy consumption by two thirds compared to a one stage process.
Thermal vapour recompression (TVR)
In a TVR compressor, part of the evaporated steam is mixed with ‘fresh’ steam from the boiler and this combined steam flow is then used as the thermal energy for that evaporation stage. The reuse of evaporated steam increases the energy efficiency of the plant.
Mixing evaporated steam and boiler steam in a TVR device usually gives a net steam which is much closer the boiling point then pure boiler steam. This makes TVR processes difficult to apply for liquids with high boiling point elevation, and unsuitable for products with high viscosities and lower rates of heat transfer.
Mechanical vapour recompression (MVR)
Where thermal energy (steam or hot water) is not available but electrical power is, a lobe, or fan compressor can be used to recompress the steam which has been generated. By compressing the evaporated steam, the temperature and pressure is increased to a point where it can provide useful energy for evaporation. This way, the same kg of steam which is evaporated from the product is reused as the thermal energy source for the same evaporation stage.
MVR compressors are a very economical technique.
Zero liquid discharge (ZLD)
Perhaps the ultimate use of evaporation technology is for ZLD systems, which combine evaporation systems with solids precipitation or crystallisation to achieve a net-zero liquid output from a process. The evaporator section concentrates the product as much as possible, typically to the point of saturation, before it is sent to the crystallisation section where the solids are suspended and separated from the saturated solution. This saturated solution (supernatant) is recycled back into the evaporator and the process is repeated or continued.
For products with a steep solubility curve additional cooling to the solid’s precipitation tank will help the solids to precipitate quicker and settle out. ZLD systems are complex and specifically designed for each application. In most cases product trials are required to establish the correct process parameters.
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