Learn Multiple-Effect Evaporator in Five Minutes

Introduction

A multiple-effect evaporator is a multi-stage evaporation process. In which, for only the first stage, external steam is used to evaporate the liquid product, and for the following stages the vapor of the liquid product is used to evaporate itself.
This is possible because each vessel in the sequence is held at a lower pressure than the last. The boiling point of a liquid decreases as the pressure decreases therefore only the first vessel which is at the highest pressure requires the external source of heat.
While in theory, it is possible to manufacture a multiple-effect evaporator with an arbitrarily large number of stages, an evaporator with more than 4 stages is rarely practical. Unless the desired product is such as liquor in chemical recovery systems where up to seven effect evaporators are used.

Working Mechanism

The vapor from the first effect becomes the heating source for the second effect. This is possible because the second effect’s boiling temperature has been lowered to 68°C (at 29 kPa), which is less than the saturated temperature of the first effect, 82°C (at 51.8 kPa). The latent heat is reused again as the vapor from the second effect becomes the heat source of the third effect, where the boiling point temperature has been lowered to 57°C (at 17.5 kPa).

Evaporators are generally rated based on their evaporation rate; the amount of water they evaporate per hour (e.g., kg/hr, ton/hr).

Applications

Originally, they were designed for concentrating sugar in sugar cane juice, now they are widely used in all industrial applications where large volumes of water must be evaporated, such as salt production and water desalination.

In the food processing industry, food products are concentrated to increase their shelf life, reduce volume and weight, reduce storage costs, reduce transportation costs, and increase value. The concentration of fresh foods, such as fruit juices, is an economical way to make use of perishable crops during peak harvest periods. Fruit juice concentrates and tomato paste are two examples of seasonal foods that evaporation preserves for year-round consumption. Lower water activity, which is a general indication of microbiological stability, preserves concentrated foods. Multiple-effect evaporation plants in sugar beet factories have up to eight effects.

In the chemical and kraft paper industries, evaporation is often used to recover valuable, nonvolatile chemicals. For example, in the kraft pulping process, water is evaporated from spent cooking liquid (i.e., black liquor), which concentrates the solids. The concentrated inorganic process chemicals can then be refined for reuse within the process. Six effect evaporators are common in the recovery of black liquor in the kraft process for making wood pulp.

In the pharmaceutical industry, evaporation is often used to create a concentrated solution, which can then be dried to create a powdered product.