A Double Effect Evaporator can efficiently use the heat from steam for the treatment of water or wastewater in countless applications.
In this article we introduce the double effect evaporator features and advantages compared to single effects evaporators and its applications in various industries.
What is a Double Effect Evaporator?
A double-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water. Water is boiled in a sequence of two vessels, each held at a lower pressure than the last.
Because the boiling temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat.
In a system with a single effect evaporator, the steam generated during processing is discarded. If the steam generated is used as a source of energy for a second evaporator, then we have a double-effect evaporation system.
The salient features of a Double Effect Evaporator are:
A double effect evaporator is an apparatus that efficiently uses the heat from steam to evaporate water or wastewater;
In a double effect evaporator, feed water is boiled in a sequence of vessels, each of which is at a lower pressure than the last;
As the boiling point of the water decreases with pressure, the vapour produced in one vessel can be used to heat the next vessel;
Only the first vessel requires a source of external heat, thus saving energy consumption;
Evaporators with more than four to five effects are rarely practical, the most common applications require double effect and triple evaporators.
Single Effect vs. Double Effect or Multiple Effect Evaporators
Evaporators may have a single unit, called single effect, and double or multiple units, called multiple-effect.
The most important difference and advantage of multiple-effect over the single-effect evaporator is the economy. In fact, double-effect and multiple-effect designs evaporate more water per kilogram of steam fed to the unit by reusing the vapor from one effect as the heating medium for the next.
Adding one evaporator to the single effect decreases the energy consumption by 50% of the original amount. Adding another effect reduces it to 33% of the total energy consumption, and so on. While in theory, multi-effect evaporators may be built with an arbitrarily large number of stages, evaporators with more than four stages are rarely practical except in systems where the liquor is the desired product such as in chemical recovery systems where up to seven effects are used.
The design of a multiple effect evaporator can be chosen among forward feed, backward feed, parallel feed. Instead, in the double effect evaporator the design is often forward feed.
Double Effect Evaporators Example
An evaporator with a double effect (or 2 stages) design is a tube-type forced circulation evaporator in which strong steam is used for the first effect to evaporate the solvent from the feed.
In the second effect, the evaporation of the concentrating feed from the first stage is used to evaporate solvent from the second stage. The evaporated solvent from the second effect is finally condensed with cooling water on the other side of the steam condenser.
By using the double effect design, evaporators can easily treat water and wastewater with capacities ranging from 1 to 3 tons per hour. Below those capacities a single effect evaporator is more suitable, while above 3 tons/hour a multi-effect evaporator is suggested.
In the picture below we can see a double effect evaporator installed in a client’s factory.
Industrial Applications for Double Effect Evaporators
Similarly to multiple effect evaporators, the most common applications for Double Effect Evaporators are in the food, chemical, pharmaceutical industries and Zero Liquid Discharge effluent treatment plants, such as for brine concentration or secondary treatment.
Double Effect Evaporators are widely used in the following areas as per the application requirements:
Industries like Chemical, Pharmaceutical, Textile, Dyeing, Breweries, Automobiles, Milk, Food industries etc, generate high amounts of wastewater.
WWTP and Desalination from RO Plants
Reject from Reverse Osmosis (RO) have a significant amount of brine that can be evaporated and concentrated by two effects evaporators.
Chemical / Pharmaceutical Industry
Double Effect Evaporators are often used to achieve the desired water quality for the production line or for treating the wastewater from the production processes.
Milk / Sugar / Food Industry
Milk, sugar, fruit juice and tomato paste can be all concentrated by Double Evaporation systems
Forward Feed Double Effect Evaporator | Two Stage Evaporation
In a forward feed double effect evaporator, both feed and steam are introduced in the first effect and the feed is passed from the second stage parallel to the vapor from the previous effect.
The water or wastewater feed is pumped into the first effect and the partially concentrated solution is sent to the second effect. The heating steam is also sent through the first stage to the second one. This process is continuous and the ends on the last evaporation stage.
The forward feed requires a pump for feeding dilute solution to the first effect. The first effect is generally at atmospheric pressure and the second effect is at lower pressure. Thus, the liquid may move without the pump from the first to the second effect by following the direction of decreasing pressure.
On the last stage of a double effect evaporator an effluent pump takes out the concentrated liquid from the last effect.
Optimal Double Effect Evaporator Design
The optimal design for a double stage evaporator design needs to account for many factors such as type of evaporator or heat exchangers, forced or natural circulation, feeding arrangement, boiling point elevation, heat transfer coefficient, fouling, and piping sizes.
Efficient Double Effect Evaporators are designed and operated according to several key criteria:
Feed Water Characteristic of Double Effect Evaporators
In any application, the most important factor to account for in the double effect evaporator design is the feed effluent characteristic, which plays a major role in deciding the configuration of the equipment. To choose the most suitable design a lab test can assess the quality of the feed by testing for the liquid components and an evaporation test can be done to assess the concentration capacity of the double effect evaporator.
Double Effect Evaporators Heat Transfer
A large flow of heat across a metallic surface of minimum thickness or high heat flux) is fairly typical. The requirement of a high heat transfer rate is the major determinate of the evaporator type, size, and cost.
Liquid-Vapor Separation in Double Stage Evaporators
Liquid droplets carried through the evaporator system, known as entrainment, may contribute to product loss, lower product quality, erosion of metallic surfaces, and other problems. Generally, decreasing the level of entrainment in the evaporator increases both the capital and operating costs. All these problems and costs considered, the most cost-effective evaporator is often one with a very low or negligible level of entrainment.
Energy Efficiency of Double Effect Evaporators
Multi-effect evaporators should be designed to make the best use of available energy, which implies using the lowest or the most economical net energy input.
What Is The Best Double Stage Evaporator for my Water or Wastewater Treatment?
For water and wastewater treatment the most suitable double stage evaporator design needs to account for the characteristics of the liquid and the distillate quality requirements.
After testing the water and wastewater in a rotary evaporator the treatment assessment report can be released and the double effect evaporator can be designed. In some cases, the distillate needs to have certain parameters that need to be respected, so testing is a must have procedure for avoiding mistakes in the treatment process.
YASA ET helps companies all around the world to find the best solution to their water and wastewater problems by manufacturing and designing single effect, double effect, and triple effect evaporators.
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