A Multiple Effect Evaporator (MEE) can efficiently use the heat from steam to evaporate water and can adopt a forward feed, backward feed or parallel feed designs for its multi-effect evaporation process.
These feed arrangements affect the final design and structure of the evaporator and the multiple effect evaporation. Before exploring these differences, we should introduce the multi effect evaporator features and applications.
Continue reading this article or watch the video below to find out more about Multiple Effect Evaporators (MEE) design, applications, advantages and disadvantages.
What is a Multiple Effect Evaporator?
A Multiple Effect Evaporator, or MEE, involves structured sequencing of evaporators to evaporate water from a liquid feed.
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 most common configuration for a MEE is a triple effect evaporator, which involves a three stages evaporation process as shown in the picture below.
Multiple Effect Evaporator (MEE) Advantages
A Multiple Effect Evaporator (MEE) advantages and features are:
A multiple effect evaporator is an apparatus that efficiently uses the heat from steam to evaporate water;
In a multiple effect evaporator, 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;
Evaporators with more than four to five effects are rarely practical, the most common applications require double effect and triple evaporators.
Three Effect Evaporator Design Feed Arrangement
A Three Effect Evaporator (3-stages) is a tube-type forced circulation evaporator in which strong steam is used for the first effect to evaporate the solvent from the feed. This multiple effect evaporator can produce three times more distillate than a single effect evaporator with the same energy consumption.
In the second effect, the vaporized solvent is used to evaporate the feed at atmospheric pressure. In the third effect, the evaporation of the concentrating feed from the second stage is used to evaporate solvent from the second stage.
The evaporated solvent from the third effect is finally condensed with cooling water on the other side of the steam condenser. In all three processes, condensate is collected in condensate receiving tanks. The pure solvent in this condensate can be reused in the subsequent process.
Applications for Multiple Effect Evaporators (MEE)
Multiple Effect Evaporators (MEE) most common applications are in the food, chemical, pharmaceutical industries and Zero Liquid Discharge effluent treatment plants.
Multiple Effect Evaporators are widely used in the following areas as per the requirement:
Effluent Treatment: Industries like Chemical, Pharmaceutical, Textile, Dyeing, Breweries, Automobiles, Milk, Food industries etc, generate high amounts of wastewater;
Desalination: Reject from Reverse Osmosis (RO) have a huge amount of water that can be evaporated by Multiple Effect Evaporators (MEE);
Chemical / Pharmaceutical Industry: Multi Effect Evaporators (MEE) are used to achieve the desired concentration for the production line;
Milk / Sugar / Food Industry: Milk, sugar, fruit juice can be concentrated in a Multiple Effect Evaporation system.
Multiple Effect Evaporators (MEE) For Industrial Wastewater
Multiple Effect Evaporators (MEE) are one of the most effective technology for the treatment of industrial wastewaters. In fact, evaporation can effectively reduce the following parameters:
Chemical Oxygen Demand (COD) - Measure of the amount of oxygen that is needed to oxidize soluble and particulate organic matter in water;
Biological Oxygen Demand (BOD) - Biological method used for the measurement of the total amount of dissolved oxygen (DO) used by microbes in the biological process of metabolizing organic molecules present in water;
Total Suspended Solids (TSS) - Suspended particles in water that are not dissolved;
Multi-Effect Evaporation is very effective in the treatment of many wastewaters, including spent cutting fluids and oily wastewaters, electroplating and landfill leachate. Often this technology requires pre-treatment that can be performed using an electrocoagulation equipment.
Forward Feed Multiple Effect Evaporator
In a forward feed Multiple Effect Evaporator, both feed and steam are introduced in the first effect and the feed is passed from effect to effect parallel to the vapor from the earlier effect. The liquid 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 continues through all the evaporation effects.
The forward feed requires a pump for feeding dilute solution to the first effect. The first effect is generally at atmospheric pressure and the subsequent effects are in decreasing pressure. Thus, the liquid may move without the pump from one effect to another effect in following the direction of decreasing pressure. Finally, to take out the concentrated liquid from the last effect, the system may need an effluent pump.
More information about forward feed evaporator can be found in this article.
Backward Feed Multiple Effect Evaporator
In backward feed Multiple Effect Evaporator, the dilute liquid is fed to the last effect and then pumped back through the successive effects to the first effect. The method requires additional pumps, generally one pump in between two effects.
Backward feed is advantageous and gives higher capacity than the forward feed when the concentrated liquid is viscous, because the viscous fluid is at higher temperature being in the first effect. However, this arrangement comes with higher material costs compared to forward feed design.
More information about the advantages and disadvantages of backward feed evaporator can be found in this article.
Forward Feed VS Backward Feed Arrangement Design
Multiple Effect Evaporators engineers need to compare forward feed vs backward feed designs advantages and disadvantages to find the optimal solution for the project.
Both designs have pros and cons that need to be evaluated:
Considerations For an Optimal Multiple Effect Evaporation Equipment Design
Many factors such as type of evaporator or heat exchangers, forced or natural circulation, feeding arrangement, boiling point elevation, heat transfer coefficient, fouling, tube size and arrangement must be carefully considered when designing a multiple effect evaporation system.
The most important parameter to consider for the Multiple Effect Evaporator design is the feed effluent characteristic, which plays a major role in deciding the number of effects. Before choosing the most suitable Multi Effect Evaporator design, it’s necessary to analyse the feed features and required evaporation criteria.
Efficient Multi Effect Evaporators are designed and operated according to several key criteria:
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: 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: 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 Multiple Effect Evaporator for Wastewater Treatment Applications?
For wastewater treatment the best Multiple Effect Evaporator design needs to account for the characteristics of the feed liquid and the distillate quality requirements.
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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