Thermal Evaporator

Thermal Evaporators

Evaporator is the name which itself describes the process equipment used to remove liquid by vaporization. In the evaporation process, concentration of a product is achieved by boiling out a water (solvent). The feed material may be a single liquid, liquid mixture, solutions or slurries, or any combination of these. Heat applied through some form through a heat transfer surface is used to produce the vaporization. The concentrated product, as discharged, is still in fluid, pump-able form.
It is a unit operation that is used extensively in processing foods, chemicals, pharmaceuticals, fruit juices, dairy products, paper and pulp, malt and grain beverages, effluents, biotechnological nutraceuticals, polymers and resins, inorganic salts, acids, bases, etc.

There are many types and variations of evaporators, and the best for a particular application depends on the product characteristics and desired results.
Evaporators can be classified in several ways:

• a) by tube position: horizontal or vertical
• b) by tube length: short or long
• c) by the direction of movement of film: rising or falling
• d) by the number of evaporators or effects. Like Multi-Effect Evaporator (MEE)

Evaporators can either work on a batch or a continuous basis. They can be designed to facilitate heat recovery (in single-stage plants by preheating installations or multistage plants by using the vapour from one stage to heat the following stage). For this purpose, the vapour is either compressed with new fresh steam or compressed mechanically. Compressed vapour (with higher enthalpy and higher temperature) is fed to the evaporator, where residual heat is used to preheat solution.
Batch-wise operating plants are used in cases where only a small amount of water is to be removed or other ingredients are to be added during the concentration, as they might have a defined dry matter level in the finished product.
Large plants are generally installed with continuous evaporators.
Evaporator installations can be either single-stage or multistage. A single-stage evaporator is a closed unit with a defined pressure and corresponding temperature. In multistage plants (up to seven stages), product from the first stage is transferred into the second stage and is heated with vapour from the first stage to the boiling point.

Typical desired results from an Efficient Evaporator

• Product concentration for further use in process or formulations
• Dryer feed pre-concentration, for lowering operation cost for Drying.
• Volume reduction for feeding to further drying in case of disposal for land filling.
• Water / solvent recovery.
• Crystallization

RAYGM supplies variety of evaporator like

• Falling film Evaporators (FFE)
• Rising Film Evaporator (RFE)
• Forced Circulation Evaporator (FCE)
• Agitated Thin Film Evaporator (ATFE)
• Multieffect Evaporators (MEE)

How to Select an Efficient Evaporator for your application:

While selecting an Evaporator following things are to be considered.

1 Steady state conditions

From the mass and energy balance equations, liquid–vapor equilibrium equation and specific relations for the product (slurry) to be evaporated, a steady state model for the evaporator system should be developed, considering one up to seven effects. From this information, it is possible to verify the decrease in vapor flowrate necessary for the operational process and an increase in the total system area, when augmenting the number of effects.

2 Retention of Key Ingredient

Key Ingredient retention in the final product should be estimated for each one of the alternative systems from the data obtained under steady state operation. When augmenting the number of effects in the evaporation system the retention of key ingredient may or may not be a function of the residence time in the evaporation system. While the number of effects increase in the evaporation system, the total residence time increases as well as the temperature at which the product is exposed, and, therefore, there is also a chance for key ingredient variation or degradation, should be avoided.

3 Economic Evaluation

The economic evaluation consists of determining the optimum number of effects and operating conditions of the system. The economic evaluation of the system should be done in two different ways. Firstly, an economic evaluation with the concept of minimizing the total (capital + operation) costs, and secondly, an economic evaluation to maximize the NPV (Net Present Value) considering the impact of the process design and operating conditions on product quality, fouling of Calandrias, CIP time requirement, CIP Chemicals disposal cost, increase in effluent load on ETP etc.

4 Optimum Operating Conditions

In the search for the optimum operating conditions of evaporation system, the system should be economically evaluated under a variable steam inlet pressure where the inclusion of key ingredient concentration as a quality parameter should be considered.

5 Optimum number of effects

The economic evaluation should be carried out by simple calculations. This is where the steady state conditions for systems with 1–7 effects are generally found, and then total cost minimization and NPV maximization methodologies were used. The search should be focused to find the number of effects that minimize the total cost and, in addition, to find the number of effects that maximize the NPV.

A properly designed evaporator shall be, at a minimum:

• Shall be designed to effectively transfer heat at a high heat flux with minimum surface area to be cost-effective for installation (Capital Cost), Operations and maintenance.
• Shall Effectively separate the vapor from the liquid concentrate and generate maximum quantity of flash vapours to use in next effects, lowering operation cost.
• Meet the conditions required by the product being processed, for e.g. reduction of effluent, concentration of product for feeding to dryer for minimizing drying cost.
• Shall Produce a product that meets the required quality of the product in terms of retention of Key Ingredients, Aroma etc.
• Shall Be energy efficient, where possible making effective use of steam with multiple-effect evaporation or vapor recompression.
• Minimize fouling of heat transfer surfaces
• Be constructed of materials that are adequate to minimize corrosion and give long life of the equipment.

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The Various types of Evaporators We supply.

Falling-film evaporator

This is the most commonly used evaporator in the industry. The product residence time in falling-film evaporators is always the same. Product is pumped into the top of the evaporation stages and passed through tubes in a thin layer (on the walls). The advantages of using falling-film evaporators is short residence or passage time of the product on the heated tubes, good heat conduction due to a thin product layer and no difference in thermal load. Depending on the design, the tubes are 4–10 m long and have diameters ranging from 25 to 80 mm. These plants can be operated with direct steam injection without vapour compression, with thermal vapour compression (steam injection) or with mechanical vapour compression.
A schematic representation of a three-effect falling-film evaporator is given below.

Advantages of falling-film Evaporators:

• The chemical properties of heat sensitive materials won’t be affected because of the short residence time of the feed slurry in equipment.
• Relatively low CAPEX + OPEX
• Large heating surface in one body can be provided.
• Low product hold-up in the entire body as product is mainly hold up in the bottom dome.
• Small foot prints save on land cost.
• Good heat-transfer coefficients at optimal temperature differences

This is the most commonly used evaporator in the industry. The product residence time in Some applications for falling-film evaporators include concentration of Herbal Extracts, Corn derivatives, dairy products (such as milk, whey, milk protein, skim milk, cream and hydrolysed milk), sugar solutions, urea, phosphoric acid and black liquor.

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Rising film evaporator

Rising Film evaporators are combination of Rising + Falling film. They are usually two pass systems. Slurry enters at the bottom of the calandria of the rising-film portion. Boiling (under vacuum and low temperature) starts as the liquid rises through the tubes. A mixture of liquid and vapor enters top dome of the Calandria and redistributed over the top of the tubes through a specially designed liquid distributors for the falling-film pass. The vapours from the rising-film pass creates thermodynamic conditions in the top dome and helps in distribution of liquid on the tubes, which ultimately increase the velocity of the liquid, which results in increases in heat transfer. The mixture of vapor and liquid from the bottom dome is pumped into external vapour separator for separation of vapours and feeding them to next calandria body or to the inlet of TVR as per system design.
Long tubes are used for the bundle design but larger temperature differences are necessary for proper operation. The long tubes allow high concentration ratios in a once-through mode with an application for viscous and foaming liquids.

The rising film evaporator’s advantages are:

• System turn down ratio is good without loss of efficiency or effectiveness.
• Suitable for foamy liquids & liquids with suspended solids.
• Relatively low residence times
• Relatively high heat transfer rates high evaporation ratio.
• Relatively low CAPEX + OPEX
• Low hold-up in the evaporator body.
• Small foot prints so less land required.
• Good heat transfer over a wide range of services.
• Suitable for evaporation of a wide range of liquid concentrations.
• Less fouling due to high tube velocities and turbulence in the tube body.

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Forced Circulation Evaporator

The feed liquid is fed into the circulating stream which is moved by a pump through a Calandria and a vapour separator vessel. The heat applied through the tube walls superheats the liquid which flashes into the vapour separator and vapours are separated from the concentrated liquid. The high velocity of the liquid effects good heat transfer values but requires a large circulation pump. Further, the high velocity reduces the tendency for fouling and scaling.

The Forced Circulation evaporator’s advantages are

• High rate of heat transfer;
• Positive circulation;
• Relative freedom from salting, scaling, and fouling;
• Ease of cleaning;
• Wide range of application.
• Best suited when treating crystalline products, corrosive products, or viscous fluids.
• Suited for vacuum service, and for applications requiring a high degree of concentration and close control of bottoms product concentration.

The Forced Circulation evaporator’s advantages are

Some of the Applications in which forced circulation is used includes urea, sodium sulfate, sodium chloride, magnesium chloride, caustic potash, citric acid, and ammonium sulfate.

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Agitated Thin Film Evaporators

Scraped surface, swept surface, agitated thin film or wiped film evaporators are very similar to scraped surface heat exchangers (SSHEs) and are designed for the evaporation of highly viscous and sticky products that cannot be otherwise evaporated easily. A rotating blade wipes the surface to promote heat transfer, to prevent deposition or charring and to allow less concentrated liquid to move to the surface. The gap between the blades and the evaporating surface can vary from zero to a few millimetres. They are used for liquids that are very viscous, heat sensitive or foul easily. The residence time in this type of evaporator can be kept very short, making it useful for heat-sensitive materials. Typically, these evaporators have only a single tube and rotor, so they have a low surface area and are a relatively expensive method of evaporation. Normally just a single scraped surface effect is used usually after concentration by another type of evaporator, Continuous falling film and rising film, as well as batch, agitated thin film evaporators, have been designed.

Advantages of an Agitated thin-film evaporator (ATFE) are:

• Short residence time in the heated zone, from seconds to minutes
• High heat-transfer coefficients due to the turbulence imparted by the rotor
• High feed-to-product ratios can be handled without recirculation.
• Ability to handle high solids concentrations, crystallized liquids and viscous materials.
• Less product decomposition due to shorter residence time and accurately controlled temperatures, resulting in higher yields.
• High recovery because of the wiping of residues by the specially designed rotor.

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Multiple Effect Evaporators

Evaporators are classified by the number of effects. In a single-effect evaporator, steam provides energy for vaporization and the vapor product is condensed and removed from the system. In a double-effect evaporator, the vapor product off the first effect is used to provide energy for a second vaporization unit. Triple- effect evaporator problems are familiar to generations of engineering students. This cascading of effects can continue for many stages. Multiple-effect evaporators can remove much larger amounts of solvent than is possible in a single effect.
In a multiple effect arrangement, the latent heat of the vapor product off of an effect is used to heat the following effect. Effects are thus numbered beginning with the one heated by steam. It will have the highest pressure. Vapor from Effect I will be used to heat Effect II, which consequently will operate at lower pressure. This continues through the train: pressure drops through the sequence so that the hot vapor will travel from one effect to the next.
Normally, all effects in an evaporator will be physically the same in terms of size, construction, and heat transfer area. Unless thermal losses are significant, they will all have the same capacity as well.
For adding more economy to system, a TVR or MVR is added to system to compress the low-pressure steam generated during liquor evaporation and is fed to designated effect as per steady flow patterns.
Evaporator trains may receive their feed in several different ways. The feed order is NOT related to the numbering of effects. Effects are always numbered according to decreasing pressure (steam flow).
Forward Feed arrangements follow the pattern I, II, III. These require a single feed pump (reduced fixed costs). They typically have reduced economy (higher operating costs) since the cold feed must be raised to the highest operating temperature. These also tend to have the most concentrated liquor, which tends to be the most viscous, in the lowest temperature effects, so there may be difficulties getting a good overall heat transfer coefficient. Backward Feed arrangements go III, II, I. These need multiple pumps to work against the pressure drop of the system; however, since the feed is gradually heated they usually have better economies. This arrangement also reduces the viscosity differences through the system and so is better for viscous solutions.
Mixed Feed arrangements offer a compromise, with the feed entering in the middle of the system (i.e. II, III, I). The final evaporation is done at the highest temperature so economies are still better than forward feed, but fewer pumps are required than in a backward feed arrangement.
Parallel Feed systems split the feed stream and feed a portion to each effect. This is most common in crystallizing evaporators where the product is likely to be a slurry.

Application of Multi-effect Evaporators.