Design features of falling film evaporators and other evaporators, factors to consider when selecting evaporators (II)

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One of the simplest and oldest designs for intermittent evaporators is the batch evaporator (Figure 1). It consists of a jacketed container (H) heated by gas or liquid. The product is metered to a specified level through a feed nozzle (F) and heated in batches to the boiling point; steam is removed until the desired concentration is achieved. Finally, the heat is removed, and the concentrate is discharged through nozzle (C) or pumped out of the tank. Intermittent evaporation is not suitable for temperature-sensitive products. First of all, the dwell time is usually very long. Furthermore, the hydrostatic head of the liquid increases the boiling point of the product at the bottom of the tank, which can be overcome by operating the tank under vacuum to lower the boiling point of the batch. Intermittent evaporators generally have a low heat transfer coefficient, with a small heat transfer area relative to the volume occupied by the container. Since product movement occurs only through natural circulation, heat-sensitive products or those containing solids tend to contaminate the heated surfaces. By placing a stirrer in the container (batch stirring), the heat transfer coefficient can be increased and fouling can be reduced. Intermittent evaporators are still used in various processes, especially those involving small batches of highly viscous and non-thermally sensitive products. Figure 1 Natural circulation tubular evaporators – natural circulation is typically used in simple applications where high product purity and stable temperature are required (while forced circulation evaporators are used for products with high viscosity, tendency to become salty, or prone to scaling). The most common natural circulation tubular evaporators are horizontal tubes, shell-and-tube (or short) vertical tubes, and long vertical tubes. The horizontal tubular evaporator, which is a type of horizontal tubular evaporator, is the oldest type of chemical evaporator (Figure 2), although other improved evaporator types are now more commonly used. It is the only type of chemical evaporator that places the heating medium inside a tube, and its main advantage is that it requires relatively little space. Horizontal evaporators are not suitable for processes where scale or salt deposits tend to form (as these will accumulate outside the tubes); they are more appropriate for processes whose final product is a liquid rather than a solid, such as syrups. In this process, the large amount of liquid stored in the evaporator allows the final density to be precisely controlled by adjusting the residence time in the evaporator. Figure 2 shows a vertical tube-type evaporator with short tubes; the tubular vertical evaporator (Figure 3) is one of the earliest types of evaporators, and it is still widely used in commercial applications today. The short-tube evaporator is characterized by a tube sheet (A) that extends throughout the entire body and a central drain pipe (B). The tube rolls between the two tube sheets, and steam is introduced, as shown in Figure 3. The liquid is inside the tube, while the heating medium is outside the tube. When the liquid boils, it rises through the pipes and then returns via the central return pipe. The condensate is discharged from any convenient location at the bottom of the tube sheet (such as C), while non-condensing gases are usually discharged from a location near the upper tube sheet (such as D). The exact positions of the feed (F) and discharge (C) can vary, but the positions shown in Figure 3 are quite typical. The operating position of the liquid is usually near the top of the top tube sheet, and the cross-sectional area of the downcomer is typically between 75% and 150% of the cross-sectional area of the tubes. The length of the pipes can range from 1 inch to 4 inches, while the diameter ranges from 30 inches to 6 feet; as is common practice, pipes with a diameter of about 2 inches and a length of about 5 feet are preferred (4). The circulation and heat transfer of this type of evaporator are strongly influenced by the liquid level. Boiling inside the tube induces circulation through the heated surface; this circulation is caused by the difference in specific gravity between the bulk liquid and the heated liquid and steam generated inside the tube. When the highest heat transfer coefficient is achieved, the liquid level is only about half of the tube length. Reducing the level below the optimal value leads to incomplete wetting of the tube walls, resulting in an increased tendency to scaling and a rapid decline in capacity. When this type of evaporator is used with products that can deposit salts or cause scaling, it is usually operated with the liquid level slightly above the optimal value and generally above the top tube sheet (3). The advantages of vertical shell-and-tube evaporators include: • Low head space • Suitable for liquids with a slight tendency to scale, as the product is in the tubes and can be easily cleaned • Dilute liquids can achieve quite high heat transfer coefficients (up to 5-10 cP) • Relatively low manufacturing costs. However, the heat transfer in vertical shell-and-tube evaporators is greatly influenced by viscosity and temperature; it is not suitable for heat-sensitive products, and it is also unsuitable for products that tend to crystallize unless agitation is provided. A major use of vertical shell-and-tube evaporators is to concentrate sugarcane juice. Figure 3 Long-tube vertical evaporator. The long-tube vertical evaporator or falling-film evaporator (Figure 4) is one of the most widely used tubular evaporators. It can be built as a separate unit, owing to its high heat transfer performance under most conditions, and also partly because of its simple structure and low cost. It is essentially a shell-and-tube heat exchanger installed on a gas-liquid separator; it requires very little space, but it needs a high head height. The diluted feed enters from the bottom of the tube sheet and flows upward through the tubes, while the heating medium flows in the shell side. At the lower part of the tube, the raw material is heated to its boiling point. Bubbles formed in the part of the tube rising from the bottom, boiling began, increasing the linear velocity and heat transfer rate. Near the top of the tube, bubbles grow rapidly. In this bubble zone, liquid droplets and bubbles rise rapidly along the pipeline and are discharged at high speed from the top, where they strike the gas-liquid separator and the foam breaks apart. Therefore, this type of evaporator (1) can be used for products that tend to form bubbles. The advantages of long-tube vertical evaporators are: • Low floor space required • Relatively high heat transfer coefficients due to local two-phase flow • Ability to handle foaming liquids. Disadvantages are: • High height requirements • Higher pressure drop in the pipes compared to falling film evaporators • The static head at the bottom of the pipes may increase the product temperature and cause heat sensitivity issues. Some common uses of long-tube vertical evaporators are the concentration of sucrose syrup, pulp black liquor from paper mills, nitrates, and electrolytic tinning solutions. Figure 4 Forced-circulation tubular evaporator: When designing tubular evaporators, the value of mechanical recirculation should be considered. At first glance, adding a pump and additional controls will increase installation, operation, and maintenance costs. However, refeeding some of the concentrate back into the feed stream can increase the heat transfer rate, enough to **reduce the size of the evaporator**, thereby lowering the total cost. An increase in the flow rate of the liquid within the pipe (usually in the range of 4–10 feet per second) can also reduce or eliminate potential scaling, thereby maintaining capacity and minimizing downtime. In a falling film evaporator, it can increase the liquid load at the bottom of the tubes, thereby improving the distillation yield. In most cases, when the feed contains solids or crystals, a forced-circulation evaporator should be used. The advantages of using a forced-circulation evaporator are: • high heat transfer coefficient • forced circulation • reduced scaling. The main disadvantages of forced circulation are: • higher cost • energy consumption of the circulation pump • longer residence time of the product in the heating zone. Typical applications of forced circulation include sodium sulfate, urea, sodium chloride, sulfuric acid, magnesium chloride, citric acid, and potassium hydroxide. Falling film evaporator (long tube): The falling film evaporator (Figure 5) is a variant of the long-tube rising film evaporator, in which the equipment is inverted with the tubular heat exchanger located at the top of the gas-liquid separator section. The feed material enters from the top of the evaporator, and a specially designed distributor ensures that it is evenly distributed throughout each tube. The distribution of the raw materials is very crucial; there are many designs for distributors, but usually they consist of porous plates placed at the top. The falling film evaporator takes advantage of gravity, resulting in a thinner film and faster flow velocity, as well as a high heat transfer coefficient and a shorter residence time in the heating zone. Falling film evaporators are particularly suitable for applications where the temperature drive between the heating medium and the liquid is small (less than 15 °F). The falling film evaporator has the ability to operate at low temperature differences and features a short residence time, which makes it suitable for heat-sensitive products. The design of the gas-liquid separator at the bottom of the tube sheet depends to a large extent on the properties of the material being used and the operating conditions. The key to a falling film evaporator is the ratio of the distillate rate to the feed rate. Using single-pass high-precision distillation cracking can reduce the flow of liquid to the less moist areas at the bottom, thereby protecting the tubes from contamination by degradation products. The main advantages of falling film evaporators are: • relatively low cost • large integrated heating area • low product retention rate • small space requirement • good heat transfer coefficient at reasonable temperature differences. The main disadvantages are: • There are requirements regarding plant height • It is generally not suitable for materials that tend to become saline or scale • Recirculation is usually required. Typical applications of falling film evaporators are the concentration of dairy products (such as whey, milk protein, skim milk, cream, and hydrolyzed milk), sugar solutions, urea, phosphoric acid, and pulp residues