Evaporation is a critical process in the food industry, used to concentrate liquid products by reducing their water content while preserving flavor, nutrients, and texture. Given the heat sensitivity of many food products, multiple-effect evaporators (MEEs) are commonly employed. These systems use a series of stages to maximize evaporation efficiency while minimizing thermal impact, making them ideal for applications like fruit juice concentration, milk powder production, and tomato paste manufacturing. This section explores the various types of evaporators and their role in optimizing food processing.
Introduction
Multiple-effect evaporation is a key technology in the food industry, used extensively for concentrating liquids like fruit juices, sugar solutions, milk, and vegetable juices. This process works by removing a portion of the solvent—typically water—thereby increasing the concentration of the remaining solution. By removing water, evaporation not only enhances microbiological stability but also cuts down on storage and shipping costs, which is why it’s essential for creating concentrated products like juices, jams, ketchup, and tomato concentrates.
At its core, the evaporator is a heat exchanger designed to separate vapors from boiling liquids. The system relies on three main components: a heating unit, a vapor separation area, and a structural body to house these elements while separating the heating and process fluids. Heat is usually supplied by steam or another hot medium that boils the solvent, thus concentrating the solute. This evaporative process is critical for further food processing steps like crystallization, coagulation, and precipitation.
Comparison between single-effect and multiple-effect
There are two main types of evaporators in use: single-effect and multiple-effect. Single-effect evaporators discard the produced vapors without using their residual heat, making them less energy-efficient. On the other hand, multiple-effect evaporators reuse the vapors as the heating medium in subsequent evaporator chambers, reducing energy consumption. These systems are more cost-effective when dealing with larger evaporation rates—typically, when the rate exceeds 1,000 kg/h. Below this threshold, a single-effect evaporator with vapor recompression is more efficient.
The primary trade-off in evaporator design is between capital costs and energy usage. While single-effect systems are cheaper to install, they are less energy-efficient compared to multiple-effect systems. For instance, a four-effect evaporator consumes about 25% less energy than a single-effect to evaporate the same amount of water. The use of a second effect can halve steam consumption. However, for every kilogram of steam used, about 30 kg of cooling water is required, which means that increasing the number of effects reduces both steam and cooling water usage.
Evaporator: (A) single-effect and (B) multiple-effect
However, with the increase in number of effects, the initial capital investment also increases. For example, a two-effect evaporator with the same capacity costs roughly twice as much as a single-effect system. As such, choosing the right number of effects becomes a balancing act, factoring in capital investment, steam consumption, and long-term operating costs. In food processing, three- and four-effect evaporators are most commonly used.
Another important concept in evaporation is boiling point elevation—the increase in boiling point temperature of a solution compared to pure water under the same pressure. The relationship between the boiling point of the solution and that of pure water is linear. While there are limitations to how many effects can be used, primarily due to high temperature gradients, increasing the number of effects continues to offer substantial gains in steam efficiency.
Types of Multiple-effect Evaporators
Evaporator design is highly dependent on the nature of the feed being processed. Factors such as the feed’s density, viscosity, boiling point relative to pressure, desired concentration, and heat sensitivity all influence the choice of evaporator. Essentially, evaporators are categorized based on how the liquid phase circulates during the process. In the food industry, several types of evaporators are commonly used: natural circulation, forced circulation, agitated thin-film, rising-film, falling-film, and rising/falling-film evaporators.
Natural Circulation Evaporators
In a natural circulation evaporator, short vertical tubes, typically 1–2 meters in length and 50–100 mm in diameter, are filled with steam. These tubes, along with the steam chest, are positioned at the bottom of the vessel. As the product is heated, it naturally rises through the tubes, while steam condenses outside of them. The liquid inside the tubes becomes more concentrated as water is evaporated.
Long-tube natural circulation evaporators offer several advantages, such as relatively high heat transfer efficiency, resistance to scaling, and minimal foaming. They are simple to maintain and require a modest initial investment. However, their food contact surface is limited, and temperature fluctuations between steam and the product need to remain small to prevent fouling. This results in lower heat transfer coefficients and reduced evaporation capacity compared to other systems. The optimal operation of a natural circulation evaporator involves stable two-phase flow, with boiling occurring at the heat exchanger outlet. Compared to forced circulation evaporators, natural circulation systems subject the process medium to less mechanical stress, as there is less shear and tensile force.
Forced Circulation Evaporators
Forced circulation evaporators come in several designs, but all feature high-rate circulation of the liquid food—either through plates or tubes, or by creating thin coatings of the product with scraper assemblies in cylindrical heat exchangers. In this system, the liquid is cycled at high speeds through a non-contact heat exchanger, with a hydrostatic head above the tubes that prevents the liquid from boiling. As the liquid passes through the heat exchanger, it is heated, and some of it vaporizes as the pressure drops in the separator, lowering the boiling point to match the pressure.
The increased liquid velocity—usually between 1.22 and 3.05 m/s—helps minimize fouling, which keeps the system running at optimal capacity and reduces downtime. Forced circulation evaporators are essential when thermosiphon circulation isn’t feasible, either due to the product’s viscosity or the presence of low volatile components in the vapor. These evaporators are more efficient than natural circulation types but tend to have longer residence times than rising and falling film evaporators.
Forced circulation evaporators are particularly useful for crystallizing salt solutions, where both the quantity and size distribution of the produced crystals can be significantly influenced by factors like heat exchanger temperature, cooling water flow rate, vessel design, vaporization rate, and residence time. In desalination processes, for instance, the temperature difference between the effects of a multiple-effect evaporator directly impacts the amount of freshwater produced. A greater temperature difference increases freshwater output while requiring fewer effects, thus reducing building and maintenance costs.
While forced circulation evaporators require expensive pumping systems, they offer advantages for materials with high heat flow densities and low evaporation rates. Furthermore, both the circulation rate and vaporization rate can be independently controlled, providing additional flexibility in operation.
Agitated Thin-Film Evaporator
Agitated thin-film evaporators are particularly effective for processing highly viscous, high-solids, heat-sensitive, or foamy compounds. These systems can concentrate liquids with viscosities up to 100 poise. The two main components of an agitated thin-film evaporator are the drying chamber with a heating jacket and the rotor with fixed blades. The rotor helps create a thin layer of liquid film on the inner surface of a metallic wall, while the external utility stream supplies heat to the outer jacket.
Due to the intense agitation from wiper blades, which distribute the feed over the cylindrical heating surface, agitated thin-film evaporators achieve much higher heat and mass transfer rates, even at low operating pressures. This type of system operates with a combination of radial drag flow (caused by the rotor blade) and gravity-driven downward flow. These evaporators are ideal for concentrating heat-sensitive foods because they have short residence times and relatively high heat transfer coefficients. Residence times typically range from a few seconds to several minutes, depending on equipment size, operating conditions, and rotor design.
Rising (Climbing) Film Evaporators
Rising (or climbing) film evaporators are designed with long tubes (2–5 cm in diameter, 10–15 m in height) that are heated from the outside by steam. Preheating the feed before it enters the evaporator helps initiate vaporization quickly, which reduces the liquid film thickness and increases its velocity. A low-viscosity liquid, almost at boiling point, is introduced at the bottom of the tubes. As the liquid moves upward inside the tubes, boiling begins, and vapor bubbles rapidly form and travel faster than the liquid. These bubbles push the liquid upward, creating a thin, rapidly moving film.
Rising film evaporators are suitable for heat-sensitive foods due to their turbulent liquid film, high heat transfer rates, and short residence times (usually between 2 and 5 minutes). The concentrated product is separated from the vapor at the upper end of the tube, and the liquid is then sent to subsequent effects in a multi-effect system, while the vapor proceeds to the condenser. These evaporators require little floor space but do need relatively high headroom. To achieve sufficient rising action against gravity, a temperature difference of at least 15°C between the liquid and the heating medium is required.
The main limitation of rising film evaporators is their inability to handle very viscous fluids well. A thicker film can impede heat transfer and slow vapor formation. Although the operation is typically one-pass, a circulating pump can be used in some cases, particularly for viscous products or those containing suspended solids or crystallizable substances, to prevent fouling of the heat exchanger surfaces. In these instances, recirculating the product back into the feed stream may be necessary to achieve the desired concentration.
Falling Film Evaporators
In contrast to rising film evaporators, falling film evaporators operate by feeding the liquid at the top of long vertical tubes. The liquid then flows downward by gravity, assisted by vapor drag. The velocity of the liquid film is faster in falling film evaporators, reaching up to 200 m/s at the end of 12-meter tubes. The distribution of the liquid is crucial to ensure that each tube has enough liquid to form a continuous film. This helps prevent dry spots, overheating, crust formation, or clogging of the tubes.
Falling film evaporators are commonly used for heat-sensitive products due to their high heat transfer rates and short residence times, which typically range from 20 to 40 seconds. These systems can achieve high heat transfer efficiency even with low temperature variations across the liquid film. In multi-effect systems, falling film evaporators can handle a significant number of effects, even up to 10 or more, depending on the temperature difference between the steam entering and the boiling temperature in the last effect.
The major advantages of falling film evaporators are their resistance to fouling and low operating costs. They can handle moderately viscous liquids and reach evaporation ratios of up to 70% without recirculation, or up to 95% with recirculation. These systems are often used in applications like citrus juice concentration, where high heat transfer rates and short residence times are required. In these systems, a multieffect evaporator with four to five effects can evaporate large amounts of water in one pass, without the need for recirculation.
Rising/Falling Film Evaporator
A rising/falling film evaporator combines the benefits of both rising and falling film evaporators into a single system. In this setup, the liquid feed is introduced at the bottom of one set of tubes, where it rises through the tubes in a rising film evaporator. The mixture of boiling liquid and vapor is then discharged and dispersed across the top of the tubes, where it enters the falling film section. From there, the liquid and vapor are directed to a vapor-liquid separator located at the bottom of the calandria. The vapor is then sent to a vacuum system or a condenser for further processing.
This combined evaporator system is preferred when a high evaporation-to-feed ratio is required, which is often the case when producing a viscous product. The integration of both rising and falling film processes enables higher efficiency and better handling of the liquid’s characteristics.
Applications of Multiple-Effect Evaporators in the Food Industry
Food products are often heat-sensitive, so to prevent excessive product degradation, evaporation operations must reduce both the boiling temperature and residence time. Thermal issues such as nonenzymatic browning, the development of a cooked flavor in milk and fruit juices, loss of carotenoid pigments like lycopene in tomato juice, and protein denaturation in milk are common problems associated with evaporation.
Tomato Paste Production
- Tomato paste production is highly seasonal, so optimizing production levels is crucial. The process typically occurs in multiple-effect evaporation systems where the temperature stays below 70°C, and the water content is reduced to a final concentration of 30 to 32 Brix. The key challenge here is the low thermal efficiency of the system, which must boil off large volumes of water from fresh juice. Evaporative units must balance energy consumption with food quality preservation.
Sugar Refining and Concentration
- In sugar refining, the heat vaporization of water is high, making the evaporation process energy-intensive. Multiple-effect evaporators are used to economize steam. The juice typically enters the evaporator with a sucrose concentration of around 12 Brix and is concentrated to approximately 65 Brix in the final effect. Increasing the number of effects, as well as adjusting temperature vapor for greater thermal efficiency, can significantly reduce energy consumption. For example, the total energy required in a system drops from 45,003 kW in a three-effect system to 24,790 kW in a five-effect system.
Fruit Juice Concentration
- Evaporation is the primary method for reducing the water content in fruit juices, with around 90% of concentrated orange juice worldwide produced through this method. Since fruit juices contain heat-sensitive vitamins and flavonoids, evaporators are designed to minimize heat exposure, operating at low temperatures and pressures. In fruit juice concentration, counter-current multiple-effect evaporation is preferred, as it handles the rapid viscosity increase during concentration more efficiently. Thin film evaporators are also used for aroma separation in tropical fruit juices like mango, guava, and pineapple.
Orange Juice Concentration
- In a triple-effect evaporator for orange juice, the first effect is the most energy-intensive, consuming 48.2% of the total exergy. The second and third effects consume 19.76% and 32.04% of the total energy, respectively. This breakdown helps in understanding where energy savings can be maximized in the concentration process.
Milk Powder Production
- Evaporators such as falling film, horizontal plate, and plate evaporators are commonly used in milk powder production. Falling film evaporators, combined with Mechanical Vapor Recompression (MVR) and Thermal Vapor Recompression (TVR), help reduce energy consumption. In milk concentration, as the milk solid content approaches 45%, the viscosity increases significantly. Typically, sterile milk is concentrated at temperatures between 40°C and 70°C under vacuum, increasing the solids content from 13% to 50%. Most modern milk evaporators combine MVR and TVR technologies in two- or three-effect arrangements.
Conclusion
Multiple-effect evaporators (MEEs) are essential in the food industry for their ability to concentrate liquids efficiently while minimizing energy use and preserving product quality. While challenges such as fouling and scaling remain, advancements in process optimization and system design continue to enhance their performance. As the demand for energy-efficient, high-quality food products grows, MEEs will remain central to sustainable food processing practices.