Evaporation in Food Engineering

Important Unit Operations in Food Process Engineering

• Preservation Processes, Refrigeration, Food Freezing, Evaporation, Membrane Separation, Dehydration, Supplemental Processes: Filtration, Sedimentation, Centrifugation, Mixing, Extrusion Processes for Foods, and Packaging Concepts are examples of processes in food engineering.

Evaporation

• Evaporation is a unit operation used to remove water from dilute liquid foods to create concentrated liquid products.
• Microbiological stability is gained from removing water from foods, aiding in decreased transportation and storage expenses.
• Tomato paste manufacture involves raising total solids from 5-6% in tomato juice to 35-37% in the paste during evaporation.
• Unlike dehydration, evaporation retains the final product in a liquid state. It also differs from distillation, as vapors are not separated into fractions.
• It uses a heat exchanger in a large chamber.
• A non-contact heat exchanger conveys heat from low-pressure steam.
• The product inside is kept under a vacuum, increasing the temperature difference and lowering the boiling temperature to minimize heat damage.
• Vapors are processed through a condenser to a vacuum system, condensing steam inside the heat exchanger, discarding the condensate
• In single-effect evaporation vapors are discarded, while in multiple-effect evaporation vapors are reused in another chamber.
• Liquid food qualities affect the evaporation process significantly.
• Concentrated liquid causes less heat transfer.
• As the liquid concentrates, the boiling point increases, lowering the temperature difference and the rate of heat transfer.
• Because food products are heat sensitive, keep heating to a minimum to prevent product degradation.
• Fouling on the heat-exchange surface reduces the rate of heat transfer.
• Equipment requires regular cleaning which causes shutdown of equipment, decreasing processing capacity.
• Product loss is caused by liquid foods foaming.
• When designing evaporation systems, keep the characteristics of liquid food is important.

Boiling Point Elevation

• Boiling point rises in contrast to pure water at a given pressure.
• Utilize Duhring's rule as simple way of estimating the increase in boiling point which establishes a linear correlation between the boiling point temperature of the liquid with solute & the boiling point temperature of water at the same pressure over moderate temperature ranges.

Using Dühring's Chart

• To find initial and final boiling points, use Dühring's chart for liquids that exert vapor pressure similar to sodium chloride solution.
• Use the steam tables to determine the boiling point of water. The boiling point of the fluid can then be found using Figure 8.3.
• Given initial concentration of 5% to final concentration of 25% solid totals at 20 kPa:
• The boiling point of water is 60°C, or 333 K according to the steam table (Appendix A.4.2) at 20 kPa.
• Based on Figure 8.3, 333 K = 60°C is the boiling point with an initial concentration of 5% total solids. Whereby 337 K = 64°C is the boiling point with a final concentration of 25% total solids.

Types of Evaporators

• Batch-type pan evaporator
• Natural circulation evaporators
• Rising-film evaporator
• Falling-film evaporator
• Rising/falling-film evaporator
• Forced-circulation Evaporator
• Agitated thin-film evaporator

Batch-Type Pan Evaporator

• It is the simplest and oldest types of evaporator.
• A steam-jacketed spherical vessel heats the product, which is either exposed to the environment or linked to a vacuum and condenser. Vacuum permits boiling at lower temperatures than atmospheric pressure, which reduces thermal damage to heat-sensitive materials.
• A small heat transfer area exists per volume causing a long residence time for the product, typically several hours.
• Mostly natural convection occurs, resulting in smaller convective heat-transfer coefficients.
• Low heat-transfer qualities limit processing capacities.

Natural Circulation Evaporators

• Inside the steam chamber are short vertical tubes, typically 1-2 m long and 50-100 mm in diameter.
• The calandria (tubes and steam chest) is in the vessel's base.
• When heated, the product rises through these tubes due to natural circulation while steam condenses on the outside.
• Evaporation happens inside the tubes, and the product gets concentrated.
• Liquid flows over the base of the vessel through a central annular section.
• To preheat liquid feed, a shell-and-tube heat exchanger is positioned outside the major vessel.

Rising-Film Evaporator

• A low viscosity liquid food is cooked inside 10-15 m long vertical tubes, which get heated from the other side using steam.
• Vapors at the heating tubes' base cause a continuous, rapid, and rising liquid film that moves up the structure.
• At least a 14 °C temperature difference between the product and heating medium is required a well-developed film.
• These evaporators achieve high convective heat-transfer coefficients.
• Liquid can recirculate, although the process is typically once-through, to get the required solid concentration.

Falling-Film Evaporator

• In contrast to the rising-film evaporator, the falling-film evaporator enables a thin liquid film move descending under gravity inside vertical tubes.
• These evaporators are difficult to design because consistently distributing liquid film throughout the downward flowing tube requires specially developed distributors or spray nozzles.
• It allows for more effects than the rising-film evaporator.
• For example:
  • Steam: 110°C
  • Boiling temperature (last effect): 50 °C
  • Total temperature differential: 60 °C
  • Rising-film evaporators need: 14 °C temperature differential. Meaning there are only 4 effects feasible
• Up to 10 or more effects are achieved by utilizing a falling-film evaporator
• Has the ability to handle more viscous liquids than the rising-film type.
• Suited for thermal-sensitive products such as juice.
• Usual residence time of 20–30 s is required, in contrast to the rising-film evaporator needing 3-4 minutes.

Rising/Falling-Film Evaporator

• Product is concentrated by circulating it across a rising-film then falling-film section
• Liquid concentrate ascends through a rising tube portion, then moves through a falling film section, eventually attaining its final concentration.

Forced-Circulation Evaporator

• Involves a noncontact heat exchanger where fluid food gets circulated through the process at a fast/high rates.
• Any liquid boilings the hydrostatic head gets removed above the tubes.
• Inside the separator, the tube bundle's absolute pressure is kept slightly under that of the tube bundle, causing the fluid to flash into vapor.
• This heat exchanger's temperature change across the heating face is usually 3-5°C.
• Axial flow pumps maintain high circulation rates 2-6 m/s versus 0.3-1 m/s for natural circulation evaporators.
• Both capital and expenditures cost very low in the evaporators which are comparable with other types of evaporators

Agitated Thin-Film Evaporator

• This is for highly vicious liquids. The feed moves over the interior of a cylinder's surface (which has heating mechanisms) by wiper blades.
• High agitation in high-pressure steam is used as the heating medium to get high wall temperatures, so that quick evaporation occurs. Resulting in higher rates of heating to occur.
• The heat-transfer area per product unit volume is kept low by the cylindrical design.
• High capital and maintenance bills with low/slow processing capacity.

Plate Evaporators

• Plate evaporators are being used.
• Plate evaporators utilize and put in place the principles of rising/falling film, falling-film, wiped-film, and forced-circulation evaporators.
• Features allows plate configuration that can be acceptable:
  • Rising/falling-film plate evaporator's design is more compact and requires less floor area.
  • Can easily inspected heat-transfer areas.
  • A falling-film plate evaporator with a 25,000-30,000 kg removal amount per hour isn't uncommon.

Apple Juice Concentration Example

• Apple juice gets concentrated in a natural-circulation single-effect evaporator.
• Operating under steady-state, the dilute juice is at a rate of 0.67 kg/s.
• The dilute juice is 11% total solids.
• The juice is concentrated to 75% solid totals.
• The heat measurements of dilute apple juice and concentrate are 3.9 and 2.3 kJ/(kg °C), in that order.
• The steam pressure reading is 304.42 kPa.
• The feed comes inside the evaporation unit at about 43.3°C .
• The product starts to boil at 62.2°.
• The combined heat-transfer coefficient is estimated to be 943 W/(m² °C).
• There is a negligible boiling-point elevation.
• The task: ascertain the quantity of focused product generated, total steam necessary, the area requirement for heat transfer, and economy of steam use.