Rising Film Evaporator

Description

Introduction to Rising Film Evaporators

A rising film or vertical long tube evaporator functions as a vertical shell-and-tube heat exchanger. The liquid enters the long tubes from the bottom and heats up as steam condenses on the tube’s outer shell. This process generates vapor inside the tube, causing the liquid to boil. The vapor pushes the liquid against the tube walls, creating an ascending force. As more vapor forms, the velocity in the tube’s center increases, forcing the liquid into a thin film that rises upwards. This rising film phenomenon gives the evaporator its name.

Applications of Rising Film Evaporators

Rising film evaporators have many uses across industries such as effluent treatment, polymer production, food processing, thermal desalination, pharmaceuticals, and solvent recovery. For example, they often serve as reboilers for distillation columns or as pre-concentrators, flash evaporators, or pre-heaters to remove volatile components before stripping.

Thermal Desalination

In thermal desalination, seawater enters the evaporator’s long tubes while steam heats it from the outside. Vapor forms inside the tubes and flows upward. This process typically runs under vacuum conditions, allowing lower temperature operation.

Juice Concentration and Food Processing

The food industry requires gentle handling of temperature-sensitive products. Rising film evaporators operate fast and efficiently, minimizing heat exposure and preserving product quality. Therefore, they suit juice, milk, and dairy concentration applications well.

Advantages and Limitations

Low Residence Time

A key advantage of rising film evaporators is their low residence time compared to plate-type evaporators. This allows operation at higher temperatures while maintaining product quality, especially for heat-sensitive materials. Moreover, they can run continuously, improving energy and time efficiency compared to batch processes.

High Heat Transfer Coefficients

These evaporators feature relatively high heat transfer coefficients, which reduce the required heat transfer area. Consequently, this lowers capital costs. Additionally, the shell-and-tube components are readily available and customizable, making them cost-effective for simple evaporation tasks. They also easily accommodate vapor separators for foaming products.

Limitations: Efficiency and Product Restrictions

Despite their advantages, rising film evaporators are less efficient than vertical or horizontal falling film evaporators, according to some studies. Falling film evaporators offer similar benefits with better efficiency. Moreover, rising film evaporators need a significant temperature difference to drive the film upward against gravity.

Another limitation is product versatility. They perform best with low-viscosity fluids that have low fouling tendencies. Plate-type evaporators better handle viscous or fouling liquids due to easier access for cleaning and maintenance.

Heat Transfer Characteristics

Heat transfers mainly by conduction and convection: steam heats the tube from the shell side, and then the tube heats the liquid and vapor inside. The long vertical tubes promote a thin, continuous liquid film along the walls, created by vapor pressure in the center rising upward. This motion creates turbulence, increasing heat transfer coefficients and improving efficiency. The evaporator’s design also influences these coefficients.

Log Mean Temperature Difference (LMTD)

Initially, heat transfer rate increases with the temperature difference until the liquid boils at a constant feed flow rate. A large temperature difference is beneficial for efficient heat transfer. However, as vapor bubbles fill the tube center, steam pressure peaks. Beyond this, any steam pressure increase reduces the temperature difference, which drives both heat transfer and the rising film. Additionally, product quality limits the allowable temperature difference. The properties of steam and the boiling liquid heavily influence this parameter.

Overall Heat Transfer Area

The heat transfer area includes the interface between the heating media and the liquid film inside the long tubes, plus any intermediate surfaces. Increasing this area improves heat transfer rates. However, larger heat transfer areas mean longer tubes and larger shells, which significantly raise construction and maintenance costs.

Residence Time

Residence time measures how long the liquid remains in the evaporator during processing. Food and beverage clients prefer short residence times to avoid nutrient loss from prolonged heat exposure. Pre-heating the liquid feed before the evaporator reduces the load and shortens residence time.

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