Introduction to evaporation

Food1040 module

Malcolm Povey

University of Leeds

The liquid feed moves on the "hot" inner walls in an evaporator. Heat is continuously transferred from the outside of the wall to the inside, thereby heating up the liquid to the boiling point where water evaporates from the liquid feed.

        

     

Evaporator basics

http://www.apv.com/anhydro/evapo/anhevap.htm

Liquid distribution

One of the most important issues influencing the effectiveness of the evaporation is the distribution of the liquid on the inner walls. To make the evaporation effective the liquid film on the inner walls should be as thin as possible. However, if the film is too thin, the product will burn on the walls, therefore it is very important to be able to control the thickness of the film also known as "the wetting rate".

http://www.apv.com/anhydro/evapo/anhevap.htm

                   

Milk and milk productsSweetened condensed milkWPC35, WPC60 and WPC80Whey and whey permeateFruit juice (tropical, citrus, temperate)Coffee, teaSugar (different types)Vitamins, pharmaceuticalsVarious teas and herb extracts

Typical applications

Various proteins e.g. blood or gelatineVarious broths e.g. chicken beefBulk chemicalsWaste water:

Spent waste water (distilleries)

•General waste water

More applications

The product to be concentrated is separated from the heating medium by a tube wall, or a flat curved plate.

The typical heating medium is steam or vapour.

The system can be evacuated in order to lower the boiling temperature.

The evaporated vapour can be reused as heating medium often in combination with either mechanical or thermal recompression. (Regeneration)

The principles of evaporation

The laboratory CFE

Heat and mass transfer are connected

Heat transfer in a CFE

q ~(θs - θp)  More specifically :- Equation 1 q = U.A. (θs - θp)  = U.A. Δθ Where A = heat exchange area = 0.251 m2  U = overall heat transfer coefficient  θs = average steam temperature in the calandria jacket θp = product temperature at boiling point (boiling set point)

steaminlet steamoutlet2

                                       

As evaporation is a very energy consuming process, the availability and relative cost of energy determine the design of the evaporation plant.Normally an evaporation plant is designed to use the energy as efficiently as possible by using more than one effect.Therefore the following technical solutions are used in order to keep the temperature of the steam high enough to run the process:

Energy consumption

•Thermal Vapour Recompression (TVR)

•Mechanical Vapour Recompression (MVR) or

•combination of both

Shown below is a figure illustrating the relation between the number of effects and energy consumption under standard conditions for a typical product.

Furthermore, the figure also shows a rough correlation between steam and electrical energy consumption per kg water evaporated, allowing the customer to estimate, whether the price of electric power in his local area justifies the use of MVR instead of TVR.

Optimising energy use

Comparison

http://www.apv.com/anhydro/evapo/anhevap.htm

Evaporator pictures and applications

Single effect recompression

•Corn Syrup

•Gelatin

•Salt

•Soybean Oil

•Steepwater

•Sugar

•Whey

      The unique liquid distribution •Short residence time •Low temperature difference

with•Single pass operation •Even shorter residence time •Larger capacities

Typical applications are:•Milk, whey and milk products •Fruit juices , Various vitamins , Pharmaceuticals •Essence recovery

Falling Film Plate & Tubular Evaporators

Falling film evaporator

http://www.swenson-equip.com/vrml.html

•The unique liquid distribution •Short residence time •Low temperature difference

Typical applications are:•Milk, whey and milk products •Fruit juices •Various vitamins •Pharmaceuticals •Essence recovery

Rising/Falling Film Plate Evaporators

                                  

APV´s rising film plate evaporators (Paravap) are particularly effective in processing high viscous liquids.Typical applications are:

•High viscous sugar •Fruit puree and pulp •High viscous pharmaceuticals •Polymers

Rising Film Plate Evaporators

Other evaporators

http://www.swenson-equip.com/index.html

Forced circulation evaporator

Evaporator Diagrams

Falling film evaporator

Forced Convection with steam jet recompression

Multiple effect evaporator

Wiped Film Evaporator