LECTURE 7EVAPORATION

Presented byMuhammad Abbas Ahmad Zaini,

PhD (Japan) CEng (UK)

It is expected that students will be able to:

Identify the different types of evaporation equipment and operation methods

Design single-effect and multiple-effect evaporators

Evaporation

Types of evaporation equipment and operation methods

Calculation methods for

Single-effect evaporators

Multiple-effect evaporators

Evaporation

Removal of water vapor from aqueous solution in the presence of heat, and more concentrated solution remains.

Application: production of concentrated juice (orange, tomato, etc.) and production of salt from seawater.

Crystallization is a kind of evaporation process.

Evaporation vs. Drying

Occur at temperature below boiling point and usually be affected by humidity

Commonly occurs at boiling point of water

Removal of small amount of water from solid material (moisture)

Removal of large amount of water from solution or mixture

DryingEvaporation

Evaporator

Industrial Evaporators

(a) horizontal-tube type; (b) vertical-tube type; (c) long-tube vertical type; (d) forced circulation type.

Types of Evaporators

Single-Effect vs. Multiple-Effect

Each individual effect will have a smaller temperature difference, thus high area of heating surfaces

Capital cost more costly

Operating cost- steam economy, only required for the first effect (1 kg steam vaporizes 3 kg water)

Small capacity but wasteful energy (1 kg steam vaporize 1 kg water)

Overall temperature drop for single-effect is somewhat equal to multiple-effect

Forward-feed evaporator

Feed is hot and final product is temperature sensitive

Evaporator Configurations for Multiple Effect

Backward-feed evaporator

Cold fresh feed, pump is required, and final product is highly viscous

Processing Factors

Liquid concentration- in evaporation, concentration of liquid increase, thus viscosity will also increase resulting in decreasing of overall heat transfer coefficient. So, additional agitation is required.

Solubility- is increasing with temperature. However, as evaporation continues, liquid solution will become more and more concentrated (saturated) and solubility will exceed its limit.

Solubility curves for some typical salts in water

Processing Factors (Cont’d)

Temperature sensitivity of materials- products very sensitive to high temperature, i.e. protein and pharmaceutical materials, may destroyed and degraded. Use vacuum instead to lower the boiling point.

Foaming and frothing- liquid solutions such as caustic solutions and skim milk. These foam and froth accompany water vapor resulting in entrainment losses.

P & T- As T increase, solution will become more concentrated, resulting in increasing of solution boiling point. Elevation or boiling-point-rise (BPR).

Scale deposition and material of construction-deposit solid materials on heating surfaces resulting in decreasing of overall heat transfer coefficient, U. Suitable material to minimize corrosion.

Processing Factors (Cont’d)

Trade-Off

Feed temperature, Tf- if cold, some of steam is used to heat up the feed to the boiling point. If feed is under pressure/vacuum and Tf is above the boiling point, additional vaporization is obtained, flash of hot feed.

Operating Pressure- if in vacuum, high ΔT can be obtained, resulting large decrease in heating surface area, A.

Steam pressure- increasing steam pressure will increase ΔT, resulting in low A and low capital cost. However the price for high-pressure steam is more costly.

In-Class Working Session

Calculation for Single-Effect Evaporator

Energy balance : Fhf+Sλs=Lhl+VHv

Heat being transferred from steam to evaporator : q= Sλs, which is equal to : q=UAΔT

Thus simply equate : Sλs= UAΔT, where ΔT= Ts –T1

Boiling Point Rise

When feed solution is no longer dilute, thermal properties of solution may differ from those of pure water.

The concentration of feed is very high, thus the heat capacity and boiling point are much higher than those for water.

The boiling-point-rise, BPR is usually given in terms of fraction of solute, x or by using empirical law such as instance Dühring’s rule for sodium hydroxide.

Dühring lines for aqueous solutions of sodium hydroxide.

Enthalpy Concentration Chart

Heat is evolved as feed solution become more concentrated.

Hold similar concepts of heat-of-solution.

Amount of heat added depends on type of substance and amount of water.

Usually given in terms of fraction of solute, x (for heat capacity) or straight away from enthalpy concentration chart such as that for sodium hydroxide-water.

Enthalpy-concentration chart for the system NaOH-water

• In order to concentrate 4536 kg/h of an NaOH solution containing 10 wt% NaOH to a 20 wt% solution, a single-effect evaporator is being used, with an area of 37.6 m2. The feed enters at 21.1 oC (294.3 K). Saturated steam at 110 oC (383.2 K) is used for heating and the pressure in the vapor space of the evaporator is 51.7 kPa. Calculate the kg/h of steam used and the overall heat-transfer coefficient.

• A single-effect evaporator is concentrating a feed solution of organic colloids from 5 to 50 wt %. The solution has a negligible boiling-point elevation. The heat capacity of the feed is cp=4.06 kJ/kg.K and the feed enters at 15.6 oC. Saturated steam at 101.32 kPa is available for heating, and the pressure in the vapor space of the evaporator is 15.3 kPa. A total of 4536 kg/h of water is to be evaporated. The overall heat transfer coefficient is 1988 W/m2.K. What is the required surface area in m2 and the steam consumption?