EVAPORATION

Evaporation is an important unit operation commonly employed to remove water from dilute liquid foods to obtain concentrated liquid products. Removal of water from foods provides microbiological stability and assists in reducing transportation and storage coasts.

Evaporation operation

-concentrating a solution by boiling the solvent

-desired product concentrated solution

--consist of heat exchanger ( capable of boiling the solution and separate the vapor phase from the boiling liquid)

The basic factors that affect the rate of evaporation are the:

rate at which heat can be transferred to the liquid,quantity of heat required to evaporate each kg of water,maximum allowable temperature of the liquid,pressure at which the evaporation takes place,changes that may occur in the foodstuff during the course of the evaporation process.

Important practical considerations in evaporators are the:

maximum allowable temperature, which may be substantially below 100°C.promotion of circulation of the liquid across the heat transfer surfaces, to attain reasonably high

heat transfer coefficients and to prevent any local overheating,viscosity of the fluid which will often increase substantially as the concentration of the dissolved

materials      increases,tendency to foam which makes separation of liquid and vapour difficult.

Purpose of Evaporation

To concentrate solution by removing the vapor from a boiling liquid solution

In the majority of cases, evaporation refers to the removal of water from an aqueous solution.

Example: concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice.

In these cases the concentrated solution is the desired product and the evaporated water is normally discarded.

In a few cases, water, which contains a small amount of minerals, is evaporated to give a solids-free water to be used as boiler feed, for special chemical processes.

Evaporation processes to evaporate seawater to provide drinking water have been developed and used.

Processing Factors

1. Concentration in the liquid

- low viscosity: high mass transfer coefficient

- high viscosity: low mass transfer coefficient

-adequate circulation and/or turbulence must be present to keep the coeffeicient from becoming too low

2. Solubility

- solubility increases with temperature

- crystallization may occur when a hot concentrated solution is cooled to room temperature

3. Temperature sensitivity of materials

- food and biological materials may be temperature sensitive and degrade at higher temperature or after prolonged heating.

4. Foaming or frothing

- food solution such as skim milk and some fatty-acid solution form a foam or froth during boiling.

5. Pressure and temperature

- high operating pressure: high boiling point

6. Scale deposition and materials of construction

- Some solutions deposit solid materials called scale on the heating surfaces.

- results in the overall heat-transfer coefficient decreases and evaporator must be cleaned.

Overall Heat Transfer Coefficient in Evaporators

The overall heat-transfer coefficient U in an evaporator is composed of:

1. The steam-side condensing coefficient, which has a value of about 5700 W/m2.K (1000 btu/h ft2.°F);

- The steam-side condensing coefficient outside the tubes can be estimated using Eqs. (4.8-20) through (4.8-26).

1. The metal wall, which has a high thermal conductivity and usually a negligible resistance;

3. The resistance of the scale on the liquid side; and the liquid film coefficient, which is usually inside the tubes.

Heat and Material Balances for Evaporators The basic equation for solving for the capacity

of a single-effect evaporator is Eq. (8.2-1), which can be written as Q=UAdeltaT

In order to solve Eq. (8.4-1) the value of q in W (btu/h) must be determined by making a heat and material balance on the evaporator shown in Fig. 8.4-1.

The feed to the evaporator is F kg/h (lbm/h) having a solids content of xF

mass fraction, temperature TF, and enthalpy hF J/kg (btu/lbm).

Coming out as a liquid is the concentrated liquid L kg/h (lbm/h) having a solids content of xL, temperature T1, and enthalpy hL.

The vapor V kg/h (lbm/h) is given off as pure solvent having a solids content of yv = 0, temperature T1, and enthalpy Hv.

Saturated steam entering is S kg/h (lbm/h) and has a temperature of TS and enthalpy of HS.

The condensed steam leaving of S kg/h is assumed usually to be at TS, the saturation temperature, with an enthalpy of hS.

This means that the steam gives off only its latent heat, λ, where:

λ = HS – hS (8.4-2)

Since the vapor V is in equilibrium with the liquid L, the temperatures of vapor and liquid are the same.

Also, the pressure P1 is the saturation vapor pressure of the liquid of composition xL at its boiling point T1. (This assumes no boiling-point rise.)

For the material balance, since we are at steady state, the rate of mass in = rate of mass out. Then, for a total balance,

F = L + V (8.4-3)

For a balance on the solute (solids) alone,

F xF = L xL (8.4-4)

For the heat balance, since the total heat entering = total heat leaving,

heat in feed + heat in steam = heat in concentrated liquid + heat in vapor + heat in condensed steam (8.4-5)

This assumes no heat lost by radiation or convection. Substituting into Eq. (8.4-5),

Substituting Eq. (8.4-2) into (8.4-6),

Fhf+Shs=LhL+VHV+Shs

Fhf+Sλ=Lhl+Vhv

The heat q transferred in the evaporator is then

Q=S(Hs-hs)= Sλ

In Eq. (8.4-7) the latent heat λ of steam at the saturation temperature TS can be obtained from the steam tables in Appendix A.2.

However, the enthalpies of the feed and products are often not available; these enthalpy concentration data are available for only a few substances in solution.

Hence, some approximations are made in order to make a heat balance. These are as follows:

1. It can be demonstrated as an approximation that the latent heat of evaporation of 1 kg mass of the water from an aqueous solution can be obtained from the steam tables using the temperature of the boiling solution T1 (exposed surface temperature) rather than the equilibrium temperature for pure water at P1.

2. If the heat capacities cpF of the liquid feed and cpL of the product are known, they can be used to calculate the enthalpies.

Effects of Processing Variables on Evaporator Operation

1. Effect of feed temperature.

The inlet temperature of the feed has a large effect on the operation of the evaporator.

If feed enter the evaporator at 311.0 K - cold as compared to the boiling temperature of 373.2 K.

About 1/4 of the steam used for heating is used to heat the cold feed to the boiling point.

Hence, only about 3/4 of the steam is left for vaporization of the feed.

2. Effect of pressure.

In Example 8.4-1 a pressure of 101.32 kPa abs was used in the vapor space of the evaporator.

This set the boiling point of the solution at 373.2 K and gave ΔT for use in Eq. (8.4-1) of 383.2 - 373.2, or 10 K.

In many cases a larger ΔT is desirable, since, as ΔT increases, the heating-surface area A and cost of the evaporator decrease.

To reduce the pressure below 101.32 kPa (to be under vacuum), a condenser and vacuum pump can be used.

For example, if the pressure were reduced to 41.4 kPa, the boiling point of water would be 349.9 K

The new ΔT would be 383.2 - 349.9 = 33.3 K.

A large decrease in heating-surface area would be obtained.

3. Effect of steam pressure.

Using higher-pressure saturated steam increases ΔT, which decreases the size and cost of the evaporator.

However, high-pressure steam is more costly as well as often being more valuable as a source of power elsewhere.

Hence, overall economic balances are really needed to determine the optimum steam pressures.

Boiling-Point Rise of Solutions

In the majority of cases in evaporation, the solutions are not dilute solutions such as those considered in Example 8.4-1.

In most cases, the thermal properties of the solution being evaporated may differ considerably from those of water.

The concentrations of the solutions are high enough that the heat capacity and boiling point are quite different from those for water.

For strong solutions of dissolved solutes the boiling-point rise due to the solutes in the solution usually cannot be predicted.

However, a useful empirical law known as Duhring’s rule can be applied.

According to this rule, a straight line is obtained if the boiling point of a solution in °C or °F is plotted against the boiling point of pure water at the same pressure for

Vapor recompression

-increase steam economy

-recompress vapor from evaporators and feed back to steam chest

Crystallizer

Crystallization- concentrating the solution to saturation and forming crystals from the solution

-mass transfer from solution to crystal surface

Crystallizers Classification

1. Precipitation Obtain by cooling a concentrated, hot solution (batch-continuous)2. Precipitation obtain by evaporating a solution (salting, draft tube, oslo)3. Precipitation obtain by adiabatic evaporation and cooling(vacuum)

Crystallization is an important operation in the chemical industry as a method of purification and as a method of providing crystalline materials in the desired size range.

• In a crystal, the constituent molecules, ions or atoms are arranged in a regular manner with the result that the crystal shape is independent of size and if a crystal grows, each of the faces develops in a regular manner.

• Energy saving is more in crystallization in comparison with distillation to obtain solid substance.

Crystallization is normally carried out either from a solution or from a melt.

Rarely, crystals are formed directly by condensation from the vapor phases.

Crystallization from solutions is important industrially.

Crystal forms:

The only logical and accepted method for the classification of crystals is according to the angles between the faces.

In this system, the types of crystal form have no relationship to the relative sizes of the faces since the relative development of the faces is not a constant characteristic of a specific material.

Any substance always crystallizes in such a way that the angle between a given Pair of faces is the same in all specimens and is characteristic of that particular substance.

The crystallization process consists essentially of two stages:

A. Formation of small particles or nuclei.

B. Growth of the nuclei.

And for crystallization to occur, saturation and supersaturation must happen.

1. Nucleation - first aggregation.

2. Growth.

Thermodynamically distinct

Want a few nuclei to grow big

Use thermodynamics to understand the required conditions

Effect of Temperature on solubility

Generally, increase in the temperature of the solution usually increases the solubility of the solute. In some cases, the temp coefficient of solubility is negative and sometimes it is zero.

KNO3 has a large positive temperature coefficient and is therefore readily crystallized by cooling a saturated solution. NaCL has a small coefficient and very little crystallization occurs on cooling and evaporation of solvent is therefore essential. (The solubility of KNO3 is more affected by temperature than NaCL).

When the stable crystal form changes as temp is altered (e.g. with hydrated salts), the curve is discontinuous, the coefficient may be positive over part of the range of temp. (Na2CO 3. 10H2O) and negative over the remainder (Na2CO3.H2O).

Factors affecting rate of crystal growth :

1- Temperature and concentration of the liquid at the crystal face:

These conditions are not generally the same as those in the bulk of the solution because

(1) Concentration gradient is necessary for the transfer of solute towards the face

(2) Temperature gradient for the dissipation of the heat of crystallization.

Thus the problem involves both heat transfer and mass transfer.

Thus the concentration gradient is the driving force for crystal growth.

2. Rate of agitation: The rate of crystallization is improved by increasing the rate of agitation. The crystallization rate initially rises very rapidly by increasing agitation but a point is reached where further increase in the agitation produces no effect on the rate of crystal growth.

3. Degree of supersaturation: This increases the crystallization rate.

4. Presence of impurities: Any foreign solid bodies act as a nucleus and enhance crystallization.

5. Viscosity: As the viscosity increases, the rate of crystal growth decreases, because the viscosity decreases the rate of diffusion of solute to the crystal surface.

Yields and Heat and Material Balances in Crystallization

Heat effects and heat balances in crystallization

When a compound whose solubility increases as temperature increases dissolves, there is an absorption of heat, called the heat of solution – occurs when the solubility decreases as the temperature increases

At equilibrium the heat of crystallization is equal to the negative of the heat of solution at the same concentration in solution.

The enthalpy H1 of the entering solution at the initial temperature is read off the chart, where H1

is kJ for the total feed. The enthalpy H2 of the final mixture of crystals and mother liquor at the final temperature is also read off. If some evaporation occurs, the enthalpy Hv of the water vapor is obtained from the steam tables. Then the total heat absorbed q in kJ is

If q is positive, heat must be added to the system. If it is negative, heat is evolved or given off.

F xF '=CM a

M h

+[F (1−x F' )−V−CM h−M a

M h

]X '

Rearrange

C=[F (1−xF' )−V ] X '−F x F'

(M h−M a

M h)X '−M a

M h

Where:

C=mass of crystals in the product magma per unit mass

Ma=molecular weight of anhydrous solute

Mh=molecular weight of hydrate cystals

Xf=mass fraction of anhydrous solute in feed

X’=solubility of the material at product temp. expressed as a weight ratio of anhydrous salt to solvent

F=total mass of feed per unit time

V=evaporation in pounds mass of solvent per unit time

-applicable to all single-stage crystalizer or multi stage, which only one liquid-solid product is withdrawn

Data needed for design

-solubility curve

-physical properties of solution (viscosity, specific heat capacity, etc.)

-variables set by the plant (quality an d quantity or utilities, composition, temp. and quantity of the feed, desired product rate and quantity.

-pilot plant data