LM 15 Evaporation

The objective of evaporation is to concentrate a consisting of a nonvolatile solute and a volatile solvent. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution or thick liquor.

Evaporation differs from distillation in that the vapor usually is a single component, and even when the vapor is a mixture ,no attempt is made in the evaporation step to separate the vapor into fractions .

Evaporation differs from crystallization in that emphasis is placed on concentrating a solution rather than forming and building crystals.

Normally ,in evaporation the thick liquor is the valuable product and the vapor is condensed and discarded.

Liquid characteristics The practical solution of an evaporation problem is profoundly affected by the character of the liquor to be concentrated.

Some of the most important properties of evaporating liquids are as follows.

Concentration As the concentration increases, the solution becomes more and more individualistic. The density and viscosity increase with solid content.

Continued boiling of a saturated solution causes crystals to form; these must be removed or the tube clog.

The boiling point of the solution may also rise considerably as the solid content increases, so that the boiling temperature of a concentrated solution may be much higher than that of water at the same pressure.

Foaming A stable foam accompanies the vapor out of the evaporator, causing heavy entrainment. In extreme cases the entire mass of liquid may boil over into the vapor outlet and be lost.

Temperature sensitivity Many fine chemicals , pharmaceutical products, and foods are damaged when heated to moderate temperatures for relatively short time .

scale Some solutions deposit scale on the heating surfaces. The over-all coefficient then steadily diminishes, until the evaporator must be shut down and the tubes cleaned.

Single- and multiple-effect operationMost evaporators are heated by steam condensing on metal tubes. Nearly always the material to be evaporated flows inside the tubes.

When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded, the method is called single-effect evaporation.

If the vapor from one evaporator is fed into the stream chest of a second evaporator and vapor from the second is then sent to a condenser, the operation become double-effect.

Additional effects can be added in the same manner.

A series of evaporators between the stream supply and the condenser is called multiple-effect evaporator.

Types of evaporators The chief types of stream-heated tubular evaporator in uses today are:1Short-tube evaporators2Long-tube vertical evaporators forced-circulationUpward-flow (climbing-film)down-ward-flow (falling-film)

Once-through and circulation evaporators Evaporators may be operated either as once-through or circulation units.

In once-through operation the feed liquor passes through the tubes only once, releases the vapor, and leaves the unit as thicker liquor. All the evaporation is accomplished in a single pass

In circulation evaporators a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and mixture passes through the tubes.

Unevaporated liquid discharged from the tubes returns to the pool.

All short-tube and forced-circulation evaporators are operated in this way.

Short-tube evaporatorsIn the older types of evaporators the tubes are short.

In the short-tube vertical evaporator shown in Fig

feed

Long-tube evaporators with upward flowA typical long-tube vertical evaporatorwith upward flow of the liquid is shown in next Fig.

Forced-circulation evaporatorsHigher coefficients are obtained in forced-circulation evaporators, an example of which is shown in fig.

Film evaporators Concentration of highly heat-sensitive materials such as orange juice requires a minimum time of exposure to a heated surface. This can be done in once-through film evaporators

Performance of tubular evaporators The principal measures of the performance of a steam-heated tubular evaporator are capacity and the economy.

Capacity is defined as the number of kilograms of water vaporized per hour.

Economy is the number of kilograms vaporizedper kilogram of stream fed to the unit.

Evaporator CapacityThe rate of heat transfer q through the heating surface of an evaporator is the product of three factors:The area of the heat-transfer surface AThe overall heat-transfer coefficient UThe overall temperature drop T

(16-1)

If the feed to the evaporator is at the boiling temperature corresponding to the absolute pressure in the vapor space. All the heat transferred through the heating surface is available for evaporation. The capacity is proportional to q

If the feed is cold, the energy is required for heating it to its boiling point.

The capacity for a given value of q is reduced accordingly , as heat used to heat the feed is not available for evaporation.

If the feed is at the temperature above the boiling point, a portion of the feed evaporates spontaneously.

The capacity is greater than that corresponding to q. this process is called flash evaporation.

Temperature differenceThe actual temperature drop across the heating surface depends on :The solution being evaporatedThe difference in pressure between the steam chest and the vapor space above the boiling liquid, and depth of liquid over heating surface.The friction loss in the tubes

In actual evaporators, however, the boiling point of a solution is affected by two factors:

The boiling point elevation

And liquid head

Boiling-point elevation and Dhrings rule For a given pressure the boiling point of the aqueous solutions is higher than that of pure water.

The increase in boiling point over that of water is known as the boiling-point elevation (BPE) of the solution.

(BPE) is best found from an empirical rule known as Dhrings rule. Which states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure.

If the boiling point of the solution is plotted against that of water at the same pressure, a straight line results.Please see Figure 16.3.

Different lines are obtained for different concentrations.

Heat-transfer coefficient The overall coefficient is strongly influenced by the design and method of operation of the evaporator.

Typical overall coefficients for various types of evaporators are given in table

typeOverall coefficientW/m2CLong-tube vertical evaporatorNatural circulation1000-2500Force circulation2000-5000

Evaporator Economy The chief factor influencing the economy of an evaporator system is the number of effects.

The economy also is influenced by the temperature of the feed.

Quantitatively, evaporator economy is entirely a matter of enthalpy balance.

Enthalpy balances for single-effect evaporator In single-effect evaporator, the latent heat of condensation of the team is transferred through a heating surface to vaporize water from a boiling solution.

The enthalpy balance for the steam side is

(16-2)The enthalpy balance for the liquor side is

(16-3)

In the absence of heat losses, the heat transferred from the steam to the tubes equals that transferred from the tubes to the liquor.

Thus , by combining Eqs. (16-2) and(16-3)

(16-4)

The heat-transfer rate q on the liquor side includes:

qf, the heat transferred to the thin liquor to change its temperature from Tf to the boiling temperature Tqv, the heat to accomplish the evaporation

The final equation for the enthalpy balance can be gotten from Eqs. (16-6) and (16-7)when the heat of dilution is negligible.

(16-8)

Problem 16.1, page 516

Multiple-effect evaporatorstriple-effect systemfeedSteam in

Methods of feedingForward feed The usual method of feeding in a multiple-effect evaporator system is forward feed.Backward Feed

See Figure 16.8 , Page 509

THE END

HW 15 is the online quiz Due date: May 6, 2013