14.6. EVAPORATOR OPERATION
In evaporation, solids may come out of solution and form a deposit or scale on the heat
transfer surfaces. This causes a gradual increase in the resistance to heat transfer and, if
the same temperature difference is maintained, the rate of evaporation decreases with time
and it is necessary to shut down the unit for cleaning at periodic intervals. The longer
the boiling time, the lower is the number of shutdowns which are required in a given
period although the rate of evaporation would fall to very low levels and the cost per unit
mass of material handled would become very high. A far better approach is to make a
balance which gives a minimum number of shutdowns whilst maintaining an acceptable
throughput.
It has long been established(18) that, with scale formation, the overall coefficient of heat
transfer may be expressed as a function of the boiling time by an equation of the form:
The rapid development of the process industries and of new products has provided many
liquids with a wide range of physical and chemical properties all of which require concentration
by evaporation. The type of equipment used depends largely on the method of
applying heat to the liquor and the method of agitation. Heating may be either direct or
indirect. Direct heating is represented by solar evaporation and by submerged combustion
of a fuel. In indirect heating, the heat, generally provided by the condensation of steam,
passes through the heating surface of the evaporator.
Some of the problems arising during evaporation include:
(a) High product viscosity.
(b) Heat sensitivity.
(c) Scale formation and deposition.
Equipment has been developed in an attempt to overcome one or more of these problems.
In view of the large number of types of evaporator which are available, the selection
of equipment for a particular application can only be made after a detailed analysis of
all relevant factors has been made. These will, of course, include the properties of the
liquid to be evaporated, capital and running costs, capacity, holdup, and residence time
characteristics. Evaporator selection considered in detail in Volume 6, has been discussed
by MOORE and HESLER(20) and PARKER(21)
. Parker has attempted to test the suitability
of each basic design for dealing with the problems encountered in practice, and the
basic information is presented in the form shown in Figure 14.15. The factors considered
include the ability to handle liquids in three viscosity ranges, to deal with foaming,
scaling or fouling, crystal production, solids in suspension, and heat sensitive materials.
A comparison of residence time and holding volume relative to the wiped film unit is
also given. It is of interest to note that the agitated or wiped film evaporator is the only
one which is shown to be applicable over the whole range of conditions covered.
14.6. EVAPORATOR OPERATIONIn evaporation, solids may come out of solution and form a deposit or scale on the heattransfer surfaces. This causes a gradual increase in the resistance to heat transfer and, ifthe same temperature difference is maintained, the rate of evaporation decreases with timeand it is necessary to shut down the unit for cleaning at periodic intervals. The longerthe boiling time, the lower is the number of shutdowns which are required in a givenperiod although the rate of evaporation would fall to very low levels and the cost per unitmass of material handled would become very high. A far better approach is to make abalance which gives a minimum number of shutdowns whilst maintaining an acceptablethroughput.It has long been established(18) that, with scale formation, the overall coefficient of heattransfer may be expressed as a function of the boiling time by an equation of the form:The rapid development of the process industries and of new products has provided manyliquids with a wide range of physical and chemical properties all of which require concentrationby evaporation. The type of equipment used depends largely on the method ofapplying heat to the liquor and the method of agitation. Heating may be either direct orindirect. Direct heating is represented by solar evaporation and by submerged combustionof a fuel. In indirect heating, the heat, generally provided by the condensation of steam,passes through the heating surface of the evaporator.Some of the problems arising during evaporation include:(a) High product viscosity.(b) Heat sensitivity.(c) Scale formation and deposition.Equipment has been developed in an attempt to overcome one or more of these problems.In view of the large number of types of evaporator which are available, the selectionof equipment for a particular application can only be made after a detailed analysis ofall relevant factors has been made. These will, of course, include the properties of theliquid to be evaporated, capital and running costs, capacity, holdup, and residence timecharacteristics. Evaporator selection considered in detail in Volume 6, has been discussedby MOORE and HESLER(20) and PARKER(21). Parker has attempted to test the suitabilityof each basic design for dealing with the problems encountered in practice, and thebasic information is presented in the form shown in Figure 14.15. The factors consideredinclude the ability to handle liquids in three viscosity ranges, to deal with foaming,scaling or fouling, crystal production, solids in suspension, and heat sensitive materials.A comparison of residence time and holding volume relative to the wiped film unit isalso given. It is of interest to note that the agitated or wiped film evaporator is the onlyone which is shown to be applicable over the whole range of conditions covered.
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