8.18

Crystallizer Controls C. J. SANTHANAM B. G. LAKATOS

(1970, 1985)

Vacuum

B. G. LIPTÁK

(1995)

(2005)

Product Feed

Flow sheet symbol

INTRODUCTION This section describes the control of industrial crystallizers. Crystallization requires supersaturation, which can be achieved by cooling, evaporation, vacuum cooling, dilution, and chemical reaction. The control of multiple-effect crystallizers is also covered. Crystallization is a widely used technique in the process industry and is used for the purpose of purification and separation of substances, as well as for the production of good-quality crystals. It is a practical method to obtain concentrated chemical substances in particulate solid form that have desirable properties such as good flow characteristics, handling convenience, suitability for packaging, and pleasant appearance. Crystallization can be carried out from solution, vapor, or melt. Here, only solution crystallization is discussed. The most commonly used methods for creating supersaturation in the solution phase include cooling, evaporation, vacuum cooling, dilution, or chemical reaction. The choice of the method depends on the solubility of the solute, feed solution composition, product specifications, and other engineering considerations. Most crystallizers in industrial use operate on a continuous basis, but batch and semibatch crystallizers are also widely used to produce fine chemicals and special-effect high-addedvalue materials such as pharmaceuticals, pigment, agrochemicals, or catalysts. This section is devoted to the subject of controlling basic continuous crystallizers. It will describe the basic crystallization techniques, while focusing on the measurement and control practices used in this industry.

THE CRYSTALLIZATION PROCESS The fundamental driving force for crystallization is the difference in chemical potential between the crystallization substance in the solid and liquid phases, but the common engineering

practice is to use supersaturation as the driving force of concentration. The crystallization process has three basic steps: (1) generation of supersaturation, (2) formation of nuclei, and (3) growth of nuclei into crystals. Supersaturation is usually expressed as the supersaturation ratio S=

c c′

8.18(1)

or the concentration difference ∆c = c − c′

8.18(2)

where c = concentration of the crystallizing substance in the solution c′ = equilibrium concentration (solubility) of the crystallizing substance in the solution In most cases, the solubility c′ of a solute increases with the increase of temperature, but there are a few exceptions to this rule. The concept of supersolubility and the existence of the metastable zone are useful in understanding the behavior of the crystallization process. Figure 8.18a shows a temperature-solubility plot for a typical salt; the plot is divided into three zones: (1) stable under saturated zone, where crystallization is not possible; (2) metastable (supersaturated) zone between the solubility and supersolubility, where spontaneous crystallization is not possible; and (3) labile (supersaturated) zone, where spontaneous crystallization is probable. The rate of nucleation and the crystal growth rate are controlled by supersaturation. The ideal process would be a stepwise procedure, but nucleation (i.e., formation of new crystals), because of the secondary nucleation, cannot be eliminated in a growing mass of crystals. The influence of supersaturation on nucleation and growth rates is shown in Figure 8.18b. Whereas the dependence of growth rate on supersaturation is linear or moderately nonlinear, the nucleation rate increases exponentially with supersaturation. 1811

© 2006 by Béla Lipták

1812

Control and Optimization of Unit Operations

Concentration Supersolubility curve B Labile supersaturated zone

C

Solubility curve A – Cooling B – Isothermal evaporation C – Vacuum cooling

A Metastable zone Stable undersaturated zone

Temperature

FIG. 8.18a The zones of saturation and solubility plots and the methods by which the state of supersaturation can be achieved.

Figure 8.18b shows three regions of supersaturation, corresponding, in principle, to the zones in Figure 8.18a: (1) metastable, where nucleation is very low and growth predominates; (2) intermediate, where nucleation becomes larger, but growth is still significant; and (3) labile, where nucleation predominates. Crystal Size Distribution

Degrees of Freedom

Industrial crystallizers usually yield a crystalline product that has a wide crystal size distribution (CSD). This CSD affects the behavior of the product in succeeding operations such as filtration, drying, transport, and storage, and is also defined

To define the maximum number of controllers, we consider the degrees of freedom for a crystallization system. The variables are 1. 2. 3. 4.

Growth or nucleation rate Metastable region

by customer specifications. Because very small crystals are difficult to filtrate, dry, or handle, crystallizer design and control usually are directed toward reasonably large crystals by minimizing nucleation. Usual industrial practice involves sufficiently low supersaturation to minimize supersaturation while being adequate for reasonable growth. At the same time, among others in the pharmaceutical industry, there is also an increasing need to produce high specific-surface micro particles. Therefore, because the desirable properties of the crystalline product may change within wide limits, the main purpose is to control the crystal size distribution. The CSD, determined by the rates of nucleation and growth, is influenced by the supersaturation, thus control of supersaturation becomes the basic element of crystallizer system control. Unfortunately, the workable degree of supersaturation is usually 0.5–1% so that it is so small that it is hard to measure directly. Besides, the on-line measurement of CSD, although it is also possible by laser diffraction, is restricted to slurries that contain solids at low concentration. As a consequence, the vast majority of crystallizers use indirect means of control. An important exception to this rule is sugar, in the case of which the level of supersaturation can be greater than usual in industrial crystallizers, and correlations can be used to measure supersaturation directly. Estimations of sugar supersolubilities are based on these empirical correlations.

Intermediate region Nucleation rate

Temperature and flow of the process fluid Temperature and flow of the cooling or heating medium Level of supersaturation Ratio of mother liquor to crystals, which can be changed by varying recycles of mother liquor to feed stream 5. Removal and dissolution rate of fine crystals The first principles-based governing equations are

Labile region

Growth rate Range of crystallizer operation

Supersaturation ratio (S)

FIG. 8.18b The influence of level of supersaturation on nucleation and the crystal growth.

© 2006 by Béla Lipták

1. Energy balance equation 2. Mass balance equation for the crystallizing substance 3. Balance equation for fines Because the system has seven variables and three equations, it has four degrees of freedom. Thus, the maximum number of automatic controllers permissible in a crystallizer system, without overdefining it, is four. The exceptions are the dilution and reaction crystallizers, where the flow of the diluent and ratio of reactants form additional degrees of freedom. The amount of fines can be estimated by using density measurement. In crystallization operations, the density sensor

8.18 Crystallizer Controls

(differential pressure or any other type), which gives an indication of the amount of crystals in the crystal slurry, appears to be an important instrument. This measurement is based on the fact that the density of the clear liquor is constant. Hence, the measurement of the differential pressure between two points in the vessel is a measure of the amount of crystals in the suspension, because any change in density is caused by a change in the amount of crystals in the crystal slurry.

EVAPORATIVE CRYSTALLIZERS This type of crystallizer produces supersaturation and, hence, crystals, by loss of solvents induced by one of the three methods: (1) indirect heating, (2) submerged combustion, and (3) spray evaporation. The first two are the dominant types and will be discussed here. Indirectly Heated Crystallizers Circulating-magma crystallizers with indirect heating are by far the most important type of crystallizers in use today: The forced-circulation (FC) and the draft-tube baffle (DTB) designs belong to this class. A typical FC crystallizer is shown in Figure 8.18c. In this design the feed is introduced into the recirculation loop. The critical design parameters in FC crystallizers are the internal recirculation rate and velocity, the crystallizer hold volume, and the speed of the circulating pump. At internal circulation less than the optimum, excessive flashing can occur at the boiling surface, causing a high level of supersaturation. If this supersaturation cannot be reduced by deposition of the solute because of the lack of adequate crystal surface area in the suspension, intensive nucleation will

occur, producing a large amount of fines and causing buildup of solids on the walls of the crystallizer. In general, recirculation rates aim at restricting the flashing at the crystallizer walls or at other boiling surfaces to approximately 1.7–4.5°C (3–8°F) whereas a magma-density range of 15–25% is typical, even though the exact optimum depends on the particular crystal system. Various designs of crystallizers are available, but the approaches of controlling the processes are similar. A possible control system is shown in Figure 8.18c where four control loops are involved: 1. Feed clear liquor is fed on level control to a feed tank. Here it is mixed with the mother liquor from the centrifuge of the crystalline product. 2. The mixed liquor is fed to the suction side of the crystallizer recirculating pump. This feed is adjusted by a level controller. (For methods of protecting the level sensor from plugging and material build-up, refer to Chapter 3 of the first volume of this handbook.) 3. The steam flow to the heat exchanger is on flow control. Once the steam rate is fixed, the production rate is also fixed, provided that the feed composition does not change. 4. Temperature control in the vessel may be achieved by controlling the evaporator chamber pressure by an air bleed. Refinements to this basic system are possible, as an interlock between the steam and circulating pump, or the addition of a density recorder. The magma density can also be used for control, rather than just for monitoring. In the control configuration shown in Figure 8.18d, the density controller

PRC

PRC

Vacuum system

PT

1813

Vacuum system

PT Air

Air LT

LT Steam

DR Steam

PDT

PDT

LIC

FT

LIC

SP FIC

FT

DIC Centrifuge

FRC

Centrifuge

FSL

FSL

Product

Mother liquor

Feed

Feed tank

Feed

Feed tank LT

LT

LIC

LIC

FIG. 8.18c The control of an indirectly heated circulating-magma-type crystallizer.

© 2006 by Béla Lipták

Product

Mother liquor

FIG. 8.18d Cascade control of crystal concentration (measured by density) by throttling heat input.

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Control and Optimization of Unit Operations

PRC

Vacuum system

PT Air LT

X

FT Steam

DIC 3

PDT

PT

L/L

%

Ratio SP FIC 2 FB

LIC FIC 1 SP

VPC 4

< Centrifuge FB

FB

NonLIC linear

Product

Mother liquor

DT FT

Feed tank

FC LT

X

Feed

FCV 2

FIG. 8.18e In this control system, feedforward, dynamic compensation and selective control of the feed are added to the basic controls of a circulatingmagma crystallizer.

serves as the cascade master of the steam flow controller, and it varies the heat input in order to keep the crystal concentration constant.

FCV-2 valve in a selective manner, they are both provided with external feedback (FB) to make sure that their integrals will not wind up when the other controller is manipulating the valve.

Advanced Controls Figure 8.18e illustrates how a number of additional features and flexibilities can be incorporated into the controls of a circulating-magma crystallizer. In this configuration the steam flow is measured and controlled on a mass basis (FIC-1) so that it might be directly related to the feed flow (FIC-2). The density of the feed stream is used to estimate the concentration of the solute. It can also be used to signal the need to increase the steam-to-feed ratio in a feedforward whenever the solvent content of the feed stream rises, i.e., when it becomes more dilute. The steam-to-feed ratio is trimmed by the feedback controller DIC-3 so as to guarantee consistent product concentration in the crystallizer. In Figure 8.18e, in order to take full advantage of the surge capacity of the feed tank, its level is not held constant but is allowed to float up and down. This level variation slowly adjusts the steam flow rate (FIC-1), which in turn changes the feed flow set point, and thereby keeps the feed tank level within limits. The lead-lag relay (L/L) provides dynamic compensation for the time constants of the process. The valve position controller (VPC-4) serves to guarantee that the crystallizer will not be starved for steam. Therefore, whenever the steam valve opening approaches 100%, the low-signal selector on the feed valve (FCV-2) blocks the control signal from FIC-2 and allows VPC-4 to reduce the feed flow as required to match the availability of the steam. Because VPC-4 and FIC-2 control the

© 2006 by Béla Lipták

Draft-Tube Baffle Crystallizer The draft-tube baffle produces larger crystals than the FC crystallizers under equivalent conditions. It consists of a closed vessel with an inner baffle forming a partitioned settling area, inside which a tapered vertical-draft tube surrounds the agitator, which enters from the top or the bottom. The agitator is of the axial-flow type and operates at low speeds. If additional classification of the crystals is desired, an elutriating leg can be fitted to the bottom cone. The draft tube is centered by support vanes to prevent body swirl and to minimize turbulence in the circulating magma. Supersaturation may be generated by either evaporation, cooling, or vacuum cooling. The baffle controls the crystal size by permitting the separation of unwanted fine crystals. Figure 8.18f shows the control of the crystal size distribution in a draft-tube baffletype crystallizer using fines removal and their dissolution. Multiple-Effect Operation Evaporative crystallizers are often used as multiple-effect systems, because such configurations improve the product size distribution because of the narrow residence time distributions. Different strategies can be employed to improve the product CSD. A successful strategy is, for instance, to permit nucleation in the first stage and only growth in the subsequent

8.18 Crystallizer Controls

PRC

1815

PT M

Vacuum system

Air

FDS – Fines density sensor SDS – Slurry density sensor LT Fines removal TT FDS SDS

LIC

Steam

Centrifuge Fines dissolution

FT

Crystalline product

FRC FSL

TIC Feed tank

Set point

LIC

LT

FIG. 8.18f The crystal size in a draft-tube-type baffle crystallizer can be controlled by removing and redissolving the fines.

stages. A simplified control scheme on a triple-effect unit is shown in Figure 8.18g. This complex system has three important features: 1. Level in each unit is an important process variable, because it determines the residence time. Level is usually controlled by throttling the makeup of the mixed liquor. 2. Steam flow to the first unit is usually on flow control. 3. Feed enters the feed tank on level control, where it is mixed with the mother liquor from the centrifuge of the crystalline product. Temperature control in the last unit is obtained by pressure control of the air bleed. In these control systems, density recorders can also be used. If boiling point elevation is sufficiently large, the detected density can also be used to directly control the effluent liquor concentration, as was the case in Figure 8.18e.

In developing the overall control system, both auxiliaries and safety interlocks must be included and designed to meet the requirements of the Fire Insurance Association (FIA). PIC PT Vacuum system

Air

I

LT

II

LIC Cond

LT

III

LIC Cond

LT

LIC Cond

FRC FT

Submerged-Combustion Crystallizers Submerged-combustion crystallizers are used on corrosive applications or in cases where the salts have inverted solubility. One possible control configuration is shown in Figure 8.18h. Here, the clear liquor is fed under flow control, while the burner fuel gas can be under either flow or pressure control, in which case a bypass controls the flow of combustion air to the burner.

© 2006 by Béla Lipták

Steam

Centrifuge Product Feed tank LT

FIG. 8.18g The basic controls of a three-effect crystallizer.

LIC

Feed

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Control and Optimization of Unit Operations

Burner combustion products

Air

FRC

Burner

Fuel

Stack

FRC

Product slurry outlet

FT Feed

Feed

Coolant inlet

FT

M Product slurry

Coolant outlet FT

FIG. 8.18h Submerged combustion-type evaporator crystallizer.

FRC Set point

In principle, cooling crystallizers operate at atmospheric pressure, and their heat is transferred to a cooling medium or to air by either indirect or direct contact. There are many types of cooling crystallizers, but the majority falls into three categories: (1) controlled-growth magma crystallizers, (2) classifying crystallizers, and (3) directcontact crystallizers.

Controlled-Growth Magma Crystallizers A variety of cradle crystallizers and scraped-surface units belong to this category. The cradle types are used in small applications and involve little instrumental control. The various scraped-surface crystallizers are used in crystallization from high-viscosity liquors or in open-tank crystallizers, as coolers to induce nucleation. Two designs of controlledgrowth magma crystallizers and their controls are shown in Figures 8.18i and 8.18j. In Figure 8.18i an evaporating refrigerant cools the shell of the unit, while the crystallization

Vapor refrigerant outlet

Liquid refrigerant inlet

PT

M

LIC LT Product

Feed

FIG. 8.18i The basic controls of a refrigerant-cooled crystallizer.

© 2006 by Béla Lipták

process takes place in the tubes. Usually several scrapedsurface tubes are used in series. The control system operation is as follows: 1. Clear liquor is fed under flow control. Because the residence time in the tubes is large and because it is required in order to obtain the required crystal size, product quality-based feedback control from the product outlet is not practical. Yet, in some applications, feedback control with dead-time compensation has been applied. 2. In Figure 8.18i, liquid refrigerant enters under level control and leaves the system as a vapor under pressure control. In other applications, the refrigerant liquid is introduced under flow control provided by a metering pump. Both two- and three-mode controllers have been used to provide pressure control. In other cooling crystallizer applications, liquid coolant is used as the heat transfer fluid. Figure 8.18j illustrates the control system of such a unit, where the cooling fluid flow is controlled in cascade by the outlet temperature of the product slurry. Classifying Crystallizers

PIC

FT

TRC

FIG. 8.18j The controls of a cooling crystallizer that is cooled by a liquid coolant.

COOLING CRYSTALLIZERS

FRC

TIT

In the Oslo “Krystal” cooling crystallizer the supersaturation is generated entirely by cooling. In this design the feed is introduced into a circulating loop, which is also being cooled. The control configuration for a cooling-type classifying crystallizer is shown in Figure 8.18k. The functions of the control loops are as follows: 1. Flow control of feed liquor is provided to maintain constant throughput. 2. Flow of coolant is controlled in cascade to maintain the outlet temperature of the process fluid at the outlet of the heat exchanger. One refinement is to interlock the coolant flow with the circulation pump motor.

8.18 Crystallizer Controls

FSL

FRC

Set point

FT

Coolant outlet

Coolant inlet

Overflow

DR LIC

Recycle tank

PDT LT

M

FRC FT Feed

fication in a cooling crystallizer. Here, the cooling occurs through a jacket under cascade control. The undersize fraction of crystals leaving the hydrocyclone with the mother liquor is returned to the crystallizer. Such operational configuration is very effective to control the size of the crystals, but it also has a tendency to cause steady-state oscillation. To minimize such oscillations requires more sophisticated control systems.

TRC

TIT

1817

Direct-Contact Crystallizers

LT

Product to centrifuge

In such units, the coolant is an evaporating refrigerant or brine, and it is in direct contact with the process slurry. Basic control methods are similar to those in controlled-growth crystallizers (Figure 8.18i). These involve constant feed rate and flow controls of the evaporating refrigerant, based on process fluid outlet temperature or on tank level.

LIC

Mother liquor

Mother liquor

FIG. 8.18k The controls of a classifying crystallizer with external circulation for cooling.

VACUUM CRYSTALLIZERS 3. Overflow from the crystallizer is on level control. Differential pressure-type level measurement is acceptable for this application. 4. Mother liquor outflow from the recycle vessel is on level control.

In vacuum crystallizers, heat input to produce adiabatic evaporation comes entirely from the sensible heat of the feed liquor and from the heat of crystallization of the crystalline product. Thus, supersaturation is generated by a combination of cooling and concentrating the liquor. The forced-circulation and draft-tube baffle crystallizer designs are both used in this operation mode.

Figure 8.18l illustrates a design that improves the crystal size distribution of the product by providing external classi-

PRC

PT

M Vacuum system

Air TT LT Set point

TIC

TIC

Coolant outlet TT

LIC

Coolant inlet

Hydrocyclon

Product slurry Feed

Feed tank LT

FIG. 8.18l The controls of a jacketed cooling crystallizer with classified product removal.

© 2006 by Béla Lipták

LIC

1818

Control and Optimization of Unit Operations

the coolant outlet. This vacuum indirectly controls the process temperature. The control system consists of (1) feed flow control, (2) product removal control based on level measurement, and (3) temperature control accomplished indirectly by manipulation of jacket coolant. Use of suspension density recorder and other refinements can also be used.

PIC

PT

Steam

M

Water LT TIC

REACTION CRYSTALLIZERS

TT Sump

LIC

Centrifuge Product Feed

Feed tank

FT

LT

FIG. 8.18m Draft-tube-type baffle crystallizer, provided with vacuum control and mother liquor recirculation.

The control strategy for a draft-tube baffle crystallizer with vacuum cooling is shown in Figure 8.18m. Here, the vacuum is on control with steam jet ejector. Air bleed is on pressure control, and both the steam and water supplies are on automatic control, which allows achieving the minimum cost of utilities. Another control configuration is shown in Figure 8.18n. Here, the vacuum in the crystallizer is maintained by throttling

FRC M

FT FT

Coolant outlet

pHIT

Air

pHE FRC

Sparger LT

FT Feed

Water

Gaseous ammonia FRC LIC

Coolant inlet

LIC LT

Product slurry outlet

FIG. 8.18n Draft-tube baffle crystallizer with combination of cooling and vacuum control.

© 2006 by Béla Lipták

Vacuum system PT

FRC

PT

PRC

pHRC Set point

Vacuum system

PRC

In reaction crystallizers, crystallization is associated with a chemical reaction that usually produces the solute directly. Chemical reactions between two components, however, may also produce some diluents, in turn causing “salting-out.” In a reaction crystallizer, once the reaction mixture is saturated with respect to the crystallizing substance, the reaction rate would determine the rate of supersaturation. In some cases, the heat of exothermic reaction may be used in evaporating the solvent, thereby producing additional supersaturation during the course crystallization. A control configuration for a reaction crystallizer is shown in Figure 8.18o, where one of the reactants is fed into the crystallizer in the gaseous phase while the second reactant is fed in the form of aqueous solution. Here, the aqueous solution feed is on flow control, the total liquid feed is on level control, while the gaseous reactant is added on pH-cascaded control. The pH metering lines should be continuously flushed when this system is used. In the case when both reactants A and B are liquids, they can be charged into the crystallizer under ratio control to maintain either the stoichiometric or any other predetermined ratio of these reactants. As shown in Figure 8.18p, the total

Crystal magma to dewatering

FT Aqueous acid Feed tank

FIG. 8.18o The controls of a reaction crystallizer that is utilizing both gaseous and liquid reactants.

8.18 Crystallizer Controls

PRC

1819

PT Vacuum system

Air LT

FFIC PDT SP FT

% B species

LIC

Product slurry to filtration

FT

Ratio SP

LIC

Feed tank LT

Feed tank

A Species LT

LIC

FIG. 8.18p Reaction crystallizer controls provide ratio control of the reactants A and B.

flow into the reactor crystallizer can be on level control and the temperature can be controlled by bleeding in air to adjust the pressure. AUXILIARY EQUIPMENT The control requirements of crystallizers are also a function of the equipment associated with them. There are four subsystems whose control should be considered in particular: 1. The feed system, including the feed liquor, recycle, and wash streams. 2. Vacuum control to maintain predetermined pressure in the system as a common means of crystallizer temperature control. 3. Dewatering system control. This includes filters or centrifuges. 4. In the case of external classification equipment, in order to improve the crystal size distribution of the product, hydrocyclones, vibrating screens, and other external classification devices can be used. CONCLUSIONS In this section some of the common methods of crystallizer control have been outlined. In the crystallization process,

© 2006 by Béla Lipták

because of the phase changes and because of the dispersed nature of the crystalline product, strong interactions exist between the process variables. Because of the complex, multiple-input/multiple-output nature of this process, the control system development should be based on a step-bystep analysis. Pilot plant studies are often recommended, because small variations in the feed liquor compositions can have tremendous influence upon nucleation and crystal growth. Also, minor differences in operating conditions can produce crystals showing significantly different properties. The developer of a control system should take these factors into consideration.

Bibliography Bamford, A. W., Industrial Crystallization, New York: Macmillan, 1966. Mullin, J. W., Crystallization, London: Butterworth & Co., 1972. Nyvlt, J., Industrial Crystallization: The Present State of the Art, Wienheim: Verlag Chemie, 1978. Perry, R. H., and Chilton, C. H., Chemical Engineering Handbook, New York: McGraw-Hill. Randolph, A. D., and Larson, M. A., The Theory of Particulate Processes, New York: Academic Press, 1988. Tavare, N. S., Industrial Crystallization. Process Simulation, Analysis, and Design. New York: Plenum Press, 1995.