12.11. INTRODUCTION AND EQUIPMENT FOR CRYSTALLIZATION

12.11A. Crystallization and Types of Crystals

1. Introduction

Separation processes for gas–liquid and liquid–liquid systems have been treated in this and previous chapters. Also, the separation process of leaching was discussed for a solid–liquid system. Crystallization is another solid–liquid separation process, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. An important example is in the production of sucrose from sugar beet, where the sucrose is crystallized out from an aqueous solution.

Crystallization is a process where solid particles are formed from a homogeneous phase. This process can occur in the freezing of water to form ice, in the formation of snow particles from a vapor, in the formation of solid particles from a liquid melt, or in the formation of solid crystals from a liquid solution. The last process mentioned, crystallization from a solution, is the most important one commercially and will be treated in the present discussion. In crystallization the solution is concentrated and usually cooled until the solute concentration becomes greater than its solubility at that temperature. Then the solute comes out of the solution, forming crystals of approximately pure solute.

In commercial crystallization not only are the yield and purity of the crystals important but also the sizes and shapes of the crystals. It is often desirable that crystals be uniform in size. Size uniformity is desirable to minimize caking in the package, for ease of pouring, for ease in washing and filtering, and for uniform behavior when used. Sometimes large crystals are requested by the purchaser, even though smaller crystals are just as useful. Also, crystals of a certain shape are sometimes required, such as needles rather than cubes.

2. Types of crystal geometry

A crystal can be defined as a solid composed of atoms, ions, or molecules which are arranged in an orderly and repetitive manner. It is a highly organized type of matter. The atoms, ions, or molecules are located in three-dimensional arrays or space lattices. The interatomic distances between these imaginary planes or space lattices in a crystal are measured by X-ray diffraction, as are the angles between these planes. The pattern or arrangement of these space lattices is repeated in all directions.

Crystals appear as polyhedrons having flat faces and sharp corners. The relative sizes of the faces and edges of different crystals of the same material may differ greatly. However, the angles between the corresponding faces of all crystals of the same material are equal and are characteristic of that particular material. Therefore, crystals are classified on the basis of these interfacial angles.

There are seven classes of crystals, depending upon the arrangement of the axes to which the angles are referred:

The relative development of different types of faces of a crystal may differ depending on the solute crystallizing. Sodium chloride crystallizes from aqueous solutions with cubic faces only. But if sodium chloride crystallizes from an aqueous solution with a given slight impurity present, the crystals will have octahedral faces. Both types of crystals, however, are in the cubic system. The crystallization in overall shapes of plates or needles has no relation to the type of crystal system and usually depends upon the process conditions under which the crystals are grown.

12.11B. Equilibrium Solubility in Crystallization

In crystallization equilibrium is attained when the solution or mother liquor is saturated. This is represented by a solubility curve. Solubility is dependent mainly upon temperature. Pressure has a negligible effect on solubility. Solubility data are given in the form of curves where solubilities in some convenient units are plotted versus temperature. Tables of solubilities are given in many chemical handbooks (P1). Solubility curves for some typical salts in water were given in Fig. 8.1-1. In general, the solubilities of most salts increase slightly or markedly with increasing temperature.

A very common type of curve is shown in Fig. 8.1-1 for KNO₃, where the solubility increases markedly with temperature and there are no hydrates. Over the whole range of temperatures, the solid phase is KNO₃. The solubility of NaCl is marked by its small change with temperature. In solubility plots the solubility data are ordinarily given as parts by weight of anhydrous material per 100 parts by weight of total solvent (i.e., water in many cases).

In Fig. 12.11-1 the solubility curve is shown for sodium thiosulfate, Na₂S₂O₃. The solubility increases rapidly with temperature, but there are definite breaks in the curve which indicate different hydrates. The stable phase up to 48.2°C is the pentahydrate Na₂S₂O₃ 5H₂O. This means that at concentrations above the solubility line (up to 48.2°C), the solid crystals formed are Na₂S₂O₃ 5H₂O. At concentrations below the solubility line, only a solution exists. From 48.2 to about 65°C, the stable phase is Na₂S₂O₃ 2H₂O. A half-hydrate is present between 65 to 70°C, and the anhydrous salt is the stable phase above 70°C.

12.11C. Yields and Heat and Material Balances in Crystallization

1. Yields and material balances in crystallization

In most of the industrial crystallization processes, the solution (mother liquor) and the solid crystals are in contact for enough time to reach equilibrium. Hence, the mother liquor is saturated at the final temperature of the process, and the final concentration of the solute in the solution can be obtained from the solubility curve. The yield of crystals from a crystallization process can then be calculated knowing the initial concentration of solute, the final temperature, and the solubility at this temperature.

In some instances in commercial crystallization, the rate of crystal growth may be quite slow, due to a very viscous solution or a small surface of crystals exposed to the solution. Hence, some supersaturation may still exist, giving a lower yield of crystals than predicted.

In making the material balances, the calculations are straightforward when the solute crystals are anhydrous. Simple water and solute material balances are made. When the crystals are hydrated, some of the water in the solution is removed with the crystals as a hydrate.

EXAMPLE 12.11-1. Yield of a Crystallization Process A salt solution weighing 10000 kg with 30 wt % Na2CO3 is cooled to 293 K (20°C). The salt crystallizes as the decahydrate. What will be the yield of Na2CO3 ∊· 10H2O crystals if the solubility is 21.5 kg anhydrous Na2CO3/100 kg of total water? Do this for the following cases: Assume that no water is evaporated. Assume that 3% of the total weight of the solution is lost by evaporation of water in cooling. Solution: The molecular weights are 106.0 for Na2CO3, 180.2 for 10H2O, and 286.2 for Na2CO3 10H2O. The process flow diagram is shown in Fig. 12.11-2, with W being kg H2O evaporated, S kg solution (mother liquor), and C kg crystals of Na2CO3 10H2O. Making a material balance around the dashed-line box for water for part (a), where W = 0, Equation 12.11-1 Figure 12.11-2. Process flow for crystallization in Example 12.11-1. where (180.2)/(286.2) is wt fraction of water in the crystals. Making a balance for Na2CO3, Equation 12.11-2 Solving the two equations simultaneously, C = 6370 kg of Na2CO3 · 10H2O crystals and S = 3630 kg solution. For part (b), W = 0.03(10 000) = 300 kg H2O. Equation (12.11-1) becomes Equation 12.11-3 Equation (12.11-2) does not change, since no salt is in the W stream. Solving Eqs. (12.11-2) and (12.11-3) simultaneously, C = 6630 kg of Na2CO3 · 10H2O crystals and S = 3070 kg solution.

2. 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. An evolution of heat occurs when a compound dissolves whose solubility decreases as temperature increases. For compounds dissolving whose solubility does not change with temperature, there is no heat evolution on dissolution. Most data on heats of solution are given as the change in enthalpy in kJ/kg mol (kcal/g mol) of solute occurring with the dissolution of 1 kg mol of the solid in a large amount of solvent at essentially infinite dilution.

In crystallization the opposite of dissolution occurs. At equilibrium the heat of crystallization is equal to the negative of the heat of solution at the same concentration in solution. If the heat of dilution from saturation in the solution to infinite dilution is small, this can be neglected, and the negative of the heat of solution at infinite dilution can be used for the heat of crystallization. With many materials this heat of dilution is small compared with the heat of solution, and this approximation is reasonably accurate. Heat-of-solution data are available in several references (P1, N1).

Probably the most satisfactory method for calculating heat effects during a crystallization process is to use the enthalpy–concentration chart for the solution and the various solid phases which are present for the system. However, only a few such charts are available, including the following systems: calcium chloride–water (H1), magnesium sulfate–water (P2), and ferrous sulfate–water (K2). When such a chart is available, the following procedure is used. The enthalpy H₁ of the entering solution at the initial temperature is read off the chart, where H₁ is kJ (btu) for the total feed. The enthalpy H₂ of the final mixture of crystals and mother liquor at the final temperature is also read off the chart. If some evaporation occurs, the enthalpy H_v of the water vapor is obtained from the steam tables. Then the total heat absorbed q in kJ is

Equation 12.11-4

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

EXAMPLE 12.11-2. Heat Balance in Crystallization A feed solution of 2268 kg at 327.6 K (54.4°C) containing 48.2 kg MgSO4/100 kg total water is cooled to 293.2 K (20°C), where MgSO4 · 7H2O crystals are removed. The solubility of the salt is 35.5 kg MgSO4/100 kg total water (P1). The average heat capacity of the feed solution can be assumed as 2.93 kJ/kg · K (H1). The heat of solution at 291.2 K (18°C) is −13.31 × 103 kJ/kg mol MgSO4 · 7H2O (P1). Calculate the yield of crystals and make a heat balance to determine the total heat absorbed, q, assuming that no water is vaporized. Solution: Making a water balance and a balance for MgSO4 using equations similar to (12.11-1) and (12.11-2) in Example 12.11-1, C = 616.9 kg MgSO4 · 7H2O crystals and S = 1651.1 kg solution. To make a heat balance, a datum of 293.2 K (20°C) will be used. The molecular weight of MgSO4 · 7H2O is 246.49. The enthalpy of the feed is H1: The heat of solution is −(13.31 × 103)/246.49 = −54.0 kJ/kg crystals. Then the heat of crystallization is −(−54.0) = +54.0 kJ/kg crystals, or 54.0(616.9) = 33 312 kJ. This assumes that the value at 291.2 K is the same as at 293.2 K. The total heat absorbed, q, is Since q is negative, heat is given off and must be removed.

12.11D. Equipment for Crystallization

1. Introduction and classification of crystallizers

Crystallizers may be classified according to whether they are batch or continuous in operation. Batch operation is done for certain special applications. Continuous operation of crystallizers is generally preferred.

Crystallization cannot occur without supersaturation. A main function of any crystallizer is to cause a supersaturated solution to form. Crystallizing equipment can be classified according to the methods used to bring about supersaturation as follows: (1) supersaturation produced by cooling the solution with negligible evaporation—tank and batch-type crystallizers; (2) supersaturation produced by evaporation of the solvent with little or no cooling—evaporator–crystallizers and crystallizing evaporators; (3) supersaturation by combined cooling and evaporation in an adiabatic evaporator—vacuum crystallizers.

In crystallizers producing supersaturation by cooling, the substances must have a solubility curve that decreases markedly with temperature. This occurs for many substances, and this method is commonly used. When the solubility curve changes little with temperature, as for common salt, evaporation of the solvent to produce supersaturation is often used. Sometimes evaporation with some cooling may be used. In the method of cooling adiabatically in a vacuum, a hot solution is introduced into a vacuum, where the solvent flashes or evaporates and the solution is cooled adiabatically. This method for producing supersaturation is the most important one for large-scale production.

In another method of classification of crystallizers, the equipment is classified according to the method of suspending the growing product crystals. Examples are crystallizers where the suspension is agitated in a tank, is circulated by a heat exchanger, or is circulated in a scraped surface exchanger.

An important difference between many commercial crystallizers is the manner in which the supersaturated liquid contacts the growing crystals. In one method, called the circulating magma method, the entire magma of crystals and supersaturated liquid is circulated through both the supersaturation and crystallization steps without separating the solid from the liquid into two streams. Crystallization and supersaturation are occurring together in the presence of the crystals. In the second method, called the circulating liquid method, a separate stream of supersaturated liquid is passed through a fluidized bed of crystals, where the crystals grow and new ones form by nucleation. Then the saturated liquid is passed through an evaporating or cooling region to produce supersaturation again for recycling.

2. Tank crystallizers

In tank crystallization, which is an old method still used in some specialized cases, hot saturated solutions are allowed to cool in open tanks. After a period of time the mother liquor is drained and the crystals removed. Nucleation and the size of crystals are difficult to control. Crystals contain considerable amounts of occluded mother liquor. Labor costs are very high. In some cases the tank is cooled by coils or a jacket and an agitator used to improve the heat-transfer rate. However, crystals often build up on these surfaces. This type of crystallizer has limited application; it is sometimes used to produce certain fine chemicals and pharmaceutical products.

3. Scraped surface crystallizers

One type of scraped surface crystallizer is the Swenson– Walker crystallizer, which consists of an open trough 0.6 m wide with a semicircular bottom having a cooling jacket outside. A slow-speed spiral agitator rotates and suspends the growing crystals on turning. The blades pass close to the wall and break off any deposits of crystals on the cooled wall. The product generally has a somewhat wide crystal-size distribution.

In the double-pipe scraped surface crystallizer, cooling water passes in the annular space. An internal agitator is fitted with spring-loaded scrapers that wipe the wall and provide good heat-transfer coefficients. This type is called a votator and is used in crystallizing ice cream and plasticizing margarine. A sketch is given in Fig. 4.13-2.

4. Circulating-liquid evaporator–crystallizer

In a combination evaporator–crystallizer, shown in Fig. 12.11-3a, supersaturation is generated by evaporation. The circulating liquid is drawn by the screw pump down inside the tube side of the condensing steam heater. The heated liquid then flows into the vapor space, where flash evaporation occurs, giving some supersaturation. The vapor leaving is condensed. The supersaturated liquid flows down the downflow tube and then up through the bed of fluidized and agitated crystals, which are growing in size. The leaving saturated liquid then goes back as a recycle stream to the heater, where it is joined by the entering feed. The larger crystals settle out and a slurry of crystals and mother liquor is withdrawn as product. This type is also called the Oslo crystallizer.

5. Circulating-magma vacuum crystallizer

In the circulating-magma vacuum-type crystallizer shown in Fig. 12.11-3b, the magma or suspension of crystals is circulated out of the main body through a circulating pipe by a screw pump. The magma flows through a heater, where its temperature is raised 2–6 K. The heated liquor then mixes with body slurry and boiling occurs at the liquid surface. This causes supersaturation in the swirling liquid near the surface, which results in deposits on the swirling suspended crystals until they leave again via the circulating pipe. The vapors leave through the top. A steam-jet ejector provides the vacuum.