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nonfunctional coatings. In the pharmaceutical industry, particles with enteric and sustained release coatings, moisture barriers, and cosmetic coats are all produced in fluidized beds. In the agricultural industry, fertilizers are produced containing time release insecticides and fungicides, and seeds are coated with protective layers. In the food industry, cereals, nuts, and other food stuffs are coated with sugar coatings. In the petroleum industry, gas oil is sprayed onto the surface of hot catalyst particles in the riser of a fluidized catalytic cracking unit. In the chemical industry, catalyst is sprayed into a fluidized bed in the production of polyolefins. Although, these processes differ greatly in the products they produce, the even distribution of a liquid onto the surface of bed particles and the subsequent formation of a coating layer on the particles is common to all these processes.

2.2Overview of Coating Process

Particle Movement in Fluid Bed Coaters. As mentioned earlier, coating can be considered to be a subset of the granulation process when the forces causing the breakup of agglomerates dominate and overwhelm the forces tending to hold particles together. With large enough particles at high superficial velocities and low spray rates, it is possible to coat particles in a dense or bubbling fluidized bed without appreciable agglomeration. However, in general, particle coating is carried out most often in some form of spouted or near-spouted bed and the solvent-solute combination should be chosen so that the liquid-to-solid transition is very fast and the resulting coating is not tacky. Two examples of spouted bed coaters are illustrated in Figs. 2(a) and 2(b). Figure 2(a) shows a conventional spouted bed coater. The liquid to be coated is introduced into the bed via a spray nozzle which can be mounted at the bottom (bottom spraying) or top (top spraying) of the bed. The fluidizing gas is fed to the conical bottom of the bed and, under the right conditions, forms a spout or central core in which solids are stripped from the dense annular bed and dragged into the upward moving central gas core. The upward movement of particles in the core, followed by their reversal in the “fountain” region, and downward movement in the dense annular bed, forms a circulation path for the solids. Figure 2(b) shows a fluidized bed with a draft tube or “Wurster Insert,” as it is referred to in the pharmaceutical industry. The purpose of the draft tube is to promote the regular circulation of particles within the bed by forcing most of the fluidizing gas up through the center of the bed, thus causing the particles to

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be entrained upwards with the gas. As the gas reaches the expansion zone above the bed, the gas slows down, and the particles decelerate and fall back into the dense bed in the annular region surrounding the draft tube. The liquid is most often introduced via a nozzle located at the bottom of the bed (bottom spraying). In this type of equipment, top spraying is possible, but rarely used. The movement of solids in beds with draft tubes can be regulated by the height of the gap between the draft tube and gas distributor plate.

The Spray Zone. The movement of particles through the spray zone is illustrated in Fig. 3. From this figure, it can be seen that the liquid spray impinges on the solid bed material as it moves through the spray zone. Upon contact with the surface of the particles, the droplets of liquid will spread over the surface of the particles and will partially coat the solid surface. The repeated motion of the particles through the spray zone allows a continuous coat of material to build up, resulting in a smooth and, hopefully, uniform coat. The wetting of the particle by the liquid drops is essential for uniform coating. If the drops do not wet the surface, but instead bead up, the resulting coat may be a discontinuous layer of tiny spheres of solute rather than the desired continuous film. The use of surfactants or plasticizers may improve the spreading, film forming potential and the wetting of liquid on the surface of the particles. An example of improved wetting using surfactant is given by Weiss and Meisen (1983) who coated urea particles with molten sulfur. Their results indicated the presence of tiny pin holes in the coat of sulfur which were eliminated by the use of surface active agents capable of changing the contact angle between the urea and molten sulfur.

The wicking of liquid into a porous solid is also of importance. Excessive wicking may give rise to a coat of varying thickness. Furthermore, to produce a coat of the desired thickness, additional coating material may be required due to the loss of coating into the particle interior.

It will become evident later on, in Sec. 2.4, that the regular movement of particles through the spray zone is essential if particles are to be coated uniformly. One primary advantage of the spouted bed is that particles circulate within the bed and through the spray zone in a fairly uniform manner, which promotes uniform coating of particles. Another advantage of spouted beds is that the liquid spray is applied to particles which are in a lean or semi-lean phase. Clearly, if particle agglomeration is to be avoided, it makes sense to keep freshly wetted particles separate from each other for as long as possible. By placing the spray nozzle at the bottom of the bed,

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particles are sprayed and then travel upwards in the core of the bed in a lean phase condition. During their upward passage, the particles are extensively dried by the flow of fluidizing gas and are relatively solvent free and nonsticky when they return to the dense annular bed. The details of coating equipment and ranges of operation, etc., will be discussed in Sec. 2.5.

Figure 3. Interaction of particles with liquid spray.

It is worth pointing out that, although the coating process may seem relatively straightforward compared to the complex phenomena at play in the granulation process, many problems are encountered in coating particles. In addition, the quality control for coatings are very tight, especially in the pharmaceutical industry. Unlike in granulation, reprocessing of offspec product is virtually impossible after coating has occurred. The prospect of dumping a batch of off-spec coated material, containing hundreds of thousands of dollars worth of active ingredient, highlights the importance of understanding and controlling the processes involved in fluidized bed coating.

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2.3Microscopic Phenomena

Atomization in Spray. From Sec. 2.2 above, it is apparent that the formation of a well-dispersed liquid spray is an essential prerequisite for uniform coating. The process of breaking up a liquid into a spray of fine droplets is termed atomization. Numerous types of atomizers are currently used, and a comprehensive description of the different nozzles available, their performance, and the atomization process in general is given by Lefebvre (1989). The most common type of atomizing nozzle used in coating operations is the dual-fluid nozzle which uses the kinetic energy of a high pressure gas (usually air or steam) to break up the liquid into a fine spray. The gas can either contact the liquid within the nozzle (internal mix) or outside the nozzle (external mix). The main advantages of dual-fluid nozzles are their good atomization, their ability to atomize high-viscosity liquids, and their resistance to blockage. However, some problems may result when using dual-fluid atomizers with highly volatile liquids due to excessive evaporation of the liquid during the atomization process. Alternatives to the use of dual-fluid nozzles are pressure or hydraulic nozzles. In these devices, the liquid used for coating is fed to the nozzle under pressure and becomes atomized by virtue of its own kinetic (pressure) energy. The hydraulic nozzle is simple to operate but generally produces a spray with a greater mean droplet size as compared to the dual-fluid nozzles. Recently, the use of ultrasonic nozzles in fluidized bed coating operations has been investigated (Olsen, 1989a,b). The liquid is atomized by an ultrasonic transducer and horn located at the tip of the nozzle. The spray produced is very fine but the throughput of such nozzles is low. The main advantage of the ultrasonic nozzle is the ability to produce a fine spray without the use of a secondary fluid, and potential applications include the ability to atomize very volatile liquids without appreciable evaporation in the nozzle.

Size Distribution of Atomized Droplets. The size distribution of droplets in a spray is a complex function of the properties of the liquid, the secondary gas (if used), and the nozzle geometry. The most reliable and often fastest way to determine this information is to experimentally measure the size distribution under the conditions of interest, and most nozzle manufacturers offer this service to their customers.

With the above statement in mind a brief summary of some important trends is given below:

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Hydraulic or Pressure (Single Fluid) Spray Nozzles

(i)Droplet size is dependent on spray rate and supply pressure, and it is difficult to adjust the spray rate independently of the droplet size (Olsen, 1989a,b).

(ii)As the flow of liquid increases so does mean droplet size.

(iii)Hydraulic nozzles tend to atomize effectively at liquid flow rates greater than 250 ml/min, Olsen (1989a,b).

(iv)Typical average droplet size is on the order of 100 µm, Hall (1991).

Dual-Fluid Spray Nozzles

(i)Flow of liquid and drop size can be controlled independently, (Olsen, 1989a,b).

(ii)Atomizing air flow rate is not critical (Nienow and Rowe, 1985), but an increase in air pressure decreases mean droplet size.

(iii)Typical [Air/Liquid] volumetric ratios for coating and granulation are 500–800 at an air supply pressure of 3 bar (Nienow and Rowe, 1985).

(iv)Liquid flow rates down to 10 ml/min are possible, and droplet diameter increases with increasing liquid flow (Olsen, 1989a,b).

(v)Typical average droplet size is 20–30 µm (Hall, 1991)

Evaporation of Atomized Droplets. The prediction of the time to totally evaporate a liquid droplet in an atomized spray is very difficult due to the complex thermal and concentration gradients present in the vicinity of the nozzle. Despite this complexity, it will be beneficial to study what happens to a single droplet of liquid when it is surrounded by a quiescent gas stream. This phenomena has been studied extensively because the time to evaporate a liquid drop has important consequences in a number of different applications; e.g., spray drying, fuel injection, and coating.

The time to evaporate a droplet of pure liquid in a stagnant gas stream was given by Marshall (1954) as:

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where T is the temperature difference between the surface of the drop and the bulk temperature of the gas. The assumption of a stagnant gas stream should hold for the case of atomized droplets, since the relative velocity between the drops and the gas will be very low and is of the magnitude of the terminal velocity of the drops. The problem with Eq. (1) is that the surface temperature of the drop is generally not known. A later analysis by Kanury (1975) solves the heat and material balance equations simultaneously to yield the following equations:

It is further assumed that the mass fraction of solvent in the vapor phase at the surface of the droplet, ys, is in equilibrium with the liquid at the drop’s surface, thus:

The surface temperature of the droplet, Ts, must first be solved using Eqs. (3) and (4), then the value of B´ can be determined and substituted into Eq. (2). As an example, these equations were solved for a number of initial droplet sizes, liquids, and gas temperatures. The results are presented in

Table 1 for the case of liquid evaporation into dry air (ybulk = 0). The results in Table 1, along with the form of Eq. (2), indicate that the time to evaporate

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Additional Factors Affecting Evaporation Times. For liquid drops containing solids, which lower the normal vapor pressure of the liquid, the net effect of the solids is to increase the time for complete evaporation, Marshall (1954). The presence of solids introduces an additional complication associated with the changing droplet surface temperature during the evaporation process. This gives rise to longer evaporation times.

For a spray of drops, two additional complications arise; namely, that there is now a distribution of drop sizes to consider and also that the heat and mass transfer to a given drop is influenced by the local phenomena occurring in the region of neighboring drops. The net result of this latter effect is that the gas no longer acts as a sink, and the gas, in the vicinity of the spray, is cooled significantly, thus, reducing the evaporation rate of drops in the spray. Local temperature profiles in the vicinity of a spray nozzle immersed in a bubbling fluidized bed were taken by Smith and Nienow (1982) who showed that significant cooling was experienced close to the nozzle.

It is evident from the discussion above that the time for complete evaporation of a drop in a spray depends on many factors in a complicated way and will be longer than that predicted by the single drop expressions given in Eqs. (1) and (2). Nevertheless, data such as that given in Table 1 and Eqs. (1) and (2) gives valuable information on the order of magnitude of the evaporation time and the role of process variables.

Consequences of Having Drops with Short and Long Evaporation Times. Combinations of small drop sizes, high bed temperatures, and volatile solvents give rise to very short evaporation times. This, in turn, gives rise to the possibility of spray drying wherein the liquid drops completely evaporate before they contact the surface of the bed material. For sprays with a large proportion of these drops, the net effect is a loss in coating efficiency, i.e., a reduction in the fraction of the liquid sprayed into the bed which ends up as coating on the solid product. Even if spray drying does not occur, the coat formed when partially evaporated drops contact the particles’s surface, may be inhomogeneous due to poor wetting characteristics.

Combinations of large drop sizes, low bed temperatures, and solvents with low volatility give rise to very long evaporation times. If the evaporation time of the drop is greater than the characteristic time for the solids to circulate through the spray zone, then some fraction of the particles will still be wet just before they re-enter the spray zone. This can cause excessive moisture buildup, and local agglomeration may occur. Little work has been done in evaluating the circulation times in large industrial coating equipment.

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However, for small scale equipment, Cheng (1993) and Xu (1993) measured the circulation times of a single tagged particle in a 150 mm diameter fluidized bed coater with a draft tube and found that typical circulation times were between 2–10 seconds. Robinson and Waldie (1978) made similar measurements in a 6" diameter conventional spouted bed and found mean circulation times in the range of 3–6 seconds.

Another problem arises when drop sizes are large compared to the size of the particles to be coated. For example, consider the coating of particles 1 mm and 100 µm in diameter with drops of 100 µm in diameter. For the 1 mm particles, the liquid drops would impinge and spread over the surface but for the 100 µm particles, the liquid drops would engulf the particles giving rise to a thick layer of liquid on the surface which favors particle agglomeration. The difficulty of getting a uniform small droplet size is the main reason for the difficulty in coating particles with diameters less than 100 µm. However, recently there have been reports of successful attempts to coat particles down to 50 µm in diameter with the use of supercritical fluid solutions (Tsutsumi et al., 1994). Due to the cohesive nature of very small particles, characterized as group C particles by Geldart (1973), normal fluidization is not possible, and the absolute lower limit for fluidized bed coating is for particles larger than about 30 µm in diameter.

2.4Modelling

Before discussing modelling, it is perhaps worth focusing on the goals of the coating process and what is meant by the “quality” of a coating. The term quality refers to one or several properties of the finished product, e.g., loading of active ingredient, in vitro dissolution characteristics, and in vivo efficacy, crush strength, appearance and shelf life. The variation of these properties within a batch of product determines the “quality” of the product. These measures of quality and the ability to obtain repeatable product performance between different batches are the goals of any coating process. At a fundamental level, we can characterize the variation in quality to be a function of two things: coating mass uniformity and coating morphology. Coating mass uniformity refers to the variation in the amount of coating material each product particle receives during a batch coating operation. Coating morphology refers to the variation in a given property for particles containing the same amount of coating material. Thus, mass uniformity is important when an active ingredient is applied in the coating, while both mass