Coating apparatus (MP-1) with inserted filters

Figure 2.21 shows its component which consists of a conical product container (no. 14) made from acrylic glass, perforated bottom with different percentage of free holes (no. 11), the Wurster column (no. 13) with a diameter of 6 cm and a heigh of 18 cm. The spraying unit (no. 15) was in a position in between the Wurster column. The partition part (no. 12) between the end of the Wurster column and the perforated plate can be varied. However, normally 0.8 cm was used in this work. Products inside the container will be fluidized from the process air from the bottom to the upper part and at the same time they will be coated with the fluid that was atomized by the air. In the region outside the Wurster column the product will flow downward and flow into the center through the partition part. The product will be blown upward again from this center with continuous coating cycles. During the upward flow of the product the drying phase was carried out by the heated process air. Because of the interchange of water content between the process air and products the process air carried more water content. Consequently, there was a temperature decrease of the process air. This process air flows through the filters and the outlet air tube by the sucking process from the ventilator (no. 19).

The important variables during a coating process were measured inside the coating apparatus. Different sensors were installed at different positions i.e. three positions for temperature (no. 1 to 3), one position for temperature and humidity using dew point (no. 4), two positions for air flow (no. 5 and 6) and one position for mass flow of coating liquid (no. 7). The signals from these 7 channels were collected at the data recording device {14} at the same time. These signals were both demonstrated on the display of the data recording device and automatically saved on a disc. This process facilitates to observe changes of process conditions which may happen during the coating process.

This permits a fast manual regulation.

As mentioned before the product container is made from acrylic glass and therefore very suitable for the visual observation of the coating process. A free downward flow of the product at the sight of the acrylic glass product container is one of the indications of a good fluidization but such limited observation could be misleading. In addition it is possible to supervise this situation by monitoring the outlet air temperature. Every product has an unique constant drying period in which the bed temperature remains relatively constant for a significant length of time. Therefore, if the outlet air temperature rises more rapidly than expected, it is an indication that fluidization is incomplete <122>.

In our work the temperature profiles of temperatures at four positions were demonstrated on the display of the data recording device. These four temperatures were measured using thermocouples {63}. These sensors should be calibrated before used, see part 2.3.3.1. Therefore values of inlet air temperature before heating, inlet air temperature after heating, product bed temperature and outlet air temperature can be observed and shown in degree celcius (°C). These temperature profiles will allow better control of the coating process. The effective drying and coating phases can be monitored. Guidelines and exemplary values are given and discussed in the literature and therefore can be used as a guideline for the preliminary experiments. After suitable temperatures for a process were chosen the control of these temperatures could easily be done.

As heated air was used to dry the product during the coating process, the drying capacity of the air must be carefully monitored. The drying capacity of the air depends upon the relative humidity (r.h.) of the incoming air. At 100 % r.h. the air is holding the maximum amount of water but if the temperature of the air is raised the relative humidity drops and the air can hold more moisture. If air is saturated with water vapor at a given temperature, a drop in temperature will force the air mass to release some of its moisture through condensation. The temperature at which moisture condenses is the dew point temperature. Dew point and vapor pressure are directly related. Thus, the drying capacity of the air varies significantly during processing. By dehumidifying the air to a pre-set dew point, one can maintain constant drying capacity and, hence, a constant process time. If absolute humidity varies during the year, changes in the relative humidity of the heated, fluidizing air will result. For this reason the air humidity may actually be lower than desirable during cold and dry seasons and a rehumidification or a wetting process may be necessary <122>.

In this present work the relative humidity of the outlet air was measured using the Hygrolog-hygrometer {33}. This was also calibrated before use, see part 2.3.3.2. This relative humidity can be calculated by using the outlet air temperature and its dew point.

This value showed the water content inside the coating column during coating process.

It can be affected by the content of water in the inlet air as mentioned before and therefore the water content of the inlet air should be controlled by using a dehumidifying unit {15}. The inlet air of room temperature was cooled down to about 5 °C by a condensing process. This air then has a humidity of about 98 % r.h., which means that this cool air contains 5 g of water in one kg of air. This cool air will then be used as inlet air into the Aeromatic MP-1 before heating. After heating this air will have a lower relative humidity whereas the absolute water content of the air remains the same as before heating. For example, if the process air was heated to 65 °C then the relative humidity of this air will be 3.5 % r.h. However, it was problematic when the process was carried out in the winter season because the relative humidity of the inlet air for the dehumidifying unit was already low. The water from the air could not be further removed and after heating in the coating apparatus, the relative humidity was extremely low. If a high relative humidity is needed for the coating process this inlet air should be wetted before use. The wetting process, however, was not possible in our laboratory because no humidifier was available. Therefore the coating process had to be carried out in the season with high relative humidity so that the dehumidifying process could be performed. The profile of the relative humidity of the outlet air shows the range which must be controlled to maintain the same coating conditions.

The spraying rate can be observed by using a mass flow rate. That means the decreasing of mass or weight (g) over the time was recorded. A digital balance {17}

connected with a digital/analog converter served for the observation. The analog (mA) values were collected in the data recording device and they were converted into digital values again. These digital values were the base for the calculation of the spraying rate over the time.

The air flow in the Wurster fluidized bed system is a combined air flow of the fluidization air and the atomizing air at the nozzle. Air and substrate velocities are not uniform across the upper part of the product bed. The velocities at the center are significantly higher than those along the walls. There is a risk that the substrate might fall down along the wall of the Wurster column, and that clusters of particles might be formed at certain process conditions in the upper part of the product bed. The terminal velocity of

particles in the upper part is limited by the height of the expansion chamber.

Unfortunately, the terminal velocity of the particles cannot be calculated. The product concentration in the mist region of the upper part must be high enough to secure adherence of all spray droplets to substrate particles. The region where the product flows down outside the Wurster column is a slightly expanded bed where the air rate is below the minimum fluidization velocity. This is the region where sticking mostly occurs, since the movement is gentle and the particles are in close contact to each other <36>.

As the velocity of the process air during the coating process is an important factor which affects the flow pattern of the product, the control of this velocity allows to regulate a desired process. In this present work the mass flow of the atomizing air and the volume stream of the process air were measured using flowmeters {21,22} and their values were shown in the form of volume of air over time (m³h^-1). These two values were separately measured at the positions demonstrated in Figure 2.21.

Moreover, the flow pattern can be affected by the perforated air distributor plate. This plate is defined by its percentage of open area. In this present work there were five different openings available i.e. 6 %, 8 %, 11 %, 13 % and 20 % {52}. These interchangeable plates provide a range of loading capacities so that batches of various sizes can be produced efficiently and with a uniform quality. To prevent channeling a plate that provides the optimum lift properties should be selected. This plate was afterwards covered with a finer screen (e.g. screen size 100 µm) which provides appropriate means of supplying air to the bed.

To move air in a fluidized bed apparatus, an exhaust ventilator mounted outside of the processing area imparted the motion and the pressure to the air by action of a turbine-wheel. The moving air acquires a force or pressure component in its direction of motion because of its weight and inertia. Thus, a negative static pressure will exist on the inlet side of the ventilator. The pressure drops (∆ P) can be determined by all the components of the complete system. As filters can cause a significant pressure drop, many process failures result from the selection of filter media with a wrong pore size.

The process failure can also occur when the filter clogs because of excessive fluidization of fine powder or when filters are improperly cleaned after finishing the process. A too fine filter will impede fluidization, causing excessive ∆ P, and a too coarse filter will cause loss of valuable product carried by the process air <122>. In our coating apparatus the pressure difference between inside the product container and behind the filters was demonstrated in millimeter of water (mm H2O) on the pressure gauge at the

control panel of the coating apparatus. The value of about 250 mm H2O or 2450 Pa shows the critical point, that means filters were blocked with fine dust or particles and the coating process had to be stopped.