Showing papers in "Chemical Engineering in 2005"

Batch drying : The 'indirect solution to sensitive drying problems

Abstract: pump. Therefore, it is essential to insert a dust filter between the dryer and the condenser to prevent the passage of particles to the condenser and into the pump. The filter has to fulfill three main tasks: • Separate and collect around 99.9% or more of the particles from the vapor • Be chemically resistant to solvents and the dust • Minimize pressure drop to maintain the vacuum inside the dryer Two types of surface-layer-formation dust filters are used in drying technology: metallic filter cartridges and fabric filters. Although fabric filters are less expensive than metallic filters, they can normally collect particles only as small as 10μm, versus 1 μm or less for metallic types. However, fabric filters can also achieve 1-μm separation when given a surface treatment, such as a membrane coating. It is important to provide sufficient filter surface to minimize pressure drop. The required surface area can be calculated by using empirical maximum vapor flowrates through the filter media (fabric filters, 100 m3/m2-h; metal filters, 300 m3/m2-h) and through the vapor piping. Sophisticated filter equations that include such parameters as vapor dust load (grams per cubic meter) and maximum pressure drop can be used for calculation, but have the disadvantage of requiring hard-to-obtain product data. Over time, the product dust forms a layer on the filter. In many applications, the layer acts like an absorbent — it becomes wet from the vapors and particles tend to build a crusty layer that blocks the pores of the filter. This reduces the vapor flow and can cause a dramatic drop in the drying speed (Figure 11). To avoid this problem, it is necessary to clean the filter regularly by blowback with pressurized gas. Initiation of the blowback cycle may be triggered when the pressure rises to a predetermined level (usually 30–70 mbars), by a specific elapsed time, or by a combination of these methods. Time-controlled filter blowback is often more efficient than the pressure drop-controlled method because it is done at regular intervals and is less disruptive to the drying process. For example, at low vacuum levels (below 100 mbars), it is important not to blow back too often, in order to avoid loss of dryer vacuum caused by the blow-back gas. For dust filters used for active pharmaceutical ingredients (API), the following limit values have shown to be useful for calculating and checking filter surfaces: • 25 m/s maximum vapor speed in vapor line • 0.5–3.0 g/m3 maximum estimated vapor dust load • 100 m3/m2-h maximum flowrate for textile filter cloth • 300 m3/m2-h maximum flowrate for metallic filters