19-08-2014, 10:20 AM
Drying is an important unit operation used in many chemical and allied industries, such as food, chemicals, pharmaceuticals, petrochemicals and plastics, fertilizers and minerals, amongst others. As drying is the final processing step in many manufacturing processes, it has a major impact on the perceived quality of the product. The choice of dryer and its mode of operation are therefore important process issues.
Drying is an important unit operation used in many chemical and allied industries, such as food, chemicals, pharmaceuticals, petrochemicals and plastics, fertilizers and minerals, amongst others. As drying is the final processing step in many manufacturing processes, it has a major impact on the perceived quality of the product. The choice of dryer and its mode of operation are therefore important process issues.
The present work is divided into two major parts: i) General selection criteria for different type of dryers, ii) Design of screw conveyor dryer for fine, cohesive and oxidative product of commercial importance and Design and optimization of heat pump and freeze dryer for drying of surgical gel.
Selection criteria: Dryer selection is predominately an art in which knowledge, experience and science all play important roles. Often there is no 'right' answer in the absolute sense as more than one option are viable, both technically and as well as economically. In the present study, the selection criteria was worked out for the different types of dryers based on feed configurations, mode of operations, product throughput, mode of heat transfer, material properties and other related aspects.
Drying of fine, cohesive and highly oxidative products: In this section, a screw conveyor dryer (SCD), in which material is simultaneously heated and dried as it is conveyed, was selected on the basis of material characteristics, space constraint and the economic feasibility. From the experimental results it was observed that drying under low pressure gives 92% moisture removal compared to 30–40% using low flow rates of nitrogen. Over the parameter range studied, the overall heat transfer coefficient and thermal efficiency were found to be in the range of 46–102W/m2K and 25–62%, respectively. Power consumption per metric ton of dried material was found to be a strong function of screw speed and material feed rate, material properties, and drive efficiency. The flow pattern and the mixing characteristics in SCD were studied using pulse input of a tracer and the dynamic step change in the input solids flow rate methods. The results obtained were beneficial for the design, control, and scale-up of extrusion and similar units. The flow in a SCD approaches plug flow as the feed rate increases, whereas, an increase in the screw speed results in a mixed flow. The combination of high feed rates with low screw speeds gave higher Pe and N values, indicating less axial mixing, and therefore better product homogeneity in terms of final moisture content and other quality attributes.
Synthesis and drying of absorbable gelatin foam: Absorbable gelatin foam (AGF) is used for rapidly arresting the bleeding from a lesion on parenchymal organs such as liver, kidney, lung, etc. In the present study, AGF was synthesized and dried using different drying techniques. The dried foam was analyzed for its volumetric shrinkage during drying, water intake capacity and rate, bulk density, flexural strength, softness and pore structure (SEM analysis). Freeze dried (FD) foam was found to be superior as compared to heat pump dried (HPD) foam. FD was found to be an expensive compared to HPD.
Both foam samples were found to be superior as compared to the commercially available AGF (such as Ab GelTM and CutanplastTM) in terms of water intake rate rehydration capacity and conformability.