Search

Food products are dried to improve their shelf-life, reduce packaging costs, lower shipping weights, enhance appearance, to attempt to encapsulate original flavor and maintain nutritional value. In recent years, increasing importance is being given for using fluidized bed (FBD) dryers to process agricultural products. Higher throughput with better quality for a range of products (having different shape, bulk density, physical and chemical properties) can be achieved in FBD drying by using medium and low grade thermal energies. The present literature review summarizes the importance of FBD drying in combination with hybrid FBD drying techniques in moisture reduction of various agricultural products. The impact of various operating parameters on the product quality, color, texture and resultant nutritional value is discussed, with possible adaption of multi-effect systems. [ABSTRACT FROM AUTHOR]

This work presents a mixed integer nonlinear programming (MINLP) formulation for the design of energy-efficient multistage adsorption dryers within constraints on product temperature and moisture content. Apart from optimizing temperatures and flows, the aim is to select the most efficient adsorbent per stage and product to air flow configuration. Superstructure models consisting of commonly used adsorbents such as zeolite, alumina, and silica-gel are developed and optimized for a two-stage, low-temperature, adsorption drying system. Results show that the optimal configuration is a hybrid system with zeolite as the first-stage adsorbent and silica-gel as the second-stage adsorbent in counter-current flow between drying air and product. A specific energy consumption of 2,275 kJ/kg is achieved, which reduces to 1,730 kJ/kg with heat recovery by a heat exchanger. Compared to a conventional two-stage dryer at the same drying temperature, this represents a 59% reduction in energy consumption. The optimal system ensures the exhaust air temperature of the first-stage regenerator is high enough to regenerate the second-stage adsorbent so no utility energy is spent in the second stage. A higher second-stage adsorbent wheel speed favors energy performance as it becomes optimized for energy recovery while the first is optimized for dehumidification. Although this work considers three candidate adsorbents in a two-stage system, the same reasoning can be applied to systems with more stages and adsorbents. The developed superstructure optimization methodology can, by extension, be applied to optimize multistage hybrid drying systems in general for any objective. [ABSTRACT FROM PUBLISHER]