Energy efficient combined superheated steam dryer/MED

Abstract

In a world of depleting fossil fuel resources, energy conscious research activities have become relevant for all industrial applications. Drying as an energy intensive process, is one example when conventional air drying is replaced by superheated steam (SHS) drying. This work presents a novel arrangement which reduces the SHS drying energy consumption. The analysis is applied to a milk powder producing plant that includes both concentration and drying processes. These are powered by a gas turbine of the simple Brayton cycle type. The raw feed is preheated before being concentrated in a single effect evaporator driven by a mechanical vapour compressor (MVC). The drying SHS in the dryer operates in a recycled mode while the evaporated water is purged and compressed in a two stage MVC equipped with an intercooler and aftercooler. The purged compressed steam is used to boost the SHS temperature from the circulating blower exit up to the required inlet temperature of the dryer. Use is made of the waste energy of the gas turbine exhaust. The balance of the recovered energy drives a multiple effect desalination (MED) unit to produce fresh water. The analysis coveres a range of dryer inlet temperatures of 150–275°C. The preheater surface area and feed concentrator shaft power per ton of powder (TP)/h are 30.4 m 2 and 34.5 kW, respectively. For the dryer, the ratio of SHS COP t/air COP t ranges from 2.51-1.94 for 150–275°C inlet temperature respectively. The overall power required/TP h is 502–423 kW, and the fresh water produced varied from 10.82–12.51 gallons/kWh for a 150–275°C inlet temperature respectively. The water produced can satisfy the requirements of about 40% of the milking heads.