Warunee Tia

Drying Kinetics and Quality of Shrimp Undergoing Different Two-Stage Drying Processes

Abstract An innovative two-stage drying concept is presented in this article. The work considered drying of shrimp using a superheated steam dryer followed by a heat pump (SSD/HPD) or a hot air dryer (SSD/AD) both from drying kinetics and dried product quality points of view. The experiments were performed using the first-stage superheated steam drying temperature of 140°C while the second-stage heat pump drying (or hot air drying) was performed at 50°C. The moisture content of shrimp at the end of the superheated steam drying stage was varied between 30 and 40% (w.b.). The effect of tempering between SSD/HPD was also investigated. Shrinkage, color, rehydration behavior, texture (toughness and hardness), and microstructure of dried shrimp were measured. The results showed that SSD/HPD dried shrimp had much lower degree of shrinkage, higher degree of rehydration, better color, less tough and softer, and more porous than single-stage SSD dried shrimp. It was also found that SSD/AD gave redder shrimp compared to shrimp dried in a single-stage superheated steam dryer. No improvement in terms of shrinkage and rehydration behavior was observed, however.

A Superheated-Steam Fluidized-Bed Dryer for Parboiled Rice: Testing of a Pilot-Scale and Mathematical Model Development

This article describes the testing of a pilot-scale superheated-steam fluidized-bed dryer for parboiled rice along with development of a mathematical model for predicting the changes in temperature of steam and moisture content of parboiled rice during drying. Based on the obtained results, it was found that the superficial velocity of steam from 1.3 to 1.5 times of the minimum fluidization velocity had no significant effect on the drying rates of rice. The energy consumption for reducing the moisture content of paddy from 0.43 to 0.22 kg/kg dry basis was approximately 7.2 MJ/kg water evaporated. Drying temperature caused the appreciable change of parboiled rice qualities as characterized by water adsorption, whiteness and pasting viscosities, white belly, and hardness. Soaking paddy at a temperature of 70°C for 7–8 h before drying was sufficiently enough for producing parboiled rice, with no white belly. The gelatinization of starch during drying resulted in higher head rice yield of the product as compared to that of raw paddy.

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

ABSTRACT A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.

INDUSTRIAL-SCALE PROTOTYPE OF CONTINUOUS SPOUTED BED PADDY DRYER

An industrial-scale prototype of spouted bed dryer with a capacity of around 3500 kg/h was constructed and tested. The prototype is shown to have a desirable feature of a spouted bed as well as the capability of continuous drying and offering consistent results throughout the testing period. Experimental results show that the prototype performs well in reducing the moisture content of the paddy and yields high product quality in terms of the milling quality. The high temperatures up to 130–160°C were applied to dry paddy from various initial moisture contents to the range of 14–25%, dry basis without significant change in quality. Thermal energy consumption, in the range of 3.1–3.8 MJ/kg water, is comparable with other commercial dryers.

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.

Simulation of a mixed air and superheated steam drying system

Numerical simulation of a cabinet-type superheated steam drying system was conducted and validated experimentally using shrimp as the test material. The experimental and simulated drying temperatures used were in the range of 140-160°C while the volume flow rate of steam of 1.04 m 3 /s was used throughout. The initial masses of the drying product were 50.7 and 31.2kg, with initial moisture contents of 4.0 kg/kg and 3.61 kg/kg dry basis, respectively; the final moisture content of the dried product was 0.25 kg/kg dry basis. The simulated moisture content of shrimp, the drying medium temperature at the outlet of the dryer, and the performance of the dryer in terms of drying time, drying rate, specific energy consumption, and drying efficiency, were verified using available experimental data. The results showed that the simulated results agreed well with the experimental results. Based on the present study, the desirable operating conditions were also evaluated for the dryer for shrimp by considering the optimized performance and good product quality.

EXPERIMENTAL INVESTIGATION OF TWO-STAGE SUPERHEATED STEAM AND HEAT PUMP DRYING OF SHRIMP

The present work proposed the two-stage drying technique concerning on the drying of shrimp using superheated steam dryer followed by heat pump dryer (SSD/HPD) at different drying conditions. The effects of drying temperatures and intermediate moisture contents (IMC) on drying characteristics and dried product quality were investigated. Shrinkage, color and texture of dried shrimp were determined. The experiments were carried out in the two stages of SSD/HPD. The first-stage superheated steam drying was performed at the drying temperatures of 160C and 180C while the second-stage heat pump drying was operated at 50C. The moisture content of shrimp at the end of the superheated steam drying stage (IMC) was varied between 30% and 40% (w.b.). For the comparison, purely SSD (superheated steam drying for the whole drying process) was also performed at the same drying steam temperature as used in the first-stage SSD. As for the effects of drying conditions, the results showed that higher drying steam temperature and higher IMC yielded better product quality only in terms of color. In addition, it was found that SSD/HPD dried shrimp had much lower degree of shrinkage, better color, softer and less tough than purely SSD dried shrimp.