Studies on Drying of Sorghum Seeds in a Fluidized Bed Dryer

Abstract

Sorghum grain ranks fifth in cereals global production. Sorghum is used for human nutrition all over the world. Drying of sorghum is necessary to avoid germination and fungal infection. Fluidized bed drying technology is used to dry the grains in the present study. This paper dealing with the drying kinetics of sorghum seeds under the operating parameters such as temperature, flow rate of the drying medium, and solids holdup. The drying rate was found to increase significantly with increase in temperature and marginally with flow rate of the heating medium and to decrease with increase in solids holdup. The drying rate was compared with various simple exponential time decay models and the model parameters were evaluated. The experimental data were also modeled using Fick’s diffusion equation. And also found the bed voidage. INTRODUCTION Sorghum is used for human nutrition all over the world. It is a major crop for many poor farmers, especially in Africa. There are several types of sorghum, including grain sorghums, grass sorghums (for pasture and hay), sweet sorghums (for syrups), and Broomcorn. Grain sorghum requires less water than corn, so is likely to be grown as a replacement to corn and produce better yields than corn in hotter and drier areas, such as the Southern US, Africa. Sorghum is also considered to be a significant crop for animal feeds. Sorghum fibers are used in wallboard, fences, biodegradable packaging materials, and solvents. Dried stalks are used for cooking fuel, and dye can be extracted from the plant to color leather. A more recent use of sorghum is for ethanol. By-products from ethanol production, such as sorghum-DDGS (distillers dried grains with soluble), are also finding a place in the market. One of the most important factors in maintaining the quality of seed is the moisture content. Too high moisture leads to deterioration in seed quality as a result of the microorganism growth and premature germination [1]. Mature grain sorghum in the field contains about 30% moisture. At moisture levels higher than 25%, the seeds are too soft to withstand the threshing action. The ideal moisture content for harvesting grain sorghum is about 20%. Harvested seeds have to be dried immediately, otherwise, because of internal heating; germination will be reduced when moisture content is high. Seeds are to be dried to a moisture content of 10-12%. Drying is a method of food preservation that works by removing water from the food, which inhibits the growth of microorganisms. [2, 3] Drying of solids is generally understood to follow two distinct drying zones known as constant rate period and falling rate period which meet at critical moisture content. The critical moisture content varies with operating parameters and with the type of drying equipment. The constant rate period is understood to have maximum drying rate. As considering the falling rate period, the rate of diffusion of moisture to surface of solids becomes the limiting factor for moisture transfer. The extent of drying zones is decided based on the type of material [4]. Fluidized beds are widely used for the drying of granular solids such as grains, fertilizers, chemicals, pharmaceuticals and minerals. This technique offers advantages such as the high heat capacity of the bed, improved rates of heat and mass transfer between the phases and ease in handling and transport of fluidized solids. The drying rate in the fluidized bed is strongly influenced by the characteristics of the material and the conditions of fluidization. [5]. Knowledge of drying kinetics is essential for sizing the dryer as well as for choosing the optimal drying conditions. Numerous mathematical models have been developed to estimate the drying kinetics. These range from analytical models, often built by regression of experimental data. In general, the drying rate in constant rate period in fluidized bed drying is modeled using (a) simple empirical correlation relating drying rate to the influencing parameters or utilizing heat and mass transfer coefficient between solids and gas in fluidized bed (or) (b) Using mass transfer models, assuming the bed to make of bubble phase, emulsion phase, and a dense phase with the exchange of mass and energy between these phases. [6]. The objective of the present study is to experimentally investigate the drying kinetics of sorghum grains in a fluidized bed dryer with respect to the operating parameters such as the temperature, flow rate of the drying medium, and the solids hold up. Although the effect of operating parameters on the drying rates are well known and the influencing parameters to respond in similar fashion qualitatively, the drying kinetics can vary quantitatively depending on the nature of the material and the drying conditions. It is further attempted to verify the compatibility of experimental drying kinetics with various simple models given in the literature, and with complex models such as Fick’s diffusion equation. A good fluidization behavior in terms of perfect mixing of the bed material was observed visibly. This was substantiated with low fluctuation in the bed pressure drop, which is an indication for smooth fluidization without formation of slugs. The minimum fluidization velocity was not found to vary with the temperature within the range of temperatures covered in the present study. [7] MATERIALS AND METHODS Sorghum seeds used in the experiment were collected from local super market. The drying experiments were conducted using a fluidized bed dryer column with the internal diameter of 0.05 m and a height of 0.5 m. The air distributor was 2 mm thick and with 2 mm perforations having 13% free area. A fine wire mesh was spot welded over the distributor plate to arrest the flow of solids from the fluidized bed in to the air chamber. Air from the blower was heated and fed to the fluidization column through the air chamber. Volumetric discharge capacity of air blower is 200 m/h. The electrical heater consisted of a multiple heating element each of 2KW rating. A temperature controller, provided to the air chamber, facilitated control of air temperature within+_5C of the set temperature for the entire operating range of 313k to 333k. Air flow was measured using a calibrated orifice meter. Table 1 shows the physical characters of sorghum grain as well as the experimental conditions covered in the present study. A known quantity of 0.125 kg sorghum seeds with initial moisture content of 20% (db) was taken in a fluidized bed, and air at the desired rate was introduced into the column. As fluidization continued, the dry bulb and wet bulb temperatures of the inlet and out let of the fluidized bed column were noted for each interval time of 1min. For these temperatures humidity was calculated using psychometric chart. Moisture ratio can be calculated using humidity values. Initially the minimum bed height at minimum fluidization velocity (Hmf) was noted. After maintaining the steady velocity and temperature, initial bed height was increased during the drying process. TABLE 1: Characteristics of the Material and the Range of Experimental Parameters Covered in the Present Study Characteristics Range of parameters Name of the material Shape of material Sphericity Size, dp(mm) Particle density(kg/m) Minimumfluidization velocity,Umf(m/s) Temperature of fluidizing air(C) Fluidizing air velocity (m/s) Solids hold up (kg) Sorghum (Sorghum Bicolor) Non Spherical 0.901 3.34 820 0.647 40,50,60 0.85,1.06,1.27 0.125,0.15,0.175 After getting steady temperatures in the outgoing air and at the final stage of drying, the increase in bed height (H-Hmf) was noted. The bed voidage at minimum fluidization velocity was given in the table. From the above equation at particular drying medium velocity and temperature, the bed voidage was calculated. Drying Kinetics The moisture ratio is calculated as