Modelling Moisture Transfer in Sorghum Stored In a Metallic Silo

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Osman, Omer Elmahi Mohammed
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An experimental study was carried out at the Faculty of Agriculture, University of Khartoum – Sudan. The main objective of the study was computer modelling of moisture contents of sorghum grain being stored in a metallic silo. The specific objectives were mathematical modelling of moisture content distribution within sorghum stored in a metallic silo, verification of the developed model by comparing the predicted results with the measured values and testing the final storage condition of the silo by comparing the grain temperatures and moisture contents with the safe storage criteria. A mathematical finite difference moisture transfer model was developed to simulate moisture content in both the radial and vertical directions in a metallic silo. The use of finite difference technique necessitated that, the grain temperatures, which were predicted with the use of Fourier’s conductive heat transfer equation had to be employed as an input data in the computer programme. Hence, in the developed moisture transfer model, both convective heat and moisture transfer were negligible. A computer programme was written using VISUAL FORTRAN V. 6.5 language and run on a PC, for simulating sorghum grain moisture contents. The computer programme predicted grain temperatures and moisture contents in the silo during the storage period using input data of initial grain temperature, initial grain moisture content, storage time, number of spatial elements in both directions and thermo-physical properties of grain and silo wall material. Other input data include: ambient air temperatures, the finite difference spatial increment in both directions, the finite difference time increment and time period over which grain moisture content was assumed to remain constant before an establishment of water vapour diffusion. To validate the model, the predicted moisture contents were compared with measured experimental data collected from sorghum of Fetarita variety stored in a metallic silo during the period of 22/4/ – 22/7/2006 i.e. 91 storage days. The experimental metallic silo was cylindrical in shape and made of galvanized zinc with 0.6 m in each of diameter and height. Initial grain temperature and moisture content were 36.5oC and 9.58% (w.b.), respectively. Ambient air and grain temperatures were recorded using thermocouples connected to a multi-channel data logger, which was configured at 24 hours intervals. Moisture content measurements were recorded at weekly intervals with calibrated moisture sensors (reethorpes) connected to a Marconi moisture meter. Under the storage conditions of the metallic silo, moisture equilibrium was unattainable and the moisture content in any part of the silo was a function of interstitial relative humidity within sorghum bulk. Therefore, the encountered moisture content variation within stored sorghum was due only to the change in the partial water vapour pressure of the interstitial air within the bulk. This established water vapour pressure besides the properties of the porous granular bed of sorghum were used to explain the moisture diffusion that took place in the metallic silo. The developed moisture transfer model predicted sorghum moisture content accurately and within the bounds of the experimental error. The standard error of estimate between measured and predicted moisture contents was 0.29 – 0.59% (w.b.). It was also found that Chung’s sorption equation was satisfactory in simulating the moisture contents within sorghum bulk stored in the metallic silo. Computer modelling could be used as a reliable, cheap and a fast technique alternative to recording moisture contents and temperatures of grains with size similar to that of sorghum and being stored in a metallic silo in order to check for the safe storage conditions of the silo for a short period of time. The time taken by the computer programme to predict grain moisture contents, during the 91 days storage period, and to give results at weekly intervals in a grid form of 91 spatial elements was 2.67 second/simulation day. No insect infestation was detected within the stored sorghum at the end of the experiment. This was due to cleanness of stored grain, short storage period and low moisture content to support such an activity.
University of khartoum