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Foreword to the first edition iii
Foreword to the second edition iv
Chapter 1: Principles of Drying 1
1.1 Introduction 1
1.2 Losses of Crops 2
1.3 Importance of Drying 3
1.4 Principles of Drying 4
Chapter 2: Moisture Contents and Equilibrium Moisture
Content Models 6
2.1 Introduction 6
2.2 Moisture Content Representation 6
2.3 Determination of Moisture Content 10
2.4 Grain Sampling 17
2.5 Equilibrium Moisture Content 18
2.6 Determination of Static Equilibrium Moisture Content 23
2.7 Static Equilibrium Moisture Content Models 27
2.8 Net Isosteric Heat of Sorption 30
Chapter 3: Psychrometry 43
3.1 Introduction 43
3.2 Psychrometric Terms 44
3.2.1 Humidity Ratio 44
3.2.2 Relative Humidity 45
3.2.3 Specific Volume 45
3.2.4 Vapour Pressure 46
3.2.5 Dry Bulb Temperature 46
3.2.6 Dew Point Temperature 46
3.2.7 Wet Bulb Temperature 46
3.2.8 Enthalpy 47
3.2.9 Adiabatic Wet Bulb Temperature 48
3.2.10 Psychrometric Wet Bulb Temperature 51
3.3 Construction of Psychrometric Chart 53
3.4 Use of Pschrometric Chart 54
3.4.1 Sensible Heating and Cooling 55
3.4.2 Heating with Humidification 56
3.4.3 Cooling with Humidification 57
3.4.4 Cooling with Dehumidification 57
3.4.5 Drying 59
3.4.6 Mixing of Air Streams 61
3.4.7 Heat Addition with Air Mixing 64
3.4.8 Drying with Recirculation 65
Chapter 4: Physical and Thermal Properties of Cereal Grains 78
4.1 Introduction 78
4.2 Structure of Cereal Grains 78
4.3 Physical Dimensions 80
4.4 1000 Grain Weight 80
4.5 Bulk Density 81
4.6 Shrinkage 81
4.7 Friction 83
4.8 Specific Heat 87
4.9 Thermal Conductivity 90
4.10 Latent Heat of Vaporization of Grain Moisture 95
4.11 Heat Transfer Coefficient of Grain Bed 99
Chapter 5: Air Flow Resistance and Fans 116
5.1 Air Flow Resistance 116
5.1.1 Non-Linear Air Flow Analysis 118
5.2 Fans 128
5.2.1 Fan Performance 131
5.2.2 Centrifugal Fan Laws 137
5.2.3 Fan Selection 137
5.2.4 Effect of Change in Fan Speed 138
5.2.5 Effect of Change in Speed and System Resistance 139
5.2.6 Fans in Series and Parallel 140
5.3 Duct Design for On-Floor Drying and Storage System 144
Chapter 6: Thin Layer Drying of Cereal Grains 150
6.1 Theory 150
6.2 Thin Layer Drying Equations 153
6.2.1 Empirical Drying Equations 154
6.2.2 Theoretical Drying Equations 155
6.2.3 Semi-theoretical Drying Equations 159
6.3 Development of Thin Layer Drying Equations 164
6.3.1 Drying Rate 167
6.4 Drying Parameters 167
6.4.1 Drying Rate Constant and Diffusion Coefficient 169
6.4.2 Dynamic Equilibrium Moisture Content 178
6.5 Finite Element Modeling of Single Kernel Drying 185
6.5.1 Finite Element Model Formulation 186
6.5.2 Finite Difference Solution in Time 192
6.5.3 Discretization of the Domain 193
Chapter 7: Deep Bed and Continuous Flow Drying 205
7.1 Introduction 205
7.2 Deep Bed Drying Models 205
7.2.1 Logarithmic Models 206
7.2.2 Partial Differential Equation Models 206
7.2.3 Comparison of Deep Bed Drying Models 208
7.3 Development of Models for Deep Bed Drying 209
7.3.1 Logarithmic Model 209
7.3.2 Partial Differential Equation Model 217
7.3.3 Method of Solution 222
7.3.4 Condensation Procedure 224
7.3.5 Sensitivity Analysis 233
7.3.6 Comparison of Simulated Drying with Experimental Results 233
7.3.7 Comparison of Direct, Indirect and Recirculating Direct Fired Drying 235
7.4 Development of Models for Continuous Flow Drying 237
7.4.1 Cross Flow Model 238
7.4.2. Fluidized Bed Drying Model 246
7.5 CFD Modeling of Fluidized Bed Drying 253
7.5.1Continuity equation 254
7.5.2 Momentum conservation equations 255
7.5.3 Energy conservation equation 256
7.5.4 User-Defined Scheme (UDS) 256
7.5.5 CFD Analysis 256
Chapter 8: Grain Drying Systems 270
8.1 Introduction 270
8.2 Solar Drying Systems 270
8.3 Batch Drying Systems 275
8.4 Continuous Flow Drying Systems 277
8.5 Safe Temperature for Drying Grain 280
8.6 Hydro-Thermal Stresses during Drying 281
8.7 Energy and Exergy Analysis 283
8.8 Neural Network Modeling 286
8.8.1 Structure of ANN model 287
8.8.2 Training of ANN model 288
8.9 Selection of Dryers 290
Chapter 9: Principles of Storage 297
9.1 Introduction 297
9.2 Principles of Storage 298
9.3 Interrelations of Physical, Chemical and Biological Variables in the Deterioration of Stored Grains 301
9.4 Computer Simulation Modelling for Stored Grain Pest Management 303
Chapter 10: Temperature and Moisture Changes during Storage 307
10.1 Introduction 307
10.2 Qualitative Analysis of Moisture Changes of Stored Grains in Cylindrical Bins 307
10.3 Temperature Changes in Stored Grains 309
10.4 Temperature Prediction 311
10.4.1 The Differential Equation of Heat Conduction in Cylindrical Co-Ordinate System 311
10.4.2 Numerical Method 313
10.5 Numerical Solution of one-dimensional heat flow 313
10.6 Numerical Solution of two-dimensional heat flow and Moisture Flow 320
10.6.1 Heat Transfer The Differential Equation of Heat Conduction in Cylindrical Co-Ordinate System 311
10.7 Simultaneous Momentum, Heat and Mass Transfer 340
10.7.1 The Energy Balance Equation 341
10.7.2 The Mass Balance Equation 341
10.7.3 The Momentum Balance Equation 342
10.4.4 Finite Difference Formulation 343
10.8 CFD Modelling of Grain Storage Systems 350
10.8.1 Continuity Equation 351
10.8.2 Momentum Conservation Equation 351
10.8.3 Energy Conservation Equation 351
10.4.4 User Defined Function 353
Chapter 11: Fungi, Insects and Other Organisms Associated with Stored Grain 358
11.1 Introduction 358
11.2 Fungi 359
11.2.1 Field Fungi 361
11.2.2 Intermediate Fungi 361
11.2.3 Storage Fungi 361
11.3 Insects 364
11.3.1 Insect Species 365
11.3.2 Grain Temperature and Moisture Content 366
11.4 Mites 367
11.5 Rodents 367
11.6 Respiration and Heating 369
11.7 Control Methods 371
Chapter 12: Design of Grain Storages 373
12.1 Introduction 373
12.2 Structural Requirements 373
12.2.1 Janssen's Equation 374
12.2.2 Rankine's Equation 377
12.2.3 Airy's Equation 379
12.3 Construction Materials 256
Chapter 13: Grain Storage Systems 394
13.1 Introduction 394
13.2 Traditional Storage Systems 395
13.3 Modern Storage Systems 395
13.3.1 Bagged Storage Systems 396
13.3.2 Silo Storage Systems 396
13.3.3 Airtight Grain Storage 398
13.3.4 Aerated Storage Systems 406
13.3.5 Low Temperature Storage System (Grain Chilling by Refrigeration) 412
13.3.6 Controlled Atmosphere Storage Systems 416
13.3.6 Damp Grain Storage Systems 420
Appendix - A Finite Difference Approximation 434
Appendix - B Gaussian Elimination Method 436
Appendix - C Finite Element Method 438
Appendix - D Computational Fluid Dynamics 441
Index
Moisture contained in a grain is an indicator of its quality and a key to safe storage and can be of two types: 'water of composition', called absorbed water, which is contained within the plant cells of which the grain kernel is composed of and adsorbed water which is present on the surface but not within the cells. The moisture content of the grains may be determined on farms, in stores and under laboratory conditions. These necessitate some standards for representation of moisture content and methods of its measurement.
Moisture content is usually expressed in per cent of moisture present in the grain, and there are two methods for expressing these percentages: (i) wet basis and (ii) dry basis.
Moisture content of a grain on a wet basis is expressed as the ratio of the weight of water present to the total weight of the grain. It is normally expressed in per cent. Moisture content on a wet basis is used for commercial designation and also universally by farmers, agriculturalists and merchants. This method of expression tends to give incorrect impression when applied to drying since both moisture content and the basis on which it is computed change as drying proceeds. For this reason moisture content on a dry basis is used in many engineering calculations and mainly used by researchers.
Moisture content on a wet basis is given by
Alternatively, moisture content on a dry basis compares the weight of the moisture present with the weight of dry matter in the grain. This can be expressed as
It may be necessary to convert moisture content from wet basis to dry basis, and vice versa. To convert moisture content from a wet basis to a dry basis, subtract each side of Equation 2.1 from 1.
This equation on simplification gives
To convert moisture content from a dry basis to a wet basis, Equation 2.2 may be rewritten as
This equation on simplification yields
2000 kg of freshly harvested paddy with a moisture content of 25% (d.b.) is dried to a moisture content of 14% (d.b.). Determine the final weight of the grain after drying.
From Equation 2.5, we can write
Here W25 + Wd = 2000 kg and Md = 0.25
Hence
Again for 12% m.c.,
Hence the final weight of the dried grain is 1792 kg.
8000 kg of paddy with a moisture content of 0.12 (d.b.) is required for a research project on grain storage. It was decided that the available freshly harvested paddy with a moisture content of 0.20 (w.b.) should be procured, and then it will be dried to a moisture content of 12% on a dry basis. How many kilograms of freshly harvested paddy are to be procured?
Here Wd + W12 = 8000 kg and Md = 0.12
Again from Equation 2.4,
For Mw = 0.20,
Again for 25% m.c. (d.b.),
Hence 8928.58 kg of freshly harvested paddy is to be procured.
Ten tonnes of rice is dried from an initial moisture content of 22.0 to 12% (w.b.) in a batch dryer using diesel fuel. Calculate (i) how much diesel is needed and (ii) cost of drying per kg. Use latent heat of vaporization of moisture = 10 MJ/kg, heating value of diesel = 42.7 MJ/l and price of diesel = Tk. 55.0/l.
The initial moisture content on a dry basis is
and the final moisture content on a dry basis is
Moisture in the grain is
and the dry matter is
The moisture removal is given by
Diesel needed is given by
Cost of drying per kg is given by
Determination of moisture content of a grain is essential to know its keeping quality. It is also important to know the moisture content during drying and storage. Price of grains depends on moisture content. Again, if the farmers sell overdried grains, they sell the dry matter of grains for the price of water. For underdried grains the farmers are offered lower prices. Also the quality of the grains will deteriorate soon during storage. These also emphasize further the need to determine the moisture content of cereal grains.
The methods of determining moisture content can be classified as (i) direct method and (ii) indirect method. Direct method involves the actual removal of moisture and its measurement. The moisture contents are expressed either on a wet basis or on a dry basis. The following are the common methods for direct determination of moisture content:
Indirect method involves the measurement of some properties related to the moisture in grains. The moisture content is expressed on a wet basis. This method is much quicker but less accurate. The instrument has to be calibrated against a primary method. The following are the common methods for determining moisture content through the measurement of some parameters of moisture contained in the grains:
Operating conditions and procedures are different for various materials. Air oven method or water oven method may be used for direct determination of moisture content, but the air oven method is commonly used for grains.
Air Oven Method, 130 ± 1°C
Water Oven or Air Oven method, 100°C
An alternative approach is to use a vacuum oven. Grain is ground and placed in the oven at 100°C and 25 mm pressure for approximately 5 h.
The grain sample should be in the oven until weight loss stops. It is practically impossible to remove all the moisture from grains without their deterioration. If the grain samples are kept too long in the oven, organic materials of the samples will be lost, and these will appear as moisture loss and give inaccurate value. So moisture content should be determined according to the standards set by the professional organization and/or government regulations.
Moisture meter employing infrared lamp is available commercially. Moisture content is measured directly by the evaporation of water from the grain sample by heating with an infrared lamp. Milling of grain sample is not essential, but this will reduce the time required for the evaporation of water from the grain sample.
This meter consists of a balance and an infrared lamp. The pan of the balance is counterbalanced by a fixed and an adjustable weight along a lever. There is also a scale calibrated in moisture content. The infrared lamp is mounted on a swivelling arm above the pan.
The procedures for measuring moisture content are as follows:
The range of moisture content that can be read is from 0 to 100%. If the sample to be tested is so wet that it cannot be milled, it should be pre-dried to reduce the time required for moisture content determination. It is interesting to note that this meter does enable the determination of high moisture contents.
In this method a sample of...
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