Introduction to food processing; drying ‐ Fundamentals
Why so many dryer types?
Key criteria for classification
Criteria for dryer selection
Different dryer types
Energy related issues in drying
• Need of food processing ‐ to avoid the spoilage of foods due to various reasons; to increase shelf life; to make food products available through out the year
• The spoilage could be due to physical damage, chemical damage, microbial attack
• Various food processing methods – Freezing, canning, preserving in syrup, food irradiation, salting, vacuum packaging, dehydration
• canning and freezing – best way to retain the taste, appearance, and nutritive value of fresh food (Cost involvement)
• Drying/Dehydration – very much cost‐effective; product takes much less storage space than canned or frozen foods; Some dehydrated products have very good rehydration properties
Removal of a liquid from a solid/semisolid/liquid to produce solid product by thermal energy input causing phase change (Sometimes converts solid moisture into vapor by sublimation eg. Freeze drying with application of heat.)
Needed for the purposes of preservation and storage, reduction in cost of transportation, etc.
Most common and diverse operation with over 100 types of dryers in industrial use
Competes with distillation as the most energyintensive
Dryer – is an equipment used in removing moisture or solvents from a wet material or product.
Hygroscopic Substance – a substance that can contained bound moisture and is variable in moisture content which they posses at different times.
Weight of Moisture – amount of moisture present in the product at the start or at the end of the drying operation.
Bone Dry Weight – it is the final constant weight reached by a hygroscopic material when it is completely dried out. It is the weight of the product without the presence of moisture.
Gross Weight – it is the sum of the bone-dry weight of the product and the weight of moisture.
Moisture Content – it is the amount of moisture expressed as a percentage of the gross weight or the bone dry weight of the product.
A) Wet Basis – is the moisture content of the product in percent of the gross weight.
B) Dry Basis 0r Regain – it is the moisture content of the product in percent of the bone dry weight.
Continuous Drying – is that type of drying operation in which the material to be dried is fed to and discharge from the dryer continuously.
Batch Drying – is that type of drying operation in which the material to be dried is done in batches at definite interval of time.
CLASSIFICATION OF DRYERS
1. Direct Dryers – conduction heat transfer
2. Indirect Dryers – convection heat transfer
3. Infra-red Dryers – radiation heat transfer
1. GW = BDW + M
2. Xm = [M/GW] x 100% (wet basis)
3. Xm = [M/BDW] x 100% (dry basis or regain)
GW – gross weight
BDW – bone dry weight
M – weight of moisture
Xm – moisture content
HEAT REQUIREMENT BY THE PRODUCT
Q = Q1 + Q2 + Q3 + Q4
Q1 = (BDW)Cp(tB – tA) kg/hr
Q2 = MBCpw(tB – tA) kg/hr
Q2 = MB(hfB – hfA) kg/hr
Q3 = (MA – MB)(hvB – hfA) kg/hr = MR(hvB – hfA)
Q4 = heat loss
Q1 – sensible heat of product, KJ/hr
Q2 – sensible heat of moisture remaining in the product, KJ/hr
Q3 – heat required to evaporate and superheat moisture removed from
the product in KJ/hr
Q4 – heat losses, KJ/hr
A,B – conditions at the start or at the end of drying operation
t – temperature in C
hf – enthalpy of water at saturated liquid, KJ/kg
hv – enthalpy of vapor, KJ/kg
Cp – specific heat of the product, KJ/kg-C or KJkg-K
Cpw – specific heat of water, KJ/kg-C or KJ/kg-K