microsoft powerpoint 4 5 drying chapter

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					Unit Operations in Food Processing

5. Drying of solids
Dr. JIA Yuanyuan

Learning on Internet
Food Processing Handbook df Unit operations in food processing Website of GEA Process Engineering Inc. ---World Leaders in Drying Technology and Equipment Dairy Processing Handbook, Chapter 17 df Animations provided by Prof. Singh

General principles
Removing relatively small amount of water in solid foods to an acceptably low value by heat .
Mechanical separations First Drying Preservation Next Cheaper. To reduce water as much as possible before drying Energy consumptive

in conjunction with other processing The earliest few techniques for preservation foods in human history

Humidity of air
Wet air = dry air + humidity 1. Steam partial pressure in air P=pa+pW Partial pressure of dry air For saturated air, pW =ps pw 2. Humidity H = 0.622 × P − pw pw 3. Relative humidity ϕ= ps For saturated air: ps ϕ = 1, H s = 0.622 × P − ps For unsaturated air: ϕ<1 ϕ↓, drying power↑ ϕ =1,no drying power
Steam partial pressure in air

A combination of heat transfer and mass transfer Heat transfer by

t Q

convection, conduction or irradiation Driving force: t-tw Q=αA(t-tw)

ps, Hw

N pw, H

Opposite direction

Mass transfer Driving force: ps-pw or Hw-H N=kHA(HW-H)

Phase equilibrium for moist solids in air
Unbound water Water content on a dry basis X, percent

Free moisture

This curve is nearly independent of temperature. X: mass of water per unit mass of bone-dry solid. w: mass of water per unit mass of wet solid. X=w/(100-w)
Bound water

Equilibrium Equilibrium moisture moisture

The difference between the total water content of the solid and X*

X*: the content of
water cannot be removed by the air has a definite relative humidity

Relative humidity H, percent

Unbound water Water content on a dry basis X, percent

Free moisture

Unbound water exerts its full vapor pressure and largely held in the voids of the solid.

Equilibrium moisture

Bound water

Relative humidity H, percent

Bound water exerts a vapor pressure less than that of liquid water at the same temperature. Water in fine capillaries; in cell or fiber walls.

Continuous air dryer
Blower Preheater Steam Wet solid Hot air

Convective drying Cross-circulation

Dryer Exhaust air

Air Condensate Dry product Carrier of heat and moisture One-through. It can also be circulated partially to save energy

Drying rates under constant drying conditions
The temperature, humidity, velocity, and direction of flow of the air across the drying surface are constant.

Equilibrium moisture

Critical moisture content is not only a property of material. It also varies with the thickness and drying rate.

Drying time under constant drying conditions
θ = θ1 + θ 2
Drying time in falling rate period Governed by diffusion of internal moisture to the surface.

Drying time in constant rate period Governed by the vaporization of surface moisture. Therefore, higher temperature, faster air velocity and lower humidity lead to higher drying rate. Decreasing the thickness of the material gives a longer critical moisture content.

Drying equipment
Spray dryers---for slurries

Nozzle atomizes the liquid into tiny drops. The ‘heart’ of the process


Very short drying time; Permits drying highly heat-sensitive materials. Example: milk powder,

Stationary nozzles


Nozzle discharging in the direction of the air flow Spray dryers often combine with cyclones to recover the powder in exhaust air NEXT PAGE

Rotating disc for atomizing milk

Milk powder manufacture

Fluid-bed dryers: for powders, granules, agglomerates,
and pellets between 50 microns and 5 mm.

Cyclone if fine particles
are present Click


Vibrating Fluid-bed dryer


Fluid-bed dryer

Click Useful when the production rate is small. Expensive to operate because of labor cost. Maybe operated under vacuum.

Many smaller try dryers in our labs.

厢式干燥器(Disc Type Drier) 小 型 的 称 为 烘 箱 , 大 型 的 称 为 烘 适用场合:任何形状的物料 优点: 对物料的适应性强。 缺点: 物料得不到分散,干燥速率低,热利用率

房,是典型的常压、间歇式、对流 干燥设备。

耙式真空干燥器: 属减压、间歇、导热式 适用场合: 热敏性物料、易产生粉末的物料、易

优点: 干燥温度不致过高 缺点: 整套设备均需密封,不易。

洞 道 式 干 燥 器 ( Series Flow

Through Rotation 连续或半连续操作 适用场合:处理量大、 干燥时间长的物料 优点和缺点同烘箱



Drum-dryers: conductive heating
Central feed Knife Steam heated drum Conveyor Effective with dilute, concentrated solutions and moderately heavy slurries, such as high heat milk powder or baby foods.

More details:

A 100% closed system

气流干燥器(pneumatic conveying dryer)
连续、对流式 干燥管----长度一般10~20m

脉冲式干燥管---颗粒变速运动, 强化了传热


晶体和小颗粒物料,尤其是热敏 性、易氧化、不宜粉碎的物料 常用的气速约为10~20/s以上, 故物料停留时间0.5秒~几秒。 浮力 曳力



最有效的干燥段:干燥速率较快 传热温度差较大 空气湿度较小 颗粒处在加速阶段 与气流的相对速度较大


干燥时间短 热效率高,可高达60% 易实现自动化、连续生产

系统的流动阻力大 物料磨损大 要求的厂房高 对除尘系统要求高

Freeze-drying or lyophilization: Principles Moisture removed
from ice directly to the vapor state by sublimation

1Torr(托)=1mmHg=133 Pa


Advantages: maintaining the structure of the food; flavor retention; higher Disadvantages:
cost; for high valued foods. long operation time; small throughout; high operation



instant coffee; backpacker meals; active enzymes; Vitamins Special for space foods

Schematic operation of freeze dryer

Control panel


Necessity of cold trap
1g water Room temperature Atmospheric pressure 100Pa,<-20℃ Volume 1 ml 1,200,000 ml
Vacuum pump Cold trap

Industrial Freeze dryer

Freeze dried fruit and vegetable

Student activities
1. Watch the video. Try to tell where is the spray dryer and where are cyclones and blowers. ated

2. Tour labs and pilot plant of Food Science and Engineering Department to see how many varieties of driers and understand their functions.

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