Unit Operations in Food Processing 5. Drying of solids Dr. JIA Yuanyuan email@example.com Learning on Internet Food Processing Handbook http://rapidshare.com/files/103853443/Food_Processing_Handbook.p df Unit operations in food processing http://www.nzifst.org.nz/unitoperations Website of GEA Process Engineering Inc. ---World Leaders in Drying Technology and Equipment http://www.niroinc.com/about/about_Niro_Inc.asp Dairy Processing Handbook, Chapter 17 http://www.ales2.ualberta.ca/afns/courses/nufs403/PDFs/chapter17.p df Animations provided by Prof. Singh http://rpaulsingh.com/animated%20figures/animationlisttopic.htm 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 tw Moisture t Q δ 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 Click Very short drying time; Permits drying highly heat-sensitive materials. Example: milk powder, Stationary nozzles Counterflow 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. Click Cyclone if fine particles are present Click http://www.youtube.com/watch?v=vQmfmsJpp9I&feature=related Click Vibrating Fluid-bed dryer Click The VIBRO-FLUIDIZER™ from GEA Niro http://www.niro.com/niro/cmsdoc.nsf/webdoc/webb7n3c3l Fluid-bed dryer Tray-dryers 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 较差、且产品质量不均匀。产量不大。 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 爆物料、排出蒸汽需回收的物料 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 Drier） 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 逆流式 错流式 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: http://www.gmfgouda.com/drum-dryer/info-drum-dryer.html#top 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 气流干燥器 适用场合： 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 1m 重力 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 气流干燥器 优点： 干燥时间短 热效率高，可高达60％ 易实现自动化、连续生产 缺点： 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 quality. Advantages: maintaining the structure of the food; flavor retention; higher Disadvantages: cost; for high valued foods. long operation time; small throughout; high operation Examples: processing-works-video.htm instant coffee; backpacker meals; active enzymes; Vitamins Special for space foods http://videos.howstuffworks.com/nasa/2205-how-space-food- Schematic operation of freeze dryer Control panel Trays 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. http://www.youtube.com/watch?v=Pt25GuAf7cI&feature=rel ated Click 2. Tour labs and pilot plant of Food Science and Engineering Department to see how many varieties of driers and understand their functions.