Introduction to Food Engineering Evaporation Introduction to Food Engineering

Evaporation Removal of water from diluted liquid foods to obtain concentrated products. Microbiological stability Reduce transportation costs, storage Evaporator Heat exchanger in large chamber Product under vacuum

Evaporator Single-effect evaporator Multiple-effect evaporator Vapor discarded Multiple-effect evaporator Vapor reused as heating medium

Boiling-Point Elevation

Duhring’s Rule Linear relationship between boiling-point temperature of solution and boiling point temperature of water at the same pressure.

Example Determine initial and final boiling point of a liquid food. The pressure in evaporator is 20 kPa. The product is being concentrated from 5 % to 25 % solids concentration. Boiling pt. of water from steam table at 20 kPa = 60 C (333 K)

From Duhring’s chart Initial 60 C Final 64 C 333 K 337 K

Types of Evaporator 1. Batch-type pan evaporator

Heat transfer per unit volume is small => long residence time, limit capacities

Types of Evaporator 2. Natural circulation evaporators 1 – 2 m vertical tubes inside steam chest

Types of Evaporator 3. Rising-film evaporator 10 – 15 m vertical tubes Film of liquid move upward Need 14 C difference between heating medium and product

Types of Evaporator Falling-film evaporator Thin liquid film move downward Distribution of liquid in uniform film by spray nozzles Handle more viscous liquids than rising-film Less residence time

Types of Evaporator 5. Rising/falling-film evaporator 6. Forced-circulation evaporator Use pump to maintain high circulation rates 7. Agitated thin-film evaporator Very viscous fluid foods Feed is spread on heating surface by wiper blades

Design of a single-effect evaporator

Mass Balance Flow Solids

Enthalpy Balance kJ/kg feed steam vapor product condensate Hv (Ts), Hv (T1) , Hc (Ts) from steam table

Rate of heat transfer q = rate of heat transfer (W) U = overall heat transfer coefficient (W/m2K) A = area, m2

Rate of heat transfer U decreases as product becomes concentrated. increase resistance of heat transfer Boiling point elevation But constant U is used -> overdesign

Steam Economy Ratio-rate of mass of water vapor produced per unit of steam consumed Typically -> 1

Example Apple juice is being concentrated. At steady state, feed = 0.67 kg/s. Concentration of the juice = 11 % total solids. The juice is concentrated to 75 % TS. Specific heats of diluted feed and concentrate are 3.9 and 2.3 kJ/kgC. The steam pressure is measured to be 304.42 kPa. Inlet feed temp is 43.3 C. The product inside the evaporator boils at 62.2 C. Assume U = 943 W/m2C, negligible boiling-point elevation. Calculate mass flow rate of concentrate product, steam requirements, steam economy and heat-transfer area.

Design of multiple-effect evap

(7.13) (7.14) (7.15) (7.16)