11b Evaporation 2

Mass Balance

Heat and Mass Transfer !

Feed In = Product Out + Vapor Out

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mf  = mp + m v H2O in Feed = H2O in Product + H2O in Vapor

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m f  (1 - xf ) = m p (1-xp) + m v !

Solids In = Solids Out . . m f  xf  = mp xp

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mi = mass flow rate of i (kg/s)

Heat Balance: an easier approach

Heat Balance

Heat Given by Condensing Steam

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xi = wt fraction of i

Heat to Raise + Feed Temperature

=

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q = ms" Hv,s = m f  cp (Tb - Tf ) + m

Heat to Boil Off Vapor

˙  f  H  f  + m ˙  H  ˙ v H v1 + m ˙  p H  p1 + m ˙  H  m m s vs = s cs  H  f  = c pf  ( T  f 

v" Hv,p

Heat Transfer Across Evaporator Wall:

" 0°C )

 H  p1 = c pp (T 1 " 0°C )

q = UA(Ts - Tb)

Use Hvs and Hcs at pressure/temperature of the steam chest steam Use Hv1 at the temperature of the boiling liquid

Steam Economy !

Improving Evaporator Performance

Used to express operating performance of an evaporator

Steam Economy =

Mass rate of water vapor produced from product Mass rate of steam consumed

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=

mv . ms

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Vapor Recompression: vapor leaving the product is compressed, thus raising its temperature. Adiabatic compression gives superheated steam at high pressure. Recycled vapors can be used as a heating medium

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Atlas-Stord

Multiple-Effect Evaporator !

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Several effects are connected in sequence. Vapor from 1 effect is used as heating medium for next. Each stage must have a lower Tb; sucessively lower pressures required to maintain (Ts-Tb) for heat transfer. Trade-off between savings in energy costs and costs of equipment.

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Forward Feed Systems: concentrated product from first effect is pumped to the second effect to be further concentrated, then on to subsequent effects

 Reverse Feed Systems: product from last effect is pumped to next to last effect, and

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Forward vs Reverse Feed

Advantages Forward Feed

Reverse Feed

Disadvantages

Inexpensive, no feed pumps between effects, lower T=less heat damage to more viscous product

Reduced ht transfer rates as product becomes more viscous, rate of evaporation falls with each effect

Best quality steam used on most difficult product to concentrate, better economy and heat transfer

Intersate pumps required, higher risk of heat damage to viscous product as liquor moves more slowly over hot surfaces

Batch-Type Pan Evaporator

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product is heated in spherical vessel in open atmosphere or connected to a condenser/ vacuum system. Residence time long due to small surface area for heating Heating usually by natural convection

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Tubular heat exchangers are better … !

Passing liquid through many tubes increases the area for heat transfer

Natural Circulation Evaporators

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Short tubes are arranged vertically in the steam chest. Heated product rises by natural convection through the tubes Concentrated liquid moves through an overflow vessel, then falls back to base of tubes

Rising Film Evaporator ! !

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Low-viscosity liquid boils inside long vertical tubes. Vapors form near bottom and cause liquid to rise and form a thin film that moves quickly up the tubes

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High heat transfer rates achieved

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Product not usually recirculated

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Need Ts - Tb > 14°C

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Residence time around 3-4 minutes

To Condenser !

Steam

Tubes:  15m x 50mm ID

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Product Out

Swenson Technology

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http://www.compevaporators.com

Wellman Process Engineering Limited 

Falling Film Evaporator Product Feed !

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Thin liquid film falls by gravity along walls of the tube

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Can incorporate more effects than rising film (10 or more)

Steam

Vacuum

Can handle more viscous materials

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Residence time 20-30 seconds

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Good for heat sensitive juices

Condensate

Product Out

Rising/Falling Film Evaporator !

Product is concentrated in rising film section, then falls as a film through subsequent section

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Forced Circulation Evaporator !

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Vapor Out !

Liquid head prevents boiling at heating surface

Concentrated Product

Low temperature rise across calandria

Dilute Product Inlet Circulating Pump

Forced Circulation Evaporators (www.niroinc.com)

 A: Product

1: Calandria

B: Vapor

2: Separator (Flash Cooler)

Liquid circulated at high rates (2-6 m/s) through heat exchanger Hydrostatic head over tubes keeps pressure lower in separator Boiling not occur in calandria; l iquid flashes in separator to from vapor

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Agitated Film Evaporator !

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Feed

Steam

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Rotor

Condensate

Concentrated Product

Vapor

Wiper blades spread viscous foods on the inside of the heating surface Agitation increases rate of heat transfer, inhibits fouling