Drying Technologies

7/30/2019 Drying Technologies

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Vacuum Drying of foods Methods of water removal

to obtain final products of thehigh quality

byreducing air pressure which results in drying at a

lower temperature than is

required at full pressure.


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States of Water


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Industry has grouped pressures

below atmospheric level into

Rough vacuum

is used in about 90% of the chemical,petrochemical, and other processingindustries.

vacuum distillation,

filtration,

crystallization,

reaction,

drying, others.

Medium vacuum

molten metals degassing, moleculesdistillation, freeze drying, and others.High and ultra-high vacuum thin films, research, and space

simulation.(Power,2005)


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Heat Transfer Mechanismsat Low Pressure

fsh TThq

xTkqC

1/1/121

44

21

ss

C

TTq

Convection

Conduction

Radiation

total pressure decreases, heat transfer coefficient (h) decreases

h : negligible, under high vacuum,.

k :lowest value at high vacuum

total pressure decreases, thermal conductivity (k) decreases


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Heat Transfer Mechanismsat Low Pressure

Heat transfer ispoorly transferredunder vacuum.

Heat is generallytransferred to the

product byconduction andsometimes byradiation.

Convection is raresince very few fluid

molecules areavailable undervacuum.


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Ejectors : Vacuum Pump


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Problem of Vacuum Drying Main problem of drying processes under vacuum .

The high operating and maintenance costs

The long drying times under continuous vacuumincrease theirenergy consumption enormously and

make the process considerably more expensive as

compared to drying at atmospheric pressure.

(Ratti, 2008)


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Advantages of EjectorsMechanical Vacuum Pump Ejector

energyconsumption very high lowmaintenance-cost very high Low(No moving part,

Easy to operate)heat convection exhaust air affects its

surrounding of theproduction machines

no heat convection

Space-requirement much more space LessEasy to Install

oil-requirement oil-lubricated, oil separator oil-freeefficency expensive energy high efficency


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Complete system of an ejector

5

6

1

4

2

3

1 Compressed fluid

2 Nozzle3 Suction pipe4 Vacuum connection5 Inlet filter6 Exhaust fluid


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Diffuser and Nozzle (Subsonic Flow)A diffuser

converts high speed, low pressure flowto low speed, high pressure flow

A nozzleconverts high pressure, low speed flow

to low pressure, high speed flow


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Total energy of High-speed flow

Common Assumptions No work or heat transfer

Neglect changes in pe

Energy Balance: Crossing out terms assumed 0

2 2

C CIN OUTIN OUT

h h2

h ke h kg 2g

e

v v

INq

0

INw

0

h ke pe 0

IN

OUTq0

OUTw0

h ke pe 0OUT


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Basic design of an ejector


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Mathematical model The main steps used to establish mathematical

modelsA. assumption proposalB. deriving the governing equations

C. providing auxiliary relation to close the governingequations

D. adopting proper mathematic approach to solve themodel


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A: Assumption proposal1. The inner wall of the ejector is adiabatic.2. The flow inside the ejector is steady and isentropic.3. The primary fluid and secondary fluid are supplied to

the ejector at zero velocity.4. The velocity at the ejectoroutlet is neglected.5. The two fluids begin to mix with a uniform pressure

at the mixing section.


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B: Governing equations


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C: Auxiliary relation1. Boundary conditions:

pressures at inlet and exit of the ejector.

the mass flow rates or the velocities of the primary andsecondary fluids

2. Initial conditions: expansion ratio entrainment ratio

cross section area of the constant-area throat tube Auxiliary relations:

Definition of Mach number

Definition of sonic velocity


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C: Auxiliary relation (Cont.)

3. Auxiliary relations: Based on isentropic assumption, the following gas

dynamic equations are frequently used in thethermodynamic models:


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C: Auxiliary relation (Cont.)

3. Auxiliary relations: (Cont.)


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D: Mathematical Model

Thermodynamicmodel

Single-phase flow

model

Constant-pressure

mixing model

Keenan and Neumann (1942)

Munday and Bagster (1977)

Eames et al.(1995)

Constant-area

mixing model

Keenan and Neumann (1942)

Grazzini and Mariani (1998)

Yapici and Ersoy (2005)

Two-phaseflow model

Sherif et al. (2000)Beithou and Aybar (2000)

Empirical/semi-empirical

model

Huang et al. (1999)Cizungu et al. (2005)Balamurugan et al. (2007)Dessouky et al. (2002)


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Thermodynamic model the model based on thermodynamic analysis is the simplest

solve the equations in one dimension

the isentropic relations and some gas dynamic equations are used to assist in the description of the relationship between

the temperature pressure enthalpy Mach number velocity

Single-phase flow assume that the inlet fluid is superheated and no phase change occurring in the ejector.

Two-phase flow ejector works as a pump

using high pressure liquid to entrain gas

or higher pressure vapor to entrain liquid


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Thermodynamic modelTwo-phase flow model

Beithou and Aybar (2000) employed a one-dimensional control volume method to develop a local

mathematical model

performance prediction of the steam-driven jet pump.

applied the conservation of mass and momentum equation as well as theentropy equation As the fluid is uncompressible,

the Bernoulli equation can be used to calculate the entrained water velocity at thenozzle.

at the mixing section, the steady-state energy equation was used to calculate thevelocity of the mixture with the assumption of

adiabatic mixing process no potential energy change.

Take the head loss into account, the Bernoulli equation was used in thediffuser to find out the pressure and velocity profiles along it.


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Thermodynamic modelTwo-phase flow model

section By usingBernoulli equation between the water tank and the exit of thewater nozzleContinuity equation

WaternozzleMixing

Diffuser


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Effect of design and operatingparameter on suction pressure


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Relationship betweenair (primary) mass flow rate and pressure drop

(Balamurugan et al. 2006)


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Relationship between

water temperature and lowest suction pressure

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Variation of the air entrainment rate

with the pressure difference

between the air entry and the throat exit:

(Kandakureet al. 2005)


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Schematic geometry of an ejector


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Effect of projection ration (LTN/DT) on the rate of entrainment


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Effect of area ratio ((DS2-DN

2)/DN2) on the rate of

entrainment for different values of projection ratios


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Effect of angle on converging section ()on rate of entrainment


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Conclusion Effect of design and operating parameter on suction pressure

Parameter Value of parameter Suction pressureLTN/DT

((DS2-DN

2)/DN2)

mT

Optimal valueOptimal valueOptimal value


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ConclusionSteam ejector Water ejector

Mostly use forVacuum ejector

Expensive Cheap

Design : GEOMETRY

Drying Technologies

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