Trygve M. EikevikProfessor

Norwegian University of Science and Technology (NTNU)

Department of Energy and Process Engineering

E-mail: trygve.m.eikevik@ntnu.no

http://folk.ntnu.no/tme

Heat Pump Assisted Drying -Freeze Drying Technology

Introduction

Open system

2

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

Quality vs. production costs3

Hot Air

Vacuum

freeze drying

Heat Pump Drying

Quality

Pro

du

ction

Cost

High

HighLow

Low

4

Comparing different drying methods

Vacuum

Freeze driers

Spray driers

Immobilisated particles

Frozen particles Solid particles

Heat pump driers

Tunnel driers

Belt driers

Drum driers

Etc.

+60oC-40oC -15oC 0oC

5

Challenges in drying systems

Optimal product quality (colour, flavour, texture, density and shrinkage) are influenced by the dryer type and dryer conditions, temperature and relative humidity

Knowledge of product characteristics such as drying curve, water activity and properties / quality attributes

To achieve high energy efficiency, high SMER-ratio

Environmental aspects

Introduction

Open system

6

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

7

Drying of cod at the clips of west Norway

8

Traditional tunnel dryers (Hordetørke)

A

C

E B

L

Lo

Recirculation air

Discharge airInlet fresh air

Side view

Product

9

Traditional tunnel dryers (Hordetørke)

A B C

Drying air

Top view

Product

10

Hot air dryers

Max. temperature

Outdoor air

E

BB

A

CC

Lo

L

11

Influence of recirculation of air

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

1,05

1,10

0 10 20 30 40 50 60 70 80

Recirculation of air (%)

SM

ER

(kg

w/k

Wh

)

0 %

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

En

eg

y a

nd

Cap

acit

y r

ed

ucti

on

(%

)

SMER

Capacity

Energy

12

Reduction in thermal efficiency

3.500

3.550

3.600

3.650

3.700

3.750

3.800

3.850

3.900

0 10 20 30 40 50 60 70 80

Recirculation of air (%)

Th

erm

al

eff

icia

ncy,

Dh

/ DY

(kJ/k

gw)

Introduction

Open system

13

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

14

Heat pump drying - Status

HP tunnel dryersHave successfully been designed, dimensioned and installed in Norway for drying of fish products (70 industrial plants in Norway)

HP fluidized bed dryersBy using drying temperatures below and above the freezing point, the rehydration, colour, taste and bulk density of food products can be regulated. Medicine, pharmaceutical and biotechnological products can be dried without loss in biological activity (5 units in Norway, 1 very large HPD plant constructed in Hungary)

Non-adiabatic HP fluidizes bed dryersHP condenser is put in the fluidized bed to increase the drying capacity (1 unit in Norway)

15

Advantages with heat pumps in combination with driers

Product qualityDrying temperatures from -20oC to +100oC secure high

quality for heat sensitive products

Environmental aspectsRecirculation of drying gas (air or inert gas)

eliminates waste gas from the dryer. Electric driven

machinery have not emissions of CO2, SO2 or NOx

Energy aspectsEnergy consumptions for a heat pump dryer are typical

20% to 35% compared to conventional dryers

16

Fluidized bed heat pump dryer

17

Tunnel dryer with heat pump

18

Schematic layout of a heat pump fluidised bed dryer

A

CB

ReceiverExternal

condenser

Condenser Evaporator

Compressor

Expansion

valve

M

Water

Fluidized

bed

chamber

Air flow

19

Water activity, aw(product in equilibrium with the ambient air)

Wate

r acti

vit

y,

aw

Moisture content, Xwb %

0

100

100

0

A

C

B

aw =p

pw’

A), B) and C) showsdifferent types of materials/products

20

Water activity vs. relative humidity

aw =pp

pw’

φ =pa

pw’

Water activity

Relativ humidity

aw > φ Product will dry (loosing moisture/weight)

aw = φ Product is in equilibrium (is not loosing or gaining weight - stable)

aw < φ Product will gain water/moisture from air (weight is increasing)

pp: vapor pressure closeto product surface

pa: vapor pressure in freeair

pw’: saturated vapor pressure for water at t

21

General drying curve with different drying phases

B

A

A’

D

C

E

Time, t (h)

Un

bo

un

d m

ois

ture

(kg

H2O

/kg

d.m

.)AB Initial drying period

BC Period with constant drying rate

CDE Period with falling drying rate

22

General drying rate curves

Dry

ing

rate

[kg

H2O

/(s m

2)]

BA

A’

D

C

E

Unbound moisture [kg H2O/kg d.m.]

Constant

drying rate

Falling

drying rate

XC

RC

23

Change in initial freezing point with reduction of water content

Apple’s enthalpy family curves

24

Enthalpy as a function of temperature and

water content

Temperature, t (oC)

En

tha

lpy,

h (

kJ

/kg

)

Zone A

Zone B

Zone C

tif

Zone A:

Atmospheric drying – unfrozen

Zone B.

Atmospheric drying – partly frozen

Zone C:

Atmospheric drying - frozen

25

Design rules for heat pump dryers

Continuous operation

Countercurrent operation (air – product)

Inlet air temperature to the drying chamber as high as possible

Avoid over dimensioning the refrigeration capacity

Provide for the air flows over product without possibility to by-pass

Optimal SMER

26

Design of heat pump dryersLengthwise countercurrent drying

φ = crosswise

φ = lengthwise

φ = 100%

SMER = (kg H2O/kWh)COP

dh/dx

lengtwise crosswise

dh dh

dx dx

27

Design of heat pump dryersAs high inlet temperature as possible

tlow

thigh

φ = 100%

SMER = (kg H2O/kWh)COP

dh/dx

high low

dh dh

dx dx

28

Design of heat pump dryersHeat pump capacity: Qo

Qohigh Qolow φ = 100%

SMER = (kg H2O/kWh)COP

dh/dx

low highQo Qo

dh dh

dx dx

29

Design of heat pump dryersThe choice of to, tc

to,high

φ = 100%

SMER = (kg H2O/kWh)COP

dh/dx

low highto to

dh dh

dx dx

to,low

Influence of COP = Qo / W

30

Mollier diagram

Absolute humidity, x [kg/kg dry air]

Te

mp

era

ture

, t

[oC

]

d

c

b

a

dhopt

dhmax

dx

dhmin

φ = 1

ts

31

Equations

Evaporator capacity

Qo = ma * (hb - hc) [kW]

Compressor power

Wcompr = (Qo / dhre) * (dhis/ηis) [kW]

Coefficient Of Performance

COP = Qo / (Wcompr + Wfan) [-]

Specific Moisture Extraction Rate

SMER = COP / (dh/dx) [kg water/kWh]

32

SMER (40/60%, R717)

0

1

2

3

4

5

6

10 11 12 13 14 15 16 17 18 19 20Temperature difference, tb - td [oC]

SM

ER

ta = -5 oCta = 10 oCta = 25 oC

ta = 45 oCta = 75 oCOpt.SMER

33

Optimal temperature difference (R717)

8

10

12

14

16

18

20

-10 0 10 20 30 40 50 60 70 80

Air inlet temperature drying chamber, ta [oC]

Op

tim

al te

mp

.dif

f., tb

- t

d [

oC

]

30/50 %

40/60 %

50/70 %

34

Optimal SMER (R717)

0

1

2

3

4

5

6

7

-10 0 10 20 30 40 50 60 70 80

Air inlet temperature drying chamber, ta [oC]

Op

tim

al S

ME

R [

kg

/kW

h]

30/50 %

40/60 %

50/70 %

Introduction

Open system

35

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

36

Phase diagram for water

Sublim

ation

Evapora

tion

Temperature, t (oC)

-30 -20 -10 0 +10

3

0

4

2

1

Pre

ssure

, p (

mm

Hg)

5

6

7

8

Vapour phase

Solid phase

Liquid

phase

37

Freeze drying, solid structure

• Vacuum freeze drying are carried out by

first freeze the product to achieve a solid

structure and than dry at pressure lower

than the triple point. The drying will occur

by sublimation – ice crystals evaporates

directly without melting

• Only water vapor will be transported in

the product and cause no internal

transport of dissolved components like

sugar, salt etc.

• Conventional convective drying at air

temperatures above 0oC will a larger part

of the water removal be done by

evaporation from the surface of the

product. Liquid will be transported from

internal out to the outer layers of the

product. This will cause a internal transport

of dissolved components.

38

Benefits with vacuum freeze drying

Tastes as fresh

• Will retain taste and nutrition's. Only water will be removed. (some light volatiles will be lost)

Look fresh

• Will retain shape and color

Lower weight than fresh product

• Freeze drying will normally remove up to 98% of the water. That means the weight of the product will be reduced with 90%

Can be stored like fresh

• Can be stored at room temperatures

39

Retention of morphologic, biochemical and immunological properties

High viability / activity of biological components

Lower temperatures, oxygen level and operational conditions than other comparable methods

High recovery of volatile components

Retention of structure, surface area

High yield

Long shelf life

Reduced weight at storage, transport and handling

Drawbacks

Higher investment costs

Higher energy consumption

Higher maintenance costs

Benefits and drawbacks compared with traditionally conservation methods

40

Vacuum freeze drying system based on heat radiation

Movable trapdoor

Condenser

during defrosting

Melt water chamber

Vacuum pump

Active condenser

Heating plates

Product trays

Rack for stacking of trays

Introduction

Open system

41

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

Advantages with Atmospheric Freeze Drying

42

• Constant or step vice temperature program

• -15oC to +30oC

• Product quality control of

• Shrinkage

• Rehydration ability

• Color

• Taste

• Aroma

• Close to vacuum freeze drying qualities

• Peas (Industrialized)

• Corn (Industrialized)

• Vegetables, fruits, berries, herbs (Tested)

• Biological active components (Tested)

43

Actual products

Fish/meat (snacks, fast food etc.)Rehydration, colour, taste, aroma

Peas, corn (bag food, quick preparation)Rehydration, formation of cracks, colour, taste

Herbs (ethereal oils, spice)Extraction level, taste, aroma, appearance, biological

effect

Berry, fruits (breakfast cereals ingredients, ”topping”)Rehydration, colour, taste, aroma

Dairy (bioactive products, aroma products)Biological activity, rancidity, taste

44

Scrimps dried at low and high temperature

Pieces of Cod dried at –5oC and +30oC45

Drying curves - temperature programPieces of Cod (5*5*5 mm)

46

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Drying time (hour)

Wat

er c

on

ten

t (%

)

Dried at -5 oC in 1 hour

Dried at -5 oC in 3 hour

Dried at -5 oC in 5 hour

Dried at -5 oC in 10 hour

Bulk density of cod after drying47

0

0,1

0,2

0,3

0,4

-20 -10 0 10 20 30 40

Temperature in drying chamber [oC]

Bulk d

ens

ity [g/m

l]

0

0,1

0,2

0,3

0,4

0 5 10 15Drying time at -5 oC [hours]

Bulk d

ens

ity [g/m

l]

Energy consumption in a heat pump dryer operated at a continued mode drying

48

COPLT

(dh/dx)LT

tLTSMER =

ttot(( )) +*

COPHT

(dh/dx)HT

tHT

ttot(( )) *

Assumptions:LT HT

Air inlet drying temperature -5C 30C

Air inlet relative humidity 40 % 40 %

Air outlet relative humidity 80 % 80 %

Surface temperature of air cooler tdp – 5C tdp – 5C

Heat pump evaporating temperature air cooler surface temperature –

2C

air cooler surface temperature –2C

Heat pump condensing temperature air inlet drying temperature

+5C

air inlet drying temperature + 5C

Heat pump refrigerant NH3 NH3

Energy consumption in a heat pump dryer 0perated at a continued mode drying

49

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12

Drying time at -5 oC air inlet temperature, [hours]

En

erg

y c

on

su

mp

tio

n [

kW

h/k

g d

rie

d

pro

du

ct]

SMER for a heat pump dryer operated at a continued mode drying

50

0

1

2

3

4

5

0 2 4 6 8 10 12

Drying time at -5 oC air inlet temperature, [hours]

SM

ER

, [k

g w

ate

r/kW

h]

51

Production line for instant products

Product

Pure/jamCold expansion

equipment

Freezing

from cold

expansion

Heat Pump

Drying

-35oC

CO2

52

Bulk density for different potatoes and extrusion with CO2

0

50

100

150

200

Extruded beate Beate Pimpernel Mandel

Bu

lk d

en

sit

y [

g/l]

53

Standard rehydration (30 sec) for different potatoes and CO2 extrusion

0

5

10

15

Extruded beate Beate Pimpernel Mandel

RE

HY

DR

AT

ION

IN

DE

X

54

Rehydration in cold water of dried products (after 2 minutes)

0

50

100

150

200

250

300

350

400

450

Cod Meat Apples Carrot

Reh

yd

rati

on

[%

]Dried at -5 oC only

Dried at +30 oC only

Ra = (M water / M dry matter) * 100

Introduction

Open system

55

Content

Closed loop system

Vacuum freeze drying

Atmospheric freeze drying

Conclusions

Conclusions

56

New generations of high quality low density dried products can be produced by heat pumps operated in a combined drying mode at drying temperatures below/above the freezing point.

There is an optimal SMER-ratio. This gives an optimal inlet temperature difference for the air cooler

SMER values for heat pumps operated in combined modes will be in the order of 1,5 to 4,5 kg H2O/kWh depending on temperature and time at freeze drying conditions.

Thank you for your attention!

Спасибо завнимание!