Spray Drying of Tomato Pulp in Dehumidified Air

8/13/2019 Spray Drying of Tomato Pulp in Dehumidified Air

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Spray drying of tomato pulp in dehumidified air:I. The effect on product recovery

Athanasia M. Goula, Konstantinos G. Adamopoulos *

Department of Chemical Engineering, School of Engineering, Laboratory of Food Process Engineering, Aristotle University of Thessaloniki,

541 24 University Campus, Thessaloniki, Greece

Received 14 October 2003; accepted 23 February 2004

Abstract

This work investigates the performance of a modified spray dryer for tomato powder preparation by spray drying of tomatopulp. A pilot scale spray dryer (Buchi, B-191) with cocurrent operation and a two-fluid nozzle atomizer was employed for the spray

drying process. The modification made to the original design consisted in connecting the spray dryer inlet air intake to an absorption

air dryer. Samples of tomato pulp with a 14% constant total solids concentration were used. Sixty-four different experiments were

conducted keeping constant the feed rate, the feed temperature and the atomizer pressure, and varying the compressed air flow rate,

the flow rate of drying rate and the air inlet temperature.

Data for the residue remaining on the walls and for the product collected in the receiving vessel were gathered. Analysis of

experimental data yielded correlations between product recovery and the variable operating conditions. The modified spray drying

system was proved advantageous over the standard laboratory spray dryer. Preliminary air dehumidification reduced residue

accumulation, allowing the product to be dried at lower air outlet temperatures.

2004 Elsevier Ltd. All rights reserved.

Keywords: Dehumidification; Powder; Product recovery; Residue accumulation; Spray drying; Stickiness; Tomato pulp

1. Introduction

Spray drying involves atomization of feed into a

spray and contact between spray and drying medium

resulting in moisture evaporation. The products to be

spray dried can be categorized into two major groups:

non-sticky and sticky products. Sticky products are

generally difficult to spray dry. During the drying pro-

cess they may remain as syrup or stick on the dryer wall,

or form unwanted agglomerates in the dryer chamber

and conveying system resulting in lower product yieldsand operating problems. Some of the examples of such

sticky products are fruit and vegetable juice powders,

honey powders and amorphous lactose powder. The

non-sticky products can be dried using a simpler dryer

design and the powder obtained is relatively less

hygroscopic and more free flowing.

The problem of powder stickiness is mainly due to the

low glass transition temperature (Tg) of the low molec-

ular weight sugars present in such products, essentially

sucrose, glucose and fructose (Bhandari, Senoussi,

Dumoulin, & Lebert, 1993; Roos, Karel, & Kokini,

1996). Spray drying is a fast process, which produces dry

product in an amorphous (glassy) from. Solids in an

amorphous state have a very high viscosity (>1012 Pas)

and as the temperature rises during drying, the viscosity

decreases to a critical value of around 107 Pas where

they first become sticky (Bhandari, Datta, Crooks,Howes, & Rigby, 1997a). This critical viscosity is

reached at temperatures 1020 C above Tg and these

temperatures decrease with an increase in water content

(Roos & Karel, 1991a). It can be, therefore, assumed

that the temperature of the particle surface during dry-

ing should not reach 1020 C above Tg (Bhandari &

Howes, 1999; Mitsuiki, Yamamoto, Mizuno, & Motoki,

1998). As a consequence, high molecular weight addi-

tives, which have a very high Tg and raise the Tg of the

feed, are usually added to the spray dryer feed to achieve

successful drying at feasible drying temperature condi-

Journal of Food Engineering 66 (2005) 2534

www.elsevier.com/locate/jfoodeng

* Corresponding author. Tel.: +30-2310-996205/995903; fax: +30-

2310-996259.

E-mail address: [email protected] (K.G. Adamopoulos).

0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2004.02.029
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tions. Maltodextrins are the most common drying aids

used at present (Bhandari, Datta, & Howes, 1997b;

Roos & Karel, 1991b).

Tomato pulp is a typical example of a product that is

very difficult to be spray dried due to the low glass

transition temperature of the low molecular weight

sugars present. The sugars found in tomato products are

mainly dextrose and levulose with a Tg of 31 and 5 C,

respectively (Bhandari et al., 1997b). A spray drying

plant capable of producing a free-flowing product that

on reconstitution compares favorably with tomato paste

has been designed featuring a co-current drying chamber

that has a jacketed wall for air-cooling. Cooling air in-

take is controlled to enable close maintenance of wall

temperature in the range of 3850 C. The paste is

sprayed into the drying air entering the chamber at a

temperature of 138150 C (Gransmith, 1971; Jayar-

aman & Das Gupta, 1995; Masters, 1979; Spicer, 1974).

The cool chamber wall will be favorable to minimize the

thermoplastic particles from sticking, as the wall will becold enough to cool and solidify the outer surface of the

thermoplastic particles coming in contact. This method,

however, was found to improve the process but not to

resolve the problem. The reason is that the cold chamber

wall will also cool the surrounding environment and

cause an increase in the relative humidity of the air close

to the wall surface.

An alternative approach to the above process has

been the Birs Tower process. The drying takes place in

a very tall tower into which the tomato juice is intro-

duced as a spray at a predetermined height. The whole

drying process relies on the time-delayed fall of the

product droplets and the very low temperature (not

exceeding 30 C) of the upward airflow. In this way,

explosion-type evaporation is avoided and the particles

are not exposed to high temperatures likely to damage

their organoleptic properties (Goose & Binsted, 1964;

Gransmith, 1971). The cost of building and operat-

ing such towers is so high today that this design of

tomato dryer is no longer realistic (Bhandari et al.,

1997b).

Another system capable of producing a free-flowing

product is a scraped surface drying chamber. This

method can be very useful for relatively less thermo-

plastic sugar such as lactose or sucrose. Karatas (1989)developed an experimental spray dryer with a chamber

wall scraper specifically to dry tomato juice. The pro-

duct recovery was up to 77% with a low inlet air tem-

perature (115 C).

Introduction of cool air at the lower part of the dryer

chamber resulting in the formation of a solid particle

surface can also reduce the stickiness of the powder

particles (Ponting, Stanley, & Copley, 1973). However, a

limited amount of air can only be introduced because

the cooling process will also raise the relative humidity

of the air that can once again aggravate the situation by

increasing the surface moisture level (Bhandari et al.,

1997b).

According to Karatas and Esin (1994), during air

drying of tomato concentrate droplets the constant rate

period was passed within about half a minute in all

cases. Upon termination of the surface drying, the dif-

fusional falling rate period exhibited discontinuities with

drying rate falling to zero occasionally and then

restarting with a drying rate different from the previous

period. This behavior is a typical consequence of case

hardening phenomena. When moisture migration is di-

rected from inside of the particle towards the surface

and becomes lower than the rate of the moisture evap-

oration from the surface, the surface becomes very dry

and hard. Hereby a thin layer which is non-permeable to

water is formed which locks in the remaining moisture

inside the particle or the drop. Drying stops at this point

and the average moisture content becomes constant.

Though there is no net drying during the case hardened

state, internal diffusion continues. Since rate of output iszero, moisture accumulates in the hardened volume and

plasticizing action proceeds outward. When the surface

layer is plasticized to a sufficient degree, drying restarts

and persists until rate of internal supply plus moisture

accumulated in the surface shell volume can meet the

drying rate. To provide a smooth drying operation any

case hardening, drying rate must be low. For this, it is

possible to operate either at low temperature or increase

the humidity of the medium, or both as to keep the

surface of the drying material plasticized (Karatas &

Esin, 1994; Szentmarjay, Pallai, & Regenyi, 1996).

Prolonged drying could, however, cause severe degra-

dation in the sensory and the nutritional properties of

the tomato.

In experiments conducted using a standard labora-

tory spray dryer with inlet air temperatures of 110140

C, outlet air temperatures of 6691 C and drying air

flow rates of 17.5022.75 m3/h, product yields ranged

from 13% to 28%, suggesting the existence of areas for

improvement in the spray drying process (Goula &

Adamopoulos, 2003). Moisture content of the tomato

powders varied from 4.16% to 11.27% wb. In all

experiments, tomato powder showed a noticeable ten-

dency to stick to internal surfaces of the dryer, as the

surface of the drying droplets remains plastic because ofthe high temperatures. Standard spray drying units do

not allow efficient drying at low temperatures at

the outlet due to the sufficiently high humidity content

of air, and, therefore, preliminary dehumidification of

air is necessary before its supply to the drying unit.

The objectives of this work were to investigate the

performance of a modified spray dryer for tomato

powder preparation under various operating conditions,

and study the effects of preliminary air dehumidification

on product recovery. The effect of system modification

on the main powder properties was investigated in

26 A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 2534


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the second part of the study (Goula & Adamopoulos,

2004).

2. Materials and methods

2.1. Materials

Medium concentrated tomato pulp with a constant

total solids mass concentration of 14 0.05%, contain-

ing 1.40 0.02% insoluble solids, 5.61 0.07% sugars,

1.53 0.03% acid, 2.20 0.02% protein and 1.10

0.01% salt, obtained from a local manufacturer, was

used as feed to the spray dryer in all experiments.

During spray drying, the feed solution held on a hot

plate was maintained at constant temperature being

continuously agitated.

2.2. Air dryer

An Ultrapac 2000 adsorption dryer (Model 0005,

Ultrafilter International AG, Haan, Germany) was

employed for the air drying process. The air was led

through the inlet of the dryer and across a prefilter,

where it was cleaned from particles. Via a shuttle valve,

the air was led into desiccant cartridges, in which it was

dried to a dewpoint of)70 C. Via another shuttle valve,

the air got into an afterfilter, in which particles from the

desiccant were retained. While one vessel with cartridge

was in the drying phase (adsorption), the other cartridge

was being dried again (regeneration). A partial stream of

dried air was expanded via an orifice and led across the

desiccant cartridge for regeneration and via a solenoid

valve and a silencer system to atmosphere.

2.3. Spray drying

A Buchi mini spray dryer (Model 191, Buchi Labor-

atoriums-Technik, Flawil, Switzerland) was employed

for the spray drying process. A peristaltic pump pumped

tomato pulp to the atomizer, and atomization was per-

formed using a two-fluid nozzle (inside diameter 0.5

mm), which used compressed air. The compressed air

flow rate was controlled by a variable area flow meter,

and cooling water was circulated through a jacketaround the nozzle. The main chamber and the cyclone

were made of thick transparent glass. Inlet drying air,

after passing through an electrical heater, flowed con-

currently with the spray through the main chamber.

Dried powder samples were collected from the base of

the cyclone.

The controlled parameters were the feed rate (feed

pump setting), the initial feed solids concentration, the

feed temperature, the atomizer pressure, the compressed

air flow rate in the atomizer, the flow rate of drying air

(aspirator setting), and the inlet air temperature. The

design of the dryer is such that the outlet air tempera-

ture, contrary to the inlet temperature, cannot be set

with a temperature regulator, but results from a com-

bination of the inlet temperature, the aspirator setting,

the pump setting, as well as the concentration of the

feed.

The modification made on the original design con-

sisted in connection of the dryer inlet air intake nipple

with the air drying unit by a flexible plastic air duct. The

compressed air was also dehumidified before its supply

to the two-fluid nozzle. The scheme of the modified

spray drying system is given in Fig. 1.

Sixty-four different experiments were conducted in

triplicate. In all experiments the atomizer pressure, the

feed rate and the feed temperature were kept at 5 0.1

bar, 1.75 0.05 g/min, and 32.0 0.5 C, respectively.

Four inlet air temperatures (Tinlet), 110, 120, 130 and 140

C (1 C), were used. The drying air flow rate (Qa) was

varied between 17.50 0.18 and 22.75 0.18 m3/h, and

the compressed air flow rate (Qc) was varied from500 20 to 800 20 L/h. Outlet air temperatures (Toutlet)

were continuously monitored. Inlet and outlet air

temperatures were read and manually logged from the

digital displays on the dryers control panel with an

accuracy of 1 C.

2.4. Spray dryer performance

The weights of the drying chamber, the cyclone and

the receiving vessel for powder were determined before

and after spray drying by an electronic balance with an

accuracy of 102 g. Product (np) and residue (nr) yieldwere determined by dividing the weight of solid mass

collected in each part of the spray dryer with the total

amount of solid mass to be spray dried. The exhausted

air carries the remaining part of feed solids away.

Fig. 1. Schematic representation of the modified spray drying system.

A.M. Goula, K.G. Adamopoulos / Journal of Food Engineering 66 (2005) 2534 27


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2.5. Statistical analysis

The data were analyzed using the statistical software

MINITAB (Release 13.32). Regression analysis was

used to fit a full second order polynomial, reduced sec-

ond order polynomials containing the three linear terms

and linear models to the data of each of the variable

evaluated (response variables). F values for all reduced

and linear models with an R2P 0:70 were calculated todetermine if the models could be used in place of full

second order polynomials to predict response of a var-

iable to compressed air flow rate, flow rate of drying rate

and air inlet temperature (independent variables). The F

value was calculated as

F SSRf SSRr=k q

SSEf=n k 1 1

where SSR is the sum of squares due to regression, SSE

is the sum of squares due to error, k is the number of

terms of the full model, q is the number of terms of thereduced model, and the subscripts f and r refer to the

full and the reduced model, respectively.

The F value indicates no significant difference be-

tween the reduced second order polynomial or linear

model and its corresponding full model (p< 0:05) if:

F


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Table 1

Experimental drying conditions

No. Tinlet (C) Qa (m3/h) Qc (L/h) Toutlet (C)

a

1 110 17.50 500 47.33 (0.58)

2 120 17.50 500 49.00 (0.00)

3 130 17.50 500 54.33 (0.29)

4 140 17.50 500 58.33 (0.58)

5 110 19.25 500 48.33 (0.29)6 120 19.25 500 51.17 (0.29)

7 130 19.25 500 56.67 (0.29)

8 140 19.25 500 62.00 (0.50)

9 110 21.00 500 49.17 (0.29)

10 120 21.00 500 51.50 (0.00)

11 130 21.00 500 58.17 (0.29)

12 140 21.00 500 63.00 (0.00)

13 110 22.75 500 51.00 (0.50)

14 120 22.75 500 53.17 (0.29)

15 130 22.75 500 59.17 (0.29)

16 140 22.75 500 64.17 (0.29)

17 110 17.50 600 48.00 (0.00)

18 120 17.50 600 50.17 (0.29)

19 130 17.50 600 55.83 (0.29)

20 140 17.50 600 59.67 (0.29)

21 110 19.25 600 49.00 (0.50)

22 120 19.25 600 51.50 (0.00)

23 130 19.25 600 57.67 (0.29)

24 140 19.25 600 63.33 (0.29)

25 110 21.00 600 50.17 (0.29)

26 120 21.00 600 53.00 (0.00)

27 130 21.00 600 59.00 (0.50)

28 140 21.00 600 64.17 (0.29)

29 110 22.75 600 52.33 (0.58)

30 120 22.75 600 55.17 (0.29)

31 130 22.75 600 60.00 (0.00)

32 140 22.75 600 65.00 (0.50)

33 110 17.50 700 49.00 (0.00)

34 120 17.50 700 51.17 (0.29)

35 130 17.50 700 57.17 (0.29)

36 140 17.50 700 62.00 (0.00)37 110 19.25 700 50.17 (0.29)

38 120 19.25 700 53.33 (0.29)

39 130 19.25 700 59.17 (0.29)

40 140 19.25 700 64.17 (0.29)

41 110 21.00 700 51.00 (0.00)

42 120 21.00 700 54.00 (0.50)

43 130 21.00 700 60.17 (0.29)

44 140 21.00 700 65.33 (0.58)

45 110 22.75 700 53.00 (0.50)

46 120 22.75 700 56.33 (0.58)

47 130 22.75 700 61.17 (0.29)

48 140 22.75 700 65.83 (0.29)

49 110 17.50 800 50.17 (0.29)

50 120 17.50 800 52.00 (0.00)

51 130 17.50 800 58.17 (0.29)

52 140 17.50 800 63.17 (0.29)

53 110 19.25 800 51.17 (0.29)

54 120 19.25 800 53.67 (0.29)

55 130 19.25 800 59.67 (0.29)

56 140 19.25 800 65.00 (0.50)

57 110 21.00 800 52.00 (0.50)

58 120 21.00 800 55.17 (0.29)

59 130 21.00 800 61.33 (0.29)

60 140 21.00 800 66.00 (0.00)

61 110 22.75 800 54.17 (0.29)

62 120 22.75 800 57.00 (0.50)

63 130 22.75 800 62.17 (0.29)

64 140 22.75 800 67.83 (0.29)

a Means of three replicates and standard deviation (in brackets).



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manufacturer, the main concern is narrowing the range

of processing variables to produce the best product

possible.

The best model predicting response of the variable

Toutlet to the process variables air inlet temperature, flow

rate of drying rate and compressed air flow rate was the

following:

Toutlet 70:6 1:15Tinlet 1:59Qa 0:00158Qc

0:00591T2inlet 0:0315Q2a

0:00407TinletQa 0:000075TinletQc 4

Eq. (4) has a R2 value of 0.988, andF,CpandSequal to

0.135, 6.3 and 0.657, respectively.

Outlet air temperature was 1924 C lower when

dehumidified air was used in the modified spray drying

system instead of undehumidified air in the standard

system. In a spray drying system, removal of moisture

from the spray depends upon the humidity of the drying

air. The drier the air the more rapid is the rate of drying,since dry air is capable of absorbing and holding mois-

ture, whereas moist air is close to saturation and so can

absorb and hold less additional moisture than if it is dry.

Hot air enters the drying chamber and due to moisture

evaporation from the spray the air temperature falls as

air passes through the chamber. The more rapid is the

spray evaporation, the more the lowering of the air

temperature, and so the drier the air, the drying air cools

more.

3.2. Product recovery

Residue data were gathered as an indication of the

process performance. Retention of product at the

chamber wall over lengthy time is undesirable. First, it is

not cost effective due to more frequent shut down of the

dryer for cleaning. Secondly, it affects product quality,

as deposits can become scorched and when dislodged,

mix in and contaminate the entire product. Accumu-

lated product receiving a more intense heat treatment

may have different properties such as moisture content,

color, and solubility and cannot considered as product.

Furthermore, deposits influence drying volume and heat

transfer processes between the chamber walls and the

moving fluids.The residues that occurred in the drying chamber and

in the cyclone (nr) varied from 20.17 to 45.83%. Figs. 2

5 show the achieved values of nr against inlet air tem-

perature and compressed air flow rate for each level of

the drying air flow rate. Each data point in the figures

represents average values of three replications. The

repeatability for nr expressed as the average standard

deviation of the three replications was 0.17%.

Residue accumulation decreased with increases of

drying air flow rate. In a spray drying system, the liquid

droplets emerge from the atomizer with velocities much

larger than that of the inlet air. Because of momentum

exchange the air inside the spray envelope is accelerated

axially to a large extent as the droplets slow toward

terminal velocities. A large pumping effect is imparted to

500600

700800110 120 130 140

20.00

25.00

30.00

35.00

40.00

45.00

50.00

nr

(%)

Qc(L/h)

Tinlet(C)

Qa: 19.25 m3/h

Fig. 3. Residue accumulation (nr) as a function of inlet air temperature

and compressed air flow rate for drying air flow rate of 19.25 m3/h.

500

600700

800110 120 130 140

20.00

25.00

30.00

35.00

40.00

45.00

50.00

nr

(%)

Qc(L/h)

Tinlet(C)

Qa: 21.00 m3/h



500600

700

800110 120 130 140

20.00

25.00

30.00

35.00

40.00

45.00

50.00

nr

(%)

Qc (L/h)

Tinlet(C)

Qa: 17.50 m3/h





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the air by the deceleration of the droplets within the

spray envelope. The resulting forward displacement of

air results in a large amount of air being entrained into

the spray from outside the spray envelope (Oakley &

Bahu, 1991; Papadakis & King, 1988). By decreasing the

air flow rate, the major part of the air entrained into the

spray comes from a recycling of the air from lower parts

of the drying chamber and so, backflow at the wall

persists up to much higher levels in the chamber. This

airflow pattern influences droplet trajectories. Smaller

droplets are drawn toward the wall by the strong

backflow circulation effect, larger ones pass back up-

ward with the backflow before evaporating, whereas

medium size droplets strike the wall before evaporating

fully, thus causing residue accumulation.

As it can be drawn from Figs. 25, residue formationdecreased with increases of compressed air flow rate.

With higher compressed air flow rate, smaller droplets

having less inertia do not reach radially outward as far

and as a result narrow spray cones are formed and rel-

atively little air is drawn inside. The trajectories are

similar in shape to those for lower compressed air flow

rates, but are narrower. Droplets strike the wall at lower

parts of the drying chamber, where their moisture con-

tent is much lower.

Wall deposits decreased by increasing the inlet air

temperature, as due to the higher temperature the

droplets are drier when they hit the wall, and depositsthat are attributable to inadequate drying of particles

decrease.

The best model predicting response of the variable nrto the process variables air inlet temperature, flow rate

of drying rate and compressed air flow rate was the

following:

nr 293 1:87Tinlet 8:98Qa 0:0137Qc

0:00738T2inlet 0:253Q2a 0:0225TinletQa 5

Eq. (5) has a R2 value of 0.979, and F,CpandSequal to


Product yield (np) varied from 36.62% to 65.86%.

Figs. 69 show the achieved values ofnp against inlet air

temperature and compressed air flow rate for each level

500600

700800110 120 130 140

20.00

25.00

30.00

35.00

40.00

45.00

50.00

n

r(%)

Q c(L/h)

Tinlet(C)

Qa: 22.75 m3/h



500600

700800

110

120

130

140

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

np

(%)

Q c(L/h)

Tinlet(C)

Qa: 21.00 m3/h

Fig. 8. Product recovery (np) as a function of inlet air temperature and

compressed air flow rate for drying air flow rate of 21.00 m3/h.

500600

700800

110

120130

140

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

np

(%)

Qc(L/h)

Tinlet(C)

Qa: 19.25 m

3/h



500600

700800

110

120

130

140

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

np

(%)

Qc(L/h)

Tinlet(C)

Qa: 17.50 m

3/h





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of the drying air flow rate. Each data point in the figures

represents average values of three replications. The

repeatability for np expressed as the average standard

deviation of the three replications was 0.21%.

As it can be drawn for Figs. 69, the effect of the

various operating conditions on product recovery is

determined by their effect on residue accumulation, since

attachment of sprayed droplets and dry powder to the

walls is the main cause of total product loss. Product

recovery increases with increases in drying air flow rate,

in air inlet temperature and in compressed air flow rate.

The best model predicting response of the variable npto the process variables air inlet temperature, flow rate

of drying rate and compressed air flow rate was the

following:

np 276 1:93Tinlet 14:7Qa 0:00817Qc

0:00735T2inlet 0:371Q2a 0:0210TinletQa 6

Eq. (6) has a R2 value of 0.983, andF,CpandSequal to


In experiments conducted under the same operating

conditions using the standard laboratory spray drying

system, residue accumulation varying from 60.08% to

70.81% was 21.4949.50% higher, whereas product

recovery ranging from 13.00% to 28.02% was 19.90

48.23% lower. Graphic comparison between the yields

of the two drying systems for each level of the air inlet

temperature is shown in Figs. 1013.The modified system decreased the residue accumu-

lation, which allowed product yield to increase. This was

due to the lower temperatures in the drying chamber

when dehumidified air was used as a drying medium

instead of undehumidified air. During spray drying,

initially the droplets are dispersed individually in a large

volume of the dryer, avoiding agglomeration although

the high moisture content particles are sticky. Towards

the lower parts of the dryer or in the recirculation

zones, the particles are already solid and should not

stick together or agglomerate. However, due to the

presence of high sugar content, the surface of the

droplets may remain plastic because of the high product

temperature, which is generally approaching the outletair temperature. There is a narrow range of outlet air

temperature for a successful drying operation. Use of

outlet air temperature below this region is not eco-

nomical, and above this region the air temperature will

be at higher than Tg 20 C, which mean that the par-ticles will exhibit sticky behavior. The extent of sticki-

ness or the consequence on structural change of the

powder depends on the difference between the temper-

ature of the product and the glass transition tempera-

ture. Thus, a small change in outlet air temperature may

have a major effect on the sticky behavior of the product

35

40

45

50

55

60

65

13.017

.818

.620

.821

.622

.323

.123

.424

.263

.663

.964

.164

.264

.464

.665

.365

.671

.0

n (%) for the standard system

np

(%

)forthemodifiedsystem

20

25

30

35

40

45

nr

(%

)forthemodifiedsystem

Tinlet: 120 C

np nr

Fig. 11. Comparison between yields of the two drying systems for air

inlet temperature of 120 C. Symbols are the same as in Fig. 10.

500600

700800

110

120

130

140

35.00

40.00

45.00

50.00

55.00

60.00

65.00

70.00

np

(%)

Qc(L/h)

Tinlet(C)

Qa: 22.75 m

3/h


compressed air flow rate for drying air flow rate of 22.75 m3/h.35

40

45

50

55

60

65

13.015

.816

.718

.619

.419

.920

.7

21.021

.865.9

66.166

.366

.466

.566

.667

.067

.271

.0


np(%)forthemodif

iedsystem

20

25

30

35

40

45

nr(%)forthemodifiedsystem

17.50 m^3/h, 500 L/h 19.25 m^3/h, 500 L/h21.00 m^3/h, 500 L/h 22.75 m^3/h, 500 L/h17.50 m^3/h, 600 L/h 19.25 m^3/h, 600 L/h21.00 m^3/h, 600 L/h 22.75 m^3/h, 600 L/h17.50 m^3/h, 700 L/h 19.25 m^3/h, 700 L/h21.00 m^3/h, 700 L/h 22.75 m^3/h, 700 L/h17.50 m^3/h, 800 L/h 19.25 m^3/h, 800 L/h21.00 m^3/h, 800 L/h 22.75 m^3/h, 800 L/h

Tinlet : 110 C

nrnp


inlet temperature of 110 C.



9/10

(Bhandari et al., 1997b). As a result, the much lower air

outlet temperatures in the modified system might result

in the formation of a solid particle surface and so, de-

crease the residue accumulation minimizing the ther-

moplastic particles from sticking.Furthermore, according to Karatas and Esin (1994),

the slow removal of water during spray drying of tomato

pulp in a medium of high humidity provides a smooth

drying operation without discontinuities, since the sur-

face of the drying material remains plasticized. How-

ever, this can also result in high residue formation. Roos

(2003) reported that the rapid removal of water results

in vitrification of liquid droplets within a short time and

formation of a solid particle surface. Rapid formation of

a dry surface layer around the particle increases the glass

transition temperature at the surface and so is the basic

requirement for successful spray drying, as solidification

of the surface does not allow formation of liquid bridges

between contacting particle surfaces or particle adhesion

on the inner surfaces of the drying equipment. As a re-

sult, use of dehumidified air as a drying medium

enhancing the rapid removal of water contributes to the

reduction of residue accumulation.

As it can be drawn form Figs. 1013, the increase in

product recovery was higher for experiments conducted

with air inlet temperature of 140 C. This was due to the

effect of air inlet temperature on residue accumulation.

In experiments conducted in the modified system, wall

deposits always decreased by increasing the inlet air

temperature. In the standard spray drying system, resi-

due formation was larger for experiments conduced with

inlet air temperatures of 110 and 140 C. Wall deposits

decreased by increasing the inlet air temperature from

110 to 130 C, whereas the increase from 130 to 140 C

resulting in much higher air outlet temperatures

(Toutlet> 80 C), led to an increase of residue accumu-lation (Goula & Adamopoulos, 2003).

4. Conclusions

An experimental spray dryer was modified for drying

tomato concentrate. The modification made on the

original design consisted in connection of the spray

dryer inlet air intake with an absorption air dryer.

Products yields ranged from 36.62% to 65.86%, whereas

residue accumulation varied from 20.17% to 45.83%.

The effect of the various operating conditions on prod-uct recovery was determined by their effect on residue

accumulation, as attachment of sprayed droplets and

dry powder to the walls was the main cause of total

product loss. Product recovery increased with increases

in drying air flow rate, in air inlet temperature and in

compressed air flow rate.

In experiments conducted under the same operating

conditions using the standard laboratory spray drying

system, product recovery was 19.9048.23% lower and

residue accumulation was 21.4949.50% higher. It ap-

pears that the modification improved product recovery.

The much lower outlet temperatures and humidities ofdrying air in the modified system resulted in the for-

mation of a solid particle surface and so, decreased the

residue accumulation minimizing the thermoplastic

particles from sticking.

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35

40

45

50

55

60

65

13.020

.721

.524

.325

.126

.026

.827

.228

.060

.160

.460

.660

.961

.261

.662

.763

.071

.0


np(%)forthemodifiedsystem

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Tinlet : 130 C

np nr



35

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13.013

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np(%)forthemodifiedsystem

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Tinlet : 140 C

nrnp





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Spray Drying of Tomato Pulp in Dehumidified Air

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