May 7, 2007 - indexes of ascorbic acid destruction and browning in the drying. Stricter quality ... Total cross section for air flow is 1.49 m2. The air humidified ...
Drying Technology
ISSN: 0737-3937 (Print) 1532-2300 (Online) Journal homepage: http://www.tandfonline.com/loi/ldrt20
OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD Dong Sun Lee & Yu Ryang pyun To cite this article: Dong Sun Lee & Yu Ryang pyun (1993) OPTIMIZATION OF OPERATING CONDITIONS IN TUNNEL DRYING OF FOOD, Drying Technology, 11:5, 1025-1052 To link to this article: http://dx.doi.org/10.1080/07373939308916881
Published online: 07 May 2007.
Submit your article to this journal
Article views: 31
View related articles
Citing articles: 16 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ldrt20 Download by: [The University of Manchester Library]
Date: 22 April 2016, At: 05:42
DRYING TECHNOLOGY. II(5). 1025-1052 (1993)
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
OPTIIIIZATION OF
OPERATING CONDITIONS OF FOOD
IN
TLWNEL
DRYING
Dong Sun lee1 and Yu Ryang pyun2 l~epartmentof Food Engineering Kyungnam University, Nasan 631-701, Korea 2~epartment of Food Engineering Yonsei University, Seoul 120-749, Korea
Key words and Phrases: ascorbic acid; browning; energy consumption; quality; radish
ABSTRACT Food drying process in tunnel dryer was modeled from Keey's drying model and experimental drying curve, and optimized in operating conditions consisting of inlet air temperature, air recycle ratio and air flow rate. Radish was chosen as a typical food material to be dried, because it has a typical drying characteristics of food and quality indexes of ascorbic acid destruction and browning in the drying. Stricter quality retention constraint required higher energy consumption in minimizing the objective function of energy consumption under constraints of dried food quality. Optimization results of cocurrent and counter current tunnel drying showed higher inlet air temperature, lower recycle ratio and higher air flow rate with shorter total drying time. Compared with cocurrent operation counter current drying used lower air temperature, lower recycle ratio and lower air flow rate, and appeared to be more efficient in energy usage. Nost of consumed energy was analyzed to be used for air heating and then escape from the dryer in form of exhaust air.
Copyright O 1993 by Marcel Dckkcr. Inc
LEE AND PYUN
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
INTRODUCTION Food dehydration is an energy intensive process in which energy cost constitutes a major portion of drying cost (Bruin 6 Luyben, 1980). Therefore energy management constitutes a very important part in food drying process and energy conservation can contribute to significant reduction of total production cost. Optimal operation of dryer is one of the feasible methods for energy saving. Food quality is another important factor to be considered simultaneously with energy conservation. Tunnel dryer is widely used for dehydrating fruits and vegetables. Operation of tunnel dryers can be varied in air temperature and velocity, tray loading, air recirculation and etc. The dryers may also be operated in cocurrent or counter current mode. Rilpatrick et a1.(1955) reviewed practical aspects of tunnel dryers comprehensively. Thompson et a1.(1981) investigated energy conservation in
?.cS~
air
Ch.UBL
-
air
9000
'b
l2000
a)
1
Front view
FIGURE 1. Schematic diagram of tunnel dryer. in mm.
Dimensions are
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1027
commercially operated tunnel dehydrator. Bertin and Blazquez(1986) modeled drying in tunnel dryer and tried optimization for maximum production capacity. So far, several attempts have been reported to optimize the food dehydration process with respect to product quality, productivity and energy cost (Brook and BakkerArkema, 1978; Doe and Menary, 1979; nishkin et al., 1982, 1983, 1984a; Thompson, 1970; Thygeson Jr. and Grossmann, 1970;). The interaction of product quality and energy efficiency needs to be elucidated to optimize operating conditions of food dehydrator(S0khansanj. 1984). In this study we are aiming at optimizing operation conditions of tunnel dryer which minimize energy consumption under satisfied product quality. AS a food material radish was chosen for being dried in typical commercial tunnel dryer.
MATERIALS AND METEOD
Figure 1 shows a typical tunnel dryer used for drying of fruits and vegetables in Korea. 10 trucks are placed in a tunnel passing intermittently with simultaneous input and output of truck. One truck has 17 trays of 84 x 84 cm and thus total tray area in a tunnel is 120 mZ. Air is forced through the heater by centrifugal fan, heated to desired temperature, and then passed between food trays by cross flow. Total cross section for air flow is 1.49 m2. The air humidified by drying food is partially exhausted at the tunnel end, while the rest of air is recirculated and mixed with fresh ambient air. The mixed air reenters into the tunnel for drying. Dryer walls are insulated by rock wool 5 mm thick.
1028
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
F O O ~Material
LEE AND PYUN
-
Radish
Radish was selected as a test food material for optimization study of tunnel drying. It is a vegetable which 18 dried commercially in appreciable amount and has a typical drying characteristics consisting of both constant rate and falling rate period (Figure 5). It also has quality change indexes of ascorbic acid destruction and nonenzymatic browning, which are common in food drying. Radishes of about 1.5 kg weight, variety .TaebackM were purchased from the market and used for determination of drying characteristics. Radish was halved longltudinally, sliced 6 mm thick and then prepared into 6 x 6 mm orthorhombic shapes. Length of cut radishes was not controlled, but was approximately 3.5 cm on the average. ~ 1 1the process optimization was done for the radishes prepared in this way.
Determination of Characteristic Drvinq Curve Characteristic drying curve was determined for prediction and simulation of radish drying under variety of conditions. The characteristic drying curve, f (Equation ( 1 ) ) proposed by Keey (1968, 1975), was determined by drying experiment under controlled conditions of constant temperature, relative humidity(R.8.) and air velocity.
Samples of 183 g were loaded on wire mesh tray of 1 5 x 23 cm dimension. It corresponds to the normal tray loading of 6.0 kg/m2. With being dried under controlled conditions of constant air temperature, relative humidity and air velocity, sample weight was continuously measured by strain gage. Relative humidity was controlled and determined by dry bulb and dew point temperature relationship. Air
TUNNEL DRYING OF FOOD
1029
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
velocity was measured by hot wire anemometer (model v-01-AO, Sogo Denshi Co. Japan). Drying rate Rd in Equation (1) can be described as a function of humidity difference potential by Equation (2):
Kg can be assumed as a function of only moisture content under constant air flow rate. Kg takes a constant value in constant drying rate period above the critical moisture content, and decreases with moisture reduction through drying. This dependence of Ka on moisture can be established by the characteristic drying'curve of Equation ( 3 ) , which was proposed, established and applied for continuous conveyor drying by Keey (1977, 1978):
The f in Equation (3) is 1 in constant drying rate period above critical moisture content and decreases into a value between 0 and 1 with moisture decrease in falling drying rate period.
Simulation of Drvinu Process in Tunnel Drver In continuous tunnel drying where inlet drying air temperature, air recycle ratio and air flow are controlled constant with respect to time, change of moisture content through tunnel can be modeled as Equation (4) (Keey, 1978):
-
L dl4 where drying.
--
-
f K ~ C(nsw tlo f L (I . M ~ ) / G ) a dz for cocurrent drying, + for counter
(4)
current
Even though tunnel drying is operated with discontinuous loading, average moisture profile along the tunnel are reported to be consistent with that of continuous loading
LEE AND P U N
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1030
(Bertin at al., 1980). In this study tunnel drying was assumed continuous in operation. L in Equation (I), dry solid flow into dryer depends on the speed of truck movement in tunnel, which is determined by total drying time. Equation ( 4 ) was solved by Runge-Kutta 4th order method to simulate moisture profile in tunnel. For cocurrent drying, humidity at tunnel inlet, Eo in Equation 4 can be calculated by Equation (5), which is derived from Equations (6) and (7) of humidity balance:.
For counter current drying Eo in Equation ( 4 ) can be calculated by Equation (a), which can similarly be derived from the relations of Equations ( 9 ) and (10):
u u l s t i o n of oualitv Chanae in Drvinq Dastruction and Nonenavmatic Browninq
-
Ascorbic
Acid
As a quality change in drying of radish ascorbic acid destruction and nonenzymatic browning were simulated and used as quality constraints in the optimization of drying. The kinetics obtained from dynamic test by Lee and Pyun (1988) were used. The functional forms of kinetic equations were similar to those of Mishkin et al. (1983, 1984b).
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1031
Moisture content (g watertg solid) FIGURE 2. Dependence of ascorbic acid destruction rate constant on moiaturs content and temperature.
Ascorbic acid destruction in radish drying a s a function of moisture content and food temperature was reported to follow Equations (11)-(14):
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
LEE AND PYUN
Moisture content (g waterlg solid) FIGURE 3. Dependence of browning rate constant on moisture content and temperature.
The dependence of ascorbic acid destruction on moisture and temperature (Figure 2) was similar to that in potato drying, nishkin et which was reported by nishkin et a1.(1984b). a1.(1984b) dilcussed the kinetics of ascorbic acid destruction from dynamic test. Nonenzymatic browning in radish drying was described to follow Equations (15)-(18):
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYlNG OF FOOD
1033
Nonenepatic browning as function of moisture content and product temperature shown in Pigure 3 is comparable to that in potato drying reported by Bendel et a1.(1955). Pood temperature through the drying, Ts is calculated from Equation (19) propoaed by Keey (1977):
Air temperature in the tunnel, TG of Equation (19) can be obtained by Equation (20):
Equations (11) and (15) were also solved combined together with Equation ( 4 ) by Runge-Kutta 4th order method.
Ener Energy consumption and energy output for the drying were calculated from energy balance on the dryer based on 1 cycle of truck turnover in tunnel operation. Energy consumption or energy input for drying process is obtained by the summation of air heating, food heating, dryer heating and air circulation energy (Equation (21)). Energy consumption or energy input +
jqcir dt
where
q heat
-
-
jqheat dt + Qfh + Qdh (21)
G (
TI CPI
4 cir ' ¶ cirr
-
( 1 - r ) CPA TA
-
r CPE TE)
(21-1) (21-2)
Qfh and Qdh were calculated from energy of heating food and dryer to wet bulb temperature and inlet drying temperature,
LEE AND PYUN
1034
respectively. Exit air temperature, TI is obtained by using Equation (20).
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
Energy output from the drying consists of energy involved in exhaust air, wall loss, dried food and dryer (Equation (22)).
--
Energy output where
qe,
gloss
(he
lqex dt +
-
Iqloss dt+ Of
h ~ )(1-r) G
V A (TM
-
TA)
+
Qd
(22) (22-1) (22-2)
Of and Qd were calculated from sensible heat of food and dryer compared to ambient temperature, respectively. Heat capacity of dryer was found to be 1299 k J P C from the construction material data. Required specific heat of radish was calculated as function of radish moisture by Equation (23).
Properties of humid air were obtained by using equations for psychrometric chart (ASAE, 1979).
Minimization of energy consumption in drying radish to final moisture of 0.15 g water/ g dry solid was attempted by Box's complex method (Box, 1965) by the procedure in Figure 4. Search regions for control variables were 50-90 OC for inlet air temperature, 0 - 1 for air recycle ratio and 107-535 kg/min for air flow rate, which gives 1 - 5 m/s air velocity in cross flow drying. Ascorbic acid retention and nonenzymatic browning level of dried product were put as implicit constraints. Initial moisture, ascorbic acid concentration and browning level were put as 16.0 g water/g dry solid, 480 mg/g dry solid, 0.077 optical density(O.D.)/g
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1035
Initial guess for control variables: inlet air temperature, air recycle ratio and air flow rate Specify constraints: 0.01 r 11.0 501 TI (OC) 1 9 0 1071 G (kg/min)s535 1, i0.15 other aualitv constraints Input data: initial condition of state variables of moisture, ascorbic acid concentration, browning level and food temperature, and ambient air conditions 4
Simulation: evaluate the state variables during drying by using drying and quality destruction kinetic model for inlet control variables Evaluate objective function of energy consumption and dependent variables of moisture content, ascorbic acid retention and browning level for drying link to complex method
I
control variables 4
Determine new guess For control variables according to complex method I
FIGURE 4. Optimization algorithm for tunnel drying of radish.
LEE AND PYUN
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1036
dry solid, respectively. As ambient conditions, temperature of 6.6 OC and wet bulb temperature of 4.0 OC were used which is average November weather conditions of Seoul area. Tray loading used was 6 kg/& which is the same as that in experiment of characteristic drying curve determination, and initial food and dryer temperature at start of drying were assumed as same as ambient temperature.
RESULTS AND DISCUSSION Gharacteristic Drvinc! Curve of Radish Drying rate curve under constant drying conditions shows constant rate above moisture content of 10.5 g/g dry solid and decreases with further decrease of moisture (Figure 5). Shape of the curve of drying rate dependence on moisture were similar for different drying conditions except that magnitude of constant and falling drying rates were dependent on drying conditions (Table I), and critical moisture content at which falling rate drying starts was appeared as 10.5 g water/g dry solid for all the experimental drying conditions. The presence of constant rate drying period and falling rate period were observed in drying of most fruits and vegetables examined by Saravacos and Charm (1962). Radish can be considered as a common food material which has typical drying characteristics. Constant rate drying coefficient, K ~ cappears not to be dependent on drying conditions with same air velocity (Table 1). Average value of RHc at air velocity of 2.3 m/s is Effect of air velocity on 15.11 g waterfi. g dry solid.AE. KHC are shorn in Figure 6 and then can be described as Equation (24):
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1037
FIGURE 5. Drying rate of radish vs. moisture content. Air temperature 60 OC; R.E.24\; Air velocity 2.3 m/s. , A : replicates
TABLE 1 Constant Rate Drying Coefficients for various Drying Conditions*
Drying Relative Humidity Drying rate temperature Humidity difference ( g water/h. (OC) (8) (kg water/ g dry kg-dry air) solid)
Constant rate drying coefficient (g/h.g dry
Average
15.11
Air velocity 2.3 m/s
LEE AND P W N
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1038
FIGURE 6. Effect of air velocity (v) on the constant drying rate coefficient(Kac). 70 OC, R.E. 23%.
Drying rate are known to be proportional to 0.8 power of air velocity for cross flow drying of slab, while that of particle drying are proportional to 0.5 power of air velocity (Keey, 1978). 0.42 in Equation ( 2 4 ) is a value close to the value of 0.5 in small particle drying. From the curves of drying rate vs. moisture content at air velocity of 2.3 m/s, f values in Equation (3) were read every 0.5 unit of moisture content and plotted as a function of moisture content in Figure 7. The f curves from different drying conditions below critical moisture content of 10.5 g water/g dry solid nearly coincide into one curve. Characteristic drying curve for radish then can be deacribed as Equation (25) and a polynomial Equation (26): when
2
when MtlO. 0.0001357M
10.5
3
f f
-
1.0
(25)
-0.01133+0.2542~-0.03951M2+0.003764M3-
(26)
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
Moisture content (gwsterlg solid) FIGURE 7. Characteristic drying curve of radish. f values from ex erimental drying curve of radish: 50 gC, R.E. 36%; o : 60 OC, R.B. 24\; x : 70 OC, R.E. 23%; A : 70 OC, R.E. 50%.
Drying rate at any moisture content in the drying of radish can be obtained by Equations (2). (3). (24). (25) and (26) for any drying conditions consisting of drying temperature, humidity and air velocity.
E f fec t of 0 uali tv Constraint on O~timizatiooof Drvinq
Moisture change, energy consumption and quality changes in tunnel drying of radish can be simulated by Equations 4 1 and 1 5 since imposing quality constraint may change optimized control variables (Karel, 1988). effect of quality constraints on the optimization of energy consumption was investigated by the procedure of Figure 4. The results for cocurrent drying are shown in Figures 8 and 9.
LEE AND PYUN
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1040
A s c o r b i c acid constraint (mg/100g d r y s o l i d )
FIGURE 8. Effect of ascorbic acid constraint of the dried product on the optimization of cocurrent tunnel drying of radish. Total drying time: 350 minutes; Initial ascorbic acid content: 480 mg/100g dry solid; Initial browning: 0.077.
-
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
300
IWI
\ 250-
0
200
0.10
0.15 0.20 ~ ~ o v n i nconstraint g (o.D. at 4 2 0 m )
0.25
FIGURE 9. Effect of browning constraint of the dried product o n the optimization of cocurrent tunnel drying o f radish. Total drying time: 350 minutes; Initial ascorbic acid content: 480 mg/100g dry solid; Initial browning: 0.077.
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
LO42
LEEANDPYUN
Optimized energy consumption increases with stricter constraints of product quality retention (i.e. higher ascorbic acid retention and lower browning level). Ascorbic acid retention higher than 330 mg/100 g dry solid or browning level lower than 0.177 compels relatively high energy conaumption steeply increased with strictness of quality constraints. Therefore quality retention constraint stricter than this level consumes too much energy only with marginal gain in dried food quality. In order to optimize energy consumption under higher quality retention constraint, combination of control variables moves toward lower drying air temperature, lower recycle ratio and slightly higher air flow rate, which causes higher energy consumption. Bigher ascorbic acid retention constraint in optimization results in lower level of browning, while lower level browning in turn provides higher ascorbic acid retention. Even though in batch drying optimal control variable profiles for product quality differ for different objective functions of quality factors different in heat sensitivity (Karel, 1988), effects of control variables on product quality retention and energy consumption seem to be more consistent in continuous tunnel drying. Por counter current tunnel drying, dependence of optimized energy consumption, control and state variables o n quality constraints were similar to Figures 8 and 9 (not shown). It is clear from Figures 8 and 9 that quality constraint and energy consumption should be compromised in practical drying situation. In this study ascorbic acid retention of 330 mg/100g dry solid and browning level of 0.177 were selected as quality constraints for further optimization from consideration of quality constraint effect on energy consumption and generally acceptable product quality.
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TABLE 2 Optimization Results of Cocurrent Tunnel Drying of Radish under Quality Constraints for Various Drying Times*
2Z Z
F a
Drying time (min.)
Optimal operating control variables Air temperature (OC)
Air recycle ratio
Air flow rate (kg dfy air/min.)
Quality of dried radish noisture Ascorbic (kg water acid(mg/ /kg dry 1009 dry solid) solid)
Energy consumption srowning ( x lo9 J ) (O.D. at 420 nm)
*Quality constraints: ascorbic acid retention 330 mg/100g dry solid, browning 0.177
Z n S]
8
0
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1044
LEE AND PYUN
Table 2 shows optimal drying conditions of cocurrent tunnel dryer under quality constraints for various total drying times. For short total drying time of 140 minutes, optimized air temperature is 90 OC of upper limit with low recycle ratio and high air flow rate. With longer total drying time air temperature and flow rate decrease, and air recycle ratio increases. Total drying time longer than 210 minutes reduces energy consumption. Level of maximum air recycle ratio allowable by total drying time and product quality constraints seems to determine energy consumption level. Thompson et a1.(1981) showed that increasing air recirculation in tunnel drying results in significant energy saving. If constraints of total drying time and quality retention permit use of high recycle ratio it can reduce energy consumption. Por total drying times longer than 210 minutes optimized air recycle ratio stays at constant value of about 0.8 and thus energy consumption also stays constant. In optimized cocurrent tunnel drying ascorbic acid content of dried radish reaches the limit of quality constraint while browning level is far below the constraint limit, 0.177. Cocurrent tunnel drying in which food and drying air moves in same direction results in contact of high moisture content food with high temperature air at tunnel inlet and then subsequent contact of lowered moisture radish with cooled air in tunnel (Figure 10). ~igh sensitivity of ascorbic acid destruction at high moisture content and high sensitivity of browning at relative low moisture (Figures 2, 3) thus produce in cocurrent operation relatively higher destruction of ascorbic acid compared with nonenzymatic browning. Optimized operating conditions of counter current drying similarly show higher air temperature, lower air recycle ratio and lower air flow rate with shorter total drying time (Table 3). Influence of total drying time on
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1045
FIGURE 10. Simulated profiles of state variables during optimal cocurrent tunnel drying of radish. Total drying time: 210 minutes. For drying conditions refer to Table 2.
energy consumption is similar to that in cocurrent operation except that counter current operation consumes less energy. Compared with cocurrent drying (Table 2) browning of dried product is closer to the constraint limit. This can be explained by opposite direction of movement of food and air flow in tunnel (Figure 11).
Socurrent Drvinq vs. Counter Current Drvinq Optimal operating conditions of counter current tunnel dryer mostly show control variables of lower air temperature, lower recycle ratio and lower air flow rate compared with those of cocurrent drying (Table 2, 3 ) . In
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TABLE 3 Optimization Results of Counter Current Tunnel Drying of Radish under Quality Constraints for Various Drying Times* Drying. Optimal o~eratingcontrol variables . time (min.) Air Air recycle Air flow temperature ratio rate (OC) (kg dry air/min.)
Energy consumption noisture Ascorbic. Browning ( x lo9 J ) (kg water acid(mg/ (O.D. at /kg dry 1009 dry 420 nut) solid) solid)
350
0.14
60.7
0.82
247.8
g
Quality. of dried radish
334.1
0.11
2.41
*Quality constraints: ascorbic acid retention 330 mg/lOOg dry solid, browning 0.177
> Z
CI
7
C Z
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
TUNNEL DRYING OF FOOD
1047
" Food temperature
."b,
0
FIGURE 11. Simulated profiles of state variables during optimal counter current tunnel drying of radish. Total drying time: 210 minutes. or drying conditions refer to Table 3.
order to satisfy quality constraints (especially low browning) in counter current drying in which low moisture radish being dried meets hot inlet air, relatively lower inlet drying temperature should be applied. To finish drying within specified total drying time with lower temperature, lower air recycle ratio ought to be applied combined with proper air flow rate. Even with use of less recycle counter current operation shows lower energy consumption than cocurrent operation (Tables 2-4). Evaporative dryer efficiencies calculated for 210 minute drying time are 64.5\ and 67.6% for cocurrent and counter current operations, respectively. These observations agree with general nature of energy spending pattern in drying process (Oplia, 1978).
LEE AND P W N
1048
TABLE 4 Energy Balance in Optimal Tunnel Drying of Radish* Energy ( x 100 J)
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
Item Air heating
Cocurrent
Counter current
2336.9(92.4)
2244.5(93.0)
Food heating
114.9(4.6)
107.8 (4.4)
Dryer heating
26.1(1.0)
23.4
Air blowing
51.112.0)
38.9 (1.6)
Energy in 11.0)
Total energy in
2529.0(100)
2411.61100)
Exhaust air
2435.0(96.7)
2334.1(96.9)
wall loss
53.712.1)
45.9(1.9)
3.5(0.1)
S.O(O.2)
2518.3(100)
2408.4(100)
Energy out Dried food
Total energy out
+ Total drying time: 210 minutes; The values in ( ) are percentage of each item divided by respective total energy in and out.
State variable changes in optimized cocurrent drying show fast moisture decrease and humidity increase in initial part of drying near tunnel inlet (Figure 10). Counter current drying shows sharp moisture decrease of radish and humidity increase of drying air opposite to food flow in middle part of the dryer (Figure 11). Food temperature stays at wet bulb temperature near tunnel inlet in both modes of operation, however counter current drying imposes high food temperature in latter part drying of low moisture radish because of incoming hot air. These profiles of
TUNNEL DRYING OF FOOD
1049
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
moisture and food temperature agree with those of general tunnel drying of fruits and vegetables (Kilpatrick et al., 1955). and result in higher ascorbic acid destruction in earlier part of cocurrent operation and higher browning in final stage counter current drying.
Enerqv Analvsis of Tunnel Drvinq Energy balance obtained by Equations (21) and (22) shows good agreement between energy input and energy output for tunnel drying process (Table 4), which confirms validity of our energy analysis. Most of energy is estimated to be consumed on air heating for drying and then escape from dryer with exhaust air. This underlines the importance of air recycle ratio in dryer operation for energy conservation. Heat recovery from exhaust air, on the other hand, might be considered as a further energy saving technique.
CONCLUSION This operation was tried product. drying of
work showed optimization scheme of tunnel drying as applied to radish drying. Energy consumption to be minimized under quality constraints of dried This kind of approach would be applied for tunnel other fruits and vegetables.
NOMENCLATURE A At a B '=A
cp CpA CpE CpG
: : : : : : : : :
Wall area o f dryer (m2) Tray area (m2) Tray area per dryer volume (m2/m3) Browning level (optical density(0.D.) at 420 nm) Ascorbic acid concentration tmg/100g dry solid) Specific heat of food (J/kg.OC) Specific heat of ambient air (J/kg.Oc) Specific heat of exhaust air (J/kg.Oc) Average seecific heat of drying air (J/kq.OC)
LEE AND PYUN : Specific heat of inlet air (J/kg.OC) : Activation energy of ascorbic acid destruction
(cal/mol) Activation energy of browning (cal/mol) : Characteristic drying curve : Air flow rate (kg dry air/min) : Reference flow rate at 2.3m/s.m2 (kg dry air/min) : Absolute humidity of ambient air (g water/kg dry air) : Absolute humidity of drying air (kg water/kg dry air) : Absolute humidity of drying air at food entrance of tunnel (kg water/kg dry air) : Saturation humidity of drying air at food temperature (kg water/kg dry air) : Absolute humidity of drying air at food exit of tunnel (kg water/kg dry air) : Humidity potential of drying(kg waterfig dry air) : Enthalpy of ambient air (J/kg dry air) : Enthalpy of exhaust air (J/kg dry air) : Drying rate constant (g/h.g dry so1id.A~) : Constant rate drying coefficient (g/h.g dry so1id.A~) : Constant rate drying coefficient at air flow rate of G, (g/h.g dry solid.AH) : Rate constant of ascorbic acid destruction (min-l) : Frequency factor in ascorbic acid destruction ~min-1) : kate constant of b r o m i n e (AEr*n/minl -,.up : kb at temperature-of Tref ( d ~ ~ ~ ~ / m i n j : rood flow into dryer (kg dry solid/min) : moisture content of food (kg water/kg dry solid) : Initia:L moisture content Of food (kg water/kg dry solid) : Final moisture content of food (kg water/kg dry solid) : Energy consumed for dryer heating ( J ) Qdh : Energy consumed for food heating ( J ) Qfh : Energy output involved in dryer after drying ( J ) Qd : Energy output involved in dried food after drying Qf (JI k;rgy consumed for heating of drying air(J/min) Pheat Energy consumed for air circulation (J/min) Pcir Energy consumed at air flow rate of G r (J/min), Bcirr Obtained from fan manufacturer. : Energy lost through dryer wall (J/min) : Energy gone with exhaust air (J/min) : Gas constant (1.987 cal/mol.K) : Drying rate (;J/h.g dry solid or kg/mZ.min) : Constant drying rate (g/h.g dry solid or kg/m2.min) : Air recycle ratio : Food temperature ( R ) : Reference temperature (338K) : Ambient air temperature (OC)
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
:
-.
TUNNEL DRYING OF F O O D TE TG T~
: Inlet air temperature (OC)
TM
: :
TWb t U
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
: exit air temperature (OC) : Drying air temperature (OC or K )
xm Xf xs z X
: : :
: :
: :
Average drying air tem erature (OC) Wet bulb temperature ( gC or R) Drying time (min) Overall heat transfer coefficient ( ~ / m ~ . ~ c . m i n ) noisture content (decimal, wet basis) Pat content (decimal, wet basis) Content of solid excluding fat (decimal, wet basis) Distance from tunnel inlet of food entrance (m) Latent heat of moisture evaporation (J/kg)
REFERENCES American Society of Agricultural Engineers, 1979, Agricultural Engineers' Handbook, pp. 341-348. Bertin, R. and Blazquez, 1.. 1986, Modeling and Optimization of a Dryer, Drying Technology, 4(1), pp. 45-66. Bertin, R.. Pierronne, F. end Combarnous, PI., 1980, Modeling and Simulating a Distributed Parameter Tunnel Drier, Journal of Food Science, 45, pp. 122-125, 137. Box, M. J., 1965, A New Method of Constrained Optimization and a Comparison with Other Hethods, Computer Journal, 8, pp. 42-52. Brook, R.C. and Bakker-Arkema, F.W., 1978, Dynamic Programming for Processing Optimization, 1. An Algorithm for Design of Multistage Grain Dryers, Journal of ~ o o d Process Engineering, 2, pp. 199-211. Bruin, S. and Luyben, K.Ch.A.M., 1980, Drying of Food Materials: a Review of Recent Developments, pp.155-215, in A.S. Mujumdar led.) Advances in Drying Vol. 1, Bemisphere Publishing Corporation, Washington D.C. Doe, P.E. and Menary, R.C. 1979, Optimization of the Bop Drying Process with respect to a-acid Content, Journal of Agricultural Engineering Research, 24, pp. 233-248. Eendel, C.E., Silveira, V.G. and Barrington, W.O., 1955, Rates of Nonenzymatic Browning of White Potato during Deh dration, Food Technology, 9, pp. 433-438. Rare?, M., 1988, Optimizing the Beat Sensitive Materials in Concentration and Drying, pp. 217-234, in S. Bruin led.) Preconcentration and Drying of Food Materials, Elsevier, Amsterdam. Keey, R.B., 1975, Batch Drying with Air Recirculation, Chemical Engineering Science, 23, pp. 1299-1308. Reey, R.B., 1975. The Process Optimization of a Conveyor Dryer, New Zealand Engineering, 30, pp. 53-57. Reev. R.B.. 1977. Process Desian of Continuous Drvino * ~iuipment,.AICEE symposium series; 73(163), pp. 1-11. Reey, R.B., 1978, Introduction to Industrial Drying operation, Pergamon Press, Oxford, pp. 198-209, pp. 259321. ~
-
Downloaded by [The University of Manchester Library] at 05:42 22 April 2016
1052
LEE AND PYUN
Rilpatrick, P.W., L0we.E. and Van Arsdal, W.B., 1955, Tunnel Dehydrators for Fruits and Vegetables, Advances in Food Research, 6, pp. 313-372. Lee, D.S. and Pyun, Y.R., 1988, Kinetics Determination of Quality Changes for the Optimization of Food Dehydration, Korean Journal of rood Science and Technology, 20(2), pp. 272-279. Mishkin, M., Saguy, I. and Rarel, M., 1982, Applications of Optimization in Pood Dehydration, Pood Technology, 36(7), pp. 101-109. Mishkin, M., Saguy, I. and Rarel, M., 1983, Dynamic Optimization of Dehydration Processes; Minimizing Browning in Dehydration of Potatoes, Journal of Food Science, 48, pp. 1617-1621. Mishkin, M., Saguy, I. and Rarel, M., 1984a, Optimization of Nutrient Retention During Processing: Ascorbic Acid in Potato Dehydration, Journal of Food Science, 49, pp. 12621266. Mishkin, M., Saguy, I. and Rarel, M., 1984b. A Dynamic Test for Kinetic models of Chemical Changes during P~ocessing: Ascorbic Acid Degradation in Dehydration of Potatoes, Journal of Pood Science, 49, pp. 1267-1274. Oplia, R.L., 1978, Energy for Food Processing. Chemtech, 8, pp. 104-107. Rha, C.R., 1975, Theory, Determination and Control of Physical Properties of Food Materials, D Reidel Publishing Co., Dordrecht, Holland, pp. 319-321. Saravacos, G.D. and Charm, S.E., 1962, A Study of the Mechanism of Fruit and Vegetable Dehydration, Food Technology, 16(1), pp. 78-81. Sokhansanj, S., 1984, Grain Drying Simulation with respect to Energy Conservation and Quality, pp.121-180, in A.S. mujumdar (ed.) Advances in Drying vol. 3, Hemisphere Publishing Corporation, Washington D.C. Thompson, T.L., 1970, Simulation for Optimal Grain Drier Design, Transactions of the ASAE, 13, pp. 844-848. Thompson, J.F., Chhinnan, M.S., Miller, M.w. and Knutson, G.D., 1981. Energy Conservation in Drying of sruits in Tunnel Dehydrators, Journal of Food Process Engineering, 4, pp. 155-169. Thygeson Jr., J.R. and Grossmann, E.D., 1970, Optimization of a Continuous Through-circulation Dryer, AICHE Journal, 16, pp. 749-754.
