journal of industrial engineering research · 2015-10-22 · 19 arif bin ab hadi et al, 2015...

Journal of Industrial Engineering Research, 1(6) September 2015, Pages: 16-24

IWNEST PUBLISHER

Journal of Industrial Engineering Research

(ISSN: 2077-4559)

Journal home page: http://www.iwnest.com/AACE/

Corresponding Author: Arif bin Ab Hadi, Department of Chemical and Process Engineering, Faculty of Engineering and

Built Environment, National University of Malaysia, 43600 UKM, Bangi Selangor, Malaysia

The Study of Temperature Distribution for Fresh Fruit Bunch during Sterilization Process. Arif bin Ab Hadi, Prof. Dato’ Ir. Dr. Abd. Wahab Mohammad, Prof. Ir. Dr. Mohd Sobri Takriff

Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, National University of Malaysia, 43600

UKM, Bangi Selangor, Malaysia

A R T I C L E I N F O A B S T R A C T

Article history:

Received 23 July 2015

Accepted 25 August 2015 Available online 12 September 2015

Keywords: FFB temperature distribution,

sterilizer cage, multiple peak, FFB

temperature profile FFB, heat transfer rate, cost of steam, heat transfer

efficiency.

The non-uniformity of temperature distribution across the fresh fruit bunch sterilizer

cage during conventional sterilization will lead to inefficiency heat transfer inside

sterilizer cage. The heat transfer efficiency during sterilization process based on FFB temperature which is crucial for process optimization is not being monitored by the

current system. A set of 6 temperature sensors was inserted at various locations inside

the cage. The temperature profile inside FFB was determined using spreadsheet heat transfer modeling tool based on temperature profile obtained from mill. The heat

transfer inside cage was calculated based on temperature profile. Important findings in

this study suggest that the efficiency of heat transfer towards FFB across cage can be obtained through temperature distribution study.

© 2015 IWNEST Publisher All rights reserved.

To Cite This Article: Arif bin Ab Hadi, Prof. Dato’ Ir. Dr. Abd. Wahab Mohammad, Prof. Ir. Dr. Mohd Sobri Takriff., The study of

Temperature Distribution forFresh Fruit Bunch during Sterilization Process. J. Ind. Eng. Res., 1(6), 16-24, 2015

INTRODUCTION

In any stage during sterilization, it is essential to ensure a sufficient heat penetration from the steam towards

the fresh fruit bunches for good sterilization. Previously, it is mentioned that USB and oil loss are used as an

indicator for sterilization efficiency. This, however, serves as a feedback indicator as the results are known much

later after sterilization takes place [3]. It is more accurate and logical to define sterilization efficiency based on

heat transfer between steam and fresh fruit bunches. By doing so, process optimization can be made possible

through determination of heat transfer rate for each individual fruit bunches.

The fresh fruit bunch (FFB) must be allowed time to heat through thoroughly and become “cooked”. With

satisfactory sterilizing the temperature reached in the center of the stalk will be found to be at least 100°C and the

time required to reach 100°C will depend on the weight of the individual bunches. It would be about 25 to 30

minutes for small bunch (3 to 6 kg) and about 50 minutes for bunches of 17 kg under normal operating conditions

[1].

Another important factor determining the sterilization cooking time is the pressure within the vessel during

the cooking period. In general, the rate of heat penetration into a bunch is proportional to the temperature

difference between the steam and the bunch [2]. The efficiency of stripping depends not only on the time reached

during a sterilization cycle but also the duration the temperature was maintained. For satisfactory conditions of

heat penetration after efficient deaeration, the time during which the temperature at bunch core is maintained

above 100°C should not be less than 35 minutes even when the temperature attained is 130°C [2]

Some mills, which have a temperature indicator located inside the sterilizer only gives the temperature

reading within the whole sterilizer at some point. However, it does not reflect the temperature of each FFB

located at different locations inside the sterilizer cages which indicates efficiency of FFB cooking process

individually. Hence, in this study, temperature inside the sterilizer cage is used as the main indicator to indicate

the above.

In a nutshell, the importance of monitoring temperature inside sterilizer cage during sterilization process is

crucial towards improving the sterilization process through process optimization.

17 Arif bin Ab Hadi et al, 2015


Objective:

The aim is to investigate the heat transfer efficiency towards FFB during sterilization by gathering and

analyzing the FFB temperature distribution data inside sterilizer cage and calculating the heat transfer rate and

steam consumption towards FFB through developed mathematical spreadsheet model.

MATERIALS AND METHODS

An experimental approach to gather data and analyze the temperature distribution and heat transfer inside

sterilizer cage at palm oil mill will be made. Based on the data gathered from mill, mathematical model

developed using spreadsheet software will be used to predict the center temperature inside FFB in order to

estimate the amount of heat transfer rate inside the sterilizer cage.

1. Mill Experimental Setup:

The experimental equipment to be used for the mill will be based on the horizontal sterilizer type as per. The

experimental work involves one of the 3 sterilizer train and one of the cages inside the train at East Oil Mill as the

experiment equipment set-up during their normal operation.

Figure 1 shows the wireless type sensor DataTrace Pro logger from MesaLab to be used for the mill

experiment. This sensor comes with a set of data logger reader, battery replacements and software for temperature

recording purposes. Using the software provided, the sensor can be configured accordingly, which includes

calibration, setting date and time for data logging etc.

Fig. 1: Wireless Sensor DataTrace Pro logger

With a set of total 6 sensors, the proposed setup to locate the sensors was done according witdh, height and

length of sterilizer cage, denoted as W, H, and L, respectively which corresponds to the x, y and z axis

coordinate. The sensors denoted as Z1, Z2, Z3, Z4, Z5 and Z6 is located at a fixed position along the z axis. A

special device which is used to locate the sensors at its required place is shown as per Figure 2a. The location of

temperature sensor denoted as T1-T36 inside the sterilizer cage from the front view is shown as per Figure 2b.

2. Mill Experimental Procedure:

The sensors were located along varying height (y axis) and the width (x axis) at fixed length (z axis). Figure

2c shows the schematic diagram for the 1st run setup. The location of sensor along the z axis on the y and x axis

(Y1, X1, Z1-Z6) which is shown by the red arrow is denoted as T1. After completing the 1st run of the first FFB

batch, proceed with next run on the next FFB batch by placing the sensors along the y axis at different x axis (Y1,

X2, Z1-Z6) as shown by the red arrow in Figure 2d. This location is to be denoted as T2.The steps are repeated

until all the readings along y axis at Y1 (Y1, X1-X6, Z1-Z6) denoted as T1-T6 is obtained inside the cage. The

remaining locations are to be repeated for Y2-Y6 (Y2 denoted as T7-12 up till Y6 denoted as T31-36).



Fig. 2: a) A device to place the temperature sensor b) Location of temperature sensor inside sterilizer cage

Fig. 2c: Schematic diagram of temperature sensor location in sterilizer cage (Y1, X1, Z1-Z6)

Fig. 2d: Schematic diagram of temperature sensor location in sterilizer cage (Y1, X2, Z1-Z6)

The main purpose of having a mathematical modeling for single fresh fruit bunch is to predict the center

temperature of FFB during sterilization process to indicate whether the fruit is sufficiently “cooked”. The

modeling work of heat transfer for fresh fruit bunch was made by applying finite differential explicit method for

two dimensions using spreadsheet. An overview of previous work which was based on heat conduction

spreadsheet model to predict time required for mesocarp to attain thermal equilibrium with the steam temperature

is given by Mohd. Halim Shah. I [7]. In this study, the prediction of the center temperature of FFB based on the



data from mill will be used in order to estimate the heat transfer rate and steam consumption at different locations

inside the cage.

The rate of heat transfer from heating steam towards FFB is obtained by using the following equation:

where Q = Mean heat transfer rate (kJ/s)

m = Mass of the FFB (kg)

Cp = Specific Heat Capacity of FFB (kJ/kg°C)

∆T= Change in FFB temperature (°C)

t = Time for the heating process (s)

The steam consumption can be determined from the heat transfer rate of the condensing steam:

where Q = Mean heat transfer rate (kJ/s)

ms = Mean steam consumption (kg/s)

hfg = Specific enthalpy of evaporation of steam

Assuming 100% heat transferred from the heating steam towards FFB, then

The heat transfer rate, Q (kJ/kg FFB), based on previous calculation were used to estimate the mean steam

consumption based on per cycle and per year basis with the above assumption. The heat transfer efficiency was

determined based on calculated overall rate inside cage and highest rate within cage. The heat transfer

efficiency, which can be translated into excess amount of mean steam consumption per year, is used to estimate

the economic value of excess steam consumption.

The unloaded cost is a basic comparison between the amount of steam produced and the cost of fuel required

to produce it [5]. The calculation for the unloaded steam cost is as below:

Where

S = Unloaded steam cost ($) per 1000lb of steam

a = Fuel cost ($/MMBTU)

Hg= Enthalpy of steam (Btu/lb)

Hf= Enthalpy of Boiler Feedwater (Btu/lb)

ηB = True boiler efficiency (ASME PTC 4.1)

RESULT AND DISCUSSION

A total of 36 trials have been conducted which consists of temperature readings of the intended locations

inside the sterilizer cage. The sensor is located in between the FFBs and the sensor probe is directed towards the

outer surface of the FFB, which represents the heating steam temperature. The data collected at each 30 seconds

interval for the whole 80 minutes of sterilization period. Figure 3 represents the temperature profile inside

sterilizer cage for all node locations (T1-T36) from top towards the bottom during whole sterilization period.

Total running sterilization time is about 80 minutes. The time starts at 0 s since initial steam admission into the

sterilizer and ends after blow down and pressure inside sterilizer reaches 0 bar g.

S



Fig. 3: Temperature profile for all node locations (T1-T36) inside sterilizer cage

The temperature pattern follows the triple peak sterilization cycle. The sterilization cycle consist of initial 2

minutes of deaeration followed by 12 minutes of first peak pressure build up to 20 bar g and exhaust, 13 minutes

of second peak up to 30 bar g and exhaust, 10 minutes of third peak up to 40 bar g, 43 minutes of holding period

and few minutes of blow down. Figure 4 and Figure 5 shows temperature profile for T1 and T36, respectively.

By referring to Figure 4 for T1, we observed that the first peak has been achieved at a time of 840 seconds or

14 minutes since initial steam admission at a temperature of 126°C. The second peak has been achieved at a time

of 1380 seconds or 23 minutes at a temperature of 134°C. The third peak achieved at 2130 seconds or 36 minutes

at a temperature of 141°C and maintains this temperature for about 44 minutes during holding period. Finally, the

temperature drops at a time of 4800 seconds or 80 minutes indicating the end of holding period during which

blow down takes place.

Figure 5 shows the temperature profile for T36 and the segregation of steps during sterilization cycle. For

comparison, we assumed the time taken to achieve intended pressure for T36 is the same as T1. Therefore, the

first peak for T36 which was achieved at a time of 840 seconds since initial steam admission for T1 corresponds

to a temperature of about 113°C. The second peak was achieved at a time of 1380 seconds corresponds to a

temperature of about 118°C. The third peak was achieved at a time of 2130 seconds corresponds to a temperature

of about 131°C.

1st peak

(126°C, 14 min)

2nd

peak

(134°C, 23 min)

Holding period

(44 min) 3

rd peak

(141°C, 36 min)

Blow down

period

Fig. 4: Temperature versus time for T1 during sterilization cycle steps



Fig. 5: Temperature versus time for T36 during sterilization cycle steps

We observe that during the initial period of sterilization, the temperature rises rapidly at the top of the cage,

suggesting the steam that is being introduced from the top of the vessel comes in contact quickly with the sensor,

hence the FFB at this location during this period. However, at the bottom, we observe a delay of temperature rise

during the same initial period of the top of the cage, indicating the contact delay between the same heating source

and the sensor, hence the FFB at this location.

By using spreadsheet model, the temperature profile inside FFB from T1 to T36 was determined based on the

mill experimental data. The model is able to predict the temperature pattern along the radius of FFB from outer

towards the inner of FFB. In this case, we wanted to predict the temperature at the center of FFB in order to

determine the rate of heat transfer and estimation of steam consumption. Figure 6 represents the temperature

profile at the center of FFB for all node locations (T1-T36) from top towards the bottom during whole

sterilization period. The temperature profile obtained is based on average weight of FFB about 17kg.

Fig. 6: Temperature profile at the center of FFB for all node locations (T1-T36) inside sterilizer cage



Fig. 7: Temperature profile at the center of FFB versus time for node location T1 inside cage

Fig. 8: Temperature profile at the center of FFB versus time for node location T36 inside cage

Figure 7 and Figure 8 shows temperature profile for T1 and T36, respectively. Since the temperature

difference across Z1-Z6 is very small and can be neglected, the temperature at Z1 is chosen in this case. From

Figure 7, we observed that the temperature at the center of FFB located at T1 which is top of the cage is estimated

based on the model to reached 100°C at 53 minutes after the start of sterilization and maintain above 100°C for

about 30 minutes before end of sterilization period. The estimated temperature value is closed to the temperature

profile obtained from previous modeling work by Chan SY [6]. However, from Figure 8, we observed that the

temperature at the center of FFB is estimated to reached 100°C at 64 minutes after start of sterilization and

maintain above 100°C for about 20 minutes before end of sterilization period.

The value of temperature change inside FFB was determined based on the average value of differential

temperature change, ∆T over the sterilization period, ∆t. In this case, the differential temperature change was

based on the selected ∆t determined previously from the model, which is 5s. Table 1 shows an example of

calculated values for temperature change per time for T1 Z1-Z6. Since the percent difference between Z1-Z6 is

very small compared to average value of Z1-Z6 (<0.1%), T1 Z1-Z6 will be represented as T1 and for similarly

for subsequent values from T2-T36.



Table 1: Calculated values for T1 Z1-Z6

Table 2: Heat Transfer Rate per unit kg FFB T1-T36

Table 2 represents a summary of calculated heat transfer rate towards FFB for T1-T36. We observed that

the heat transfer rate at the top decreases with time during sterilization. Towards bottom of the cage, the rate

increases with time which indicates the delay effect of steam travel path from top towards the bottom of the

cage. The total heat transfer rate during sterilization at the top is the highest and decreases towards the bottom of

cage.

The heat transfer rate inside the sterilizer cage which is taken as the mean value, Qmean per unit kg FFB

based on average value for each location (T1-T36) is calculated as per below:

Qoverall = = 284.12 kJ/kg FFB

The mean heat transfer calculated is used to estimate the heat transfer efficiency within sterilizer cage as per

below:

Heat Transfer Efficiency = = = 95.5%

This value indicates the efficiency of heat transfer within the cage is 95.5%. The result indicated that the

efficiency of heat transfer toward FFB inside sterilizer cage is estimated about 95% at the bottom as compared

to the top. The ideal heat transfer value was taken based on the highest heat transfer rate within the cage.

Table 3: Steam Consumption per cycle (kg/cycle) T1-T36



Table 4: Steam Consumption per year (kg/year) T1-T36

Table 3 and 4 shows the estimated steam consumption calculated per cycle and year basis. The total mean

steam consumption was calculated based on the average of total FFB mass per sterilizer of 24.5 ton FFB,

specific enthalpy of condensing steam hfg at 40 psig = 2140kJ/kg (from steam table) and the total steam

consumption per year was calculated based on assumption that mill runs an average of 15 cycles per day, 25

days per month and 10 months per year.

The heat transfer efficiency, which also can be translated into amount of steam consumption per cycle and

per year was determined based on difference between the ideal and mean steam consumption rate value within

the sterilizer cage as per below:

(7.2863 x 106 - 6.9609 x 10

6 kJ per cage)/(2.14 x 10

3 kJ/kg) = 152.056kg of steam per cycle

(152.056 kg of steam/cycle)(15cycle/day)(25days/month)(10months/year) = 5.7021x 105 kg per year

This value can also be translated to economic value on the amount of excess steam consumption per year

due to inefficient heat transfer within the sterilizer cage. The estimation of total amount of cost saving calculated

based on excess steam consumption data:

($US6.81/1000lbsteam)(2.2046lb/kg)(5.7021x105kg steam/year) = $US8560.75/year = RM30818.70/year

Conclusion:

Based on the results obtained in this study, it is shown that temperature distribution inside sterilizer cage

has shown proven potential towards optimizing steam consumption based on calculated heat transfer efficiency

inside cage. The result indicated that the efficiency of heat transfer toward FFB inside sterilizer cage is

estimated about 95% efficiency for the bottom as compared to the cage top. The efficiency towards heat transfer

which has been translated into excess steam consumption and economic value has proven to bring added value

towards optimization of sterilization process.

REFERENCES

[1] Palm Oil Factory Process Handbook Part 1, PORIM, 1985.

[2] Mongana Report, First Volume, 1955

[3] N Ravi Menon, Innovation Potentials in Palm Oil Mill Design. Palm Oil Engineering Bulletin, 104: 9-12

[4] Frank, P., Incropera, David P. Dewitt, 2002.Fundamentals of Heat and Mass Transfer, 5th

edition. John

Wiley & Sons

[5] Swagelok Energy Advisors, Inc. 2011. Knowing the cost of steam. Document No.31

[6] Chan, S.Y., 1985. Modeling and Simulation of the Sterilised of Fresh Oil Palm Fruit Bunches (FFB),

University Malaya: Master Thesis.

[7] Mohd Halim Shah, I., A.A. Mustapa Kamal, M. Noor Azian, 2009. A system approach to Mathematical

Modeling of Sterilization Process in Palm Oil Mill. European Journal of Scientific Research.

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