Ref: C0689
A review on exergy analysis of food production processes
Z. Deniz ALTA and Can ERTEKİN*, Department of Farm Machinery, Faculty of Agricultural
Engineering, Akdeniz University, 07070, Antalya, Turkey
Abstract
Exergetic analysis provides a tool for a more realistic view between energy losses to the environment and internal irreversibilities in the process. This paper investigates the impact of
food production processes in terms of energy and exergy utilization. The main defined energy utilization mechanisms in food production in this study are labour and energy consumption
by industrial processes to perform mixing, cooling, heating etc. The aim of this work is to
extract valuable guidelines for designing sustainable food processes from a thermodynamic
perspective. The present literature review also summarizes the using of exergy analysis in
food production unit operations and facilities, discovers it benefits and abilities, and identifies
prospects for future researches.
Keywords: energy, exergy, food, production, process
1
Introduction
The design of an energy-intensive system for lower cost and high efficiency is one of the essential approaches for sustainable development. Due to the high prices of energy, environmental concerns, increasing World population and decreasing fossil fuel resources, the optimum application of energy and energy consumption management methods are vital
(Aghbashlo et al. 2013). Energy analysis based on the first law of thermodynamics is a basic
and traditional approach to estimate various energy conversion processes. However, it provides no information about the irreversibility aspects of processes. Due to the deficiencies
and shortcomings of energy analysis, the exergy analysis which provides a more realistic
view of the systems and processes has been widely used, recently (Dincer and Rosen 2007;
Çengel and Boles 2008). It is a powerful to study and optimize various types of energy systems.
The main objective of exergy analysis of the systems is to provide a clear picture of the process, to quantify the sources of inefficiency, to distinguish the quality of energy consumption,
to select optimal operation conditions and reduce the experimental impacts. It can be successfully used to choose the appropriate component design & operating procedure. Many
engineers and scientist suggest that the thermodynamic performance of a process is best
evaluated by performing an exergy analysis (Rosen and Dincer 2001). This study tried to
summarize some applications of exergy analysis for food production processes and systems
as an instance.
* Corresponding author; Tel: 90 242 3102481; Fax: 90 242 2274564;
E-mail address: [email protected] (C. Ertekin)
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2
Exergy analysis and exergy concept
The three balance equations (mass, energy, and exergy balance) were used to find the heat
input, rate of exergy destruction, and energy and exergy efficiencies for a general steadystate and steady-flow process (Figure 1).The law of conservation of mass and energy states
that mass and energy can neither be created nor destroyed, whereas the second law of
thermodynamics deals with the quality of energy to cause change, degradation of energy
during a process, entropy generation and the lost opportunities to do work.The total mass of
material entering the plant must equal the total mass of material leaving the plant. And in
similar, the total energy in the materials entering the processing plant, plus the energy added
in the plant, must equal the total energy leaving the plant. Exergy is not subject to a conservation law, but is destroyed due to irreversibilities during any of the thermal processes, including drying, heating, boiling, freezing etc. for food industry. It means that reducing the irreversibilities in a production system will increase the exergy (i.e. availability) and hence the
efficiency of the system (Dincer 2002).
Exergy
destruction
Exergy
Exergy
Figure 1: Schematic of flows in a unit operation
The mass balance equation can be expressed in the rate form as;
where
is the mass flow rate, and the subscript in stands for inlet and out for outlet.
The general energy balance can be expressed below as the total energy inputs equal to
total energy outputs;
Exergy is defined as the maximum amount of work that can be produced by a stream of matter, heat or work as it comes to equilibrium with a reference environment. The general exergy
balance can be written as follows;
Exergy destruction associated with the irreversibilities (entropy generation) within the system
boundaries and exergy losses associated with the transfer of the exergy (through material
and energy streams) to the surroundings are given by;
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where ex is the specific exergy of the components, such as water, product, and air.
Here, in a similar way, exergy efficiency is defined as the ratio of total exergy output to
total exergy input;
Maximum improvement in the exergy efficiency for a process or system is obviously achieved when the exergy destruction or irreversibility is minimized.
3
Application of exergy analysis to the food production process and facilities
The study of process engineering is an attempt to combine all forms of physical processing
into a small number of basic operations, which are called unit operations. The essential concept is therefore to divide physical food processes into basic unit operations, each of which
stands alone and depends on coherent physical principles. Because of the dependence of
the unit operation on a physical principle, or a small group of associated principles, quantitative relationships in the form of mathematical equations can be built to describe them. The
equations can be used to follow what is happening in the process and to control and modify
the process if required (Earle 1966).
Important unit operations in the food industry are fluid flow, heat transfer, drying, evaporation,
boiling, freezing, contact equilibrium processes (which include distillation, extraction, gas
absorption, crystallization, and membrane processes), mechanical separations (which include filtration, centrifugation, sedimentation, sieving, pitting, peeling, slicing etc.), size reduction
and mixing.
Application of exergy analysis of food processing plants has been reported quite few studies
since the early 1980s.
The investigation of heat recovery and exergy balance in tomato paste plant was first studied
by Forciniti et al. (1985). Sogut et al. (2010) also studied on energetic and exergetic performance evaluation of the quadruple-effect evaporator unit (QEEU) in the same sector. It was
found that the highest exergy destruction occurs in the first effect because of high temperature differences between steam and tomato paste inlet. Automatic control system was suggested to eliminate the pressure loss caused from the collection of steam into a large tank
before entering the evaporator unit.
Waheed et al. (2008) carrying out energy and exergy studies conducted in an orange juice
manufacturing industry. The exergy analysis revealed that, the pasteurizer was responsible
for most of the inefficiency (over 90%) followed by packaging (6.60%). It was suggested that
the capacity of the pasteurizer could be increased to reduce the level of inefficiency of the
plant.
Sorguven and Ozilgen (2012) analyzed the production of strawberry-flavored yogurt, which
involves utilization of solar energy by plants to produce agricultural goods; feed consumption
by herbivores to produce meat and milk; fossil fuel consumption by industrial processes to
perform mixing, cooling, heating, etc. The extremely high exergy losses were accounted in
the study and the direction for the development of new technology in food processing was
suggested to decrease waste of energy and carbon dioxide accumulation in the atmosphere.
Fadare et al. (2010) presented the energy requirements and exergy inefficiencies for processing of malt drink for a Nigerian brewery. The results are showed that, the pasteurization
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process alone accounted for over 50% inefficiency of the overall system. Increasing the capacity of pasteurizer unit and utilization of process heat integration between pasteurizer and
other units were suggested to reduce the load on the boiler and also improve the energy efficiency of the system.
Bozoglan and Hepbaslı (2010) observed the exergy efficiency of an olive oil refining plant
and determined the exergetic destructions in each devices in the plant. The functional exergetic efficiency of the plant is obtained to be 12%, while the exergy efficiency on the exergetic fuel–product basis is found to be about 65%.The maximum value of the exergy destruction rate was found in boiler, followed by distillation unit and stream generator. To increase the exergy efficiency of the plant, examination of reverse osmosis and PH control
system to prevent corrosion in the boiler and usage of more efficient pumps were suggested.
Ozilgen and Sorguven (2011) assessed energy and exergy utilization and carbon dioxide
emission during production of soybean, sunflower, and olive oils. It was emphasized that,
agriculture is the most energy and exergy intensive process and emits most of the carbon
dioxide, and diesel is the dominant energy and exergy source. The most energy intensive
process was found as olive oil production, whereas the soybean agriculture results in a rather large CO2 emission. Decreasing the diesel consumption with good agricultural practices
and employing biodiesel were suggested.
Most of the studies in literature has been found regarding the exergy analysis of sugar production plants. Tekin and Bayramoglu (1998) applied the exergy analysis to the industrially
important process of sugar production from sugar beets. The process is partitioned into five
main units, namely, raw juice production, juice clarification, juice concentration, sugar refining
and steam-power production, and two auxilliary units, namely; vacuum system and hot water
processing-storage system. 74.4% of relative exergy losses occurs in steam-power system
due to the irreversible nature of combustion and also by means of waste exergies of stack
gases. An improved thermal recuperation, e.g. preheating of combustion air with hot combustion gases, and oxygen enrichment of the combustion air were suggested for significant
reductions in exergy losses.
Bayrak et al. (2003) investigated the energy and exergy analyses of an sugar production
plant which composed of four stages including sherbet production, sherbet distillation, sherbet thickening and refinery (sherbet crystallization). It was concluded that the exergy loses
took place mostly during the sherbet production process. It is generally suggested that the
irreversibility, mostly stem from the finite temperature differences at the production stages,
should be reduced to conduct more productively the sugar production process.
Other studies were carried out in sugar cane factories, such as the work by Raghu Ram and
Banerjee (2003), who evaluated two evaporation system designs using exergy analysis. Sahin et al. (2010) studied improvements on a sugar production plant and determined energy
and exergy efficiencies of the system and sub-systems. Taner and Sivrioglu (2013) presented the methods and equations for energy and exergy analysis of sugar factories. The importance of the application of these analyses to the sugar production processes for the increase of productivity was highlighted.
Ensinas et al. (2009) applied the exergy balance analysis in an integrated sugar, ethanol and
electricity process representing the current situation of sugarcane factories in Brazil. Production process was divided into eight sub-systems; juice exraction, juice treatment, juice evaporation, sugar boiling, fermentation, distillation, cogeneration system, condensate tank and
water cooling system. It was found that, the cogeneration system is responsible for 63% of
the total irreversibility generated in the base case with an exergetic efficiency of 18%. The
exergetic efficiency calculated for the whole plant in the base case, which represents a typical sugar and ethanol plant in Brazil, was 35%.Using of boilers with higher thermal efficiencies together with the process steam demand reduction was suggested.
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Table 1: Some of the recent studies and the most important results obtained using exergy analysis to
evaluate the food processing operations performance
_________________________________________________________________________
Author(s): Tekin and Bayramoglu (1998)
Product(s): Sugar
Operation units: Raw juice production, juice clarification, juice concentration, sugar refining and steampower production, and two auxilliary units, namely; vacuum system and hot water processing-storage
system.
Aim: To apply exergy analysis to sugar production process from sugar beet.
Outcome: The stream-power system is responsible for the highest exergy losses.
Author(s): Bayrak et al. (2003)
Product(s): Sugar
Operation units: Sherbet production, sherbet distillation, sherbet thickening, and refinery (sherbet crystallization)
Aim: To apply energy and exergy analyses to sugar production stages by using the operational data.
Outcome: The exergy losses took place mostly during the sherbet production process
(ηI=96.8% ηII=49.3%) because of the irreversibility in the sub-operation stages, which are vapour production, circulation sherbet mixing and bagasse compression.
Author(s): Raghu Ram and Banerjee (2003)
Product(s): Sugar
Operation units: Evaporation
Aim: To apply pinch analysis to evaporators in a sugar factory
Outcome: The amount of steam consumption will reduce by 9 T/h and exergy losses are reduced by
48% of its original value if the existing quadruple effect is modified to a quintuple effect
Author(s): Waheed et al. (2008)
Product(s): Fruit juice
Operation units: Fruit reception, bin house, sorting, cleaning, grating, crusher, screw finisher, centrifuge, holding tank, pasteurizer, packaging, warehouse
Aim: To determine energy consumption pattern and exergy destructions in an orange juice production
process
Outcome: The major exergy loss took place at the pasteurizer with an inefficiency of over 90%
Author(s): Ensinas et al. (2009)
Product(s): Sugar and etanol
Operation units: Juice exraction, juice treatment, juice evaporation, sugar boiling, fermentation, distillation, cogeneration system, condensate tank and water cooling system
Aim: To apply the exergy balance analysis in an integrated sugar, ethanol and electricity process
Outcome: Improvements in the cogeneration system were found to be the most important to reduce
exergy destruction
Author(s): Sahin et al. (2010)
Product(s): Sugar
Operation units: Sugarcane slaughter and sherbet production, sherbet distillation, sherbet thickening,
and refinery (sherbet crystallization)
Aim: To apply the exergy analysis in sugar production processes
Outcome: The best values of the energy and exergy efficiencies were obtained as 95·1% for the juice
production process and 74·3% for the juice concentration process respectively.
Author(s): Sogut et al. (2010)
Product(s): Tomato paste
Operation units: Evaporator unit
Aim: To evaluate the performance of quadruple-effect evaporator unit (QEEU) by using exergy analysis based on actual operational data
Outcome: The highest and lowest exergetic improvement potential occurs in first and third effect of
QEEU system, respectively.
Author(s): Fadare et al. (2010a)
Product(s): Malt drink
Operation units: Silo house, brew house, filter room, and packaging house
Aim: To estimate the energy requirements and exergy inefficiencies for processing of malt drink
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Table 1: (continued)
_________________________________________________________________________________
Outcome: The exergy analysis revealed that the packaging house operation was responsible for most
of the inefficiency (92.16%) and the most exergy loss took place in the pasteurizer, which accounted
for 59.75% of the overall system inefficiency.
Author(s): Bozoglan and Hepbaslı (2010)
Product(s): Olive oil
Operation units: Steam generators, several tanks, heat exchangers, a distillation column, flash tanks
and several pumps
Aim: To identify improvements in olive oil refinery plants’ performance
Outcome: The maximum exergy destruction rate takes place in the boiler with 490.7 kW, followed by
the distillation unit and the steam generator with 127 and 60.5 kW, respectively.
Author(s): Ozilgen and Sorguven (2011)
Product(s): Soybean oil, sunflower oil and olive oil
Operation units: Agriculture of olive, sunflower and soybean plants, conveying, cleaning, grinding,
mixing, decantation, pressing, extraction and phase, refining, bottling, pasteurization, packaging and
all transportations
Aim: To assess energy and exergy utilization and carbon dioxide emission during production of three
different vegetable oils.
Outcome: The most energy intensive process was found as olive oil production, whereas the soybean
agriculture results in a rather large CO2 emission.
Author(s): Sorguven and Ozilgen (2012)
Product(s): Strawberry-flavored yogurt
Operation units: Milk production, non-fat yogurt production, strawberry agriculture, sugar beet agriculture, sugar production, strawberry jam production and flavored yogurt production
Aim: To investigate the impact of food production processes on the environment in terms of energy
and exergy utilization and carbon dioxide emission.
Outcome: 53% of the total exergy loss occurs during the milk production and 80% of the total work
input is consumed during the plain yogurt making.
Author(s): Taner and Sivrioglu (2013)
Product(s): Sugar
Operation units: Sugarcane slaughter and sherbet production, sherbet distillation, sherbet thickening,
refinery, drying, power supply
Aim: To present the methods and equations for energy and exergy analyses of sugar factories
Outcome: For the profitability of enterprises, to maximum saving in energy must be achieved and minimization of exergy losses is needed.
_________________________________________________________________________
Another issue mostly studied on is deal with exergy analysis of drying processes of several
food products. Drying is one of the oldest methods of preserving food and mostly used in
food processing operations. Utilization of high amount of energy in the drying industry makes
drying one of the most energy-intensive operations with a great industrial significance (Dincer
2002). In the drying industry, the goal is to use a minimum amount of energy for maximum
moisture removal for the desired final conditions of the product. Several studies have been
conducted on exergy analysis of food drying process. The drying process was thermodynamically modeled by (Dincer and Sahin 2004) and drying of different products such as wheat
kernel (Syahrul et al. 2003), pistachio (Midilli and Kucuk 2003), plum (Hepbasli et al. 2010),
red pepper slices (Akpinar 2004), potato (Aghbashlo et al. 2008), apple slices (Akpınar et al.
2005), broccoli florets (Icier et al. 2010), pasta (Ozgener 2007), carrot slices (Aghbashlo et
al. 2009), milk (Jin and Chen 2011), white cheese (Erbay and Koca 2012). In this paper,
using of exergy analysis in drying operations and facilities was not explained in detail. It
could be helpful to glance at the research written by Aghbashlo et al. (2013) in this regard.
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Conclusions
This paper summarized some aspects of exergy analysis and its applications in food production processes. Literature survey showed that exergy analysis has been used in a variety of
applications for analyzing of production systems and could help to overcome problems in
many fields including efficiency improvement, energy resource utilization, and find the causes, locations, and magnitude of irreversibilities for food production systems. We hope that
this study will be a reference for the future researches.
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Nomenclature
T
Subscripts
in
out
0
Specific exergy (kJ/kg)
Energy rate (kJ)
Exergy rate (kJ)
Exergy destruction (kJ)
Mass flow rate (kg/h)
Temperature (K)
Exergy efficiency (%)
Inlet
Outlet
Reference (dead) state
kPa)
(303.15 K and 101.325
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