Process integration in the food industry
Prof. François Maréchal (École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne)
Process integration, initially developed for the chemical processes, is of big interest in the
food and agro production sites provided it is adapted to the specificities of such industry. When
adopting a system approach, food processing is converting raw materials into products, the driver
of the production being energy while the support of production is mostly water. The process
units in the food industry are either processing solids (dryers, roasters,...) or liquids (cooking,
fermenting, ...). In addition, the food production requires side operations like packaging, cold
storage, cleaning in place and air conditioning that have to be considered simultaneously with
the process itself. The temperature levels is relatively limited and is most of the time crossing the
ambient temperature. Food and agro processes are therefore good candidates for the integration
of combined heat and power production (CHP) and of industrial heat pumps. The use of water
as a support media in the process makes also the food and agro processes, interesting candidates
for a combined heat and water recovery. Finally, as the feedstock is a biomass resource, the
process results in biomass waste production that can be converted into energy resources (heat
and power) for the process. Process integration is an essential tool for the energy efficiency of
food and agro processes with the goal of converting conventional processes (Fig. 1(a)) into an
integrated system (Fig. 1(b)) that maximises the energy usage efficiency and the use of renewable
energy resources. In addition, due to the multiplication of similar plants that food processing
industry offers a high potential for replication of solutions.
Industrial food and agro symbiosis system
Heat pumps and refrigeration
Heat recovery
Biomass + solar
Biomass
Raw materials
Raw materials
Waste
Waste management
Fossil resources
A
B
C
CO2
A
B
C
Exergy
A
B
C
Products and by-products
Products& by-products
Key performance indicators
Key performance indicators
Fossil resources
CIP
Packaging
Conditioning
Processing
Cooling & refrigeration
CIP
Packaging
Conditioning
Processing
Cooling & refrigeration
Heat
Heating
Electricity
Heating
Conversion
Electricity
Cogeneration
Conversion
Food or agro process
Waste
A
B
C
Heat losses
CO2
Costs
(a) Before Integration
A
B
C
Exergy
Waste
Heat losses
A
B
C
Costs
(b) After Process integration
Figure 1: The process integration approach in the food and agro industry
The rational use of energy in the food and agro processes requires first a method to define
the energy requirement of the production. A top down approach [4] is used to identify the
major energy consumers on the production site. The 80-20 rule is used to identify the most
important process unit operations that explain the overall energy bill. From this analysis, the
process unit requirements are defined in terms of heat transfer (heating and cooling), water
consumption, utility and electricity requirement as well as in waste and water productions.The
systematic development of process unit integration models including the detailed analysis
of the energy usage by exergy analysis and the definition of the process integration interfaces
can be realised and embedded in energy management tools [4]. These models can then be
deployed in data bases and offered to be used in software (e.g. www.pinchlight.ch or the energy
technologies data base of the OSMOSE platform (EPFL)).
Considering the discontinuous or batch operation nature of most of the food and agro processes, time averaging technique [3] is used to set the heat recovery target of the plant. This
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is fully justified if one realises that most of the heat exchange requirement have a temperature
range compatible with conventional heat storage technologies. This requires however the adaptation of the ∆min contributions assumptions that have to be associated to the type and the
availability in time of the different heat transfer requirements. The heat cascade allows to target
the heat recovery in the system. This technique can be combined with producer/consumers
models for the water recovery to set the combined heat and water recovery targets. The resulting
Grand composite curves are used to assess the integration of the energy conversion system. The
optimal scheme for heat pumping and CHP are highly integrated and requires the generation
of system configurations where several heat pumps (including refrigeration) are integrated ([1])
with the cogeneration and the process. Those schemes are obtained by applying optimisation
models (mixed integer linear programming) of the heat cascade that allow the simultaneous
targeting of the heat recovery and the energy conversion in the process ( [2]).
The use of mathematical techniques also allows engineers to transform the heat recovery
targets into practical solutions. In the food industry, this means mainly to identify the heat
transfer fluids that have to be used to avoid a direct exchange between process units. In her
thesis, Becker ([1]) has proposed the incorporation of restricted matches constraints in the
targeting procedure to identify the temperature enthalpy profiles of the heat transfer fluids and
to calculate and optimise their flow in the integrated system considering at the same time the
integration of the utility streams.
The mathematical model is formulated as a multi period mixed integer linear programming problem ([1]) to define the size of the heat storage tanks in the system. The resolution
of such problem gives not only the size of the units but also the strategy of the operation. This
makes the food processing processes actors of the electrical grids management since the combined
heat and power production system including the heat pumps will have the possibility of dephasing the heat supply and the electricity production/consumption, therefore taking opportunity of
the price of electricity on the market.
The end of this analysis defines the energy target of the total food production site. It also indicates opportunities for energy savings by system boundaries extension and by the development
of industrial symbiosis concepts. The complete list of streams to be considered in the heat
exchanger network design is therefore available and engineers will develop and optimise the heat
exchanger network design in a multi-period frame work. Considering that such problem becomes at the same time a scheduling problem, mathematical programming techniques will
therefore be applied to generate, validate and operate the final energy system configuration of
the process.
References
[1] Helen Carla Becker. Methodology and Thermo-Economic Optimization for Integration of
Industrial Heat Pumps. PhD thesis, STI, Lausanne, 2012.
[2] Francois Marechal, Anurag Kumar Sachan, and Leandro Salgueiro. 5.3 Application of Process
Integration Techniques in the Brewing Industry. In J. Klemes, editor, Handbook on process
integration. Woodhead Publishing Ltd, 2013.
[3] B Linnhoff, G. Ashton, and E. Obeng. Process integration of batch processes. IChemE
Symposium Series, 109:221–237, 1988.
[4] D Muller and F Marechal. Energy management method in the food industry. In Handbook
of water and energy management in food processing. Woodhead Publishing Ltd, 2008.
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