Energy Efficiency in the Food and Beverages Industry

Power Quality & Utilization Guide Application Guide For Food & Beverage Industry

Quentin Rosier — Laborelec February 2010

Steam

Vapor

Condensate Feed

Energy Efficiency

Concentrate

Energy Efficiency www.leonardo-energy.org

1. Introduction This Application Guide provides a detailed overview of available measures for energy efficiency in the Food & Beverage processing industry. It is based on examples from theory and practice. As the food and beverage industry is a vast sector, examples for a subsector are treated in this application guide, namely for the subsector of fruit and vegetables. The content of this study is mainly based on the potential energy savings in the food industry. Nowadays and as in the past, the aims that the food industry tries to reach are:

•

To extend the shelf life by preservation techniques which inhibit microbiological or biochemical changes and thus allow time for distribution, sales and home storage

•

To increase the variety in the diet by providing a range of attractive flavors, colors, aromas and textures in food; to change the form of the food to allow further processing (e.g. the milling of grains to flour)

•

To provide the nutritional quality of the food

•

To generate income for the manufacturing company

This study will not try to be complete and describe in detail every operations mentioned in the next chapter. We will try to describe a wide range of the most significant processspecific energy efficiency measures. As much as possible, we will reinforce the theoretical explanation with practical study cases.

2. Overview of the food processing technology In the food industry, heat has an important influence on food processing because it is the most convenient way of extending the shelf life of foods. Indeed, heat will destroy enzymatic and microbiological activity or remove water to inhibit deterioration. One way to classify food processes is in the following four main categories:

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Application Guide for Food & Beverage www.leonardo-energy.org 1. Processing at ambient temperature - Raw material preparation (cleaning, sorting, grading and peeling) - Size reduction - Mixing and forming - Separation and concentration of food components - Fermentation and enzyme technology - Irradiation - Processing using electric fields, high hydrostatic pressure, light or ultrasound

2. Processing by application of heat Heat processing using steam or water - Blanching - Pasteurization - Heat sterilization - Evaporation and distillation - Extrusion Heat processing using hot air - Dehydratation - Baking and roasting Heat processing using hot oils - Frying Heat processing by direct and radiated energy - Dielectric, ohmic and infrared heating

3. Processing by the removal of heat - Chilling - Controlled- or modified-atmosphere storage packaging - Freezing - Freeze drying (lyophilisation) and freeze concentration

4. Post-processing operation - Coating and enrobing - Packaging - Filling and sealing of containers - Materials handling, storage and distribution 3

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3. Process-specific energy efficiency measures 3.1. Energy efficiency measures for PEELING Peeling is used in the processing of many fruits and vegetables to remove unwanted material and to improve the appearance of the final product. There are five main methods of peeling: 1. Flash steam peeling Food is fed into a pressure vessel which is rotated at 4-6 rpm. High pressure steam (1,5 bar) is injected and all food surfaces are exposed to the steam by the rotation of the vessel. The surface layer is heated rapidly but the product is not cooked. Texture and color are therefore preserved. The pressure is then instantly released which causes the steam situated under the surface of the food to “flash off”. Water spray is then needed to remove any remaining traces.

Figure 1 – Flash steam peeling installation

€ Heat recovery on the discharge steam Use of condensing heat exchanger systems to heat facility or process water

2. Knife peeling Stationary blades are pressed against the surface of the rotating fruits or vegetables to remove the skin. Alternatively, rotary blades may rotate against stationary foods. (e.g. citrus fruits)

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Application Guide for Food & Beverage www.leonardo-energy.org 3. Abrasion peeling The food is placed into a rotating bowl made of an abrasive surface (carborundum) which will remove the skin. Carborundum rollers may be used as well. Waste peels are washed away by a large amount of water. Advantages: + Low energy costs (process operated at room temperature) + Low capital costs + No heat damage + Good appearance of the food

Limitations: - Irregular product surfaces (e.g. eyes in potatoes) may require hand finishing - Higher product loss than flash peeling (25% instead of 8-18% losses for vegetables) - Heat recovery on the waste diluted products is difficult - Relatively low production flow as all pieces of food need to contact the abrasive surfaces

€ Multi-stage abrasive peeling The significant amount of usable product usually lost during the process can be reduced by using multi-stage abrasive peelers. The product will be routed through a series of progressively milder abrasive drums.

CASE STUDY A Food processing company in Pennsylvania (USA), has used a multi-stage abrasive peeler on its potato chip processing line since 2001. The new peeling process was estimated to reduce potato usage by 354,000 pounds per year while maintaining the same production rate (Food Engineering 2003). The savings in reduced potato costs were estimated at $31,860 per year. Additional reported benefits included less potato waste for disposal as well as fewer quality problems with downstream processes such as slicing and frying.

4. Caustic peeling The food is dipped in a heated caustic solution (100-120°C) to soften the skin which is then removed by high-pressure water (wet caustic peeling) or with rubber discs or rollers (dry caustic peeling). Product losses are of the order of 17% and this peeling method consumes generally less energy and water than steam-based peeling methods. 5

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€ Energy savings with dry caustic peeling methods Wet caustic methods generate wastewater with a very high pH and organic which leads to high wastewater treatment costs. In contrast, dry caustic methods require only fresh water to remove residues of peel and caustic. CASE STUDY In a demonstration project at a peach peeling and canning facility, dry caustic peeling methods generated nearly 90% less wastewater and had over 50% less organic loading than wet caustic peeling methods (U.S. EPA 1999).

5. Flame peeling This technique has been developed for onions. The product is introduced into a furnace heated to 1000°C and the outer “paper shell” and root hairs are burned off. The burned skin is removed by high-pressure water.

3.2. Energy efficiency measures for BLANCHING The main function of blanching is to destroy enzymic activity in vegetables and some fruits, prior to further processing. The food is heated rapidly to a pre-set temperature, held for a time at this temperature and then cooled rapidly to near ambient temperatures. The two most common methods of blanching involve passing food through an atmosphere of saturated steam or a bath of hot water.

3.2.1. Steam blanchers The conventional steam blancher consists of a mesh of conveyor belt that carries food through a steam atmosphere in a tunnel (typically 15 m long and 1-1,5 m wide). The cooling section employs a fog spray to saturate the cold air with moisture. This reduces the evaporative losses from the food and reduces the amount of effluent produced. Air cooling is employed as well. Typically the equipment processes up to 4500 kg/h of food.

Figure 2 – Steam Blancher/Water Cooling

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Figure 3 – Steam Blancher/Air Cooling

Advantages + Smaller loss of water-soluble components + Smaller volumes of waste particularly with air cooling instead of water cooling + Easy to clean and to sterilize

Limitations - Limited cleaning of the food, so washers also required - Irregular blanching if the food is pilled to high on the conveyor - Some loss of mass in the food

€ Common energy efficiency features of modern steam blanchers

♦

Steam seals, which help to minimize steam leakage at the blancher entrance and exit - Use of water spray curtain to condense escaping steam: energy efficiency improvement of 19% - Food enters and leaves the blancher through rotary valves or hydrostatic seals: energy efficiency improvement of 27% - Steam re-used by passing through a Ventury valve and use of hydrostatic seals: energy efficiency improvement of 31%

♦

Insulation of the steam chamber walls, ceiling and floor

♦

Forced convection of steam throughout the product depth using internal fans or steam injection which increase the heating efficiency of the product and helps to reduce the blanching time. Sometimes, in forced convection installations, it is possible to recover and to re-circulate the steam that does not condensate during the first pass.

♦

Process controls which optimize the steam flow based on such variables as product temperature, blanching time and product depth.

♦

Recovery of condensate for use in water curtain sprays or for product cooling

♦

Heat recovery on the exiting condensate if internally recycling is not permitted

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Energy Efficiency www.leonardo-energy.org € Heat and hold techniques In traditional blanching, the products are continuously heated by the medium until the specified core temperature is reached. In heat and hold blanching, the products is exposed at just the minimum amount of steam required to heat the surface at the necessary temperature for blanching (heat section). Afterwards, the product enters in an adiabatic holding section in which the heat at his surface is allowed to penetrate to his core, which raises the entire product to the required blanching temperature without the use of additional steam. ⇒

Blanching time reduced by up to 60%

⇒

Blanching energy efficiency improved to 68-91%

⇒

Product blanched: 6-7 kg/kg steam (conventional: 0,5 kg/kg steam)

3.2.2. Hot-water blanchers In the hot-water blanchers, the food is held in hot water at 70-100°C for a specified time and then removed to a dewatering-cooling section. • In the reeler blancher, food enters a slowly rotating cylindrical mesh drum which is partly submerged in hot water. The food is moved through the drum by internal flights. • Pipe blanchers consist of a continuous insulated metal pipe where hot water is recirculated through the pipe and food is metered in. Advantages + Lower capital cost + Better energy efficiency than steam blanchers Limitations - Purchase water and waste water treatment are higher - Risk of contamination by thermophilic bacteria

Figure 4 – Hot-Water Blancher cooler

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€ Use of a heat exchanger Up to 70% of the heat is recovered (see figure 8)

€ Recirculated water-steam mixture A recirculated water-steam mixture is used to blanch the food, and final cooling is by cold air. ⇒

Effluent pollution is negligible

⇒

Water consumption is about 1m³ per 10t of product

⇒

Product blanched: 20 kg/kg steam (conventional: 0.5 kg/kg steam)

3.3. Energy efficiency measures for PASTEURIZATION Pasteurization is a mild heat treatment in which food is heated to below 100°C to reduce the number of viable pathogens so they are unlikely to cause disease. Therefore, pasteurization aims to extend the shelf life of food for several days or months with minimal changes in the sensory characteristics or nutritive value.

There are three main types of pasteurization used today: •

Lower Temperature/Longer Time (e.g. milk at 63°C for 30 min, less often used)

•

High Temperature/Short Time (e.g. milk at 71.7°C for 15 s)

•

Ultra High Temperature (or flash pasteurization) (e.g. milk at 100°C for 0.01 s)

3.3.1. Pasteurization of packaged food Some liquid food (for example beers and fruit juices) are pasteurized after filling into containers. Hot water is normally used for glass containers to avoid risk of thermal shock whereas plastic or metal containers use both steam-air mixtures or hot water because there is little risk of thermal shock.

Pasteurizers may be operated in batch or continuously. - The batch equipment consists of a hot water bath in which packaged food is heated. Cold water is then pumped in to cool the product - The continuous version consists of a long narrow bath fitted with a conveyor belt to 9

Energy Efficiency www.leonardo-energy.org carry containers through the heating and cooling stages. - The tunnel design consists of a number of heating zones where automated water sprays heat containers gradually until pasteurization is achieved. Water sprays then cool the containers. - Steam tunnels allow faster heating, shorter residence times and smaller equipment. Temperatures in the heating zone are gradually increased by reducing the amount of air in the steam-air mixtures. Cooling operation is realized by water sprays or bath immersion.

€ Recirculation of water Savings in energy and water consumption are achieved by recirculation of water between the preheated sprays, where water is cooled by the incoming food and cooling zones where water is heated by the hot products. See figure below for an example.

Cooling

Heating

Pre-heating

Food

3.3.2. Pasteurization of unpackaged liquids

Large scale pasteurization usually employs plate heat exchangers.

Pasteurizing operations: 1.

Food is pumped to a “regeneration” section, where it is pre -heated by food that has already been pasteurized.

2.

It is then heated to pasteurizing temperature in a heating section and held for the time required to achieve pasteurization.

3.

The pasteurized product is then cooled in the regeneration section (and simultaneously pre-heats incoming food)

4.

Finally the product is cooled by cold water in a cooling section (and chilled water if needed).

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Figure 5 – Pasteurizing using a plate heat exchanger

Advantages of heat-exchangers over in-bottle processing: + more uniform treatment + simpler equipment and lower maintenance costs + lower space requirements and labour costs + greater flexibility for different products + greater control over pasteurization conditions

€ Heat recover ratio improvement The choice of a pasteurization installation is often function of a budget which is fixed in advance. The heat recovery capacity is not taken into account at all. Important energy savings can be realized by increasing the surface of the heat exchanger (regeneration section).

⇒ Up to 97% of the heat can be recovered.

€ Compact Immersion Tube liquid heating technology The Compact Immersion Tube (CIT) consists principally of a combustion chamber and a heat exchange tube coiled inside the reservoir of a hot water circuit. 11

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Exhaust from the combustion chamber circulates in the heat exchange tube, which transmits the heat to the water in the reservoir. The hot water is then circulated to another heat exchanger for use in the pasteurization process. ⇒ CIT heat exchangers reportedly use up to 35% less energy than centralized water heating systems

Hot water tank

Pasteurizer

Boiler Fume

Figure 6 – Pasteurization by compact Immersion Tube

3.4. Energy efficiency measures for HEAT STERILISATION Heat sterilization refers to the process in which food is heated at a sufficiently high temperature and for a sufficiently long time to destroy microbial and enzyme activity. Sterilized food has a longer life expectancy and lower rates of disability.

Sterilization can be achieved through application of heat, chemicals, irradiation, high pressure or filtration. In this chapter, we will describe the heat sterilization procedures.

3.4.1. In-container sterilization Four major types of heat-sterilization containers are used in heat sterilization processing: metal cans, glass jars or bottles, flexible bags, rigid trays.

Before processing the filled containers, it is necessary to remove air to prevent: - strain on the container due to the heated air expansion - internal corrosion and oxidation of the food

Air removal can be achieved with a vacuum pump or by steam flow closing, where a blast of steam (0,4 bar) carries air away from the surface of the food immediately before container is sealed.

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Heating methods: 1. Heating by saturated steam Latent heat is transferred to food when saturated steam condenses on the outside of the container. After sterilization, the containers are cooled by water sprays. Steam is rapidly condensed and as the foods cool more slowly than the atmosphere, the container is placed in a pressurized atmosphere to equalized the pressure and to prevent strain on the containers (pressure cooling, until 100°C). Afterwards the over-pressure of air is removed and cooling continues to 40°C.

The inconvenience of this method is the low rate of heat penetration to the thermal centre, resulting in long processing times and low productivity.

Figure 7 – Heat sterilization by saturated steam

2. Heating by hot water Foods are processed in glass containers or flexible pouches (bags) under hot water with an over pressure of air.

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Figure 8 – Heat sterilization by hot water

3. Heating by flames Sterilization at atmospheric pressure using direct flame heating of spinning cans (flame temperature of 1770°C). The high internal pressures limit this method to small cans. Example of application: mushroom, sweet corn, green beans, pears, cubed beef + short processing time + high quality food + energy consumption reduction by 20% in comparison with conventional canning sterilization

3.4.2. Ultra high-temperature (aseptic processes)

If the product is sterilized before it is filled into pre-sterilized containers, higher processing temperatures for a shorter time are possible.

Example of applications: milk, fruit juices and concentrates, cream, yoghurt, wine, salad dressing, egg, ice cream mix, cottage cheese, baby foods, tomato products, fruit and vegetables, soups and rice desserts.

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Figure 9 – Time-temperature conditions for UHT and canning

€ Sterilizer insulation All exposed surfaces of sterilizers should be properly insulated to minimize heat losses. Furthermore, insulation should be checked regularly for damage or decay and repaired when needed. ⇒

The typical payback time for insulating sterilizers where the temperatures of exposed surfaces are greater than 75°C is 2 years.

€ Heat recuperation on the cooling down cycle Energy efficiency can be improved by using the heat in the cooling down sector to preheat the containers in the pre-heat sector. Another option is to re-use this heat to heat process water or cleaning water.

3.5. Energy efficiency measures for EVAPORATION & DISTILLATION Evaporation and distillation aim to separate specific components to increase the value of the food. In both type of operation, heat is used to remove one or more components from the food by exploiting their differences in vapor pressure (volatility).

3.5.1. Evaporation Evaporation or concentration by boiling, is the removal of water from liquid food by boiling off water vapor. It is used to produce a more concentrated product. 15

Energy Efficiency www.leonardo-energy.org 3.5.1.1. NATURAL CIRCULATION EVAPORATORS

♦

OPEN- OR CLOSED-PAN EVAPORATORS

Pan evaporators can be heated directly by gas, by electrical resistance wires or heated indirectly by steam through internal tubes or an external jacket. For vacuum operation, they are fitted with a lid.

€ Liquid foods preheating

The energy efficiency of basic evaporator can be increased by using the steam coming off the evaporator pan to pre-heat the liquid food (the sap in the schema below) before it enters the pan

♦

Short-tube evaporators

It consists of a tube-and-shell heat exchanger disposed mainly vertically. Feed liquor is heated by steam condensing on the tubes.

♦

Rising film evaporators

The feed at near boiling is fed to the bottom of the Calandria. It is then pumped inside the tubes. Steam is provided on the shell side. Liquid and vapor are separated in the vapor separator at the top. ⇒

Multiple effect or vapor recompression system are

used to achieve higher steam economy

♦

Falling film evaporators

This type of evaporator is generally made of long tubes (4-8 meters length) which 16

Application Guide for Food & Beverage www.leonardo-energy.org are surrounded by steam jackets. The feed liquor is introduced to the top of the tube bundle and the force of gravity supplements the forces arising from expansion of the steam, produce a very high flow rate and short residence time. This evaporator is applicable to highly viscous solutions or very heat sensitive food. The steam is fed on the shell side. The concentrate is collected at the bottom. ⇒

Multiple effects evaporators can achieve steam economy Feed

Figure 10 – Falling film evaporator

3.5.1.2. FORCED CIRCULATION EVAPORATORS ♦

Plate evaporators

They are similar in construction to the heat exchangers used for pasteurization and ultra high-temperature sterilization. The mixture of vapor and concentrate is separated outside the evaporator.

Steam

Vapor Concentrate

Condensate Feed

Figure 11 – Plate evaporator

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Energy Efficiency www.leonardo-energy.org 3.5.1.3. MECHANICAL (OR AGITATED) THIN-FILM EVAPORATORS This type of evaporators consists of a steam jacket surrounding a high-speed rotor, fitted with short blades along its length. Feed liquor is introduced between the rotor and the heated surface. The evaporation takes place rapidly as a thin film of liquor is swept through the machine by the rotor blade.

Figure 12 – Mechanical film evaporator

3.5.2. Distillation

Distillation is a method of separating fluids in a mixture based on differences in their volatilities in a boiling liquid mixture. When a food that contains components having different degrees of volatility is heated, those that have a higher vapor pressure (more volatile components) are separated first. These are termed “distillate” and components that have a lower volatility are termed “bottoms” or residues. In order to enhance the separation of these components and equilibrium conditions between the liquid and vapor phases, a proportion of the distillate is added back to the top of the column (reflux) and a portion of the residues is vaporized in a reboiler and added to the bottom of the column. Columns are filled with either a packing material or fitted with perforated trays, both of which increase the contact between liquid and vapor phases. 18

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Figure 13 – (a) Schematic diagram of a continuous distillation column and (b) Internal plates in the column to promote cross-flow

€ Vapor recompression The evaporated vapor passes through a compressor (or high pressure or a steam ejector) where the pressure of the vapor is increased by a factor of 1,2 to 2,0. The increased pressure of the vapor enables it to provide energy and temperature difference required for evaporation.

Figure 14 – Mechanical vapor recompression evaporation

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CASE STUDY Vapor recompression of the distillation column to heat up the bottom products (application shown used in a plant of the chemical industry sector, France – similar for food production). Colonne 2236

CMV

PAC

P condensor Distillate TOP Refluxe TOP

Feed

Refluxe Bottom Thermo Oil BOTTOM

Residues

Description of the installation: The distillation column is heated up with thermal oil through a heat exchanger located on the reflux loop. The condensation of the vapor is realized by cold water on the top of the column. Optimization: To increase the vapor pressure (and therefore the temperature) by a mechanical compression to heat up the bottom refluxes. Description of the energy savings calculation: Because we do not dispose of enough technical information about the vapor distilled, we will approximate the mechanical vapor compression (MVC) to a heat pump. Indeed, the consumption of a MVC is nearly the same as a heat pump except for the extra consumption of the compressor and the investment cost of a heat exchanger (evaporation side).

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M’

T1

Cp

Feed

t/h 2,5

°C 42

kJ/kg.K 1,47

Distillate

1,9

67

1,47

Top refluxes

11

67

1,47

Top

12,9

76

1,47

Residues

0,7

96

1,76

Bottom refluxes

160

96

1,76

Bottom

151

96

1,76

Bottom heating with a heat pump η carnot

0,6

COP (Pcd/Pelec)

5,8

Tev

71

°C

Tcd

111

°C

5

°C

∆T heat exchanger PAC Pelec PAC

136

Electrical consumption with a Heat Pump Electrical cost with a Heat Pump

kWe

1.192

MWhe/

78.648

year €/year

6.866

MWh

Bottom heating with thermal oil Estimation of the energy consumption with thermal oil Heating cost with thermal oil

219.728

€/year

Potential energy saving valorization Thermal oil price

32

Operating hours

8.760

€ h/year

Savings Heating cost with thermal oil Electrical cost with a Heat Pump Financial savings

219.728

kWth

78.648

€/year

141.080

€/year

2.500

€/kWe

Investment Heat pump cost Total investment

340.078

Payback time

2,4

21

€

years

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€ Heat recovery from vapor or liquid product Energy saving can be realized by recovering the heat contained in vapors (or liquor products) to preheat the incoming feed liquor or to raise steam in a boiler.

CASE STUDY

Distillation column CBAT Distillate

Ta2 = 108°C Residues Tr2 = 55°C Ta1 = 50°C

Tr1 = 123°C Feed

Heat recovery on the residues of the distillation column to preheat the incoming feed – application shown used in the chemical industry, France – similar for food production. Description of the installation: The distillation column is heated up with thermal oil through a heat exchanger located on the reflux loop. The condensation of the vapor is realized by cold water on the top of the column. Optimization: To preheat the incoming feed by recovering the heat contained in the bottom products. Data

Feed Residues

22

M’

T1

T2

Cp

t/h 14 11

°C 50 123

°C 108,8 55

kJ/kg.K 1,26 1,38

Application Guide for Food & Beverage www.leonardo-energy.org Energy saving analysis Potential energy saving valorization Thermal oil price Operating hours Savings Calorific energy saving Annual energy saving Financial savings Investment Heat exchanger cost (40 m² steel) Total investment Payback time

32 8.760

€ h/year

287,1 2.515

kWth MWhth/year

80.473

€/year

26.000 52.000

€ €

0,6

year

€ Multiple effects evaporation Several evaporators (or “effects”) are connected together. The evaporated vapor from one effect is used directly as the heating medium in the next effect. However, this vapor is present at a lower temperature (pressure). Therefore the pressure in the following effects has to be progressively lowered in order to decrease the boiling temperature of the product and maintain a sufficient temperature difference with the product to evaporate. The number of effects used in a multiple effects system is determined by the savings in energy consumption compared with the higher capital investment required and the operating cost of increasingly higher vacuum in successive effects (generally, three to six effects are used).

Figure 15 – Multiple effects evaporation schema

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Figure 16 – Principle of multiple effects evaporation

Steam consumption with vapor compression and multiple effect evaporation

⇒

Number of effects

Vapor recompression [kg of steam / kg of evap water]

WITHOUT

WITH

1

1.1

0.6

2

0.6 3

0.4 0.4

0.3

CASE STUDY Condensate evaporated vapor recovery to fill in the cleaning water tank of a dairy factory –Belgium.

River

Live Stea

55° Condensate evaporated vapor Feed

Soda &

Soap

Ground

Acid

solution

water

tank 60°C

tank 80°C

tank 10°C

Concentrated Feed

Description of the installation: The condensate evaporated vapor going out of the evaporation installation has a temperature of 55°C and is thrown away in the river after cooling. Two tanks are used for cleaning purposes and are heated respectively at 60°C and 80°C. Ground water at 10°C is used to fill in those tanks. 24

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Optimization: To reuse the condensate evaporated vapor which is at a temperature of 55°C to fill in the tanks. Data Supplement water for the Soda & Acid tank Supplement water for the Soap tank

18 10

Operating days

365

Total supplement of water which has to be

10.220

m³/day m³/day days/year m³/year

heated up Groundwater tank temperature

10

°C

Condensate evaporated water temperature

55

°C

Average cleaning water tank temperature

70

°C

Natural gas price

30

€/MWhp

Operating hours

8.760

Annual energy saving

540

h/year MWhth/

Financial savings

16.000

year €/year

Piping, valve, regulation

12.000

€

Installation

12.000

€

Total investment

24.000

€

Payback time

1,5

year

3.6. Energy efficiency measures for EXTRUSION Extrusion is a process which combines different types of operations including mixing, cooking, molding, trimming, shaping and forming. An extruder consists of a flighted Archimedes screw which rotates in a tightly fitting cylindrical barrel.

Raw materials are fed into the extruder barrel and the screw then conveys the food along it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, it fills the barrel and the spaces between the screw 25

Energy Efficiency www.leonardo-energy.org flights and become compressed. As it moves further along the barrel, the screw molds the material into a semi-solid, plasticized mass. Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel. The under pressure food emerges from the die and expands to the final shape and cools rapidly as moisture is flashed off as steam.

♦

If the food is heated above 100°C, the process is known as extrusion cooking. This process combines the effect of heat with the act of extrusion. Heat can be added to the shaft of the screw, by a steam or electrical heaters surrounding the barrel or by direct injection of steam which is mixed with the paste in the screw.

♦

Low pressure extrusion at temperature below 100°C is called cold extrusion (the food remains at ambient temperature).

Basically, there are two different kinds of extruders in the feed industry: - single-screw extruder - twin-screw extruder

Figure 15 – Principle of a single screw extruder with grooved plastification barrel and barrier- screw with shearing- and mixing parts

3.7. Energy efficiency measures for DEHYDRATION (or DRYING) Dehydration (or drying) can be defined as the application of heat under controlled conditions to remove the majority of the water normally present in a food by evaporation (or by sublimation in the case of freeze drying). The main purpose of drying is to extend the shelf life of foods by a reduction in water activity. 26

Application Guide for Food & Beverage www.leonardo-energy.org Mechanical separations, membrane concentration, evaporation and baking are therefore excluded unit operations which normally remove less water than dehydration.

3.7.1. Hot-air driers 1. Bin driers These are large, cylindrical or rectangular containers fitted with a mesh based. Hot air passes up through a bed of food at relatively low velocities. This type of drier can be several meters high. à Mainly used for “finishing” (3-6% moisture content)

2. Cabinet driers (tray driers) These consists of an insulated cabinet fitted with shallow mesh or perforated trays, each of which contains a thin layer of food (2-6 cm deep). Hot air is blown over and/or through each tray à Used for small-scale production (1-20 t/day)

3. Conveyor driers (belt driers) Continuous conveyor driers are up to 20 m long and 3 m wide. Food is dried on a mesh belt (5-15 cm deep)

⇒

Foods are dried to 10-15%

⇒

Used for large-scale drying (5,5 t/h)

Figure 15 – (a) Conveyor drier and (b) Three-stage conveyor drier

4. Fluidised-bed

driers

The hot air is blown through the bed at a sufficient velocity to cause the food to become suspended and vigorously agitated (fluidized). 27

Energy Efficiency www.leonardo-energy.org The maximum surface area of food is therefore exposed for drying. Vibrating beds are extremely effective in keeping the material in a live fluidized state during this transition phase.

Figure 16 – Fluidised-bed driers

5. Pneumatic driers In vertical driers, the air flow is adjusted so that lighter and smaller particles, which dry more rapidly, are carried to a cyclone separator more rapidly than are heavier and wetter particles, which remains suspended to receive the additional drying required. The pneumatic ring driers allows products that require longer residence times to recirculate

Figure 17 – Pneumatic ring driers

until it is adequately dried

6. Rotary driers A rotating drum is fitted internally with flights to cause the food to cascade through a steam of hot air as it moves through the drier.

Figure 18 – Rotary drier

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7. Spray driers Pre-concentrated food (40-60% moisture) is atomized to form fine droplets and then sprayed into a flow of heated air at 150-300°C in a large drying chamber

8. Sun and solar drying Sun drying (without solar equipment) is the most widely practiced agricultural processing operation worldwide. Solar drying use more sophisticated methods and collect solar energy to heat air which in turn is used for drying

3.7.2. Heated-surface (or contact) driers

The contact driers have two main advantages over hot-air drying: - no need to heat up large volumes - drying can be realized in the absence of oxygen (and therefore prevent for food oxidation)

Typically the heat consumption is 2.000-3.000 kJ per kg of water evaporated compared with 4.000-10.000 kJ per kg of water evaporated for hot-air driers.

1. Drum driers (roller driers) A thin layer of food is spread on the surface of the rotating steel drum which is heated internally by pressurized steam at 120-170°C. Before the drum has completed one revolution, the dried food is scraped off by a blade which is in contact with the drum surface uniformly along his length.

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Figure 19 – Single drum and double drum driers

€ Insulation of cabinets and ducting Any hot surfaces of drying equipment that are exposed to air, such as burners, heat exchangers, roofs, walls, ducts and pipes should be fully insulated to minimize heat losses. € Recirculation of the exhaust air through the drying chamber Check if

a higher outlet temperature can be tolerated by the product

and a lower

evaporative capacity is acceptable. Indeed, the reinjection of the exhaust air directly into the inlet air stream will raise the humidity of incoming air and reduce its drying capacity. € Exhaust air heat recovery To recover the heat from the exhaust air from the dryer to preheat the inlet air stream using heat exchangers or thermal wheels or fore-warming the feed material.

CASE STUDY Heat recovery on the drying tower to preheat the inlet air in a dairy factory – Belgium. HEAT EXCHANGER 90°C

55°C

48°C

20°C

Heat transfert : 525 kW

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Exhaust Air

V’= 55.000 m³/h

Inlet Air

V’= 55.000 m³/h

Application Guide for Food & Beverage www.leonardo-energy.org Description of the installation: The drying tower uses hot air at a temperature of 200°C. This inlet air comes from outside and is heated with steam. The exhaust air at the outlet of the drying tower has a temperature of 90°C. Optimization: To preheat the inlet air (until 48°C if the inlet air temperature is 20°C) by recovering the heat in the exhausting air. Data Exhaust air temp before the heat exchanger Exhaust air temp after the heat exchanger Incoming air temp before the heat exchanger Incoming air temp after the heat exchanger Exhaust air/Incoming air flow

Natural gas price Operating hours Calorific energy saving Annual energy saving

90 55 20 48 55.000

30 7.500 525 3.937

Financial savings

118.12

°C °C °C °C m³/h

€/MWhp h/year kWth MWhth/ year €/year

5 Heat exchanger cost (390 m²)

107.00

€

Transport + installation

0 160.00

€

Total investment

0 267.00

€

0 Payback time

2,3

year

€ Direct fired dryers Use of direct flame heating by natural gas and low NOx burners to reduce product contamination by the products of combustion. Direct fired dryers are generally more energy efficient than indirect heated dryers because they remove the inefficiency of first transferring heat to air and then transferring heat from air to the product. ⇒

à 35% to 45% more energy efficient 31

Energy Efficiency www.leonardo-energy.org

€ Mechanical dewatering Mechanical dewatering of the food prior to drying can reduce the moisture load on the dryer and save significant amounts of energy. Mechanical dewatering methods include: - filtration - use of centrifugal force - gravity - mechanical compression - high velocity air

⇒

For each 1% reduction in feed moisture, the dryer energy consumption can be reduced by up to 4%

€ Drying in two stages For example, fluidized beds followed by bin drying or spray drying followed by fluidized bed drying. € Process controls Automatic control of air humidity by computer control.

3.8 Energy efficiency measures for OVENS The ovens permit to realize the baking operation which use heated air to alter the eating quality of foods. Ovens are classified into direct or indirect heating types.

3.8.1. Direct heating ovens In directly heated ovens, air and the products of combustion are recirculated by natural convection or by fans.

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Advantages: + short baking times + high thermal efficiency + good control over baking conditions + rapid start-up, as it is only necessary to heat the air in the oven However, care is necessary to prevent contamination of the food by undesirable products of combustion. Microwave and dielectric ovens are another example of direct heating ovens.

3.8.2. Indirect heating ovens

Different techniques for heating the oven can be applied:

•

Steam tubes heat air in the baking chamber and are either heated directly by burning fuel or supplied with steam from a remote boiler

•

Combustion gases are passed through banks of radiator tubes in the baking chamber

•

Fuel is burned between a double wall and the combustion products are exhausted from the top of the oven

•

Electric ovens are heated by induction heating radiator plates or bars

Different kind of continuous and semi-continuous ovens exist: a) Revolving hearth oven

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Energy Efficiency www.leonardo-energy.org

b) Reel oven

c) Multi-cycle tray oven

d) Tunnel oven

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Application Guide for Food & Beverage www.leonardo-energy.org

(f)

(a)

(c)

(b) (e) (d)

(g)

(a) cold supply air (b) hot combustion air (c) hot-zone integrity air (d) hot-oven heat exchanger exhaust (e) oven exhaust air plus product evaporation

€Heat recovery on the exhaust air of the convection oven Figure 20 – Example Heat recovery for convection oven

The heat from the exhaust air from the convection oven (e) and the exhaust gas from the oven chamber (d) (indirect-fired ovens) can be used to preheat the incoming fresh air (a). A case study including a heat recovery heat exchanger has been done in the dehydration paragraph in this document (§ 3.7). Heat recovery could also be applied to heat process water.

3.9. Energy efficiency measures for FRYING Frying is a unit operation which is mainly used to alter the eating quality of a food. 3.9.1. Equipment There are two type of friers: ♦

Shallow-frying equipment consists of a heated metal surface, covered on a thin layer of oil 35

Energy Efficiency www.leonardo-energy.org

♦

Continuous deep-fat friers consists of a stainless steel mesh conveyor which is submerged in a thermostatically controlled oil tank. They are heated by electricity, gas, fuel oil or steam.

Oil is continuously recirculated through external heaters and filters to remove particles of food that would burn and affect the quality of the product.

3.9.2. Heat and oil recovery systems

The heat contained in the escaping fryer exhaust gases can be recovered by heat exchangers mounted in the exhaust hood (economizer) and used to preheat incoming food or oil or to heat process water. Conditioning of the exhaust gas is required however, to remove fats and to reduce

♦

fouling of the heat exchanger.

Oil recovery systems remove entrained oil from the exhaust air and return it to the oil tank.

♦

Fryer exhaust gas can be reused as combustion air into the burner chamber. By this way, in addition to recover the exhaust heat, smoke and other products of oil degradation are prevented from being discharged into the atmosphere.

Waste heat exchanFrying oil heat exchanNatural Gas

700 °C

Oi

Economiser

FRYE Filter Figure 21 – Heat and oil recovery system

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Application Guide for Food & Beverage www.leonardo-energy.org

CASE STUDY A global manufacturer of frozen potato products, installed a special system for recovering heat from exhaust gases on the potato frying line, England - 1995. Fryer exhaust gases were first saturated with water vapor using turbine washers and then condensed in a vertically heat exchanger which allowed condensate, fat and fatty acids to drain into a container below the heat exchanger. The heat exchanger was used to pre-heat air for the facility’s potato chip dryers, to heat water used in potato blanchers, and to provide facility hot water. Exhaust gases exiting the vapor condenser passed through a scrubbing tower and were discharged to the atmosphere.

€ Using spent fryer oil as fuel The frying process can generate significant amounts of spent oil, which can be used as diesel engine fuel at facilities that have diesel cogeneration units or diesel backup power generators. Oil has to be properly filtered to remove contaminants and special modifications are required to the fuel injection system. Using oil as bio-diesel reduces solid waste while reducing the company’s necessary purchases of diesel fuels.

CASE STUDY A Japanese Food Company that produce deep-fried vegetables and shellfish decided to install a diesel co-generation system in 1997 that burns a mixture of spent vegetable oil and marine gas oil. The ratio used was 70% of vegetable oil and 30% of marine gas oil. The spent vegetable oil consumption was 32 to 42 tons per month. As of 2002, the system was running with no major problems and was able to run with fuel and maintenance costs that were 50% less than a co-generation system running on marine gas oil alone (CADDET 2002). The system was also reported to reduce both emissions of sulfur oxides (SOx) and the smoke density of the exhaust.

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Energy Efficiency www.leonardo-energy.org

3.10. Energy efficiency measures for CHILLING & FREEZING

•

Chilling is the unit operation in which the temperature of a food is reduced to between -1°C and 8°C. It is used to extend the shelf life of fresh and processed foods by reducing the rate of biochemical and microbiological changes. Chilling is often combined with other unit operations (e.g. fermentation, pasteurization).

•

Freezing is the unit operation in which the temperature of a food is reduced to below its freezing point and a proportion of the water forms ice crystals.

Chilling equipment is classified by the method used to remove heat:

♦

Mechanical refrigerators:

A refrigerant circulates between the four elements of the refrigerator (evaporator, compressor, condenser, expansion valve), changing state from liquid to gas and back to liquid.

In the evaporator, the liquid refrigerant evaporates under reduced pressure, and in doing so absorbs latent heat of vaporization and cools the freezing medium. This is the most important part of the refrigerator, the remaining equipment is used to recycle the refrigerant.

- Cryogenic chilling In cryogenic systems, Nitrogen or CO2 is sprayed directly onto the product. When the refrigerant expands through the spray nozzle, it changes to approximately equal parts (by weight) of solid and vapor. As the liquid droplets touch the product’s surface, the liquid changes to a vapor that extracts heat from the food. The cold vapor realized the cooling of the product as well (around 15% for CO2 systems and 50% for Nitrogen systems).

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€ CONDENSER UNIT energy efficiency measures

♦

SPEED REGULATION OF THE FANS with variable frequency drive (VFD)

CASE STUDY Company specialized in potatoes, mashed potatoes and French fries production Belgium. Description of the installation:

The condenser’s fans of the cooling installation MK1, MK3 and MK4 are regulated to work at a specific condensing temperature with two speeds motors. Optimization: To regulate the condensing temperature by regulating the condenser’s fans with a VFD. Description of the energy savings calculation: Based on a simulation, the condenser’s load has been calculated in function of the wet bulb temperature. Indeed, the condensers have been designed to work at full load during the summer and therefore they are running at partial load most of the time. In combination with a standard temperature profile for one year, the energy consumption has been calculated for a 2 speeds motor’s fan and a variable frequency drive motor’s fan. The yearly difference gives the potential energy saving. 39

Energy Efficiency www.leonardo-energy.org

Energy savings Condenser’ fans MK1 power

2x 30 kW

Estimation of the energy saving

25 MWh/

Condenser’ fans MK3 power

2x 30 kW

Estimation of the energy saving

25 MWh/

Condenser’ fans MK4

2x 11 kW

Estimation of the energy saving

9,1 MWh/

Electricity price

98 MWh/

Total electrical energy saving

59 MWh/

Financial savings

5.700 €/year

Total investment for a variable frequency drives Payback time

28.360 € 5 years

♦

HEAT RECOVERY to heat water or air processes

♦

MINIMAL CONDENSING TEMPERATURE

As a rule of thumb, a reduction of the condensing temperature of 1°C reduce the energy consumption by around 3%.

♦

Others points to check out :

•

Temperature difference at condenser is not too high (max 15°C)

•

Condensers are well ventilated

•

Condensers are clean

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€ EVAPORATOR energy efficiency measures

♦

EVAPORATOR’S FANS REGULATED with variable frequency drives

CASE STUDY Company specialized in potatoes, mashed potatoes and French fries production Belgium. Description of the installation: In the French fries cooling tunnel, the evaporator’s fans are always running at full speed. Optimization:

During the short regular break of the process (fries cutting, technical problems,…), the speed of the fans could be reduced with a variable speed drive.

Description of the energy savings calculation: The existing energy consumptions have been measured on site. We can consider that the energy consumption with VFD is linear with the power consumption for an operating load between 50 and 100%.

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Energy Efficiency www.leonardo-energy.org Energy savings Fans power

198

Operating hours

6.500

kWe h/year

Existing motor frequency

50

Hz

Proposed motor frequency

15

Hz

Energy consumption at lower frequency

5.3

kWe

Break due to fries cutting

1,46

%

Break due to technical problems

4,29

%

Break due to other parameters

1,44

%

Total percentage with no production

7,19

%

Total hours with no production

467,35

h/year

90

MWh/

Total energy saving

year Electricity price

97

€/MWh

Financial savings

8.800

€/year

Total investment for a variable frequency drive

3.800

€

Payback time

♦

0,4

year

Others points to check out: •

The efficiency of the defrost system

•

Temperature difference at the evaporator not too high (max 7°C)

€ COMPRESSORS energy efficiency measures

♦COOLING CAPACITY REGULATION with a variable frequency drive CASE STUDY Company specialized in potatoes, mashed potatoes and French fries production Belgium. Description of the installation: The factory disposes of several cooling installations. One of them consists of two screw compressors which are operating nearly at full load all the time. The flow regulation is done in multi-stage regime. Optimization: 42

Application Guide for Food & Beverage www.leonardo-energy.org

To decrease the wearing of the compressors and at the same time to reduce their electrical energy consumption, we will run one of the compressors at full load and the other at 90% of his nominal load with a variable frequency drive (VFD). This measure will have no significant consequences on the cooling capacity of the installation. Description of the energy savings calculation: The existing energy consumptions have been measured on site. We can consider that the energy consumption with VFD is linear with the power consumption for an operating load between 50 and 100%. Energy savings (only for the screw compressor MK3) Condenser’ fans MK1 power

2x 30 kW

Estimation of the energy saving

25 MWh/

Condenser’ fans MK3 power

2x 30 kW

Estimation of the energy saving

25 MWh/

Condenser’ fans MK4

2x 11 kW

Estimation of the energy saving

9,1 MWh/

Electricity price

98 MWh/

Total electrical energy saving

59 MWh/

Financial savings

5.700 €/year

Total investment for a variable frequency drives

Payback time

28.360 €

5 years

♦ HEAT RECOVERY Heat from the oil or air cooling system can be recovered to heat air or water process

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Energy Efficiency www.leonardo-energy.org

4. References - Eric Masanet, Ernst Worrell, Wina Graus, Christina Galitsky, 2008. Energy efficiency improvement and cost saving opportunities for the fruit and vegetable processing industry. ENERGY STAR guide sponsored by the U.S. Environmental Protection Agency. - Région Wallonne, 2008. Economies d’énergie dans l’industrie alimentaire – Les récupérations de chaleur dans le process. Cahier technique n°7. - Région Wallonne, 2008. Economies d’énergie dans l’industrie alimentaire – La réfrigération. Cahier technique n°5. - Serge Guégan, 2008. Food Intelligence – The World Food & Beverage Companies Top 100. France. - X. Serrano. The extrusion-cooking process in animal feeding Nutritional implications

- P J Fellows, 2000. Food Processing Technology, Principles and Practices, Second Edition.

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