27712338 energy efficiency in the food and beverages industry

8/9/2019 27712338 Energy Efficiency in the Food and Beverages Industry

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Power Quality & Utilization GuideApplication Guide For Food & Beverage Industry

Quentin Rosier — LaborelecFebruary 2010

Steam

Vapor

Energy Efficiency

Concentrate

Condensate Feed



Energy Efficiencywww.leonardo-energy.org

1. IntroductionThis 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 thesubsector of fruit and vegetables. The content of this study is mainly based onthe 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 w

ill reinforce the theoretical explanation with practical study cases.

2. Overview of the food processing technologyIn the food industry, heat has an important influence on food processing becauseit is the most convenient way of extending the shelf life of foods. Indeed, heat will destroy enzymatic and microbiological activity or remove water to inhibitdeterioration. One way to classify food processes is in the following four maincategories:

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Application Guide for Food & Beveragewww.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 hydrostaticpressure, 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 Heatprocessing 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




3. Process-specific energy efficiency measures3.1. Energy efficiency measures for PEELING Peeling is used in the processing ofmany 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 ste

am peeling Food is fed into a pressure vessel which is rotated at 4-6 rpm. Highpressure 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 isthen 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 systemsto 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 & Beveragewww.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 amountof 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 usuallylost 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-sta

ge 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 whilemaintaining 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 C) The food is dipped in a heated caustic solution (100-120°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 areof the order of 17% and this peeling method consumes generally less energy and water than steam-based peeling methods. 5




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 pee

ling and canning facility, dry caustic peeling methods generated nearly 90% lesswastewater 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° and the outer “paper shell” and root hairs are burned off. The burned C skin is removed by high-pressure water.

3.2. Energy efficiency measures for BLANCHING The main function of blanching isto 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 atthis temperature and then cooled rapidly to near ambient temperatures. The two m

ost 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 saturatethe 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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Application Guide for Food & Beveragewww.leonardo-energy.org

Figure 3 – Steam Blancher/Air Cooling

Advantages + Smaller loss of water-soluble components + Smaller volumes of wasteparticularly with air cooling instead of water cooling + Easy to clean and to s

terilize

Limitations - Limited cleaning of the food, so washers also required - Irregularblanching if the food is pilled to high on the conveyor - Some loss of mass inthe 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 i

mprovement of 19% - Food enters and leaves the blancher through rotary valves orhydrostatic 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 Efficiencywww.leonardo-energy.org Heat and hold techniques In traditional blanching, theproducts are continuously heated by the medium until the specified core temperature is reached. In heat and hold blanching, the products is exposed at just theminimum amount of steam required to heat the surface at the necessary temperature for blanching (heat section). Afterwards, the product enters in an adiabaticholding 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° for a specified time C and then removed to a dewatering-coolingsection. • 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 metalpipe where hot water is recirculated through the pipe and food is metered in. Advantages + Lower capital cost + Better energy efficiency than steam blanchers Li

mitations - 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 toblanch 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 PASTEURIZATIONPasteurization 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 daysor 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° for C 30 min, less often used) • High Temperature/Sho

rt Time (e.g. milk at 71.7° for C 15 s) • Ultra High Temperature (or flash pasteurization) (e.g. milk at 100° for 0.01 s) C

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 littlerisk 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 narrowbath fitted with a conveyor belt to 9



Energy Efficiencywww.leonardo-energy.org carry containers through the heating and cooling stages.- The tunnel design consists of a number of heating zones where automated watersprays heat containers gradually until pasteurization is achieved. Water spraysthen 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 operat

ion is realized by water sprays or bath immersion.

Recirculation of water Savings in energy and water consumption are achieved byrecirculation 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 Food

Heating

Pre-heating

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 itis pre -heated by food that has already been pasteurized. 2. 3. It is then heated to pasteurizing temperature in a heating section and held for the time required to achieve pasteurization. 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 isoften function of a budget which is fixed in advance. The heat recovery capacityis 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




Exhaust from the combustion chamber circulates in the heat exchange tube, whichtransmits 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 sy

stems

Hot water tank Boiler Fume

Pasteurizer

Figure 6 – Pasteurization by compact Immersion Tube

3.4. Energy efficiency measures for HEAT STERILISATIONHeat 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 d

isability.

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 ablast 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 andas 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 airis 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 pressur

es 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 produ

cts, 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 regu

larly for damage or decay and repaired when needed. ⇒ The typical payback time for insulating sterilizers where the temperatures of exposed surfaces are greaterthan 75° is 2 years. C

Heat recuperation on the cooling down cycle Energy efficiency can be improvedby 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 & DISTILLATIONEvaporation and distillation aim to separate specific components to increase thevalue of the food. In both type of operation, heat is used to remove one or mor

e 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 Efficiencywww.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 orheated 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. Feedliquor 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 & Beveragewww.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 iscollected at the bottom. ⇒ Multiple effects evaporators can achieve steam econom

yFeed

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 pasteurizationand 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 Efficiencywww.leonardo-energy.org 3.5.1.3. MECHANICAL (OR AGITATED) THIN-FILM EVAPORATORSThis type of evaporators consists of a steam jacket surrounding a high-speed rotor, fitted with short blades along its length. Feed liquor is introduced betweenthe rotor and the heated surface. The evaporation takes place rapidly as a thinfilm 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 differencesin 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 residue

s is vaporized in a reboiler and added to the bottom of the column. Columns arefilled with either a packing material or fitted with perforated trays, both of which increase the contact between liquid and vapor phases. 18




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 bottomproducts (application shown used in a plant of the chemical industry sector, France – similar for food production).Colonne 2236

P condensor Distillate TOP Refluxe TOP

CMV

PAC

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 vapordistilled, we will approximate the mechanical vapor compression (MVC) to a heatpump. Indeed, the consumption of a MVC is nearly the same as a heat pump exceptfor the extra consumption of the compressor and the investment cost of a heat exchanger (evaporation side).

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M’ Feed Distillate Top refluxes Top Residues Bottom refluxes Bottom Bottom heating with a heat pump η carnot COP (Pcd/Pelec) Tev Tcd T

eat exc

anger PAC PelecPAC Electrical consumption wit

a Heat Pump Electrical cost wit

a Heat Pump t/

2,5 1,9 11 12,9 0,7 160 151

T1 ° C 42 67 67 76 96 96 96

Cp kJ/kg.K 1,47 1,47 1,47 1,47 1,76 1,76 1,76

0,6 5,8 71 111 5 136 1.192 78.648 ° C ° C ° C kWe MW

e/ year /year

Bottom heating with thermal oil Estimation of the energy consumption with thermal oil Heating cost with thermal oil 6.866 219.728 MWh /year

Potential energy saving valorization Thermal oil price Operating hours Savings Heating cost with thermal oil Electrical cost with a Heat Pump Financial savings

Investment Heat pump cost Total investment 2.500 340.078 /kWe 219.728 78.648141.080 kWth /year /year 32 8.760 h/year

Payback time

2,4

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 CBATDistillate

Ta2 = 108°C Residues Tr2 = 55°C Ta1 = 50°C Feed Tr1 = 123°C

Heat recovery on the residues of the distillation column to preheat the incomingfeed – 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. Opt

imization: To preheat the incoming feed by recovering the heat contained in thebottom products. Data M’ Feed Residues t/h 14 11 T1 ° C 50 123 T2 ° C 108,8 55 Cp kJ/kg.K 1,26 1,38

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Application Guide for Food & Beveragewww.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) Totalinvestment Payback time

32 8.760 287,1 2.515 80.473 26.000 52.000 0,6

h/year kWth MWhth/year /year year

Multiple effects evaporation Several evaporators (or “effects”) are connectedtogether. 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 inenergy consumption compared with the higher capital investment required and theoperating cost of increasingly higher vacuum in successive effects (generally, t

hree 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 1 2 3 1.1 0.6 0.4

WITH 0.6 0.4 0.3

CASE STUDY Condensate evaporated vapor recovery to fill in the cleaning water ta

nk of a dairy factory –Belgium.

River

Live Stea Soda & Acid 55° Condensate evaporated vapor Feed tank 60° C Soap solution tank 80° C Ground water tank 10° C

Concentrated Feed

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

. C 24




Optimization: To reuse the condensate evaporated vapor which is at a temperatureof 55° to fill in the C tanks. Data Supplement water for the Soda & Acid tank Supplement water for the Soap tank Operating days Total supplement of water whichhas to be heated up Groundwater tank temperature Condensate evaporated water te

mperature Average cleaning water tank temperature Natural gas price Operating hours Annual energy saving Financial savings Piping, valve, regulation Installation Total investment Payback time 18 10 365 10.220 10 55 70 30 8.760 540 16.000 12.000 12.000 24.000 1,5 m³/day m³/day days/year m³/year ° C ° C ° C /MWhp h/yearMWhth/ year /year year

3.6. Energy efficiency measures for EXTRUSIONExtrusion is a process which combines different types of operations including mixing, cooking, molding, trimming, shaping and forming. An extruder consists of aflighted 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 barreland the spaces between the screw 25



Energy Efficiencywww.leonardo-energy.org flights and become compressed. As it moves further alongthe 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 expandsto the final shape and cools rapidly as moisture is flashed off as steam.

If the food is heated above 100° the process is known as extrusion C, 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 thebarrel or by direct injection of steam which is mixed with the paste in the screw.

Low pressure extrusion at temperature below 100° is called cold C extrusion (thefood 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 byevaporation (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 & Beveragewww.leonardo-energy.org Mechanical separations, membrane concentration, evaporation and baking are therefore excluded unit operations which normally remove lesswater than dehydration.

3.7.1. Hot-air driers 1. Bin driers These are large, cylindrical or rectangularcontainers 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 longand 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 foodto become suspended and vigorously agitated (fluidized). 27



Energy Efficiencywww.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 lighterand 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 until it is adequately driedFigure 17 – Pneumatic ring driers

6. Rotary driers A rotating drum is fitted internally with flights to cause thefood 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 finedroplets and then sprayed into a flow of heated air at 150-300° in a large drying chamber C

8. Sun and solar drying Sun drying (without solar equipment) is the most widelypracticed 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 comp

ared 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 ofthe 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 pip

es should be fully insulated to minimize heat losses. Recirculation of the exhaust air through the drying chamber Check if a higher outlet temperature can betolerated 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 dairyfactory – Belgium.HEAT EXCHANGER 90° C 55° C Exhaust Air 48° C 20° C Inlet Air V’= 55.000 m³/h

V’= 55.000 m³/h

Heat transfert : 525 kW

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Application Guide for Food & Beveragewww.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 airtemperature is 20° by recovering the C) heat in the exhausting air. Data Exhaustair temp before the heat exchanger Exhaust air temp after the heat exchanger In

coming air temp before the heat exchanger Incoming air temp after the heat exchanger Exhaust air/Incoming air flow 90 55 20 48 55.000 ° C ° C ° C C ° m³/h

Natural gas price Operating hours Calorific energy saving Annual energy saving Financial savings

30 7.500 525 3.937 118.12 5

/MWhp h/year kWth MWhth/ year /year

Heat exchanger cost (390 m²) Transport + installation Total investment

107.00 0 160.00 0 267.00 0

Payback time Direct fired dryers

2,3

year

Use of direct flame heating by natural gas and low NOx burners to reduce productcontamination by the products of combustion. Direct fired dryers are generallymore energy efficient than indirect heated dryers because they remove the ineffi

ciency of first transferring heat to air and then transferring heat from air tothe product. ⇒ à 35% to 45% more energy efficient 31




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 controlof air humidity by computer control.

3.8 Energy efficiency measures for OVENSThe ovens permit to realize the baking operation which use heated air to alter t

he eating quality of foods. Ovens are classified into direct or indirect heatingtypes.

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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b) Reel oven

c) Multi-cycle tray oven

d) Tunnel oven

34




(f)

(a)

(b) (e) (d)

(c)

(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 ovenFigure 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 FRYINGFrying is a unit operation which is mainly used to alter the eating quality of afood. 3.9.1. Equipment There are two type of friers: Shallow-frying equipmentconsists of a heated metal surface, covered on a thin layer of oil 35




Continuous deep-fat friers consists of a stainless steel mesh conveyor which issubmerged in a thermostatically controlled oil tank. They are heated by electric

ity, gas, fuel oil or steam.

Oil is continuously recirculated through external heaters and filters to removeparticles 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 heatexchangers mounted in the exhaust hood (economizer) and used to preheat incomingfood 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 tothe 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 oildegradation are prevented from being discharged into the atmosphere.

Waste heat exchanFrying oil heat exchanNatural Gas Oi 700 ° C

Economiser

FRYE Filter Figure 21 – Heat and oil recovery system

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CASE STUDY A global manufacturer of frozen potato products, installed a specialsystem for recovering heat from exhaust gases on the potato frying line, England- 1995. Fryer exhaust gases were first saturated with water vapor using turbinewashers and then condensed in a vertically heat exchanger which allowed condens

ate, fat and fatty acids to drain into a container below the heat exchanger. Theheat 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 tothe 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 vegetableoil and 30% of marine gas oil. The spent vegetable oil consumption was 32 to 42tons 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 alsoreported to reduce both emissions of sulfur oxides (SOx) and the smoke densityof the exhaust.

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3.10. Energy efficiency measures for CHILLING & FREEZING

•

Chilling is the unit operation in which the temperature of a food is reduced tobetween -1° and 8° It is used to extend the shelf life of fresh and processed CC. 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 tobelow 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, andin doing so absorbs latent heat of vaporization and cools the freezing medium.This is the most important part of the refrigerator, the remaining equipment isused 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 extractsheat 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 regulatedto 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 combinatio

n 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’sfan. The yearly difference gives the potential energy saving. 39




Energy savings Condenser’ fans MK1 power Estimation of the energy saving 2x 30 kW 25 MWh/

Condenser’ fans MK3 power Estimation of the energy saving

2x 30 kW 25 MWh/

Condenser’ fans MK4 Estimation of the energy saving

2x 11 kW 9,1 MWh/

Electricity price Total electrical energy saving Financial savings Total investment for a variable frequency drives Payback time

98 MWh/ 59 MWh/ 5.700 /year 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° reduce the energy C 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 consumptionshave been measured on site. We can consider that the energy consumption with VFDis linear with the power consumption for an operating load between 50 and 100%.

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Energy Efficiencywww.leonardo-energy.org Energy savings Fans power Operating hours Existing motorfrequency Proposed motor frequency Energy consumption at lower frequency Breakdue to fries cutting Break due to technical problems Break due to other parameters Total percentage with no production Total hours with no production Total energy saving 198 6.500 50 15 5.3 1,46 4,29 1,44 7,19 467,35 90 kWe h/year Hz Hz kWe% % % % h/year MWh/ year Electricity price Financial savings Total investment f

or a variable frequency drive Payback time 97 8.800 3.800 0,4 /MWh /year year

Others points to check out: • • The efficiency of the defrost system Temperaturedifference at the evaporator not too high (max 7° C)

COMPRESSORS energy efficiency measures

COOLING CAPACITY REGULATION with a variable frequency drive CASE STUDY Companyspecialized in potatoes, mashed potatoes and French fries production Belgium. De

scription 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




To decrease the wearing of the compressors and at the same time to reduce theirelectrical 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 Estimation of the energy saving 2x 30 kW 25 MWh/

Condenser’ fans MK3 power Estimation of the energy saving

2x 30 kW 25 MWh/

Condenser’ fans MK4 Estimation of the energy saving

2x 11 kW 9,1 MWh/

Electricity price Total electrical energy saving Financial savings Total investment for a variable frequency drives

98 MWh/ 59 MWh/ 5.700 /year 28.360

Payback time

5 years

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

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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 Protecti

on 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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