Topic 4 FDFTECENG4A. Topic 4: Apply principles of evaporation to a production process

Transcription

1 Topic 4: Apply principles of evaporation to a production process 1

2 Contents Overview 3 Process of evaporation 4 Principle of evaporation 4 Stages in the evaporation process 5 Evaporation equipment 7 Safe operation and maintenance 11 Environmental impact of evaporation processes 11 Effect of evaporation on product 14 Physical property changes 14 Flavour changes 14 Preservation effect 15 Performance and testing 16 Tests for determining solids in a product 16 Factors affecting the rate of evaporation 16 Factors affecting performance 17 Summary 21 Glossary 22 Answers to activities 23 2

3 Overview This topic is about the ways that liquid foods are evaporated in the food processing industry. It looks at different types of evaporation equipment and their operation. It shows the factors that influence the rate of evaporation. It looks at what happens to foods when water is evaporated from them. You will learn about: what evaporation means stages in the process types of evaporation equipment operating capacity of evaporation equipment safety and maintenance of evaporation equipment how evaporation can affect product quality At the end of this topic you will find a Glossary to help you with the meaning of new words. 3

4 Process of evaporation Many foods contain a lot of water. Reducing the amount of water saves space, weight and packaging. This is usually done by boiling off some of the water in an evaporator. Some of the reasons for evaporation in the food industry are: concentration of a liquid before further processing, for example milk or sugar crystallisation reduction in volume to reduce packaging and transport costs, for example orange juice, tomato paste concentration of a food for preservation, for example condensed milk, jam. Principle of evaporation In the process of evaporation some of the water is evaporated from liquid foods while still leaving them in a liquid form. For evaporation to be economical it must be done rapidly. The most rapid way is to make the water evaporate at the boiling point. However the nature of the food must also be taken into account. This is because too high a boiling point during evaporation may lead to product degradation, for example loss of vitamins or change of flavour. Boiling temperature of liquids There are two effects pressure and concentration to consider. Effect of pressure: The pressure on water is due to the deep layer of air that surrounds the earth. It is called atmospheric pressure and is equal to 101.3kPa. At sea level water boils at C. At lower pressures the boiling point of water is lower. At the top of a mountain the pressure is less because the depth of air above the mountain is less and so the boiling point is lower. At the top of Mt Kosciusko water boils at about 93 0 C and at the top of Mt Everest it boils at about 70 0 C. At higher pressures, for example in a retort, water boils at a higher temperature. At a processing temperature of C the pressure in a retort is kpa. 4

5 Effect of concentration: As a liquid becomes more concentrated its boiling point rises. A 10% solution of sodium hydroxide boils at C while a 30% solution of sodium hydroxide boils at C. For sugar syrups the boiling points are: % sucrose boiling point ( 0 C) As the concentration of dissolved solids increases, the solution becomes more viscous. This is the basis of confectionary making. Soft confectionary products are made by boiling syrup to C C while hard confectionary products are made by boiling syrup to C C. Stages in the evaporation process There are three stages in the evaporation process: Heat exchange: The first stage involves exchange of heat from the heat source to the liquid being evaporated. The heat must be from a source at a suitable temperature and this is condensing steam in most cases. The steam comes either directly from a boiler or from a previous stage of evaporation in another evaporator. Evaporation of the liquid: As the liquid becomes more concentrated the boiling temperature rises due to the effect of increasing concentration of dissolved solids. Increasing dissolved solids lead to increased viscosity and poorer circulation of liquid and so the total rate of heat transfer will drop. It is possible to lower the boiling point by reducing the pressure on the liquid. This is done by attaching a vacuum pump to the evaporator to pump out air. Separation of the vapour that is formed from the liquid: A condenser removes the bulk of the volume of the vapours by condensing them to a liquid. Condensers for the vapour may be either surface or jet condensers. Surface condensers provide sufficient heat transfer surface, pipes for example, through which the condensing vapour transfers latent heat of vaporization to cooling water circulating through the pipes. In a jet condenser, the vapours are mixed with a stream of condenser water, sufficient in quantity to transfer latent heat from the vapours. 5

6 Activity 1 1 What are the advantages of evaporating liquid foods? 2 What are the three stages in the evaporation process? 3 What happens to the boiling point as a liquid becomes more concentrated? Check your answers at the end of this topic. 6

7 Evaporation equipment There are many types of evaporation equipment depending on the use in the manufacturing process. The following discussion looks at different types of commonly-used evaporation equipment. Single-effect evaporator This type has three sections: a heat exchanger an evaporating section where the liquid boils and evaporates a separator in which the vapour leaves the liquid and passes off to the condenser or to other equipment. All three sections may be contained in a single vertical cylinder (see Figure. 1). In the centre of the cylinder there is a steam heating section, with pipes passing through it in which the evaporating liquors rise. At the top there are baffles, which allow the vapours to escape but check liquid droplets which may accompany the vapours from the liquid surface.. Figure 1: Evaporator 2004 Earle: Unit Operations in Food Processing Web edition: Fig 8.1 7

8 Multiple-effect evaporator In this type one evaporator is treated as a low pressure boiler. The steam produced is used for further heating in another following evaporator, called another effect. They use less heat than single-effect evaporators. Figure 2: Double effect evaporator forward feed 2004 Earle: Unit Operations in Food Processing Web edition: Fig 8.2 Vacuum evaporator A vacuum pump attached to a closed tank pumps out air and steam formed by evaporation. Steam is condensed and removed before it enters the vacuum pump. Film evaporators Film evaporators spread the liquid in a thin layer over a large heated surface area at reduced pressure. The liquid moves rapidly over the hot surface so contact time is short. The film may be formed by centrifugal force or by a combination of pumping, gravity and scrapers. 8

9 Rising film evaporator This has a vertical heat exchanger section with long tubes and a steam liquid separator. Liquid is pumped into the base and passes up the tubes, forming steam. The higher up the tube the more steam forms. The slugs of steam improve heat transfer by forcing the liquid into a thin film between the slug and the wall. At the top of the tubes the steam and concentrate go to the separator. The liquid drops to the bottom and the vapour passes out the top. Falling film evaporator The liquid enters at the top and flows down the tubes. The concentrate leaves at the bottom. The vapour, as it forms, is sucked back up the tube by a vacuum system attached to the top. Because vacuum is being used evaporation can be carried out at a lower temperature than in the rising film evaporator. Forced circulation evaporators The major resistance to heat flow in an evaporator is usually in the liquid film, not the condensing steam. Pumps or impellers can be fitted in the liquid circuit to improve the circulation of the liquid and increase its velocity of flow across the heating surfaces. Forced circulation is used particularly with viscous liquids. Forcedcirculation evaporators are used for processes where crystals are formed. They keep crystals suspended in solution to prevent scaling of the equipment. Plate evaporators A plate heat exchanger can be adapted for use as an evaporator. The vapours have a much larger volume than the liquid. Spacings between the plates have to be increased and passages provided for the vapours. Jacketed pans This is usually done at atmospheric pressure on a batch basis. The heating in the jacket may be by steam or hot liquid. Scrapers or paddles may be fitted to provide agitation. 9

10 Basket evaporators This is a variation of a conventional evaporator in which the steam chest is contained in a basket suspended in the lower part of the evaporator and recirculation occurs through the space around the basket. Figure 3: Evaporators (a) basket type (b) long tube (c) forced circulation 2004 Earle: Unit Operations in Food Processing Web edition: Fig 8.3 Variations to the process There are three types of variations that can occur during the process: Flavour recovery: When concentrating fruit juices some critically important flavours evaporate with the steam, mostly in the early stages. Flavour recovery may be performed along with the main concentrating process. Flavours are collected in a concentrated form and then separated from the main condensate. Alternatively, some of 10

11 the raw juice is mixed with the concentrate to replace the lost flavours. Viscous products: If the viscosity of a product becomes very high the rate of circulation will become very low. A scraped surface heat exchanger may be required to maintain a high rate of heat transfer while keeping product damage to a minimum. Foaming: Foaming can be a problem during evaporation of some liquids. Foam breaking mechanisms such as spinning blades are needed for these liquids. Safe operation and maintenance The manufacturer s operating manual will give instructions for the safe operation and maintenance of evaporation equipment. Here is an example of some possible instructions for using an evaporator: 1 Wear gloves, safety glasses, and protective clothing when working near evaporators. 2 Steam lines are very hot even when steam pressure is zero. Wear heatsafe gloves when adjusting steam pressure or touching any steam lines. Condensed steam discharge lines which go down the drain are hot. Take care when working near them. 3 Check that evaporators are filled with liquid before turning on the steam. Turn on the cooling water for the condensers before turning on the steam. 4 Monitor levels in each effect to make sure the effects do not run dry. Environmental impact of evaporation processes A multiple effect evaporator has the advantage over a single effect evaporator in that the heat is used over and over again which reduces energy use. Also, condenser and cooling water requirements will be less in a multiple effect evaporator. Another practice is to take the vapour and, after compressing it, return it to the steam chest of the evaporator from which it came, resulting in economy of steam. Reduction in the use of energy and resources leads to a reduction in waste, emission and pollution, and also saves money. There may be company environmental management manuals or similar documents which detail procedures for waste treatment and disposal, as well as energy conservation measures. 11

12 Activity 2 1 Give three examples of equipment that is used for evaporation in food processing and what foods they are used for. 2 Name a heat sensitive food and describe what method could be used to evaporate it. 3 Name two problems that could occur with evaporation of some foods and how they are overcome. Check your answers at the end of this topic. The following questions are based on your workplace. Attempt the questions and check the feedback with your supervisor. What processes in your workplace involve evaporation? 12

13 What types of evaporation equipment are used in your workplace? Identify practices related to evaporation processes in your workplace that: demonstrate the wise use of environmental resources reduce the harmful impact on the environment. 13

14 Effect of evaporation on product Physical property changes Evaporation results in an increase in boiling point of the solution. The boiling point may rise considerably, for example jam is boiled to about C and sugar confectionery to about C or higher. Evaporation results in an increase in the amount of solids present. For example an evaporator could be used to produce a 35% solids tomato concentrate from a 6% solids raw juice. Viscosity of a fluid will often increase as the concentration of the dissolved materials increases. For example as sugar syrup is heated to make confectionery it becomes more and more viscous. Evaporation may result in crystallisation of a supersaturated solution, for example when making fondant (used in chocolate centres). Sugar syrup is heated to the proper temperature. Then the supersaturated solution is cooled carefully. Crystallisation is induced by the beating. Flavour changes If evaporation temperature is high cooked flavours develop. It is better to concentrate foods at as low a temperature as possible. This is done in a vacuum evaporator. Sugar cane juice is concentrated this way before crystallisation. Many foods are more affected by heat than sugar is, for example fruit juice, tomato paste and milk. For such foods short time, one pass or film evaporators are used. Some products develop flavours which are acceptable or even desirable, for example jam, sugar confectionery and meat sauce. Evaporation is usually done in jacketed pans. 14

15 Preservation effect The removal of water helps to preserve food. Evaporated milk is made by removing about 60% of the water from whole milk, making the lactose concentration about 11.5%. Condensed milk has even more water removed than evaporated milk and so inhibits bacteria even more. The high sugar concentration in some evaporated foods inhibits the growth of some kinds of bacteria. It may also prevent the growth of some survivors of heat treatment. Sweetened condensed milk has very high sugar content and increased %salt, both of which tie up moisture, making it unavailable to any but osmophilic micro-organisms. Activity 3 1 Name three changes that occur in the physical properties of a liquid when it is evaporated. 2 How does evaporation help to preserve a liquid food? Check your answers at the end of this topic. 15

16 Performance and testing Tests for determining solids in a product There are four types of tests that can be used: Refractive index: a measure of how much the speed of light is reduced inside a substance. For example, if a substance has a refractive index of 1.4, it means that light travels at 1 / 1.4 which equals 0.71 times the speed in air or in a vacuum. A refractometer: an optical instrument used to determine the refractive index of a substance or some physical property of a substance that is directly related to its refractive index. Degrees Brix: is defined as soluble solids per 100g (not per 100 ml) and is a measure of all soluble solids such as pigments, acids and sugar. A Brix scale measures refractive index through a scale calibrated for sucrose solutions. For example tomato sauce with a refractive index of has soluble solids expressed as 26 0 Brix. Baume: also gives a measure of soluble solids. (1.8 Brix = 1 Baume) Factors affecting the rate of evaporation The factors that affect the rate of evaporation in a process are the: rate at which heat can be transferred to the liquid quantity of heat required to evaporate each kg of water maximum allowable temperature of the liquid pressure at which the evaporation takes place changes that may occur in the foodstuff during the course of the evaporation process. 16

17 Factors affecting performance The technical information supplied with an evaporator may include heat transfer area, maximum evaporation rate, maximum steam consumption, maximum steam pressure and maximum vacuum. It will give an indication of expected performance. Poor performance may be due to such factors as: Leaks: Air has to be vented to keep the pressure from increasing. Water leaks dilute the product. Fouling of surfaces: Fouled heat transfer surfaces decrease evaporation rate. Insulation thickness: Wet or damaged insulation reduces efficiency and should be repaired or replaced. Example: evaporation process what is the rate of evaporation? A climbing film evaporator is required to concentrate 540 kg of tomato juice from 12% solids to 28% solids. The quantity of tomato juice feed per hour is 360 kg. Heat transfer rate is 1.32 x 10 5 Js -1. The maximum allowable temperature for tomato juice is 57 C so the juice is fed to the evaporator at 57 C. Steam is used in the jacket of the evaporator at a pressure of 170 kpa (abs) and at this temperature the latent heat of vaporisation is 2366 kj kg -1. For 100 kg solids (kg) liquids (kg) total (kg) Feed Product Evaporation 57 Heat required per kg of feed for evaporation = 0.57 x 2366 x 1000 = 1.34 x 10 6 J Rate of evaporation = heat transfer rate/ heat required = (1.32 x 10 5 )/ (1.34 x 10 6 ) = 0.1 kg s -1 or 360 kg h -1 17

18 Example: evaporation process how much steam is required? How much time is required? A single effect evaporator is required to concentrate 1000 kg of a solution from 10% solids to 30% solids at a rate of feed is 250 kg /hour. Based on latent heat of vaporisation and sensible heat calculations: Heat required by the solution = 4.57 x 10 8 J h -1 Heat available per kg of steam = 2.34 x 10 6 J Quantity of steam required per hour = heat required/ heat available = (4.57 x 10 8 )/ (2.34 x 10 6 ) = 195 kg h -1 Time required = weight of solution/ rate of feed =1000 / 250 = 4 hours Activity 4 1 Name a factor that can cause poor performance in evaporators. 2 Name three factors that increase the rate of evaporation in evaporators. 18

19 5 3 What is the rate of evaporation if the heat transfer rate is 8.42 x 10 Js -1 and the heat required per kg of feed is 7.43 x 10 6 J? 4 If the heat available per kg of steam = 26 x 10 6 J how much steam is required per hour? 5 How long will it take to evaporate 500 kg of a solution if feed rate is 200 kg /hour? Check your answers at the end of this topic. 19

20 The following questions are based on your workplace. Attempt the questions and check the feedback with your supervisor. What parameters are set to achieve a target total solids level for an evaporation process in your workplace? What tests are carried out to determine solids in these processes? What methods are used to monitor performance of evaporation processes in your workplace? What can go wrong with evaporation equipment used in your workplace and what can be done about it? 20

21 Summary This topic has shown you some of the ways that liquid foods are evaporated in the food industry. You have seen how evaporation equipment works and learned about some of its safety and maintenance features, as well as its operating capacity. You can see what happens to foods when some of the water is removed from them and the reasons for doing so. You have seen how processing parameters are set to achieve a product of a particular solids and what tests are carried out to check the solids in the final product. 21

22 Glossary 0 Brix soluble solids per 100g crystallisation drying evaporation refractive index solubility solute supersaturated solution the formation of a solid of a specific shape (a crystal) from a liquid by cooling or evaporation the removal of water from a solid or liquid to form a solid product the reduction of the water content of a liquid a measure for how much the speed of light is reduced inside a substance the amount of solute that will dissolve in 100 g of solvent at a given temperature the solid material that is added to a liquid and dissolves to form a solution a solution that has more solute dissolved in it than is normally possible at that temperature You should add other new words and their meanings to this Glossary. 22

23 Answers to activities Activity 1 1 Reducing the amount of water saves space, weight and packaging. 2 Heat exchange, evaporation of the liquid, separation of the vapour that is formed from the liquid 3 Boiling point increases Activity 2 1 Vacuum evaporator (eg heat-sensitive foods such as fruit juice), film evaporator (milk), forced circulation evaporator used for processes where crystals are formed (whey). There are many other examples. 2 Tomato juice (use a climbing film evaporator) 3 Flavours that evaporate are collected in a concentrated form and separated from the main condensate; use of a scraped surface heat exchanger if the viscosity of a product becomes very high; foam breaking mechanisms such as spinning blades for products that foam Activity 3 1 An increase in boiling point, increase in viscosity, crystallisation 2 High sugar and/ or low moisture inhibit the growth of some microorganisms. Activity 4 1 water leaks, fouled surfaces, damaged insulation (there may be others) 2 higher heat transfer rate, lower pressure, lower heat required for the liquid to evaporate kg/h kg/h 5 2.5h 23