07.09.2013 1 GENERAL DESCRIPTION OF APPLIED TECHNOLOGIES MECHANICAL TECHNOLOGIES PRECIPITATION [1, 2] Precipitation is used to eliminate dissolved substances or colloidal particles out of a liquid by adding heat or a chemical agent that encourage the particles to stick together. There are four basic stages in the liquid-solid separation process: pH adjustment, coagulation or flocculation, clarification or thickening and optional filtration. The process flow scheme is given in Figure 1. Sedimentation and filtration are discussed in the following chapters. Filter press Treated stream Settling tank Balance tank Solid for recovery /disposal pH adjustment and flocculant addition Feed stream Figure 1:Liquid-Solid separation by precipitation Coagulation/Flocculation In the precipitation process, chemical precipitants, coagulants and flocculants are used to increase particle size through aggregation. Small colloidal particles (<1μm) aggregate to large particles, which settle down as sediment or can be floated or filtered. Coagulants and flocculants modify the surface tension to improve wetting, contact and aggregation. Coagulation implies aggregation caused by compression, or collapsing of the electrical double layers that surround the tiny, colloidal particles. Aluminium and iron salts (Al 2 (SO 4 ) 3 , FeCl 3 ) and lime are the most common used coagulants. The flocculation process involves the physical binding or bridging of particles through the use of a polimer flocculant. Synthetic organic polymers that may be cationic, anionic or non-ionic in character (used alone or combined with metals salts) are generally used to promote flocculation. Settling or sedimentation occurs naturally as flocculated particles settle down or it can be forced via a combined precipitation. Heat precipitation [3-5] The separation/ coagulation of several substances such as proteins can take place by means of heat precipitation. In the case of whey generated during cheese production, acid or neutral whey must be heated to at least 90ºC and maintained at that temperature for at least 10 minutes to achieve maximum precipitation. The critical variables that affect this process have been duly described. Traditional whey cheeses are produced by heat treatment of whey leading to precipitation of whey proteins.
Transcript
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GENERAL DESCRIPTION OF APPLIED TECHNOLOGIES
MECHANICAL TECHNOLOGIES
PRECIPITATION [1, 2]
Precipitation is used to eliminate dissolved substances or colloidal particles out of a liquid by adding heat or a chemical agent that encourage the particles to stick together. There are four basic stages in the liquid-solid separation process: pH adjustment, coagulation or flocculation, clarifi cation or thickening and optional filtration . The process flow scheme is given in Figure 1. Sedimentation and filtration are discussed in the following chapters.
Filter press
Treated stream
Settling tank
Balance tank
Solid for recovery /disposal
pH adjustment and flocculant addition Feed stream
Figure 1:Liquid-Solid separation by precipitation
Coagulation/Flocculation
In the precipitation process, chemical precipitants, coagulants and flocculants are used to increase particle size through aggregation. Small colloidal particles (<1µm) aggregate to large particles, which settle down as sediment or can be floated or filtered. Coagulants and flocculants modify the surface tension to improve wetting, contact and aggregation.
Coagulation implies aggregation caused by compression, or collapsing of the electrical double layers that surround the tiny, colloidal particles. Aluminium and iron salts (Al2(SO4)3 , FeCl3) and lime are the most common used coagulants.
The flocculation process involves the physical binding or bridging of particles through the use of a polimer flocculant. Synthetic organic polymers that may be cationic, anionic or non-ionic in character (used alone or combined with metals salts) are generally used to promote flocculation. Settling or sedimentation occurs naturally as flocculated particles settle down or it can be forced via a combined precipitation.
Heat precipitation [3-5]
The separation/ coagulation of several substances such as proteins can take place by means of heat precipitation. In the case of whey generated during cheese production, acid or neutral whey must be heated to at least 90ºC and maintained at that temperature for at least 10 minutes to achieve maximum precipitation. The critical variables that affect this process have been duly described.
Traditional whey cheeses are produced by heat treatment of whey leading to precipitation of whey proteins.
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Heat precipitation is also used for the coagulation of the protein contained in the fish by-products during fish meal and fish oil production.
SEDIMENTATION/ CENTRIFUGATION [24-26]
Sedimentation and centrifugation are technologies used for separating immiscible particles from a solution according to size, density and viscosity of the medium. The separation takes place due to natural gravity or by the application of centrifugal forces. When the time is not a limiting factor and the density differences are large, the separation can take place by gravity. If the time is a limiting factor and the densities are similar, then centrifuges can be used in order to speed up the separation process. Rotating the mixtures generates the centrifugal forces and this strength of this force will depend on the speed and rotation radius.
Sedimentation
The separation can be performed in batch or in continuous way. The batch one occurs in a vessel where the heavier particles fall to the bottom. The sedimentation time can be reduced is the surface of the vessel increased and the height decreased.
Centrifugation
The centrifuges can be classified in three groups:
• Tubular/disc bowl centrifuges (see Figure 2.)for separating immiscible liquids different specific gravity or clarification of liquids. Tubular bowl centrifuges consist of a Tubular Bowl that can generate centrifugal force of 16,000 times the force of gravity. This centrifuge is generally used for continuous separation.
• Solid bowl/nozzle valve discharge centrifuges, for clarifying liquids by the removal of small amount of solids.
• Conveyor bowl/reciprocating conveyor centrifuges, for dewatering sludge.
Figure 2: Tubular disc bowl centrifuge
Another system of classification is the rate or speed at which the centrifuge is turning. Ultracentrifugation is carried out at speed faster than 20,000 rpm. Super speed ultracentrifugation is at speeds between 10,000 and 20,000 rpm. Low speed centrifugation is at speed below 10,000 rpm.
Centrifugation is applied in dairy industry for the recovery of casein and in lactose and whey protein processing.
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It is also applied in the production of blood plasma, blood meal and red blood cell plasma in case of meat by products.
FILTRATION
Filtration is used to remove solids from a liquid stream. Those solids include clays and silts, natural organic matter, precipitates from other treatments, Fe and Mn and microorganisms. The separation is done by means of a porous medium, screen or filter cloth, which retains the solids and allows the liquid to pass through. The filters can be layers of sand, gravel, and charcoal or activated carbon, which help to remove even smaller particles. Filtration clarifies water and enhances the effectiveness of disinfections.
It can be applied alone or as a pre-treatment before the application of another technology.
Filtration equipment can operate naturally or forced by the application of pressure (pressure filtration) to the feed side or by the application of a vacuum (vacuum filtration) to the filtrate side.
ELECTROSTATIC SEPARATION
Electrostatic precipitators are used to handle large volumes of gases from which aerosols must be removed. They have the advantage of low pressure drop, high efficiency for small particle size, the ability to handle both gases and mists for high volume flow, and the relatively easy removal of the collected particulate. The four steps in the process are: • Place a charge on the particle to be collected • Migrate the particle to the collector • Neutralize the charge at the collector • Remove the collected particle At the very high DC voltages used, 25 to 100 kV, a corona discharge occurs close to the negative electrode. The gas close to the negative electrode is thus ionized upon passing through the corona. As the negative ions and electrons migrate toward the collector electrode they, in turn, charge the passing particulate. The electric field then draws the particulate to the collector where it is deposited. The collected material is removed, usually by hitting the collector, a process which is called rapping. If a mist is being collected, then the material runs down the collectors and is removed at the bottom. Figure 3. shows a full-scale precipitator with plate type collectors.
Figure 3: Electrostatic precipitator [60]
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EMULSION SEPARATION [61]
There are generally two methods for demulsification; Chemical and physical. Chemical Demulsification involves addition of a chemical demulsifying agent to the emulsion that engage in molecular interaction within the system in order to enhance phase separation. This method has the disadvantage of cost and addition of a chemical which might be needed to be separated in further parts of the process. These implication are usually avoided by application of physical methods which include the already mentioned gravity settling and electrostatic coalescence. The principal idea behind the later method is to enhance phase separation through electrically-aided charging, migration, collision, and thus coalescence of dispersed phase droplets whitin the system. This is a processes is usually in a cell called the coalescer with dimensions proportional to nature of the process but usually much smaller than a traditional gravity settler. The environmental , cost and processing time friendly nature of this process makes it an ideal option in many phase separation application when the separation of water-in-oil emulsions are needed.
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DIFFUSIONAL TECHNOLOGIES
DISTILLATION [1, 9-12, 64]
By distillation the components of a liquid mixture can be separated by the difference in volatility. When the liquid mixture boils, the concentration of each component in the vapour is proportional to its volatility. The remaining liquid is rich in the less volatile substances and the vapour in those that are more volatile.
The classical types of distillation are batch distillation and continuous distillation.
Batch distillation
In small plants, volatile products are recovered from liquid solution by batch distillation. The mixture is charged to a tank (called still) and heated to bring the liquid to the boiling point and then vaporise part of the batch. In the simplest method of operation, the vapours are passed directly from the still to a condenser (Figure 4). The vapour leaving the still at any time is in equilibrium with the boiling liquid. Compositions of both phases change with time, concentrations of more volatile components decrease in liquid and vapour phases. Batch distillation with only a simple still-condenser system does not give a good separation unless the relative volatility is very high. In many cases, to improve the separation a column is connected to the still. The vapour leaving the column is condensed and part of the condensate is returned to the top of the column. This returned liquid is called reflux. The use of reflux increases the purity of products.
Batch still
Steam
Product receiver
Condenser
Figure 4: Batch distillation
Continuous distillation
For large-scale production continuous distillation (rectification) is normally used.
In the flash distillation process the liquid mixture is separated by vaporising one of the components in a tank (flash drum). The feed stream is pumped through a heater, and then the pressure is reduced as the feed flows through a valve and into the flash drum. It is used when the liquids to be separated have significant differences in boiling points. The applicability of this type of distillation is limited when high purity is desired and the components are heat sensitive.
To produce nearly pure products a fractionating column is used. The flow-diagram of the continuous fractionation is shown in Figure 5. The column is fed continuously with the liquid mixture to be distilled. The products are removed from the condenser and reboiler, called top and bottom products, respectively. The liquid in reboiler is partially converted to vapour by heating. This vapour is brought into intimate countercurrent contact with a descending liquid stream in the column. This liquid is obtained by condensing the vapour leaving the top of the column, and returning a part of the liquid as reflux to the top of the column. The feed is usually a liquid at the boiling point, and it is added to the liquid flow in the lower section of the column. The upper section of the column, above the feed, is called rectifying section. Here the vapour stream is enriched in low boiling component and close to pure top product is obtained. The lower section of the column, below the feed, is the stripping section, in which the liquid is enriched in the high boiling component. The liquid withdrawn from the reboiler is the bottom product. For production highly pure products a tall column and a large reflux is required.
Fractionation of multi-component mixtures requires more column.
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The columns contain plates (e.g. bubble-cap plates, sieve plates, valve-trays) or packings (e.g. Raschig rings, Pall rings or regular packings) to contact the vapour and liquid phases.
Special distillation processes
If the components form an azeotropic mixture the complete separation is impossible. For such systems a solvent is added to alter the relative volatility of the original components. If this component forms an azeotrope with one or more components of the feed, the process is called azeotropic distillation. The azeotrope forms the top or bottom product in the column and is later separated into added solvent and feed component. Usually, the material added forms a low-boiling azeotrope and is taken at the top. An example of azeotropic distillation is the use of heptane or cyclohexane to separate ethanol and water, which form a minimum-boiling azeotrope with 95.6 weight percent alcohol.
Reboiler
Top product Reflux
Distillation column
Bottom product
Condenser
Feed stream
Figure 5: General diagram of the column distillation
There is also the so-called steam distillation where hot steam helps to release the desired molecules from the material. This is widely applied for the production of essential oils that will then escape from the material and evaporate into the steam. It is very important that the temperature is carefully controlled not to burn the essential oils. The steam which then contains the essential oil, is passed through a cooling system to condense the steam, which forms a liquid from which the essential oil and water is then separated.
The steam is produced at greater pressure than the atmosphere and therefore boils at above 100 °C, which facilitates the removal of the essential oil at a faster rate, preventing damage to the oil.
Another type of distillation is the reactive distillation in which distillation is combined with a chemical reaction. This distillation offers some advantages over the conventional distillation, especially for equilibrium limited reactions such as esterification and ester hydrolysis reactions.
Distillation is normally applied for the separation of solvent mixtures for recovery and reuse, and for the removal of volatile compounds from aqueous feed streams. It is broadly applied for the production of essential oils, alcohol production from wine pomace and from starch rich solid by-products.
ABSORPTION [64, 65]
In gas absorption a soluble gas or vapour is absorbed from its mixture with an inert gas by means of a liquid in which this component is soluble.
The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discharged or reused. Sometimes a solute is removed from a liquid mixture by bringing the liquid into contact with an inert gas. This process is the reverse of gas absorption, is called desorption or gas
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stripping. In many cases, stripping is used to recover the solute in more concentrated form and regenerate the absorbing liquid. Plate or packed columns are used to enable intimate contact between gas and liquid. Counter current flow is normally used with the gas entering at the bottom and the liquid at the top (Figure 6.).
Gas Liquid
Gas Liquid
Figure 6: General scheme of absorption
Absorption columns are often operated under pressure to give increased capacity and higher rates of mass transfer. Low temperatures are favourable for absorption of the gas components. By stripping the temperature may be increased or the total pressure reduced, or both these changes may be made.
Both absorption and desorption may be accompanied by chemical reactions in the liquid phase. Major applications are absorption of CO, CO2, CS2, H2S, O2, O3, NO, NO2, NH3, Cl2, Br2, COCl2, HCl, HBr, SO2, SO3 and olefins.
Air stripping is a proven technology for removal volatile organic compounds (VOCs) from wastewater. The air (off-gas) is further treated to recover (e.g. carbon adsorption), or destroy (e.g. incineration) the VOCs in the air stream.
EXTRACTION [64, 65]
Extraction can be defined as a process of removing substances from a solid or a liquid mixture by means of a solvent. (Mechanical pressing, which is often called extraction is described among the mechanical separation processes.). The EC Council directive 88/344/EEC (ammended version published in Official Journal on 03.12.1997) states the regulations regarding the use of extraction solvents in the production of foodstuffs and food ingredients within EC. This directive can be used as an example of the current legal situation concerning the permitted residual solvent levels. The solvents are classified according to their acceptability from the point of view of safety to the consumer. Water, to which substances regulating acidity or alkalinity may have been added, and ethanol are authorised as extraction solvents in the manufacture of foodstuffs or food ingredients. Solvents to be used in compliance with good manufacturing practice for all uses are: propane, butane, carbon dioxide, nitrous oxide, ethanol, ethyl acetate and acetone. For other extraction solvents (e.g. hexane, methanol, methyl acetate) conditions of use are specified by this directive.
Solvent extraction
Solid-liquid extraction (leaching) [66, 67]
Leaching is the separation of a solute from a solid phase by dissolving it in a liquid phase. It consists of two stages: contacting solvent and solid to effect a transfer of solute, and then the separation of the solution from the remaining solid. Throughout the extraction the solvent may be percolated through an unagitated bed of solid particles, or the particles are dispersed into the solvent and are later separated from it. Both method may be used either batch or continuous.
Small volumes of raw materials (e.g. isolation of flavours, fragrances, pharmaceuticals) are generally processed in batch extractors. Grinding of raw material to a specific particle size is the initial step of the process. The pre-treated plant material is put into water or some common organic solvents (e.g. hexane, ethyl acetate, ethanol). At the end of a prescribed amount of time, after dissolution of the valuable
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components the exhausted solid residue is filtered off. Following extraction, simple evaporation of the solvent is often sufficient to obtain the final product, but legislation requires the removal of organic solvents to a prescribed residual level. Vacuum is often applied. The type of solvent system used varies for different plants and determines the character of the extract. Resinoids are obtained with hydrocarbons, absolutes are prepared with ethanol.
Huge volumes of oil and sugar production demand the use of fully mechanised and largely automated, very large-scale continuous extraction units with capacities of up to 4000 ton seed/day in oil industry, and up to 10000 ton sugar beet/day in sugar industry. Figure 7. depicts the extraction and solvent recovery system. Continuous extractors transport huge volumes of solid material and solvent, bring them into intimate contact for substantial periods of time. There are many types of industrial extractors used for oil recovery (e.g. De Smet, Lurgi, Crown, Carrousel) and sugar production (e.g. tower extractor(Buckan-Wolf, BMA), drum extractor (RT), trough extractor (DDS)). Solvent recovery and recycle is essential in the process. From oil solution (called miscella) the solvent is recovered in multistage evaporation. The extracted solid material (meal) is treated in desolventizer/toaster unit, where the solvent is evaporated. The evaporation is enhanced by the injection of live steam.
Figure 7: The extraction and solvent recovery system
Liquid-liquid extraction [68]
Liquid-liquid extraction is an operation by which a solute species is removed from a solution by contact with an immiscible solvent. It is a mass-transfer operation wherein both phases are liquids. Each phase will have a different concentration with respect to the solute. The phases are allowed to separate and the solution is mixed again with the solvent in the same way to produce a further change in composition. Continuous countercurrent operation allows for more complete removal of the solute, furthermore solvent is re-used so that less solvent is needed than in single stage extraction. This principle can be used for purifying liquid mixtures.
Having decided on liquid-liquid extraction as the unit operation to reach a given separation it is necessary to select suitable equipment for the task. The choices are: a mixer-settler, a series of mixer-settlers, a column (which may be agitated or pulsed), or a centrifugal device.
Liquid-liquid extraction is used when the separation cannot be effectively done by distillation, where the compounds in the solution have nearly the same boiling point or decomposition occurs upon heating. Solvent extraction can be a separation process of a solute from a dilute solution, especially if the solute of interest is less volatile than the main component of the feed mixture. There is growing interest in producing commodity chemicals from renewable resources, e.g. by fermentation of various forms of biomass. The products, such as carboxylic acids, could be recovered by extraction from the dilute and complex solutions.
SUPERCRITICAL FLUID EXTRACTION [21, 22, 69]
Supercritical fluid extraction is a solvent extraction of compounds present in a solid or semisolid matrix that takes place above the critical point. The critical point is where gas and liquid status no longer exists due to pressure (critical pressure, Pc) and temperature (critical temperature, Tc) conditions. The region above critical point is the supercritical region (red squares in Figure 8) and the new phase is known as supercritical fluid (SCF).
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Figure 8: Pressure-temperature diagram for CO2
SCFs have special physico-chemical properties (Table 1.), behaving somehow as liquids (density and solvating power), and as gas (transport and compressibility).
One powerful aspect of SFE is the ability of control which components of a matrix are extracted and which not. This is accomplished through precise control of several key parameters such a temperature, pressure, flow rates and processing time. Yields can be comparable to extractions performed by traditional techniques, and product purity is high. Decomposition of materials almost never occurs due to the relatively mild processing temperatures.
There are various solvents used in SFE for different extraction processes, but more of the 90% of extractions use CO2 as solvent due to its unique properties (SCCO2):
� CO2 has the critical temperature and pressure in relatively low values (31º C and 74 bars) that preserve product quality.
� Non-toxic gas, non-hazardous and leaves no undesirable solvent residues.
� No flammable or explosive and quite chemically inactive
� Rather cheap even at high purity levels
� Very easy to eliminate from obtained extracts and its use does not represent any environmental problem
� CO2 extracted products exhibit excellent storage stability
Mobile phase Density (kg/m 3) Viscosity (Pa*s) Diffusivity (cm 2/s)
Gas 1 10-5 10-1
Supercritical fluid 200-700 10-4 10-3- 10-4
Liquid 1000 10-3 10-5
Table 1: Physical properties of supercritical fluids
In the supercritical state, the polarity of CO2 is similar to that of the liquid pentane, so it is a good solvent for lipophilic compounds. Difficulties of supercritical CO2 in extracting polar compounds can be solved with the use of modifiers, as methanol, ethanol or water, that are added to the fluid in a static or dynamic way. These modifiers change CO2 density and solvating power. CO2 is considered as GRAS (Generally Recognised As Safe) by the US FDA (Food and Drugs Administration), so it is accurate for working with human consumption products.
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Supercritical fluid extraction (SFE) is carried out either to isolate a desired compound of higher value, or remove undesired components from a raw material to obtain a product with improved properties. Solid materials are processed in large scale batch units. The separation of mixtures that are in liquid state under the operation conditions can be carried out in a countercurrent procedure.
The basic arrangement of a multipurpose extraction unit for processing solid materials is shown schematically in Figure 9. The plant is equipped with three extractors, which can be connected in series or in parallel. While two of them is in operation, the third is depressurised, emptied and refilled with the raw material. Usually, the extractors are designed for operation with baskets in order to allow quick and easy raw material handling. The supercritical fluid, generally carbon dioxide, is stored in a working tank. The liquid CO2 is cooled to 0 - 5°C in a pre-cooler before the pump. It is compressed to a desired pressure in the pump. The high pressure liquid is then heated to the extraction temperature and fed into an extraction vessel, which is maintained at this temperature.
Figure 9: Multipurpose supercritical extraction unit
The CO2 passes through the raw material and extracts the soluble components. Following extraction, the pressure is reduced through a control valve to precipitate the extract. The precipitated material is collected at the bottom of the separator. Reduction of pressure causes cooling of the fluid and so the separator is heated. A series of separation vessels at successively lower pressure may be employed, which allows a simultaneous fractionation of the extract. The heavier (less volatile) components are collected in the first separator. The lighter (volatile) components are precipitated at lower pressures in the second separator. Addition of more separators gives further fractions. Plants which process highly viscous, sticky extracts (which may cause plugging), are provided with two separators switched in parallel for the first separation step. In the second (or last) separator the CO2 is evaporated and then liquefied in a condenser. The liquid CO2 is recycled to the storage-tank. There are standardized extraction units on the market offered by many companies (e.g. NATEX, SEPAREX, UHDE).
Some of the current industrial applications of supercritical fluid extraction are:
• Coffee and tea decaffeinating
• Spices extraction
• Paprika oleoresin extraction
• Essential oils extraction
• Food grease removal
Agro-food by-products appear as a good raw material for many active substances, and SCCO2 can be considered a clean technology suitable for upgrading processes.
Supercritical extraction can be applied alone or in combination with conventional extraction. This is the case for example of the extraction of astaxanthin from the shells and heads of crustaceans using ethanol as co-solvent.
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ADSORPTION
Adsorption is the concentration of a solute at the surface of a solid, when solid particles are contacted with gaseous or liquid mixtures. The solid is termed the adsorbent and the solute being adsorbed is the adsorbate.
Most adsorbents are highly porous materials, and adsorption takes place primarily on the walls of the pores or at specific sites inside the particles. The internal surface area is often 500 to 1000 m2/g.
In adsorption, the solid is usually held in fixed bed and fluid is passed continuously through the bed until the solid adsorbent is nearly saturated. The flow is then switched to second bed, and the saturated bed is replaced or regenerated. A typical system used for adsorption of solvent vapours is shown in Figure 10.
Steam Inert gas
Vent
Condenser Feed
Clean gas
H2O
Solvent
Figure 10: Vapour-phase adsorption system
Activated carbons, in either granular or powdered form, have been widely used as adsorbents in gas or water treatment plants to remove taste and odour causing contaminants. Activated carbons are prepared from carbonaceous raw materials (e.g. wood, lignite, coal, nutshells etc.) by a thermal process involving dehydration and carbonisation followed by application of hot stream.
Usually, reactivation of spent activated carbon is done by heating it to about 920-950°C in a steam-ai r atmosphere. This operation can be carried out in multiple hearth furnaces or rotary kilns. Adsorbed organics are burned off and carbon is reactivated ( the cycle can be repeated up to 30 or more times).
Drying of gases is often carried out by adsorbing the water on silica gel, alumina or other inorganic porous solids. The zeolits, or molecular sieves are used in separation gases with low dew points. Adsorption from liquids is used to remove organic components from drinking water or aqueous wastes, coloured contaminants from sugar solutions and vegetable oils, and water from organic liquids.
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ION EXCHANGE [8]
Ion exchange is a reversible chemical process, which applies the ability of an exchange resin to take up defined ions from a solution and liberate an equivalent amount of equally charged ions into the liquid.
Ion exchangers are filled with particles of granulated artificial resin whose molecular structure contains acid and alkaline groups. Through ion exchange the resins are spent and they have to be regenerated from time to time to eliminate the absorbed ions. The regeneration process provokes new environmental problems, since heavily concentrated wastewaters or used resins have to be treated or dumped.
The ion exchange materials can be naturally occurring inorganic zeolites, hydrous oxides or phosphates, or synthetic organic resins with attached functional groups. The last ones are the most used due to the possibility to tailor them for specific applications.
Ion exchanger resins can be classified as cationic exchangers, when positively charged mobile ions available for exchange, and anionic exchangers, when the exchangeable ions are negatively charged. Both resins can be produced from the same basic organic polymer. Resins can also be classified as strong or weak acid cationic exchangers or strong or weak base anionic exchangers.
The removal efficiency of the desired substance depends on the following factors:
� The concentration of the substance to be removed
� The frequency of the ion exchanger regeneration
� Feed stream quality
� Chemical degradation or physical change due to the age of the resin
The ion exchange process can be performed in batch or in continuously percolating column. In the first case, the resin and the solution are mixed in a tank, the exchange is allowed to come to equilibrium and then the resin is separated from the solution. The degree to which the exchange takes place is limited by the preference that the resin has for the ions in solution. Batch regeneration of the resin is chemically insufficient, that is why batch processing by ion exchange has limited applicability.
Passing the solution through a column containing a bed of exchange resins (see Figure 11.) is like treating it several times in batch tanks. The columns can contain cationic or anionic exchangers, or a combination of both. With this method it is possible to perform a more selective exchange of ions since the different resins can be combined upon the needs. Almost all the industrial applications of ion exchange use fixed-bed column systems.
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Figure 11: Ion exchange column
Ion exchange is mainly applied for water softening (elimination of Mg2+ or Ca2+) or to eliminate mercury or heavy metals from low contaminated wastewater. The application of ion exchange technology is only economically feasible for contaminant concentrations up to 2 g/l. It is mainly applied in the metal industry but it has some non-metallic potential applications such as the treatment of chloro-alkali brines, and tannery effluents.
Ion exchange technology is used for demineralisation of whey in order to produce demineralised whey powder and lactose.
EVAPORATION [1]
Evaporation uses a heat input to vaporise and remove one or more components from a liquid feed stream. The general flow diagram is shown in Figure 12. It reduces the volume of the original feed stream and concentrates non-volatile substances that are dissolved in it.
In the majority of cases the volatile solvent is water. Normally, in evaporation the concentrated solution (a thick liquor or a slurry) is the valuable product, and the vapour is condensed and discarded or reused. As the concentration of the solute increases the density and viscosity of the solution increases. The boiling point of the solution may also rise considerably as the solute content increases. Foaming may occur during vaporization, especially when organic substances are processed. Many foods and the food ingredients are damaged when heated.
The evaporation can take place under vacuum conditions if the liquid to evaporate is heat sensitive to reduce the boiling point and consequently product damage by heat.
Most evaporators are heated by steam condensing on metal tubes. Plate-type heating surfaces (e.g. Alfa-Luval, APV, Blake-durr) are applied increasingly in the sugar industry. The solution to be evaporated flows inside the tubes. The most important types of evaporators are the long-tube vertical evaporators (with climbing-film (Figure 13.), falling-film (Figure 14.) or forced circulation) and the agitated-film evaporators. Heat-sensitive materials are treated by simple passage through the tubes. If the solvent is water a multi-stage evaporator unit may be used (Figure 15). The evaporators are connected in series so that the vapour from one stage serves as the heating medium for the next one. So the heat in the original steam is reused in subsequent stages and only the vapour from the last stage is sent to a condenser. Generally, the dilute solution is fed into the first stage and the liquor is removed from the last stage.
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Purified solvent
Condenser
Heat Exchanger
Concentrated solutes
Feed stream
Figure 12: General evaporation flow diagram
Figure 13: Rising film evaporator [59]
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Figure 14: Falling film evaporators [59]
Steam
Condensate
Vapour to condenser
Feed Thick liquor
I II III IV
Fig. 15: Flow sheet for multiple effect evaporator
Natural evaporation using solar energy is applied, for example, in Southern European countries in animal farming companies, slaughterhouses and sausage meat industry as well as in the processing of vegetables like oil mills and fruits processing. The liquid stream is pumped on panels. The elements that pilot the process are wind, atmospheric humidity and temperature. The process can be helped by ventilation.
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It is commonly applied as a pre-treatment for reducing the moisture content of some products and concentrate them prior a drying step. This is the case of the concentration of the soluble in the stick water for the production of fish meal and fish oil.
Evaporation is applied as well for the de-watering of salt streams, the concentration of saline effluents (e.g. wastewater from fish and meat industring), the recovery of salts from brines and saline waters (using solar evaporation). It is also applied for concentration highly contaminated wastewater like oil mill wastewaters, or effluents from regeneration of ion exchange resins.
Evaporation MVR-system is normally used at the potato starch factories in Europe for concentration of potato fruit juice after protein coagulation. Then the concentrate can be used as fertilizer and the condensate as clean water in washing potatoes or in the refineri treatment after in slugdeblanket system airaration.
CRYSTALLIZATION
Crystallisation is the formation of solid particles within a formerly homogeneous phase. Crystallisation from liquid solution is the most important industrially. Solidification from a liquid melt by freezing is also used for purifying some certain materials.
Crystallisation from a solution may be induced by supersaturation of the solution. Supersaturation may be generated by three different methods. If the solubility of the solute increases strongly with increase in temperature, a saturated solution becomes supersaturated by simple cooling the solution. If the solubility is relatively independent of temperature, supersaturation may be generated by evaporating a part of the solvent. When the solubility is very high neither cooling nor evaporation can be used effectively. In this case supersaturation may be generated by adding a third component. The third component may be an anti-solvent that forms with the original solvent a mixture, in which the solubility of the solute is sharply reduced. The third component may also be a reactant that forms an insoluble substance with the original solute.
In the formation of a crystal two steps are required: the birth of new particle called nucleation and growth of this particle to macroscopic size. The final particle size and particle size distribution of the product are largely affected by the degree of supersaturation and the crystallisation technique applied.
Mother liquor is separated from the crystals by filtration or centrifuging, and the residue on the surface of crystals is removed by washing with fresh solvent.
For the production of lactose from whey, the lactose is crystallised in crystallisation tanks. It is also applied for the production of natural sweeteners from the pomace generated during fruit and vegetable production.
DRYING (de-hydratation) [13]
Drying refers to the almost complete removal of water from the material- whether solids, liquids or slurries- treated to water content less than 5%.
There are many types of drying techniques, from the traditional solar drying (spreading thin layers of the material in an open sunny place) to the sophisticated freeze dryers, flash dryers and spray-dryers.
Three basic methods of heat transfer are used in industrial dryers in various combinations: convection, conduction and radiation. Dryers used in industrial processes employ mainly forced convection and continuous operation with the exception of the indirectly heated rotary dryer and the drum dryer (conduction and batch use). With forced convection equipment, indirect heating frequently employs a condensing vapour such as steam in an extended surface tubular heat exchanger or in a steam jacket where conduction is the method of heat transfer.
07.09.2013 17
Drying can take place in batch or continuous operation:
• Batch dryers: usually related to small production or operations that require flexibility. The batch type forced-convection dryers find the widest possible application of any dryer used today.
• Continuous dryers: For the drying of liquids or liquid suspensions, the evaporator of choice is usually either a drum dryer or a spray dryer. Drum dryers are usually steam heated and can be divided into two broad classifications: single drum and double drum. Spray drier are explained in more detail bellow.
Other drying method broadly applied in food preservation and in the drying of extracts is lyophilisation or freeze-drying.
Freeze drying (lyophilisation) [14-16]
Lyophilization or freeze-drying is the process of removing water from a product by sublimation and desorption. This process is performed in a lyophyliser, which consists of a drying chamber with temperature controlled shelves, a condenser to trap water removed from the product, a cooling system to the shelves and condenser, and a vacuum system to reduce the pressure in the chamber and condenser to facilitate the drying process. It is applied to preserve temperature labile food products that contain water or solvents.
At atmospheric pressure water can have three physical states: solid, liquid or gaseous. Below the triple-point (see Figure 16) only the solid and the gaseous states exit. The principle of freeze-drying is based on this fact. The process is divided into two physical processes: the freezing of the material below its solidification temperature and the removing of the ice or solvent crystals at very low temperatures and pressures.
Triple point
Figure 16: H2O Pressure-temperature phase diagram
The process can be performed in batch mode, using trays that are placed in the drying tunnel (static freeze drying), or can operate continuously (dynamic) using continuous air freezing belts (Figure 17.). In addition, fluidised bed systems and freezing channels are used.
The disadvantages of freeze drying is the high costs since vacuum is needed during the process and the low temperature that need to be achieved (-40 to –60ºC) makes necessary the installation of a refrigeration plant.
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Figure 17: Left standard freeze-drying, Right schematic of a belt system for dynamic freeze-drying
Depending on the product and the packaging system, freeze-dried foods are shelf-stable at room temperature for up to ten years or more if canned, and between 6 months to 3 years if stored in a poly-bag container. The main determinant of degradation is the amount and type of fat content and the degree to which oxygen is kept away from the product.
The process is used for drying and preserving a number of food products, including meat, vegetables, fruits, and instant coffee products. Lyophilisation is used as well for the purification of many pharmaceutical and nutraceutical products. The dried product will be about the same size and shape as the original frozen material but the weight reduction can be up to 92%. The products have excellent stability and optimal reconstitution when placed in water. Freeze-dried products will maintain nutrients, colour, flavour, and texture often indistinguishable from the original product.
Freeze drying is used after water extraction of collagen from the skins and the bones generated during fish processing and from hides from meat processing. The collagen produced can reach a price in the market of 12-15 €/kg if the quality is high enough for cosmetic purposes.
Spray-drying [13, 17]
Spray drying is the transformation of a liquid into a powder in one step. Due to the fact that both the mass and the energy are transferred in a short time and that the product can be maintained at low temperatures (mostly under 80°C) for a short time (3 to 30 s), i t makes it perfect for food applications, where the product is often very sensitive to temperature, and where it is very important to maintain the natural properties of the products (colour, aroma, etc).
The main process steps are:
• Atomisation of the liquid into a spray of small droplets. By atomising the feed into fine droplets a very large surface area per unit mass is generated with a very short path for the heat and mass transfer that will take place in the drying chamber. This step is of prime importance to efficient drying. The atomisers used are: rotary atomizer, nozzle type and two-fluid nozzle or pneumatic type.
Different atomizers produce different particle size distribution and spray patterns, which will determine the properties of the particle that will result at the end of the process. The characteristics of the produced spray will determine also the design of the rest of the process.
07.09.2013 19
• Drying: during this step, the droplet created by atomisation comes in contact with hot air in the drying chamber. This contact causes two kinds of transfer: a heat transfer from outside to the inside of the particle and a mass transfer of moisture in the opposite direction.
Different designs of chambers exist in the market. The first distinction is made between the co-current and counter current chambers.
• Discharge of powder and collection: Most of the particles (approximately 90%) can be separated in the drying chamber, however, in order to improve the yield of collection, the separation is aided by means of an external system. These systems can be cyclones (main advantage is the separation of the fines from the exhausted air, for their recirculation in the system to allow agglomeration when it is needed) and bag-filters (they can be used alone or after the cyclones to improve the separation. The advantages are the cleaning of the exhausted air in one step bellow the environmental standards, and minimal losses of the end product).
The main designs of spray dryers for the food industry are revised in the following paragraphs:
Conventional single stage spray dryer
The basic system is a single stage co-current dryer with rotary or nozzle atomization. With nozzle atomization, co-current as well as counter-current mixed-flow can be obtained.
In the co-current mode, standard, non-agglomerated powders are made with an average particle size range of 30 to 125 micrometer.
One variation of this design is the counter current spray dryer, where the only difference is that the nozzle is situated at the bottom of the chamber, leading to the so called fountain nozzle mixed flow mode. This makes the residence time of the particles in the chamber bigger, thus allowing the use of lower temperatures. However, it presents the disadvantage that the particles are in contact with the hot air once they have lost part of their moisture. In general the co-current system is therefore preferred in the food industry. In the fountain nozzle mixed flow mode, a free-flowing product with 100 to 250 micron average size can be obtained.
The Figure 18 represents the conventional single stage co-current spray dryer with rotating disk or nozzle atomisation.
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Figure 18: Conventional single stage co-current spray dryer
Another modification of this basic design is the addition of an instantizing step directed to render an agglomerated powder. For this, purpose there are two possible configurations: a conventional single stage spray drier with re-circulation or a fluidised spray dryer.
Conventional single stage spray dryer with recircul ation
In this case the fines are separated in a cyclone and are re-circulated to the chamber near to the feed nozzle, thus allowing the fine particles to agglomerate together with the droplets that are being formed. Due to this modification the product yield increases and its functional properties are improved.
Fluidised spray dryer
In the fluidised spray dryer a typical spray dryer is integrated together with a fluidised bed dryer (Figure 19).
The first part of the system, the SPRAY DRYER, is responsible for the droplet formation and for the initial moisture removal. At the end of this process, the particles enter the fluid bed slightly wet. This allows the particles to stick to each other, forming thus agglomerates.
The second part, the FLUIDIZED BED, has longer residence times, which allows the reaching of lower moisture contents and provides the agglomerated powder with additional functional properties, like improved flow ability, low dust content and superior re-hydration characteristics, needed for the production of the so called instant foodstuffs.
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Agglomerated (instant), free flowing dustless powders are produced in this dryer with average particle sizes ranging between 100 to 350 microns.
Figure 19: Spray dryer with fluid bed attachment (two-stage drying)
Spray dryers usually incorporate one or two fluid beds (static and vibrating) for the final drying and cooling of the agglomerated powder. This is mainly to the necessity of the production of powders that may be easily reconstituted such as instant products.
Spray drying is widely applied for the production of food products such as:
• products from animal blood such as blood meal, blood plasma and blood cells
• whey products such as lactose, whey protein and whey powders
• soluble and refined fibres from apple pomace and potato pulp. Only as soluble fibre-refined fibre non soluble fibre- will stock in nozels.
Flash-drying [18, 19]
This method is used for drying solids that have been dewatered or have low moisture content. Flash dryers are known as pneumatic dryers and they are the simplest gas suspension dryers. A single operation provides mixing; heat and mass transfer to dry the solid.
The feeds to be dried should have the following characteristics:
• Granules, powders or crystallized
• Small and homogenous particle size to allow transfer without segregation and build up
• Reasonable dry and not sticky.
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The surface or the unbound water is removed effectively in a very short time, 0-3 seconds.The drying process is very fast and not suitable for diffusion controlled drying processes.
The system is designed depending on the feed and the product characteristics, operational safety requirements and heating sources. The system can be a closed cycle flash drying (see Figure 20) for the evaporation of organic solvents rather than water. The drying gas is inert, typically nitrogen, and the solvent evaporated is condensed.
Figure 20: Closed Cycle Flash drying system
For larger particles longer drying times may be required to reach the desirable moisture content. Some examples are:
• Extended residence time flash drying system
• Ring dryer
• Two-stage flash drying system
• Agitated flash dryer.
Flash drying systems are very fast and suitable for heat sensitive or easily oxidized substances. It is applied for the production of fibres, soluble and refined in mixture, from apple pomace and potato pulp.
Drum drying (heat inside the drum) can be used for drying of potatofibre
MEMBRANE SEPARATION [6, 7]
A membrane separation system separates an incoming stream into two different streams: permeate and concentrate (see Figure 21). The permeate is the fluid that has passed through the membrane while the concentrate is the fluid that contains the constituents that were not able to pass through the membrane.
Membrane Feed
Concentrate
Permeate
Figure 21: Principles of membrane separation
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The membrane itself is essentially a barrier, which separates two phases and restricts selectivity the transport of various constituents of the incoming stream. The type of membrane determinates the nature of the constituents that are separated.
The membranes can be classified as homogenous and heterogeneous, symmetric or asymmetric in structure, solid or liquid, can carry a positive or negative charge or be neutral or bipolar.
Common to all membrane filtration processes is that it is a pressure driven technique, in which a solution is forced through a porous membrane to achieve a selective separation. The permeate passes the membrane according to the molecular size and the membrane pore size.
Some types of membranes commonly used are explained below:
• Microporous: The membrane’s performance is the same as for fibre filters and separates depending on the particle size and pore diameter by a sieving mechanism. These membranes are made of materials such as ceramics, graphite, metal oxides, polymers, etc. The pores in the membrane vary between 1 nm-20 µm.
• Homogeneous: This is a dense film through which the mixture of substances is transported by pressure, concentration or electrical potential gradient. With these types of membranes, chemical species of similar size and diffusivity, can be separated efficiently when their concentrations differ significantly.
• Asymmetric: This comprises a very thin skin layer (0.1-1.0 µm) on a highly porous thick substructure (100-200 µm). The thin skin acts as the selective membrane. Its separation characteristics are determined by the nature of membrane material or pore size, and the mass transport rate is determined mainly by the skin thickness. The porous sub-layer acts as a support for the thin, fragile skin and has little effect on the separation characteristics.
• Electrically charged : These are necessarily ion-exchange membranes consisting of highly swollen gels carrying fixed positive or negative charges. These are mainly used in the electro-dialysis.
• Liquid : A liquid membrane utilizes a carrier to selectively transport components, such as metal ions at relatively high rate across the membrane interface
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Units are generally compact and their modular construction means that they can be scaled up or down easily. Flat, spiral, tubular and hollow fibre modules are applied (Figure 22). Membrane separation processes have commonly been applied as an end-of-pipe technology but it can also be very effective as a part of a re-use/recycle loop integrated into the manufacturing process.
Schematic diagrams of plate and frame membrane module and spiral-wound membrane module
Schematic diagrams of tubular membrane module and capillary membrane module
Diagram of hollow-fibre membrane system
Figure 22: Different membranes systems (www.tifac.org.in)(7)
In general, membrane separations are classified according to the pore size, as follows:
Decreasing membrane pore size
Microfiltration
Ultrafiltration
Nanofiltration
Reverse osmosis
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• Microfiltration (MF) is the most widely applied membrane process. It is a sterile filtration (sieving mechanism) with pores of 0.1-10.0 µm that removes particulate matter and microorganisms from liquids when they are going to be used for food purposes. Typically, micro-filtration concentrates solids and oils from liquids and /or slurries.
MF membranes are made from natural or synthetic polymers such as cellulose nitrate or acetate, polyamides, polypropylene, etc., and also inorganic materials such as aluminium and glass. The selected material depends on the application. In order to select one or another it is important to take into account its mechanical strength, temperature resistance, hydro-phobity, hydro-phility, permeability and costs.
The most common applications are the degreasing and recovery of colloidal particles, concentration of fruit juices and alcoholic beverages, fermentation, preparation of sterile water for pharmaceuticals and chemical industry. There are potential applications for the future in biotechnology for concentration of biomass and separation of soluble products, and in non –sewage water treatment.
• Ultrafiltration (UF) is a filtration process that can filter out macromolecular substances such as proteins, sugars, polymers or very fine colloidal material. The driving force for the transport is a pressure differential. The separation depends on the charge of the particles.
UF depends strongly on the physical properties of the membrane such as permeability and thickness and on the system variables, such as feed consumption and concentration, temperature and pressure.
The materials used for the production of these membranes are polymeric materials, polysulphone, polypropylene, and inorganic materials such as ceramics and carbon based.
It is applied for the separation of biological products such as proteins from fermentation broth, removal of turbidity from beverages such as beer or cider, oil, water and emulsion separations, and the separation of fats, oils or greases in the food industry (for posterior animal feed or disposal). When it is used to treat oily waters, pre-treatment is required to remove free oil and any solids above 1 mm.
UF is applied to whey for the posterior production of lactose, casein and whey protein concentrates. For the last one, it is the most common technology applied and it can be applied alone or in combination with reverse osmosis.
UF may be applied in the future for the following applications, among others:
� Ultra-filtration of milk
� Ultra-filtration of potatofruitjuice (native protein)
� Bio-processing: separation and concentration of bioactive substances
� Refining oils
� Sugar beet-juice purification
• Nano-filtration (NF) generates a purified solvent from a stream containing solutes. It retains molecules with a molecular weight of about 200 g/mol such as sugars, bacteria, proteins and bivalent ions such as Ca2+, Mg2+ and SO4
2+.
It is applied where high removal of organic substances and moderate removal or inorganic substances is needed. It is not effective with small molecular weight organics compound.
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NF uses semi-porous membranes, which retain larger molecules, but allow the passage of small oils droplets. The materials are normally of cellulosic acetate and aromatic polyamide type.
NF is applied for the demineralisation of whey but having the disadvantage of losses of lactose.
• Reverse osmosis (RO) generates a purified solvent (most often water) from a stream containing dissolved solutes. It retains dissolved solids, bacteria, viruses and other germs. The pore diameter ranges from 0.5-1.5 nm. More than 95-99% of inorganic salts and charged organics will be rejected due to the charge repulsion established at the membrane surface.
Reverse osmosis membranes are normally made of polymers, cellulosic acetate and polyamide and they are asymmetric or skinned membranes and thin film composite membranes (TFC). The support material is commonly polysulphone based.
The main advantages of RO are that it involves no phase change and the ability to remove organic contaminants and 95-99% of inorganic salts with minimal chemical requirements.
It is currently applied in dairy industry for lactose production from whey. Is used for dewatering of potatofruitjuice before proteincoagulation and evaporation.
Another membrane technique is Electro-dialysis (ion exchange).
Electro-dialysis (ED) is based on a membrane separation in the presence of an applied electrical potential. In ED, low molecular weight ions migrate in an electrical field across cationic and anionic membranes. These membranes are alternately arranged between the cathode and the anode within a stack. Prior to membrane separation a pre-filtration stage is necessary to remove large suspended solids.
ED membranes are usually made of cross-linked polystyrene that has been sulphonated.
ED membranes are applied in dairy industry for the demineralisation of whey.
The use of ED has increased in the last years and there are many potential uses of ED separation processes in the future, such as de-acidification of fruit juices, heavy metal recovery, recovery of organic acids from salts, pH control without the addition of acids and bases.
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Table 2. summarises the characteristics of each of the membrane techniques explained above:
Separation of salts and micro-solutes from solutions
Electro- dialysis
Cation and anion exchange membrane
Sulfonated cross-linked polystyrene.
Electrical potential gradient
Desalting of ionic solutions
Table 2: Summary of general characteristics of membrane techniques
Membrane technology is used for concentration, purification and fractionation, and it is very effective for both recovery and re-use of raw materials, products and water. It is applied, for example, for the concentration of liquids such as cheese whey, demineralisation of whey, whey fractionation and water purification. It is also used for the production of blood plasma in the meat sector.
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CHEMICAL MODIFICATIONS
NEUTRALIZATION
Neutralization of wastewater is often utilized in the following cases:
• Prior to discharge of wastewater into a receiving water
• Prior to discharge of industrial wastewaters to the municipal sewer system (it is more economical to neutralize small streams, than to process the large volume of combined domestic and industrial sewage)
• Prior to chemical and biological treatment.
Neutralization of acidic wastes
The following methods and reagents are commonly used.
The liquid stream is passed through a limestone bed. Up flow type arrangement is preferable used. For wastewaters containing H2SO4, limestone beds should not be used in concentration of H2SO4 exceeds 0.6%. Because the limestone becomes covered with an insoluble coat of CaSO4. Presence of metallic ions (e.g. Al3+, Fe3+) in the wastewater also reduces effectiveness of the limestone bed by coating of particles with precipitated hydroxides.
Slurried lime neutralization is the most common method. Stepwise addition of lime is recommended. In the first stage the pH is raised to a value of 3.0-3.5, and in the second (fine tuning) it is adjusted to desired value. Two agitated vessels, connected in series are used for contacting.
Caustic soda (NaOH) offers an advantage with respect to uniformity of the reagent, ease of storage and feeding, rapid reaction rate, and the end products are soluble. However, caustic soda is more expensive than lime.
Sodium carbonate (Na2CO3) is not as reactive as caustic soda and presents frothing problems owing to release of carbon dioxide.
Neutralization of alkaline wastes
Strong acids, like H2SO4 (the most common) and HCl can be used to neutralize alkaline wastewaters. The reaction rates are essentially instantaneous. Two-stage neutralization system can be used. Flue gases containing 14% or more of CO2 are used for neutralization of alkaline wastewaters. The CO2 forms carbonic acid in water, which reacts with the base. Reaction rate is slow but sufficient for neutralization (pH:7-8). Using a simple spray tower is satisfactory.
CALCINATION
The calcination is the chemical process of subjecting substances to heat with access of air whereby they are converted into a powder like matter, like lime in appearance. The term calcined is applied to any substance that has been exposed to a roasting heat.
Calcination is used in order to produce construction materials out of mollusc’s shells.
HYDROLYSIS [23]
Concept of hydrolysis means- breaking up of a chemical compound influenced by water. The most common hydrolysis occurs when a salt is dissolved in water. It involves the splitting of a bond between the components and the addition of the hydrogen cation (H+) and the hydroxide anion (OH -) from the water molecule; a salt breaks up to an anion and a cation. The addition of strong acids, bases or steam will often bring about hydrolysis where ordinary water has no effect.
Some industrially important hydrolysis reactions are the synthesis of alcohols from olefins (e.g., ethanol, CH3COOH, from ethane, CH2CH2) in the presence of a strong acid catalyst, the conversion of starch to sugars in the presence of a strong acid catalyst, and the conversion of animal fats or vegetable oils to glycerol and fatty acids by reaction with steam. The catalytic action of certain enzymes, hydrolases, allows the hydrolysis of e.g. proteins, fats, oils, and carbohydrates.
Meat and fish gelatin is obtained by hydrolysis of collagen from by-products generated during meat and fish processing.
Hydrolysis can be used for production of peptide/hydrolysate based on denatured potato protein.
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BIOCHEMICAL MODIFICATIONS
PASTEURISATION
When the aim is to use the side product for food purposes it is necessary to apply a thermal treatment such as pasteurisation. For example, it is usual to apply HTST (High Temperature Short Time) pasteurisation, as the one used in dairy industry, for proteins and LTLT (Low Temperature Long Time) pasteurisation and long retention time for those products that are thermo- sensitive. HTST process is known to result in higher quality products.
FERMENTATION [20]
Fermentation is an energy producing metabolic process in which breakdown products of the organic substrate serve both as hydrogen donors and hydrogen acceptors. Fermentation proceeds under anaerobic conditions and is carried out by bacteria and yeasts. The carbon oxidised in the course of fermentation is released in the form of CO2. The products of microbial fermentation, depending on physiochemical conditions and the particular microorganisms involved, can be very different(see Figure 23).
Glucose Pyruvate
Yeast fungi Ethanol
Clostridia
H2 + CO2
Acetate
Butyrate
Acetone
Butanol
2-Propanol
H2 + CO2
Acetate
Ethanol
Acetoin
Butanediol
Coli-Aerogenes-Group
HCOOH
Propiono bacteria
Lactobacilli
Succinate
Propionate
Lactate
Educts Species group Metabolites
Fig. 23: Overview of products and species involved in the main fermentation processes [62]
Fermentation is industrially performed in containers, bioreactors or fermenters.
Fermentation is carried out by bacteria and yeasts and the product formed identifies the type of fermentation; for example, alcoholic and lactic fermentation are those that result in the formation of alcohol and lactic acid, respectively.
o Lactic fermentation: this fermentation is carried out by lactic acid bacteria which convert sugars to lactic acid (homolactic fermentation).
It is also possible for lactic fermentation to convert glucose to a mixture of lactic acid and other products (heterolactic fermentation). All fermented dairy products are produced by homo-lactic fermentation.
07.09.2013 30
o Alcoholic fermentation: this fermentation is carried out by yeast which, under anaerobic
conditions, convert sugar to carbon dioxide (CO2) and ethanol.
This type of fermentation is used in the production of wine and beer, and in the production of alcohol from molasses and other vegetable solid by-products. When the raw materials are not sugars (e.g. cellulosic materials), they have to be hydrolysed before fermentation can commence.
In the dairy industry, increasing use of ultrafiltration and other membrane separation systems will lead to the production of permeate instead of the conventional whey (the dairy by-product of cheese making). These processes could yield copious streams of proteinless by-product rich in lactose and minerals. The lactose has to be removed before the stream can be disposed of, and one of the best ways of doing so is via fermentation processes.
Under investigation is the possibility of extracting astaxanthin from shrimp waste by lactic fermentation combined with enzymatic hydrolysis.
Fermentation can be used for removal of protein (protease) and starch (amylase) in potato pulp and then can used fermentation for production of soluble fibre on the cellulosis etc.
The use of organic waste in biogas plants
The anaerobic treatment of organic (waste) substrates is already widely used, particularly in agriculture, for the treatment of animal excrement, in municipal sewage treatment and generally in the treatment of effluent heavily contaminated with organic material. On a much smaller scale, domestic waste is also subjected to anaerobic biological treatment. Many forms of organic waste produced by the food industry are suitable substrates for a biogas plant. Usually, treatment is conducted by way of co-fermentation together with liquid manure or clarification sludge. Figure 24 provides an example of a basic flow diagram for a co-fermentation plant for treating solid, pasty and liquid waste.
Anaerobic degradation of the organic substrates proceeds through a number of stages (see Figure 25). Carbohydrates, fats and proteins are converted step by step finally to CO2, CH4 and a residue of partially degraded organic material, whereby, apart from fermentation, anaerobic respiration processes, such as denitrification and methanogenesis, also play an important role.
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CHP plant
Flat bunker
Comminution
Sanitisation
Fermentation
Solid waste Liquid waste
Clarification sludge
Sludge buffer
Fe-separator
Pulper
Drum screen
Suspension tank
Tank
Dewatering Biogas storage tank
Post-composting
Biofilter
Interfering material I
Interfering material IV
Interfering material III
Interfering material II
Interfering material V
electricity Secondary fertiliser
thermal energy
Exhaust air
water
Projekt: Bezeichnung:
Stoffstrom- und Energiebilanzen Grundfließbild Radeberg
Freigegeben Auftraggeber: Projekt Nr.: Bearbeitet von: Erstellungsdatum
cand.-Ing. L. Schulken 31.05.2002
Rack
Figure 24: Simplified basic flow diagram of the BVR co-fermentation plant, Radeberg.
07.09.2013 32
Polymeric substrates Proteins Carbohydrates Fats
Amino acids Sugars
Organic acids Alcohols
Hydrogen Carbon dioxide
Methane
Hydrolysis
Fermentative Bacteria
Acetogenic Bacteria
Methanogenic Bacteria
Processes Substances Organisms
Acid formation
Generation of acetic acid
Generation of methane
Fatty acids
Acetic acid
Fermentative Bacteria
Fig. 25: Steps in the anaerobic degradation of organic substrates. [63]
Nowadays the utilization of organic waste is an integral part of the cyclic flow of materials in an economy based on the principle of long-term sustainability. With the technology currently available it is possible to utilize in fermentation plants a very wide range of substrates, solid, pasty or liquid, from homes and industry, as well as from agriculture. In respect to scale and technical standards, plant design can also be greatly varied to meet a wide range of conditions and requirements. Such plants are particularly suitable for decentralised use.
Specific yields of biogas from different substrates vary greatly. The quality of the residue from biogas production and its suitability as a fertiliser depends greatly on the quality of the substrates used, particularly on any persistent contamination with toxic substances, such as heavy metals. If the substrates used are largely free of persistent contamination, the residue produced will be a high quality fertiliser very suitable for use in agriculture, gardening and landscape restoration measures (see Figure 26).
07.09.2013 33
Fig. 26: Comparison of threshold and target values for heavy metal content in clarification sludge used in
agriculture with the values for spent sludge from the Radeberg co-fermentation plant, Germany.
ENZYMATIC HYDROLYSIS
Using added enzymes to hydrolyse food proteins is a process of considerable importance used to improve or modify the physicochemical, functional, and sensory properties of the native protein without jeopardising its nutritive value, and often protein absorption is improved. These enzyme-based processes occur under mild conditions over a series of stages and do not produce hydrolytic degradation products via racemization reactions observed with both acid and alkaline hydrolysis.
Enzymatic hydrolysis has several distinct advantages over other processing methods for recovering the protein from food wastes, including:
• The unique specificity of action of the enzyme, making it possible to control the characteristics of the end products.
• Digestion under mild condition, avoiding extremes of pH and temperature which could compromise the nutritive quality of the final product.
• Subsequent deactivation of the enzyme by heating, thus making its removal unnecessary.
• Attractive product functional characteristics such as solubility, dispersibility, foaming capacity and foam stability among others.
• No destruction of amino acids, so the protein tends to retain its nutritive value better than in traditional acidic or alkaline hydrolyses.
A flow sheet for the enzymatic hydrolysis of food wastes is given in Figure 27.
0
200
400
600
800
1000
1200
1400
1600
Zn Pb Cr Cu Ni
[mg/
kg D
S]
German target values
EU target values ~2025
Values from the Radeberg co-fermentation
2500
0
1
2
3
4
5
6
7
8
9
1 0
C d H g
07.09.2013 34
Figure 27. Enzymatic hydrolysis
FOOD WASTES
ENZYMATIC REACTION
COLLECT SUPERNATANT
CENTRIFUGATION
INACTIVATION OF HYDROLYSIS
HOMOGENIZATION
DEHYDRATION
PROTEIN HYDROLYSATE
07.09.2013 35
REFERENCES
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07.09.2013 36
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[41] www.fiebig-naehrstofftechnik.de/
[42] www.hemostat.com
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