Drying in Emerging technologies
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Emerging Technologies in Drying
- 1. Concentration of product
Thermal concentration The concentration of liquid product streams is primarily intended to reduce the cost of storing, packaging, handling and transport by the reduction of the volume . Furthermore, the concentration promotes durability because water contributes significantly as a main component of liquid foods for growth of microorganisms. The reduction of the water content by concentration thus results in a reduction of the microbial load and enhances the durability of the product.
Typical products which are concentrated in the food industry are fruit juices, jams and marmalades, milk for the production of condensed milk, whey and lactose as a precursor to dry, sugar syrup, malt and glucose syrup, vegetable juices, purees and pastes. In dry countries , drinking water is generated by the evaporation of sea water , the concentrated salt solution and the remaining salt is a by-product . The conventional evaporation method used for concentrating foods includes batch and Beck evaporators, natural circulation evaporator (Robert evaporator, Vogelbusch evaporator), forced circulation evaporator, rising or falling film evaporator, thin film evaporator, plate evaporator, flash evaporator, but also the concentration by freezing is possible. To increase the energy efficiency of conventional evaporator which is formed during the evaporation vapors (vapor-saturated air) can be directly used (in uncompressed form) in the next stages of a multistage evaporation as a heating medium again . The vapors can be condensed by mechanically or thermal process, before they are used in the subsequent or the same evaporation stage again.
Conventional Concentration of Whey
The resulting sweet whey from cheese production is a concentrated by evaporation and then spray dried and fed to the whey powder production. The whey is evaporated to a solids concentration of 58%. Before the evaporation process , fat and cheese dust (or pieces of cheese) are removed using a Seperator from the whey. The evaporation and concentration of whey is done in a five-step thermal vapor compressor (BV). The whey comes with a solids concentration of 5% in the first stage of evaporation. The feed is first preheated by heat exchanger in all evaporator stages to 81.5 ° C. The mass flow rate at the evaporator inlet is 13,000 kg / h. The whey contains after the first round of the five-stage BVs a solids concentration of 35% and goes out of 1,800 kg / h from the evaporator. This concentrate is stored in a concentrate tank and cooled to 5 ° C.
The 35% concentrate is diluted with fresh whey (5% TS) to a concentration of 15%. Then the 15% whey goes through a mass flow rate of 13,000 kg / h evaporation plant again and is concentrated to a solid concentration of 58%. The mass flow of 58% concentrate is 3100 kg / h. The concentrate passes after the final evaporation stage of the second pass in a tank and the concentrate is cooled to 25 ° C. From the concentrate tank, the whey concentrate then passes on to the next process step, the drying. Drying takes place in a spray drier and the final product is whey with a solids concentration of 97.5 to 98.5%. This powder is sold at 0.7-0.8 € / kg and used as a feed additive or food for cheese or baked goods.
- Figure 1 Simplified flow diagram of the five-stage is in BVs
- Figure 2 Simplified flow diagram of the 5-stage BVs for the concentration of whey
- Potential ET for the concentration
The table below shows the list of identified ET for the concentration of food.
- Table 1 List of Identified ET for concentration of food
- New Technologies
- A)Membrane Distillation
- General Description
The MD is a thermal process in which molecules can diffuse through only vapor through a porous hydrophobic membrane. The liquid feed is in direct contact with one side of the membrane but prevent the penetration of the hydrophobic properties of the liquid into the pores of the membrane by the prevailing surface tension (interfacial tension). This results in the liquid-vapor phase boundary surfaces of the openings of the membrane pores. The driving force of the MD is a vapor pressure differential between the feed side and the permeate side of the membrane. There different types by which this difference can be achieved.
- Direct contact membrane distillation (DKMD)
When DKMD flowing on the permeate side of the membrane in direct contact with an aqueous solution having a lower temperature than the feed, in the opposite direction of the feed flow direction. Due to the difference in temperature of the feed and the permeate there is a vapor pressure difference, start by which volatile molecules of the warmer feeds to evaporate and thus can penetrate the membrane in the vapor state. The vapor then condenses on the colder liquid-vapor phase boundary on the permeate side of the membrane again.. The disadvantage of the DKMD is that this configuration has the highest MD heat losses by thermal conduction of the membrane. An essential advantage lies in the DKMD the ease of use when compared with the other configurations.
- Figure 3 Direct-Contact-membrane distillation (DKMD)
- Process model and technology analysis MD
In order to select a suitable membrane for the MD, it is first important to know the components of the feed.
- Table 2 List of The ingredients of whey in mg per 100 g of liquid and powdered whey
The three main components of the liquid whey are carbohydrates (in the form of lactose), proteins (in the form of whey protein), and fat. Since the whey is degreased before concentration , the majority of the fat falls off as an essential component and does not need to be taken into account in the further specific membrane selection. The whey protein, which consists of albumins and globulins, according to is the "highest quality protein of nature" and is unmatched by any other known protein in its quality exceeded In industry, it is often the case that the proteins in the whey for example by Ultrafiltration (UF) are separated from the remaining components as a pure and concentrated protein concentrate . These whey proteins are valuable food additives and (protein shakes for athletes) are used including in baby food or diet and sports drinks.
- Energy consumption MD
The use of the MD in the industry can be assumed that a plurality of MD modules are connected in series to achieve the desired solids concentration and the feed is not recycled. Furthermore, a series connection of modules is useful in order to achieve the required by the industry flow rate per unit of time can.
- Figure 4 Flow diagram MD for the concentration of whey including solar thermal integration - a variant (WRG between feed and concentrate)
- Figure 5 Flow diagram MD for the concentration of whey including solar thermal integration - c variant (use straight from the fermenter)
- 2.Drying of Whey
Thermal Drying In the thermal drying, volatile substances, especially moisture (water) is removed from a solid, semi-solid or liquid product by the application of heat to achieve a solid product stream. The drying or removal of water from the product is carried out on the one hand to improve the durability and storage stability and, secondly, to facilitate handling and to reduce transport costs. Often the drying of the product is necessary to achieve the desired quality.
In the thermal drying, two processes run simultaneously. On the one hand, the transition heat to the product to be dried in order to evaporate the water on the surface can take place. The heat transfer from the environment to the product by means of convection, conduction or radiation or a combination of several. On the other hand, the water from the interior of the product must be transported to the surface (diffusion) in order to also be able to evaporate subsequently. By heat conduction, the heat transferred to the surface is brought into the interior of the product. The transport of the water to the surface is carried out either by diffusion in the liquid (by a concentration gradient), or in the vapor state (when the water begins to evaporate already in the interior or by a hydrostatic pressure difference when the rate of evaporation inside is higher than the transport rate of the steam to the surface or into the environment of the product). But can also both mechanisms may be responsible for the transport of moisture. The drying rate is certainly dependent upon both the rate of evaporation at the surface as well as the transport speed of the water from the interior to the surface.
The whey powder preparation is effected by spray drying. Whey is dried at 25 ° C a whey concentrate with a solids concentration of 58%, a mass flow rate of 1900 kg / h and an inlet temperature. The whey product comes in powdered form from the spray dryer from having a solid concentration of 97.5 to 98.5%, a temperature of about 85 ° C and an amount of 1176 kg / h. The dryer exhaust air first enters a cyclone and then into a filter for dust removal. The temperature of the exhaust air before it enters the filter is about 85 ° C. The dedusted exhaust air is cleaned or after the filter in a WT, where the heat of the exhaust air for preheating of the supply air of room temperature is used at about 72 ° C. The preheated air is then heated above a WT in the combustion chamber at ~ 184 ° C and reaches this temperature in the spray tower.8.64 Nm ³ natural gas consumed per 100 kg of product.
- Figure 6 Conventional Drying of whey
Emerging Technologies in Drying
- Table 3 Tbale shows emerging technologies used for drying
- New Technologies
- A) PULSE COMBUSTION DRYING
- General information on Pulse Combustion Drying (PCD)
The designation Pulse Combustion (PC),means pulsating ignition or combustion, comes from the periodic (pulsating) combustion of solid, liquid or gaseous fuels. ,They are conventional burners as opposed to the PC, supplied continuously to ensure that a continuous fuel combustion takes place. By a periodic combustion, in pressure, speed and, to a certain degree, temperature waves travel from the combustion chamber via an exhaust pipe into the drying apparatus.
There is flow of both air and fuel into the combustion chamber through valves, where they form an explosive mixture. The ignition of the mixture takes place via a spark plug, leading to explosive combustion and a rapid increase in temperature and pressure in the combustion chamber. In the meantime, through the closed valve and the rapid pressure increase in a flow of exhaust gases is led into the exhaust pipe. As long as the pressure increase caused by the combustion in the combustion chamber is larger than the pressure loss due to the outflow of the exhaust gases through the exhaust pipe, the pressure rises in the combustion chamber. When the pressure increase caused by the combustion is then lower than the pressure loss due to the escape of the exhaust gases decreases, the pressure in the combustion chamber and a part of the exhaust gas flow in the exhaust pipe flows back to the combustion chamber. The pressure drop in the combustion chamber, the valves and open air and fuel can flow in again. This new mixture is ignited either the spark plug or on contact with the hot exhaust gases back-flown and the cycle begins again .
- Figure 7 Principle of Pulse Combustion Drying
In the PC, either mechanical or aerodynamic valves can be used. In mechanical valves optional rotary valves or mechanical membranes is used. A rotary valve consisting of two superimposed discs, which are provided with identical openings . While a disk is static, the other and the result is a periodic overlap of the openings and air or fuel is allowed to flow into the combustion chamber rotates. Do not overlap the openings, the valve is closed. The rotational speed of the rotary valve has the oscillation frequency, which is formed in the PC to be adjusted or determine this. Through the use of rotary valves Gasströmungsoszillationen can be achieved with high acoustic parameters. The valves must be open when in the combustion chamber, a vacuum prevails, in order to put the system in an acoustic resonance can. Although rotary valves are mechanically more claimable as diaphragm valves, but they have disadvantages compared to aerodynamic valves due to the use of moving parts that can wear out in high temperature zones
- Positive effects of PCD
The pulsating combustion (PC) in comparison to conventional, continuous combustion has some advantages:
- improve the heat and mass transfer rates by a factor of 2-5 (eg when used in combination with a dryer)
- better combustion intensity by a factor of up to 10
- higher combustion efficiency with low excess air values
- reduced exhaust emissions (in particular NOx, CO and soot) by a factor of up to 3
- better thermal efficiency of up to 40%
- less space for the incinerator
- Process Model of PCD
The process model created here consists of a spray dryer connected to a pulsating burner and a solar thermal system. Spray drying has been selected, as it is already used in the dairy considered to dry the whey concentrate, and this method is therefore suitable for the production of whey powder. In addition, so no new dryer needs to be purchased, since the existing can be used, resulting in lower investment costs. For the integration of solar thermal energy, there are two ways in principle. One hand, a preheating of the feed and on the other hand, preheating of the combustion air is possible. If the solar thermal energy to preheat the feed used, decreases at a constant feed rate, that energy needs, which must be entered by the exhaust gases into the dryer to evaporate the predetermined amount of water can. Thus, this leads to lower fuel consumption.
- Figure 8 Process model PC including solar thermal energy for the production of whey powder
The preheating of the combustion air would also lead to lower fuel consumption, but it is likely that the fuel savings are due to a solar thermal feed preheater higher than for preheating the combustion air as the heat transfer between two fluids is better (between water from solar storage tank and feed) as between liquid (water from the solar storage) and gas (burner supply). Thus preheating the feed is taken into account for the process model.
- Energy consumption of PCD
The specific thermal EE-consumption of PC in combination with the spray-drying (without solar thermal integration) is a burner efficiency between 90 and 99% from .765 to 0.842 kWh / kg H2O. This provides a gas requirement of 5.15 to 5.67 Nm ³ / 100 kg of product. That is, although the conventional spray drying process in which the exhaust heat recovery considered a dairy carried out of the dryer, the gas consumption is the PC where no heat recovery occurs also, assuming the worst burner efficiency of 90% at still from 2.97 to 3.49 Nm ³ / 100 kg of product with the gas consumption of 8.64 Nm ³ / 100 kg of product in the conventional drying. In a burner efficiency of 90-99% of the thermal demand of the PE-PC is (without integration of solar thermal energy) from 0.895 to 0.985 kWh / kg H2O. The thermal primary energy demand, compared with the thermal primary energy demand of conventional spray drying of dairy consideration of 1.502 kWh / kg H2O, therefore, is around 34 to 40% below.
The integration of ST, is, as mentioned above, to preheat the feed to 55 ° C. 3 SD in the calculations (100%, 60% and 40%) were considered. Solar energy is free of charge and the thermal energy can be used freely after a certain payback period. By preheating the feed with solar thermal energy decreases, depending on the SD, the gas demand and thus the specific fossil energy demand. In Table 7.3, only the thermal energy consumptions are therefore shown that by fossil fuels (natural gas) must be covered. In addition, the required collector area is still, depending on the SD at an average of 729.3 kWh solar NE per m² collector area and year, expressed in m².
Source: Ulrike Herzog (2012)Möglichkeiten des Einsatzes von „Emerging Technologies“ in der Lebensmittelindustrie zur thermischen Energieverbrauchs-senkung und zur Einbindung von Solarthermie