|LOW TEMPERATURE EVAPORATOR MODULE (LTEM)
Introduction of cogeneration has encouraged Sugar Industry to reduce their process steam consumption by employing innovative equipment's and technology. The evaporators use 2 to 2.5 bar pressure steam in four to five effects. A concept of novel process employing low pressure vapours from pan and evaporators in multiple effects plate falling film evaporator. Using this technology, we are able to use available enthalpy and temperature of vapours for evaporation before letting it go to condensing and cooling systems. By eliminating evaporation process above 100 degree C, this concept aims to increase plant capacity utilization, lowering cost of evaporation, enhancing power generation potential. Low temperature evaporator module has potential to reduce scaling, inversion, colour formation and heat losses.
- Installation at Dalmia Chini Mills, Nigohi, U.P.
- Installation at Dalmia Chini Mills, Jawaharpur, U.P.
- Installation at Colony Sugar, Pakistan
- Installation at DSCL, Ajbapur, U.P.
The distinct advantages of Low Temperature Evaporator Module (LTEM) are given below:
- Improves final product quality due to low exhaust pressure at evaporator station.
- Increase plant capacity utilization.
- Increase imbibition water.
- Increase power generation from turbine.
- Reduce steam use in juice heating.
- Reduce process sugar losses.
- Reduce steam and juice flash heat losses.
- Minimize colour in juices.
- Reduce entrainment losses in pans.
- Reduce inversion at high temperature.
- Reduce power losses in turbine.
- Reduce complicated bleeding schemes.
- Low operating cost due to very low wetting rate i.e. in the range of 3-5 L/h-cm.
- PATENTED TECHNOLOGY *
Using this technology the following equipment that gets eliminated from the existing process are Exhaust/ Vapour Condensate Heat Recovery, Insulation of Evaporator and Heaters, Semi-Kestners, Falling Film and Vapour Cells, Entire Juice Heater, Chain driven by Steam and Condensate Flashing above 100°C.
The salient features of Low Temperature Evaporator Module (LTEM):
- Low Retention Time Features: Low Temperature Evaporators offer the low residence time which help assure a high product quality.........
The salient features of Low Temperature Evaporator Module (LTEM):
- Low Retention Time Features: Low Temperature Evaporators offer the low residence time which help assure a high product quality that means it reduces sucrose inversion, sugar losses and colour formation. In comparing the different types of plate evaporators that are available, Low Temperature Evaporator Module designs provide the shortest residence time.
- Operating with Low Temperature Difference (Effective ΔT = 3-4 K): Low Temperature Evaporator Module (LTEM) operates at very low temperature difference which also helps to assure a high product quality when concentrating heat sensitive products. In comparing the different types of plate evaporators, Low Temperature Evaporator Module designs provide the lowest possible ΔT.
- Higher OHTC Value (W/m2K): Q = U x A x ΔT: Higher evaporation rate with same available heating surface and delta T requires lower heating surface. The OHTC is 4000 W/m2K in the 1st effect of Plate FFE compare to 2400 W/m2 K in conventional Robert body. The overall heat transfer coefficient (U-value) is higher not only due to the smaller wall thickness but also because of the plate characteristics and the flow pattern. Depending on the effect, the U-value can be higher than for tube bundle evaporators, with the same temperature difference: from 25% in the thin juice section up to 250% in the thick juice section.
- Higher Heating Surface Density: The heating surface density in LTEM is 170-240 m²/m³ due to compactness while a Robert body has range of 40-50 m²/m³ .
- Wetting Rate: The evaporators are generally operated at a minimum wetting rate of 20 - 25 L/cm-hr for the cane sugar industry, while the optimum wetting rate of the Low Temperature Evaporator Module is approximately 4-5 L/cm-hr for the same. Due to the low wetting rate required by the Low Temperature Evaporator Module the corresponding circulation pumps in plate pack are also of smaller design than those of the tube-type falling film evaporator.
- Hydro-Static Head Effect: The liquid has to be heated to the above the boiling point when submerged under the head of liquid. This has the most significant effect at low absolute pressure, in which case it seriously reduces the temperature difference for heat transfer. A significant advantage of Low Temperature Evaporator Module is the absence of hydrostatic effect on evaporation.
- Expansion Option: An interesting expansion option is available involves minimising evaporation stages by installing a Low Temperature Evaporator Module. With the greater heating surface density and higher HTC, an effective increase in capacity of 100% is possible.
- Dimensions / Weights: Typically stainless steel plates with a wall thickness of 0.6 mm are used in the sugar industry. Tube bundle evaporators require a wall thickness of 1.2 - 2 mm. This means a reduction of approx. 50%, which results in lower weight in addition to better heat transfer. The design of the Low Temperature Evaporator Module makes it possible to achieve a heating surface density which is up to five times higher. The overall height and weight are also reduced as a result of the compactness.
- Fully Automatic Process Control: There are two basic types of control for a falling film evaporator: inlet and outlet control. The control parameter is the liquid level in the lower vessel. In addition, the circulating volume flow is controlled and if the level falls short of the minimum coverage, emergency water is added.
Cleaning cycle of the evaporator can be controlled by the checking the temperature difference across the evaporator or by the calculating the HTC by reverse calculation. If delta T increases from the desired level, evaporator has to be taken in cleaning immediately either in manual or auto mode. Irrespective to these pressure drop across the filters to be monitored and auto switchover to standby recirculation pump in case of operating pump failure is the additional safety considered in automation.
- Energy Efficiency: As a general rule, the larger the system, the more it will pay back to increase the thermal efficiency of the evaporator. The main factors that will affect the selection of the technique are detailed below.
- Evaporation Rate: The higher the capacity of the evaporator, the more the designer can justify complex and expensive evaporation systems in order to provide high energy efficiency. As the energy cost is increasing we can justify the use of more heating surface but plate technology is able to provide highest rate of evaporation for same conditions among all available technologies.
- Steam Pressure: The availability of steam at a medium pressure of about 12 kPa permits the efficient use of multi-effect evaporators. The LTEM can be applied across three or four effects. This is the simplest and least costly technique for enhancing evaporator efficiency.
Conclusion and Scope: It is possible to reduce steam consumption below 25% on cane and exhaust pressure below 1 bar A by introduction of LTEM. Reduced process steam demand and its pressure eliminate power loss or higher fuel consumption typically experienced during season due to cane processing in conventional plants.
LTEM is a good option and ideal one to save energy, at the same time it will provide for satisfactory results even at extremely low heating steam pressures or at a less active temperature gradient. More compact design of evaporators can be developed by incorporating two or more effects in single body. We can now design power plant from regular cycle efficiency of 25% to 35% and set aim for 50%. Similarly more power enhancement possibilities can be derived with operating the pan at low vapour pressure in the range of 0.5-0.7 Bar (A) by developing new pans.