analise dinâmica de um chiller de absorção de brometo de lítio ...

98Figure 1. Schematic diagram of the cooling system forethanol fermentation using a single-effect absorption chiller.capi@ of Figure 1, are: generator, absorber, condenser,evaporator, solution heat exchanger, solution pump, expansionvalve and throttling valve. Firstly, the weak solutionof lithium bromide is pumped from the low pressureregion in the absorber to the high pressure region inthe generator where heat is supplied by the waste heatsource (point 11) and the refrigerant water is separatedfrom aqueous lithium bromide solution by evaporation.The superheated steam (point 7) goes through the condenserwhere it is condensed on the surface of a coolingcoil. The saturated water, that leaves the condenser, isthrottled through the expansion valve to the lower pressurein the evaporator, where it evaporates by absorbingheat and provides cooling capacity (point 18). The saturatedsteam from the evaporator (point 10) flows tothe absorber where it is absorbed by the strong solutionoriginating from the generator. The mixing processintheabsorberisexothermicandneedstorejectheattransferring it to the water from the condenser. Theweak solution, which leaves the absorber, is pumped tothe generator and goes through the solution heat exchangerwhere it is heated, and the refrigeration processbegins again.One constraint of the single-effect water/lithium bromideabsorption chiller is the crystallization of the absorbent(lithium bromide) and the point of the cyclemost vulnerable in this regard is the stream entering theabsorber (point 6), because of its highly concentratedsolution and low temperature.The absorption chiller has a nominal cooling capacityof 3000 kW and it chills two fermentation vats. Thecooling capacity of the absorption chiller was selectedconsidering the availability of waste heat at the plant.This study only shows the refrigeration of one fermentationvat, thus the mass flow rate of the chilled waterfrom the evaporator is divided into two streams. Thevolume flow rate and the temperature of some streamsin the absorption chiller are shown in Table 1. The configurationof the water loop inlet in the absorption chilleris: condenser (point 15) and evaporator (point 17) usethe water from the cooling tower, the absorber uses thewater that leaves the condenser (point 16) to reject heat.Usually commercial absorption chillers use inverse configuration(from absorber to condenser) in order to avoidcrystallization. As the operation temperature rangedoes not present risk of crystallization, it is reliable tooperate under the configuration described above, allowinghigher COP (Herold et al. 1996). The generator isheated by the contaminated condensate from the juiceevaporation process during sugar production (point 11).The water used to cool the fermentation vat is a mixtureos chilled water (point 18) and water from coolingtower (point 23). The simulation of the cooling tower isbeyond the scope of this analysis.Table 1. Volume Flow Rate and Temperature of SomeStreams in the Absorption Chiller.Point ˙V T( m 3 h −1 ) ( ◦ C)1 26 –11 350 9613 800 T 1615 800 T 2317 450 T 23The ethanol fermentation (region slowromancapii@ inFigure 1) is a biochemical process where the sugars aretransformed into alcohol by the industrial yeast Saccharomycescerevisiae. The carbon source used is canemolasses. The fermentation time ranges from 4 to 6hplus the wait time (1 to 3h) and there are two fermentationprocesses per day for each vat. At the end of thefermentation, all sugars have been consumed and thefinal fermentation product, called wine, has an ethanolconcentration of 60 to 90 kg m −3 .The heat released during the ethanol fermentation isremoved by means of a plate heat exchanger, where thecold stream (point 19) is provided by both streams, waterfrom the cooling tower (point 23) and from the evaporatorof the absorption chiller (point 18). The mustfrom the fermentation vat (point 21) flows to the fermentationheat exchanger with a constant volume flowrate and the flow rate of the cold stream (point 19) isused as a manipulated variable to control the fermentationtemperature.The heat exchangers of the process are in a counterflow arrangement. Plate heat exchangers are used at thesolution heat exchanger of the absorption chiller and fermentationheat exchanger, while those of the absorptionchiller are shell and tube heat exchangers. The absorptionchiller heat exchangers are made of copper while the112 / Vol. 13 (No. 3) Int. Centre for Applied Thermodynamics (ICAT)