Thermodynamic Performance Analysis For Recycling Of Dilute Alkali From Mercerizing With Mechanical Vapor Recompression Evaporation Technology

The mercerization process in textile industry consumes great amount of alkali, and dischargewasted lye with high content of alkaline. If disposed directly and inappropriately, these waste lyewill not only lead to great waste of lye, but also increase the difficulty and cost of sewagetreatment, and cause serious environmental pollution. In this paper, mechanical vaporrecompression (MVR) evaporation technology is deployed to process waste lye. After evaporated,the concentration of alkali in the recycled lye will meet the requirements of mercerization process,so the condensate water can be discharged under industrial standards. The introduction of thesteam compressor technology help collect and compress the second steam as recycling heatingsteam in the system, which fully exploits the remaining latent of the second steam and save greatamount of cooling water. Compared to traditional multi-effect evaporation system, energyefficiency of the new system was effectively improved, ensuring very promising applications inthe future.Firstly, this paper introduced the basic treatment process of waste alkali liquid, andfocused on the MVR evaporation technology. A mathematical model of the MVR evaporationsystem with first law of thermodynamics is set up. The influence rules of temperature differencebetween cold and hot fluid, the evaporation temperature, the solution concentration of dischargelye on evaporation, compressor power consumption of unit evaporation are given byan established mathematical model and experimental study. The results show that: the greater thetemperature difference will leads to the increase of evaporation and compressor powerconsumption of unit evaporation. In this case, requirements on the temperature difference forlower compressor power consumption and higher evaporation are inconsistent. So It’s hardto tell the appropriate temperature difference from the first law; The increase of evaporatingtemperature can make the system decreases in the power consumption which is conducive to thecompressor operation; With the increase of the solution concentration of discharge lye,compressor power consumption of unit evaporation remains unchanged, and the evaporation is ina decrease trend. The solution concentration of discharge lye is chosen to meet the minimumrequirements for recycling process that18%is appropriate.Secondly, another mathematical model is set up based on the second law of thermodynamics, and the differences between thermal and exergetic analysis are compared. Calculation resultsindicate that the total exergetic efficiency of the system is4.8%, which is mainly contributed bythe compressor, the evaporator, the water pump and emission section with exergetic loss of36.9%,31.2%,19.0%and9.2%. Such exergetic efficiency is relatively low, compared to the high thermalefficiency of the system achieved by taking full advantage of the heat of secondary steam. Giventhat the loss of exergy from the compressor and evaporator takes68%of the total amount, moreattention and emphasis should be put to reducing the irreversible loss of them. Besides, the resultsof theoretical analysis and experimental study showed that the system can have the peak exergeticefficiency when temperature difference is in the range of5-7℃, reducing energy consumption andimproving system performance. Meanwhile, the total exergetic efficiency of the system canimprove by increasing evaporating temperature and reducing the solution concentration ofdischarge lye.