Study On Key Technology Of High-salinity Wastewater Condensed Based On Optimal Utilization Of Low Temperature Heat In Refinery

Due to the high operating costs and the large demand of heat, the technique of zero-discharge of saline wastewater from the domestic petrochemical refinery is still blocked. This paper shed light on the new technique of concentrating high-salinity wastewater with the low-temperature waste heat as a driving power for LT-MED. And the key parameters of the process were optimized. Through studying the technology of combining low-temperature waste heat recovery optimization with multiple effect evaporator, this paper aimed to realize concentrating high-salinity wastewater with low-temperature waste heat as a driving power for multiple effect evaporator, This research will not only enrich the ways to deal with the low-temperature heat, but also a impetus for the energy conservation and emission reduction among the domestic petrochemical enterprises.In the research, the types, distribution characteristics and utilization status of low-temperature waste heat sources were first investigated. The results showed that low-temperature waste heat sources in Refinery A were mainly distributed in the atmospheric and vacuum distillation unit, catalytic cracking unit, delayed coking unit and diesel hydrotreating unit, etc. And some of the low-temperature waste heat was left unutilized, and most of it were cooled by the recycling cooling water. In Refinery B, an old oil-refining enterprise, the low-temperature waste heat was mainly used in gas fractionation unit and lithium bromide refrigeration. Then, through the analyses, it was proved to be feasible to concentrate high-salinity wastewater with the low-temperature waste heat as a driving power for MED. By establishing appropriate evaluation method, the optimal heat source was selected. And the heat exchanger network was designed by using pinch technology.And then, the saline wastewater quality in the refinery was analyzed. It was found that the reverse-osmosis water was the main source of the saline wastewater. But due to the different nature of wastewater, the reverse-osmosis water could directly enter the MED, while the wastewater from the electro-desalting unit had to first enter the pre-treating unit before entering the MED.In the MED system, the relationship of operating pressure-concentration-the optimal heat transfer temperature was stimulated, and the relation model of operating pressure、 concentration and the best heat transfer temperature was established. Taking triple effect evaporator as an example, the influence that the pressure exerted on the evaporation process was studied.. The results showed that the heat transfer area was less with higher-pressure initial steam, and in order to achieve the same concentration ratio, the mass of high-pressure steam should be increased. Under the condition that the final effect of effluent concentrations were invariant, in order to get the same effects, the optimal pressure difference was35kPa. If the mass of initial steam was constant, the difference of operating pressure between the effects was correspondingly larger when the concentration ratio was bigger.The effect of different operating parameters of MED on the system was optimized by thermodynamic modeling. The results indicated that low-temperature heat source of higher temperature could produce larger mass of steam and higher GOR too. However, larger effect numbers could inevitably lead to the higher cost of water desalination and total investment. Thus, the effect number should be4. When the effect numbers were the same, the mass of desalinated water and GOR increased correspondingly with the increase of the temperature of the last effect. When the temperatures of last effect were the same, the larger effect number could bring about larger GOR. However, when the concentration ratio was over100, the change of the flow of solution was smaller and the heat exchange area was bigger. Thus the concentration ratio should not be chosen too high. Steam properties affected the MED performance. When the steam temperature were higher, the heat exchange area declined sharply. At the same time, the yield of the desalinated water increased, yet the change of GOR was smaller. When the effects were the same, desalination water yield was higher when the steam pressure was higher. When the pressures were the same, the heat exchange area went up as the effect numbers increased. However, in MED with the same effect numbers, the heat exchange area was smaller when the steam pressure was higher.Finally, the low-temperature heat recovery of the refinery and the key parameters of MED were optimized. The whole technology of driving MED with the low-temperature heat recovery was analyzed according to the systematic power cycle theory. The results indicated that the low-temperature heat source of higher temperature could produce larger mass of steam. When the temperature of low-temperature heat source was determined, it was advantageous to gain larger mass of pure water and bigger GOR with steam of higher pressure; considering irreversible energy loss, the same low-temperature heat source could produce larger mass of steam when the pinch temperature difference was lower in system. When the temperature difference between the low-temperature heat source and the working fluid was smaller, the irreversible energy loss was smaller and thus larger mass of steam could be produced, and the heat transfer area was higher, too. When the initial temperature of the heat exchange medium was high, the mass of steam was high, and the thermal efficiency was high, too. In the permit condition, heat exchange medium can choose a high heat exchange situation. Further analyses indicated that higher ambient temperature brought about higher thermodynamic efficiency of RC system and lower loss, however the performance of the system was also worsen. Comparing among different working fluids, since the application of the ammonia water system as working fluid lowered the irreversible heat loss, the ammonia water system could be one of the working fluids to recover the low-temperature waste heat.