Design Method Of Heat Integrated Water Networks Considering Non-isothermal Mixing

This paper studies the effect of stream non-isothermal mixing on system energy target, and establishes effective stream mixing rules and control methods in heat integrated water networks. A conceptual method is proposed for designing heat integrated water networks with simple structure and good energy performance.Non-isothermal mixing shares the common features of mixing, such as reducing stream number and simplifying network structure. Moreover, as a way of direct heat transfer, no heat transfer area is needed. Minimum approach temperature is allowed to be set as small as possible, so heat that otherwise can't be recovered through traditional indirect heat transfer may be recovered through direct heat transfer, thus increases heat recovery potential. However, improper non-isothermal mixing also gives rise to energy penalty. Non-isothermal mixing rules are presented after careful study of different sorts of non-isothermal mixing and their energy impact. Utility consumption won't decrease after homogeneous mixing, and it may increase, decrease, or keep the same after heterogeneous mixing. When energy penalty occurs, indirect heat transfer should be employed before mixing to reduce the temperature difference. Non-isothermal mixing rules can precisely predict the effect of non-isothermal mixing on the system energy target and avoid energy penalty by designing new mixing temperatures.'Separate system'method is improved in this paper from two aspects for network generation. Separate systems can be created by either fixing the hot or fixing the cold composite curve and streams in them needn't keep the minimum approach temperature; instead, their heat transfer driving force can be set larger so as to reduce heat transfer area while not causes energy penalty. Based on non-isothermal mixing rules and improved separate system method, a systematic approach is proposed to design energy efficient water networks. Design initial water network with the help of'two dimensional grid diagram', then estimate whether energy penalty arises from non-isothermal mixing streams using non-isothermal mixing rules and avoid penalty if it happens. Finally, accomplish heat integrated water networks design with improved separate system method. This new approach not only makes full use of direct heat transfer to obtain a simpler network structure, but also avoids the possible energy penalty and keeps the minimum water and energy targets.