Access Type

Open Access Dissertation

Date of Award

January 2018

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

Yinlun Huang

Abstract

Sustainable development has become a key concern in industries, largely due to natural resource depletion, global competition, and environmental pressure. Despite the efforts for sustainability improvement, still over a half of energy consumption is wasted in manufacturing sectors, where the chemical industry is responsible for an energy efficiency lower than it should be. Many attempts have been made to recover the thermal energy using heat integration techniques. Although process work is more expensive than process heat, no efficient solution has been studied to recover mechanical energy yet. In chemical plants, many process streams need to be pressurized or depressurized in different operational stages. Therefore, the energy of these streams can be recovered by a new class of exchange, which is called work exchange.

From the thermodynamics point of view, in heat integration, temperature is a state variable and the temperature difference is the driving force for heat transfer. In work integration, pressure is a state variable. A system reaches a mechanical equilibrium if at every point within a given system there is no change in pressure with time, and there is no movement of material. Work integration through direct work exchangers could contribute significantly to mechanical energy recovery through synthesizing work exchange networks (WENs), where work exchangers are operated in a batch mode, while compressors and expanders as utility units are operated in a continuous mode; these render WENs a type of sophisticated hybrid network system.

This research focuses on a new type of process integration for effective work integration through WEN synthesis. The concept of work integration has been studied and a mathematical modeling and analysis method is introduced to predict the maximum amount of mechanical energy that can be feasibly recovered using direct work exchangers prior to WEN configuration development. A thermodynamic model-based synthesis approach is developed to design a cost-effective heat-integrated work exchanger network (HIWEN), in which direct work exchangers may work under different operating conditions. Note that direct work exchangers have been used widely for seawater reverse osmosis (RO) desalination, where liquid streams are pressurized or depressurized. This type of unit, however, cannot be directly used for mechanical energy recovery involving streams in gas phase in chemical process systems. Thus, investigation of direct work exchangers that can be operated for mechanical energy recovery involving gas streams has been performed. A CFD-based model is developed to conduct various simulations to study the design of such a device, and its operational behavior under different operating conditions. The findings from this dissertation can have great potential for improvement of energy efficiency in manufacturing sectors.

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