Improvements for mass-exchange networks design

Water Footprint and Water Pinch Analysis in Ethanol Industrial Production for Water Management

Fuel ethanol is considered to be a clean alternative fuel to meet increasing energy demands and mitigate environmental pollution. Faced with challenges in terms of energy security and environmental pollution, China is vigorously developing fuel ethanol. However, ethanol-manufacturing is a water-intensive industry; it consumes large volumes of fresh water and generates a corresponding amount of waste water. Expansion of this industry can reduce water quality and cause water stress. This study aims to combine the water footprint (WF) with a water pinch analysis technique to manage water consumption and sewage discharge systematically in an ethanol plant. A well-operated cassava ethanol plant in China was chosen as a case study. The WF of industrial ethanol production was evaluated. The total WF was 17.08 L/L ethanol, comprised of a 7.69 L blue water footprint (BWF), and a 9.39 L gray water footprint (GWF). The direct WF was 16.38 L/L ethanol, and the indirect WF was 0.70 L/L ethanol. Thereafter, a water pinch analysis was conducted, and the optimal direct water reuse scheme was studied. After the water network was optimized, the BWF was reduced by 0.98 L/L ethanol, while the GWF was reduced by 1.47 L/L ethanol. These results indicate that the combined use of WF and pinch analysis can provide the starch-based ethanol industry with an effective tool to improve its water management.

Analysis and Optimization of Open Circulating Cooling Water System

Open circulating cooling water system is widely used in process industry. For a system with a fixed structure, the water consumption and blowdown usually change with the varying parameters such as quality and temperature. With the purpose of water saving, it is very important to optimize the operation strategy of water systems. Considering the factors including evaporation, leakage, blowdown and heat transfer, the mass and energy conservation equations of water system are established. On this basis, the quality and temperature models of makeup and blowdown water are, respectively, developed. The water consumption and discharge profiles and the optimal operating strategy of the open recirculating cooling water system under different conditions are obtained. The concept of cycles of temperature is proposed to evaluate the temperature relationship of various parts of the open circulating cooling water system. A mathematical relationship is established to analyze the influence of the water temperature on the makeup water rate of the system under the condition of insufficient cooling capacity of the cooling tower. In addition, the co-influences of quality and temperature parameters on the system are analyzed.

Flowrate targeting for threshold problems and plant-wide integration for water network synthesis

Water reuse/recycle has gained much attention in recent years for environmental sustainability reasons, as well as the rising costs of fresh water and effluent treatment. Process integration techniques for the synthesis of water network have been widely accepted as a promising tool to reduce fresh water and wastewater flowrates via in-plant water reuse/recycle. To date, the focus in this area has been on water network synthesis problems, with little attention dedicated to the rare but realistic cases of so-called threshold problems. In this work, targeting for threshold problems in a water network is addressed using the recently developed numerical tool of water cascade analysis (WCA). Targeting for plant-wide integration is then addressed. By sending water sources across different geographical zones in plant-wide integration, the overall fresh water and wastewater flowrates are reduced simultaneously.

A holistic framework for design of cost-effective minimum water utilization network

Water pinch analysis (WPA) is a well-established tool for the design of a maximum water recovery (MWR) network. MWR, which is primarily concerned with water recovery and regeneration, only partly addresses water minimization problem. Strictly speaking, WPA can only lead to maximum water recovery targets as opposed to the minimum water targets as widely claimed by researchers over the years. The minimum water targets can be achieved when all water minimization options including elimination, reduction, reuse/recycling, outsourcing and regeneration have been holistically applied. Even though WPA has been well established for synthesis of MWR network, research towards holistic water minimization has lagged behind. This paper describes a new holistic framework for designing a cost-effective minimum water network (CEMWN) for industry and urban systems. The framework consists of five key steps, i.e. (1) Specify the limiting water data, (2) Determine MWR targets, (3) Screen process changes using water management hierarchy (WMH), (4) Apply Systematic Hierarchical Approach for Resilient Process Screening (SHARPS) strategy, and (5) Design water network. Three key contributions have emerged from this work. First is a hierarchical approach for systematic screening of process changes guided by the WMH. Second is a set of four new heuristics for implementing process changes that considers the interactions among process changes options as well as among equipment and the implications of applying each process change on utility targets. Third is the SHARPS cost-screening technique to customize process changes and ultimately generate a minimum water utilization network that is cost-effective and affordable. The CEMWN holistic framework has been successfully implemented on semiconductor and mosque case studies and yielded results within the designer payback period criterion.