Design of batch operations: Systematic methodology for generation and analysis of sustainable alternatives
Introduction
The economic and industrial activities related to chemicals-based products are continuously increasing all over the world. Consequently, concerns about the sustainability of the modern society are growing everyday and demands for improving the industrial plant operations and designs are also increasing. The use of green chemicals and process operability at sustainable conditions are two factors, among many others, that should be taken into account in the retrofitting of the already existing processes. Important questions to ask in this respect are—how should we improve an existing process (that is, generate sustainable retrofit alternatives) without too much effort? What should be done in order to make the process more sustainable? Which are the process points that can significantly improve the process performance? In order to address these and related questions, it would be useful to develop systematic methods and tools, which enable the generation of more sustainable alternatives and enhance the ability of the process to adapt to future needs.
Much work has been done on developing optimization models for scheduling of batch operations. Chakraborty and Linninger (2002) introduced the concept of combinatorial process synthesis for developing plant-wide recovery and treatment policies for batch manufacturing sites. The authors executed a flowsheet generation step combined with multi-objective optimization in order to obtain the operating policies with optimal trade-off among the conflicting objectives, cost and environmental impact. Montagna (2003) proposed new alternatives in the retrofit model of multiproduct batch plants. This model considered the inclusion of intermediate storage tanks, which can be simply added or the replacement of existing units that can be sold. This allowed the author to obtain more efficient and real world solutions, even though it required working with a more complex model. In a recent review paper, Póvoa (2007) covered various approaches for design of batch plants and retrofit design problems. However, all these approaches only cover retrofit analysis for scheduling of batch operations but do not consider further improvements that can be achieved in terms of sustainable design alternatives.
Also, methodologies taking into account environmental aspects, have been developed. For example, Zhao-Ling and Xi-Gang (2000) formulated and solved a multi-objective optimization problem for the optimal design of batch processes with waste minimization, while Lee and Malone (2000), suggested a production planning method for a batch process including solvent recovery to minimize the disposed solvent wastes.
Recently, due to concerns about the time consumed and the complexity of the problems considered in earlier solution approaches, systematic methodologies to generate new design alternatives have been proposed. Halim and Srinivasan (2006) presented an intelligent system for waste minimization assessment of batch processes (BATCHENVOPExpert). Recently, Halim and Srinivasan (2008) also proposed an intelligent simulation–optimization framework for identifying comprehensive sustainable alternatives for batch processes. An AP-graph-based approach has been used to identify the root cause for process waste generation and for identifying new design alternatives. Simon, Osterwalder, Fischer, and Hungerbuhler (2008), on the other hand, proposed a decision support framework for retrofitting chemical batch processes based on indicators, heuristics and process models. Their framework considered the identification of improvement opportunities in a batch plant by considering first the product market situation.
Recently, Carvalho, Matos, and Gani (2008) proposed a generic and systematic methodology for identifying feasible retrofit design alternatives for continuous chemical processes. The methodology determines a set of mass and energy indicators from steady-state process data, establishes the operational and design targets, and through a sensitivity based analysis, identifies the design alternatives that can best match a set of design targets. An indicator sensitivity analysis is performed to define the design targets and process synthesis/design methods and tools are employed to generate sustainable process alternatives. A computer-aided tool, called SustainPro has also been developed to facilitate the calculations needed for the application of the methodology.
The objective of this paper is to present a methodology that covers batch as well as continuous operations, building on the concepts developed earlier for continuous operations. In this extended methodology, several new features have been introduced. When a process involves batch operations a flowdiagram is created in order to represent the sequence of operations. An algorithm to generate a similar flowdiagram to represent batch operations has been developed. A decomposition technique to take into account the accumulated mass present in each operation has been developed and added to the methodology in order to address/identify problems related to the batch operations. New indicators have also been developed to consider the effects of sizing (equipment) parameters, the energy spent in each batch operation and the time spent on the batch operations. With these additions, it is now possible to determine and evaluate sustainable design alternatives for continuous, semi-continuous and batch processes, or, mixed-mode operations. The sustainable design alternatives generated through the extended methodology, are evaluated through a set of sustainability metrics (Azapagic, 2002), safety indices (Heikkilä, 1999) and the WAR algorithm parameters (Cabezas, Bare, & Mallick, 1999).
The application of the extended systematic methodology for the batch process is illustrated through two case studies. The first case study, involves a simple batch process where consumption of water needs to be reduced without sacrificing process performance qualities (Wulff, Gitz, & Wenzel, 2007). Through this case study the application of the different steps of the methodology is illustrated. The second case study involves insulin production (Petrides, Sapidou, & Calandranis, 1995), which is quite a complex and relatively big process. Again, application of the main steps of the methodology is illustrated.
Section snippets
Indicators
For all types of processes (batch and/or continuous mode of operation), a set of five mass indicators and three energy indicators can be calculated. The mass indicators are: material value added (MVA), energy and waste cost (EWC), total value added (TVA), reaction quality (RQ) and accumulation factor (AF). The energy indicators are: demand cost (DC), total demand cost (TDC) and the reusable energy factor (REF). The mass and energy indicators are calculated for all the closed- and open-paths
Operation indicators
There are three operation indicators, the total free volume factor (TFVF), the operation time factor (OTF) and the operation energy factor (OEF). With these indicators it is possible to have an analysis of the performances of the batch operation in terms of time, volume and energy. In the text below, the operational (batch) indicators are explained in more details.
Compound indicators
A set of compound indicators, which allow the identification of the compound causing a bottleneck in a given operation, have been developed. There are three different compound indicators: the free volume factor (FVF), the time factor (TF) and the energy factor (EF). The TF and the EF are applied for each accumulation-path and their calculations are dependent on the type of operation, such as, mixing, reacting and separating operations.
The compound indicators are explained below.
Methodology
The main steps of the extended methodology are described below. The work-flow for the extended methodology is organized in terms of seven steps, as shown in Fig. 2. In the text below, each step is explained.
Case studies
The application of the new extended methodology is highlighted through two case studies. The first case is related to a simple process that requires a reduction of water consumption and comes from a laundry process in Denmark. The second case study involves insulin production, which is a quite complex and relatively big process.
Discussion and conclusions
The development of a systematic and generic indicator-based methodology for analysis of continuous and batch processes and for generating sustainable (design) improvements, has been presented and highlighted with two case studies. This approach is able to trace and then locate possible problems related to the compounds being handled in the process across the flowsheet (open- and closed-paths) and also for the operation time (accumulation-path). The methodology is systematic and generic in
Acknowledgement
The authors gratefully acknowledge financial support from Fundação para a Ciência e a Tecnologia (under Grant No. SFRH/BD/24470/2005).
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