Feasible products and cost indicators for vapour-liquid equilibrium separation processes

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, South Africa in fulfilment of the requirements for the degree of Doctor of Philosophy.The aim of process synthesis is to select process equipment and interconnections in order to achieve some performance goal, such as the composition of one or more products, in an optimal manner. This work focuses on the synthesis and optimization of vapour-liquid equilibrium (VLE) separation processes. The regions of feasible products for a range of VLE separation processes (including simple distillation, simple condensation and processes comprised of one or two flash stages and mixing) are determined and analyzed. Binary and ternary systems, both ideal and non-ideal, are considered. It is shows .at the products of VLE separation processes are not inherently restricted; limitations in product composition are therefore introduced by the choice of separation equipment. The products on the boundary of the region of feasible products are often found to be associated with infinite costs. Therefore costs and, product composition should he considered simultaneously in process synthesis. This work introduces a pair of variables which are proposed as cost indicators for VLE separation processes. These variables, which are called "capacity variables", pertain to distillation columns and a wide range of other VLE separation processes, allowing their comparison. The variables incorporate both capital and energy costs of a process and are thus related to total annual costs. They accommodate costs of both boiling and condensing. They ate simple to calculate from the mass balance equations. The capacity variables are analogous to mean residence time, which serves as a cost indicator for processes invoi ving reaction. The capacity variables are evaluated for a range of processes separatins binary and ternary mixtures and a range of thermodynamic systems. The capacity variables reflect the impact of the operating parameters, such as product specifications and reflux ratio, on process costs. In distillation processes, the optimum reflux ratio is found to be close to the beuristic value of 1.2 times the minimum reflux ratio. The capacity variables are proposed as useful tools for the initial stages of process synthesis.Andrew Chakane 201

A systematic framework for assessing the applicability of reactive distillation for quaternary mixtures using a mapping method

Reactive distillation (RD) is a useful process intensification technique used in the chemical process industries as it offers important advantages such as energy and cost savings, relative to conventional technologies. However, industrial application of RD is still limited by the complexity of designing and understanding such a complex process. While simple, robust shortcut design methods that require only basic information (such as the relative volatility of components) exist for conventional distillation, such methods for evaluating the applicability of RD are not yet established. This work fills this gap by presenting a new systematic framework for assessing the RD applicability based on a mapping method. The method enables RD designs to be screened using only relative volatilities and chemical equilibrium constant as input data. The evaluation focuses on reactions involving four components (A + B ⇌ C + D) with various boiling point orders, which are of most industrial importance. The proposed systematic framework is validated through its application to five case studies, (trans-)esterifications presenting various separation challenges due to the formation of azeotropes. This novel approach offers a valuable aid for engineers in taking an educated go/no-go decision in the very initial stages of conceptual design, before performing any rigorous simulations of RD flowsheets

Effect of boiling point rankings and feed locations on the applicability of reactive distillation to quaternary systems

Reactive distillation (RD) offers major benefits such as costs reduction and energy saving, but the understanding and design of RD processes usually demand complex tasks that include extensive studies and rigorous simulations. To reduce this complexity and the time required, this study applies a novel mapping method to quickly provide insights into the RD applicability to reversible quaternary systems (A + B ⇌ C + D). Generic cases are used to produce applicability graphs (i.e. plots of reflux ratio vs number of theoretical stages) and multiple RD column configurations. The systems are assumed to have ideal properties and fixed key parameters (i.e. relative volatilities and chemical equilibrium constants). This study focuses on quaternary reactions considering different boiling point rankings and feed locations. Using the mapping method, quick results are achievable regarding the preliminary economic ranking of RD processes, and the optimal feed locations with reduced energy requirement (i.e. lower reflux ratio). Ultimately, this study provides a much better understanding of the effect of boiling point orders and feed locations on the RD applicability and conceptual design, being a valuable tool in early techno-economic evaluations