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Unformatted text preview: Chapter 5 : Selection of Separation Technologies and Synthesis of Separation Flow Sheets CHBE 241 Department of Chemical and Biological Engineering The University of British Columbia 2007 1 Introduction I Separations are a major part of modern chemical processes, typically accounting for 50% or more of the total capital and operating costs. I In a chemical process, a mixture of compounds must be separated into the appropriate product stream (or recycle or waste stream). This is accomplished by exploiting di erences in physical properties . I The rst step in choosing a separation technology is to gather information about the physical properties of the components to be separated and then to answer the following questions: 1. How do the components to be separated di er in physical properties? 2. Is the di erence between the components large? 3. Can the di erence be feasibly exploited? 4. Do the components go to the correct product stream? By answering these questions, we can choose the best physical property di erences on which to base our design. 2 Mixtures and Phases I In separation processes, multicomponent mixtures are separated into streams of di ering composition. I The phase of the streams plays a signi cant role in how a separation processes actually work. I Phase homogeneous, physically distinct, and mechanically separable portion of matter. I Solids, liquids, and vapours are all phases. Supercritical uids and plasmas are phases that are less commonly encountered. A phase is homogeneous. Within a phase, the chemical composition and physical properties are uniform. It may be a single component, or a phase may be a multicomponent mixture, with the species distributed evenly at the molecular level. Sugar solution in water a multicomponent mixture but a single phase. Emulsions, like oil-vinegar salad dressing, or bubbly liquids, like carbonated soda, or suspensions, like muddy water, are not single phases, because the components in the mixture are not completely and uniformly distributed at a molecular level. 3 Mixtures and Phases (2) A phase is physically distinct. Vapours, liquids, and solids di er in fundamental ways. Vapours much less dense than liquids. Usually liquids are less dense than solids. Vapours are highly compressible (their density changes a lot with pressure). Liquids and solids are almost incompressible. Vapours and liquids adopt the shape of their containers, whereas solids retain their shape. Phases are mechanically separable. One phase can be separated from another by using mechanical force and mechanical devices. Example: Diesel fuel spilled in a pond will oat on top of the water and can be removed by using a skimmer. Dirt particles in muddy water can be ltered out, producing clari ed water....
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