Procedure for a conceptual design of a separation process 1. Definition of the separation problem 2. Accumulation of data of the substances involved 3. Survey of the molecular physical chemical properties 4. Accumulation of information on the separation problem: 5. Selection of possible separation processes. Ch 11 General Considerations

Procedure for a conceptual design of a separation process (contd.) 6. Design steps for each separation process selected 7. Comparison of different processes, evaluation. 8. Decision for a special process 9. Re-design of the selected process, optimization. General Considerations

Composition of the feed mixture; Composition of the products; Specifications which have to be met; Throughput; Special boundary conditions, e.g. throughput not constant, environmental restrictions, possible risk minimization. Definition of The Separation Problem

Chemical structure; Molecular weight and other characteristic molecular physical-chemical data; Thermodynamic state, vapour pressures; Chemical, thermal stability; Miscibility with the other compounds involved and with other important solvents (water, ethanol,...); Handling, toxicity, value; Any other information on the pure substances and mixtures which could be of importance for the solution of the separation problem. Accumulation of Data on Substances

Survey of the molecular physical chemical properties of the pure substances with the goal to define appropriate separation principles and separation processes. Accumulation of Data

Property Differences Equilibrium properties: Vapor pressure Solubility Distribution between immiscible liquid phases Melting point Chemical reaction equilibrium Electric charge (isoelectric point) Surface sorption Rate properties: Diffusivity Ionic mobility Molecular size Molecular shape

Accumulation of information on the separation problem: usual solutions carried out solutions published solutions patented solutions of the separation problem or similar separation problems. Accumulation of Data

Selection of Possible Separation Processes According to basic principles of the process: Separational baseMode of operation Phases in equilibrium Surface phenomena Field effects Size Molecular velocity Chemical properties Combinations Single unit process Combined process units without a barrier batch continuous hybrid with a barrier b, c, h

Classification of Unit Operations Distillation - vapor pressure Extraction - distribution between solid and fluid phases Extraction - distribution between fluid immiscible phases Crystallization - melting point or solubility Adsorption - surface sorption Reverse osmosis - diffusivity and solubility Membrane gas separation - diffusivity and solubility Ultrafiltration - molecular size Ion exchange - chemical reaction equilibrium Dialysis - diffusivity Electrodialysis - electric charge and ionic mobility Liquid membranes - diffusivity and reaction equilibrium Chromatographic separation - depends on stationary phase In red / blue letters: unit operations can be carried out with supercritical fluids

Define the separation task for each single process step within the separation process, Set up mass and energy balances Selection of separation equipment Design of the separation equipment: Calculation of separation characteristics: number of theoretical stages, height, diameter; or number of transfer units, height of a transfer unit, Separational analysis: variation of process conditions, limiting conditions Selection of a first set of separation characteristics, Constructive design of separation equipment. Control procedures. Design Steps For Each Separation Process

Comparison of different processes, evaluation. Decision for a special process Re-design of the selected process, optimization. Making decisions

Computer programs: Flowsheeting, Expert systems Functional analysis: A process is split into functional units with special tasks. Example: Depropanizer: Separation of C3 from C4: separation of low volatiles (C4+) from the C3-cycle gas; Methods for Process Design

Methods for Process Design II Evolutionary design: Begin with a simple solution of the task. Carry out optimization with respect to other conditions, e.g. minimal energy need. Morphological design: Definition of important process goals, which can be used for evaluation of every solution. Procedure: Define single tasks, find out all possible solutions to carry out the tasks, evaluate all possible combinations. Heuristic rules: Generalisations on the basis of very many observations. Rules of thumb

Rules of Thumb 1. Favor distillation - unless relative volatilities are less than The easies separation first 3. Component with the highest mole fraction to be removed first - if not selectivities vary widely 4. With distillation, remove the component of highes volatility first 5. Mass separation agents are to be removed in the immediate following step - unless they are of use in subsequent steps 6. Do not use a second mass separation agent to remove or recover a mass separation agent - unless it is really profitable For Process Synthesis