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Sequencing Separation Trains CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 4 – Sequencing Separation Trains September 11, 2012 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Process Design/Retrofit Steps Assess Primitive Problem Detailed Process Synthesis Algorithmic Methods Development of Base-case PART II Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization Plant-wide Controllability Assessment
Lecture 4 – Introduction • Almost all chemical processes require the separation of chemical species (components), to: Purify a reactor feed Recover unreacted species for recycle to a reactor Separate and purify the products from a reactor • Frequently, the major investment and operating costs of a process will be associated with separation equipment • For a binary mixture, it may be possible to select a separation method that can accomplish the separation task in just one piece of equipment. • More commonly, the feed mixture involves more than two components, involving more complex separation systems.
Lecture 4 – Objectives Be familiar with the more widely used industrial separation methods and their basis for separation. Understand the concept of the separation factor and be able to select appropriate separation methods for liquid mixtures.
Example: Butenes Recovery
Example: Butenes Recovery 100 -tray column C 3 & 1 -Butene in distillate Pentane withdrawn as bottoms 2 -C 4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4 Propane and 1 -Butene recovery n-C 4 and 2 -C 4=s cannot be separated by ordinary distillation ( =1. 03), so 96% furfural is added as an extractive agent ( 1. 17). n-C 4 withdrawn as distillate.
Separation is Energy Intensive • Unlike the spontaneous mixing of chemical species, the separation of a mixture of chemicals requires an expenditure of some form of energy • Separation of a feed mixture into streams of differing chemical composition is achieved by forcing the different species into different spatial locations, by one or a combination of four common industrial techniques: The creation by heat transfer, shaft work, or pressure reduction of a second phase that is immiscible with the feed phase (ESA – energy separating agent) Introduction into the system of a second fluid phase (MSA – mass separating agent). This must be subsequently removed. Addition of a solid phase upon which adsorption can occur Placement of a membrane barrier
Common Separation Methods
Common Separation Methods
Common Separation Methods
Separation Method Selection • The development of a separation process requires the selection of: • Separation methods ESAs and/or MSAs Separation equipment Optimal arrangement or sequencing of the equipment Optimal operating temperature and pressure for the equipment Selection of separation method depends on feed condition: • Vapor • Liquid • Solid Partial condensation, distillation, absorption, adsorption, gas permeation (membranes) Distillation, stripping, LL extraction, supercritical extraction, crystallization, adsorption, and dialysis or reverse osmosis (membranes) If wet drying, if dry leaching
Separation Method Selection • The separation factor, SF, defines the degree of separation achievable between two key components of the feed. This factor, for separation of component 1 from component 2 between phases I & II, for a single stage of contacting, is: C = composition variable, I, II = phases rich in components 1 and 2. • (8. 1) SF is generally limited by thermodynamic equilibrium. For example, in the case of distillation, using mole fractions as the composition variable and letting phase I be the vapor and phase II be the liquid, the limiting value of SF is given in terms of vapor-liquid equilibrium ratios (K-values) as: (8. 2), (8. 3)
Separation Method Selection • For vapor-liquid separation operations that use an MSA that causes the formation of a non-ideal liquid solution (e. g. extractive distillation): (8. 5) • If the MSA is used to create two liquid phases, such as in liquid extraction, the SF is referred to as the relative selectivity, β , where: (8. 6) • In general, MSAs for extractive distillation and liquid-liquid extraction are selected according to their ease of recovery for recycle and to achieve relatively large values of SF.
Equal Cost Separators Liquid-Liquid Extraction should NOT be used when α for ordinary distillation is greater than 3. 2 Extractive distillation should NOT be used when α for ordinary distillation is greater than 2 Ref: Souders (1964)
Summary – Separation Trains On completion of this part, you should: Be familiar with the more widely used industrial separation methods and their basis for separation. Understand the concept of the separation factor and be able to select appropriate separation methods for liquid mixtures.
Other Business • Next Lecture – September 20 – Sequencing Ordinary Distillation Columns (SSLW p. 216 -223)