CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 4 – Sequencing Separation Trains September 9, 2008 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel Sequencing Separation Trains

Process Design/Retrofit Steps Assess Primitive Problem Development of Base-case Plant-wide Controllability Assessment Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization Detailed Process Synthesis - Algorithmic Methods SECTION B

Algorithmic Methods

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

100-tray column C3 & 1-Butene in distillate Propane and 1-Butene recovery Pentane withdrawn as bottoms n-C4 and 2-C4=s cannot be separated by ordinary distillation (  =1.03), so 96% furfural is added as an extractive agent (   1.17). n-C4 withdrawn as distillate. 2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4

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

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 Partial condensation, distillation, absorption, adsorption, gas permeation (membranes) Liquid Distillation, stripping, LL extraction, supercritical extraction, crystallization, adsorption, and dialysis or reverse osmosis (membranes) Solid 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: (5.1) C = composition variable, I, II = phases rich in components 1 and 2. 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: (5.2)

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): (5.4) If the MSA is used to create two liquid phases, such as in liquid- liquid extraction, the SF is referred to as the relative selectivity, β, where: (5.5) 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 Ref: Souders (1964) Extractive distillation should NOT be used when α for ordinary distillation is greater than 2

Summary – Separation Trains  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. On completion of this part, you should:

Other Business Comment on Homework Overlap –Exercise A.1. and SSL: 4.1 are the same problem. –No need to turn in a second copy, unless you have discovered an error in the original. –If you need to make corrections to this homework assignment, then turn it in Thursday during lab. Homework –SSL: 5.1, 5.2, 5.3 (5.3 utilizes Lab Homework Exercise A.2) –Due Tuesday September 16 Next Lecture –Sequencing Ordinary Distillation Columns (SSL pp )