CHEN 4460 – Process Synthesis, Simulation and Optimization

Sequencing Separation Trains CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University Lecture No. 4 – Sequencing Separation Trains September 13, 2011 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

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

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100-tray column C3 & 1-Butene in distillate Propane and 1-Butene recovery Pentane withdrawn as bottoms n-C4 and 2-C4=s cannot be separated by ordinary distillation (=1.03), so 96% furfural is added as an extractive agent (  1.17). n-C4 withdrawn as distillate. 2-C4=s withdrawn as distillate.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

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

C = composition variable, I, II = phases rich in components 1 and 2. • (8.1) 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: • 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)

(8.5) 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): • 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: • (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 • Homework • SSLW: 6.1, 6.2, 6.3 (6.3 utilizes Lab Homework Exercise A.2) • Due Tuesday September 20 • Next Lecture – September 20 • Sequencing Ordinary Distillation Columns (SSLW p. 216-223) • Lab Tonight • I am unable to be there tonight due to the Omega Chi Epsilon initiation ceremony. Vivi and Charlotte will be there though. • Impromptu Class Poll • Who is still here for the summer 2012 semester? • Any interest in spending 6-8 weeks in either Mexico or Canada?

CHEN 4460 – Process Synthesis, Simulation and Optimization