Guidelines for Separation System By: Dr. Muhammad Syarhabil bin Ahmad

Feed Separation (may be required) Purifying reactor feed from Catalyst poison Significant amt. of inert Substances that disrupt the reaction and produces byproducts Substance that reacts with reagent and corrodes the reactor

Examples Removal of propane from a feed of propylene for polymerization. (distil) Dehydrogenation of ethanol. Aqueous ethanol is concentrated to its near- azeotropic cond. (distil) Remove small amt. of water during vinyl chloride production to prevent corrosion. (adsorption)

Phase Separation of Reactor Effluent Need to separate into each component or phases. Heterogenous or homogeneous Phases Liquid Vapor Solid How to separate? By chg T, P, Dry, Centrifuge, Decant etc.

Industrial Separation Operation Heat transfer (ESA) ex. P etc Introducing 2 nd fluid (MSA) (needs to recover the solvent & recycled) Addition of solid for selective adsorption (solids need to be treated and regenerate adsorbing capabilities) (MSA) Membrane barrier.(Permeation produces mechanical energy loss, so ? ESA or MSA) ESA = Energy separating agent, MSA mass separating agent

Selecting Separation Methods 1)Types of separation methods 2)ESA/MSA 3)Separation equipment 4)Optimal arrangement or sequencing equipment 5)Optimal operating temperature and pressure for the equipment

Types of Separation Method Flash Distillation Liquid-liquid extraction Membrane Crystallization Etc…..

Consideration for the Selection of a Separation Method Phase condition Separation factor Reason of separation

Vapor Feeds For vapor or readily converted to vapor 1) Partial condensation/partial vaporization 2) Distillation (cryogenic cond.) 3) Gas absorption 4) Gas adsorption 5) Gas permeation using membrane 6) Desublimation

Liquid Feeds 1) Flash or partial vaporization 2) Distillation 3) Stripping 4) Azeotropic distillation 5) Extractive distillation 6) Liquid-liquid extraction 7) Crystallization 8) Liquid adsorption 9) Dialysis, reverse osmosis, ultra filtration and pervaporization with a membrane 10) Supercritical extraction

Rule of Thumb (heuristics) For liquid- Distillation is cheapest and most favored Attempt to condense vapor into liquid. Then use liquid separation.

Solid Separation Crystallization Melt crystallization Precipitation Desublimation Drying Filtration/centrifugation for wet cake Cyclone separation

Separation Factor (SF) SF = C I 1 /C I 2 C I J = composition of component C II 2 /C II 2 J in I. (composition in terms of mole fractions, mass fractions or concentration) The bigger the SF the better separation between the phases of I and II. In distillation using mole fractions in vapor I and liquid II. SF = y2/y1= y1/x1 = K1 =  1,2 K-values (vapor and liquid X2/X1 y2/x2 K2 equilibrium ratios  = relative volatility, where SF > 1.0

For Ideal Gas Law Can look up from vapor pressure data to calculate SF. SF =  12 = P 1 /P 2, P = vapor pressure

Other SF Values SRK (Suave-Redlich-Kwong) (Non ideal) Use PR (Peng-Robinson) (Non ideal) Modified Raoults Law ( Azeotropic & extractive distillation) When using Mass separating Agent (MSA) it is referred as relative selectivity

Selection of Equipment Absorption, Stripping and Distillation – use trayed or packed columns Liquid-liquid Extraction- mixers, centrifugal extractors. Adsorption – activated carbon, molecular sieves, silica gel, activated alumina Membrane separation- synthetic glassy or polymeric polymers. Thickness 0.1  -1.0 

Cont’d Leaching – reducing size of materials and mix with solvent Crystallization – vacuum evaporating draft tube crystallizer Drying – dryers include tunnel,belt, rotary, spray, drum etc…

Favorable Sequences 1)Remove unstable, corrosive or chemically reactive early in sequences 2)Remove final prod. One by one as distillates in sequences 3)Remove the component with highest molar % early in the sequences 4)In the order of decreasing volatility 5)Lastly - highest purity products 6)Favors near equimolar distillate and bottoms in each column

Sequencing Ordinary Distillation Columns for Nearly Ideal Fluid Mixtures Relative volatility or SF >1.05 Not excessive reboiler duty Tower pressure not approach critical T Overhead vapor can be partially condensed to produce reflux Bottom T not too high for decomposition Azeotrope not preventing desired separation Column pressure drop tolerable – esp under vacuum

Sequencing Ordinary Distillation Columns for Non-Ideal Fluid Mixtures Identify azeotropes- binary? ternary? Identify alternaters- extractive distillation towers or azeotropic distillation towers Select entrainer- homogeneous azeotropic distillation towers-no distillation boundary -induce liquid-phase splitting as in heterogeneous azeotropic distillation Identify feasible distillate and bottom product composition

Separation Systems for Gas Mixtures 3 categories 1)Sharp splits to produce nearly pure products- ex. Cryogenic, adsorption difficult for mixtures 2)Enrichment for selected species- ex. O 2 and N 2 enrichment – membrane permeation 3)Purification – adsorption or absorption with chemical reaction Ex. Separation of air into N 2 and O 2 enriched product- a) Membrane most economical b) Adsorption moderate rate c) Cryogenic distillation at high rates.

Separation Sequences for Solid- Fluid Systems Example:meta & para xylenes- b.p diff. = C but m.p diff = 64 0 C, thus crystallization used. Phtalic anhydride-separate from Naphtalene or o-xylene s.m by desublimation & distillation to form a melt From 10%wt MgSO 4 (aq)-reduce water, then centrifuge,vacuum crytallizer, dried more in hydroclone, another centrifuge and dried to 2wt% moisture using direct heat rotary dryer

The End