LECTURE 2 Separation method selection (Distillation) Associate Prof Dr Rashmi Walvekar Lesson outline ❑ Industrial separation process ❑ Factors influencing separation ❑ Selection of feasible separation process Learning outcomes At the end of the lecture student are able to: • explore the issue of how to choose the right separation process, or narrow down the number of choices for a given separation. • heuristic approach using simple rules of thumb and direction guidelines that can be followed to arrive at the separation process sequence to be adopted in the final plant design. Introduction ✓ selection of separation process sequence is complicated ✓ no STANDARD procedure ✓ optimum separation scheme depends on time and money allocated in the development and analysis. Also depends on how skilled is the process engineer. ✓Complexity increases with increase in complexity of feed mixture Example: binary mixture requires 1 unit multicomponent mixture requires “separation sequence train” Industrial separation processes Fig 1: Schematic flow sheet of a naphtha steam cracker. Example of separation train (series of separation units) Selecting separation method 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 largely 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 Factors influencing choice of separation process 1. Economics: ✓ economic value of the product and scale of operation constraints include: • market strategy and timing, reliability, risks associated with innovation, and capital allocation Two extreme cases are: 1. The product is of high unit value with a short market life expectancy 2. The product is a high-volume chemical with many producers in a highly competitive market Factors influencing choice of separation process 2. Feasibility: ✓ separation process selected should have potential to produce a desired product. ✓ need for extreme processing conditions (T and P). ✓ each separation unit produces different product (each unit has different working principle). Factors influencing choice of separation process 3. Product stability: ✓ avoid damage to the product (thermal damage) ✓ thermal damage - denaturation, formation of unwanted color, polymerization etc. ✓ example: Distillation (operate under vacuum to reduce reboiler temperature) ✓ minimize holdup time at high temperature Factors influencing choice of separation process 4. Design reliability: ✓ most important factor – plant should work properly to product desired product that is profitable. ✓ only apply well understood processes that are highly acceptable by industries -> technologically mature process for commercial use Selection of feasible separation processes 1. Classes of processes: ✓ clear distinction between ESA, MSA and rate controlled processes should be done. ✓ energy consumption increases from ESA→MSA→rate controlled. • multistage rate-governed separation process should only be considered when it gives a significantly better separation factor than an equilibrium based process. • a mass-separating-agent process should only be considered when it provides a better separation factor than an energy-separating-agent process. Selection of feasible separation processes 2. Initial screening: ✓ define the problem and select most feasible separation process. ✓ depends on feed conditions, mixture properties to be exploited and characteristic of separation process. • Feed conditions: flow rate, composition • Product conditions: purity, recovery Selection of feasible separation processes Table 1:Factors that influence the selection of feasible separation operations. Selection of feasible separation processes Table 2: Ease of scale-up and staging of the most common separation operations. Selection of feasible separation processes 3. The separation factor(SF): defines the degree of separation achievable between two key components of he feed This factor, for the separation of component 1 from component 2 between phases I and II, for a single stage of contacting, is defined as: SF = C 1I / C 2I C 1II / C 2II Where, C = composition variable, I, II = phases rich in components 1 and 2. ✓ The value of the separation factor is limited by thermodynamic equilibrium, except in the case of membrane separations that are controlled by relative rates of mass transfer through the membrane. Selection of feasible separation processes ✓ Operations employing an ESA are economically feasible at a lower value of SF than those employing an MSA. ✓ MSA is used based on ease of recovery for recycle and to achieve large SF. ✓ Exploit the molecular properties in the most economical manner Fig 2: Relative separation factors for equal-cost separation operations. (Adapted from M. Souders, Chem. Eng. Prog., 60(1964)2 75–82) Separation of homogeneous liquid mixtures • Strengths of distillation ✓ small equipment requirement ✓ easy staging ✓ economics of scale ✓ energy costs ✓ design and scale-up reliability Distillation is the most frequently used separation process in practice Separation of homogeneous liquid mixtures • Limitations of distillations: ✓ low relative volatility ✓ Feed composition ✓ extreme conditions ✓ small capacities ✓ product degradation ✓ column fouling Example 1. Specification for Butenes Recovery Design for Butenes Recovery System 100-tray column C3 & 1-Butene in distillate Pentane withdrawn as bottoms 2-C4=s withdrawn as distillate. Furfural is recovered as bottoms and recycled to C-4 Propane and 1-Butene recovery 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. Sequencing of Ordinary Distillation Columns 21 Use a sequence of ordinary distillation (OD) columns to separate a multicomponent mixture provided: in each column is > 1.05. The reboiler duty is not excessive. The tower pressure does not cause the mixture to approach the TC of the mixture. Column pressure drop is tolerable, particularly if operation is under vacuum. The overhead vapor can be at least partially condensed at the column pressure to provide reflux without excessive refrigeration requirements. The bottoms temperature for the tower pressure is not so high that chemical decomposition occurs. Azeotropes do not prevent the desired separation. Algorithm to Select Pressure and Condenser Type Number of Sequences for Ordinary Distillation Equation for number of different sequences of P − 1 ordinary distillation (OD) columns, NS, to produce P products: [2(P − 1)]! Eq (2.1) Ns = P ! (P − 1)! P # of Separators Ns 2 1 1 3 2 2 4 3 5 5 4 14 6 5 42 7 6 132 8 7 429 Example 2.1 Sequences for 4-component separation Example 2.1 – Sequences for 4-component separation Identifying the Best Sequences using Heuristics 26 The following guidelines are often used to reduce the number of OD sequences that need to be studied in detail: Remove thermally unstable, corrosive, or chemically reactive components early in the sequence. Remove final products one-by-one as distillates (the direct sequence). Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed. Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of other components. Sequence separation points to leave last those separations that give the highest purity products. Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column. The reboiler duty is not excessive. Class Activity Example 2.2 Arrange in the order of increasing boiling point Design a sequence of ordinary distillation columns to meet the given specifications. Possible solution ▪ Variation in relative volatility and molar percentage. • First column should separate C3, the most volatile. • Second column should be for separ ation of nC4 and iC5 as LK and HK respectively. • Two most difficult splits are iC4/nC4 and iC5/nC5, so two separate columns for these separation Example 2.3 The table below gives the data for a ternary separation of benzene, toluene and ethyl benzene. Using the vapour flowrate equation, determine whether direct or indirect sequence should be used. solution Hence, the direct sequenceshould be used. NOTE: High V, High Capital and Op. Costs!! Example 2.3 (homework) In a large chemical plant methylcyclohexane is separated from toluene by distillation. For the construction of a new plant it is considered whether the use of extractive distillation or extraction could provide economically more attractive options. A detailed evaluation requires firstly the characterization of the current separation between methylcyclohexane and toluene. a. Calculate from the given data the relative volatility for the separation of methylcyclohexane and toluene by distillation. Consider the mixture to behave (almos)t ideal. b. Which of the given solvents might be feasible to enhance the relative volatility sufficiently to make extractive distillation economically attractive? c. The same question for extraction. Assume that the solvents and the methylcyclohexane/toluene mixture are (almost) immiscible. Besides methylcyclohexane (45%) and toluene (45%) the feed also contains 5% of cyclohexane as well as benzene. d. What would be the optimal arrangement of separation steps for distillation, for extractive distillation and for extraction to separate this mixture into its four pure constituents? What distillative separation might become problematic and why? e. Which of the arrangements seems, considering the number of operations (columns), the most economical solution. f. Why is the counting of the number of operations sufficient to get a first impression about the most economical route? What effects are not taken into account? Complex Columns for Ternary Mixtures In some cases, complex rather than simple distillation columns should be considered when developing a separation sequence. Ref: Tedder and Rudd (1978) Regions of optimality As shown below, optimal regions for the various configurations depend on the feed composition and the ease-of-separation index: • ESI = AB/ BC ESI 1.6 ESI 1.6 Regions of optimality 1. If 40 to 80% is middle product and nearly equal amounts of overhead and bottoms are present, then favor design V. 2. If more than 50% is middle product and less than 5% is bottoms, then favor design VI. ESI 1.6 3. If more than 50% is middle product and less than 5% is overheads, then favor design VII. 4. If less than 15% is middle product and nearly equal amounts of overheads and bottoms are present, then favor design III. 5. Otherwise, favor design I or II, whichever removes the most plentiful component first. Regions of optimality ESI 1.6 1. If more than 50% is bottoms product, then favor design II. 2. If more than 50% is middle product and from 5 to 20% is bottoms, favor design V. 3. If more than 50% is middle product and less than 5% is bottom, favor design VI. 4. If more than 50% is middle product and less than 5% is overhead then favor design VII. 5. Otherwise, favor design III. 6. Thermally coupled designs I11 and IV should be considered as alternatives to designs I and 11, respectively, if less than half the feed is middle product. In addition, designs 111, IV, VI, and VII should be considered for separating all mixtures where a low middle product purity is acceptable. Sequencing of V-L Separation Systems 45 When simple distillation is not practical for all separators in a multicomponent mixture separation system, other types of separators must be employed and the order of volatility or other separation index may be different for each type. If they are all two-product separators and if T equals the number of different types, then the number of possible sequences is now given by: T Ns =T P −1 Ns Eq (2.2) For example, if P = 3, and ordinary distillation, extractive distillation with either solvent I or solvent II, and LL extraction with solvent III are to be considered, then T = 4, and applying Eqns (2.1) and (2.2) gives 32 possible sequences (for ordinary distillation alone, NS = 2).