Sequencing Distillation Columns CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University Lecture No. 5 – Sequencing Ordinary Distillation Columns September 18, 2012 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel
Lecture 5 – Objectives • Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near-optimal sequence. • Be able to apply systematic methods to determine an optimal sequence of distillation-type separations.
Sequencing OD Columns • 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.
(8.9) Number of Sequences for OD • Number of different sequences of P –1 ordinary distillation (OD) columns, NS, to produce P products:
Best Sequence using Heuristics • 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 should not be excessive.
Class Exercise Design a sequence of ordinary distillation columns to meet the given specifications.
Exercise – Possible Solution Guided by Heuristic 4, the first column in position to separate the key components with the greatest SF. • Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of other components.
Exercise – Possible Solution a = 1.5 a = 3.6 a = 2.8 a = 1.35
Complex Columns • 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): • ESI = AB/ BC • ESI 1.6 • ESI 1.6
(A) Sequencing V-L Separation • 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: • 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, T = 4, and applying Eqns (8.9) and (A) gives 32 possible sequences (for ordinary distillation alone, NS = 2).
Example: Butenes Recovery Propane Butane Butene Pentane • For T = 2 (OD and ED), and P = 4, NS = 40. • However, since 1-Butene must also be separated (why?), P = 5, and NS = 224. • Clearly, it would be helpful to reduce the number of sequences that need to be analyzed. • Need to eliminate infeasible separations, and enforce OD for separations with acceptable volatilities. 1-Butene and 2-Butene are structurally very different, whereas, the optical isomers are much closer related and are difficult to separate by distillation
Example: Butenes Recovery • Splits A/B and E/F should be by OD only ( 2.5) • Split C/D is infeasible by OD ( = 1.03). Split B/C is feasible, but an alternative may be more attractive. • Use of 96% furfural as a solvent for ED increases volatilities of paraffins to olefins, causing a reversal in volatility between 1-Butene and n-Butane, altering separation order to ACBDEF, and giving C/B = 1.17. Also, split (C/D)II with = 1.7, should be used instead of OD. • Thus, splits to be considered, with all others forbidden, are: (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Set distillate and bottoms column pressures using Estimating Annualized Cost • For each separation, CA is estimated assuming 99 mol % recovery of light key in distillate and 99 mol % recovery of heavy key in bottoms. The following steps are followed: • Estimate number of stages and reflux ratio by WUG method (e.g., using Aspen Plus “DSTWU Column”) • Select tray spacing (typically 2 ft.) and calculate column height, H • Compute tower diameter, D (using Fair correlation for flooding velocity, or Aspen Plus Tray Sizing Utility) • Estimate installed cost of tower (e.g. Peters & Timmerhaus) • Size and cost ancillary equipment (condenser, reboiler, reflux drum). Sum total capital investment, CTCI • Compute annual cost of heating and cooling utilities (COS) • Compute CA assuming ROI (typically r = 0.2). CA = COS + r *CTCI
Butenes Recovery – 1st Branch (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Butenes Recovery – 2nd Branch (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Butenes Recovery – 3rd Branch (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Butenes Recovery – 4th Branch (A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II
Example: Butenes Recovery • Lowest Cost Sequence
Summary – Sequencing On completion of this part, you should: • Understand how distillation columns are sequenced and how to apply heuristics to narrow the search for a near-optimal sequence. • Be able to apply systematic B&B methods to determine an optimal sequence of distillation-type separations.
Other Business • Homework • SSLW: 8.1, 8.2, 8.3 • Due Tuesday September 25 • Next Lecture – September 25 • Review of Non-Ideal Thermodynamics (SSLW 223-230)