054402 Design and Analysis II

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LECTURE 4: SEQUENCING OF SEPARATION TRAINS

Daniel R. Lewin

Department of Chemical Engineering

Technion, Haifa, Israel

Ref: Seider, Seader and Lewin (1999), Chapter 5

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

Assess Primitive Problem

Plant-wide Controllability Assessment

Development of Base-case

Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization

Steps in Process Design and Retrofit

Detailed Process Synthesis -Algorithmic Methods

SECTION B

Section B: Algorithmic Methods

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 those costs associated with the 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. However, more commonly, the feed mixture involves more than two components, involving more complex separation systems

Instructional Objectives

When you have finished studying this unit, 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.
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..

Example 1. Specification for 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Pentane withdrawn as bottoms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n-C4 withdrawn as distillate.

2-C4=s withdrawn as distillate.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

Design for Butenes Recovery System

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)
the introduction into the system of a second fluid phase (MSA – mass separating agent). This must be subsequently removed.
the addition of a solid phase upon which adsorption can occur
the placement of a membrane barrier

Common Industrial Separation Methods

Common Industrial Sep.Methods (Cont’d)

Selecting Separation Method (1)

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 methodlargely depends of 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.

(5.1)

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)

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:

(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, b , where:

(5.5)

For vapor-liquid separation operations that use an MSA that causes the formation of a non-ideal liquid solution (e.g. extractive distillation):

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.

Relative volatilities for equal cost separators

Ref: Souders (1964)

Sequencing of Ordinary Distillation 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.

Algorithm to Select Pressure and Condenser Type

(5.7)

Number of Sequences for Ordinary Distillation

Equation for number of different sequences of P 1 ordinary distillation (OD) columns, NS, to produce P products:

Example 2 – Sequences for 4-component separation

Identifying the Best Sequences 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 is not excessive.

Class Exercise

Design a sequence of ordinary distillation columns to meet the given specifications.

Guided by Heuristic 4, the first column in position to separate the key components with the greatest SF.

Class Exercise – Possible Solution

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

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:

(5.8)

Sequencing of V-L Separation Systems

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.

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 (5.7) and (5.8) gives 32 possible sequences (for ordinary distillation alone, NS = 2).

Example 3 (Example 1 Revisited)

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.

Example 3 (Example 1 Revisited)

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 method 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, CA

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 FUG method (e.g., using HYSYS.Plant “Shortcut Column”).
Select tray spacing (typically 2 ft.) and calculate column height, H.
Compute tower diameter, D (using Fair correlation for flooding velocity, or HYSYS Tray Sizing Utility).
Estimate installed cost of tower (see Unit 6 and Chapter 9).
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

(A/B…)I, (…E/F)I, (…B/C…)I, (A/C…)I , (…C/B…)II, and (…C/D…)II

1st Branch of Sequences

2nd Branch of Sequences

3rd Branch of Sequences

4th Branch of Sequences

Lowest Cost Sequence

Next week: Azeotropic Distillation

Separation Trains - Summary

On completing this unit, 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.
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..

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