RESIDUE CURVE MAPPING - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/22098/14/14_chapter 5.p… · compositions. Triangular diagrams arc often used to describe the equilibrium

()pltnuzallOll o/ Desqpi & ~a11c>n in Reactuie Dlsl!llallon 65

CHAPfER V RESIDUE C URVE MAPPING

RESIDUE CURVE MAPPING

Application11 of residue curve mapping to RD processes

have recently been reported, but n generalized and

systematic approach is still missing for the case of reactive

feeds outside the conventional composition ranges. This

chapter is addressing these design aspects. The reaction

invariant space, defined in terms of transformed

composition variables, is divided into sub-regions

characterized by separation boundaries. A reasibihty

analysis of the RD process i11 performed based on the

location of the reacting mixture and init ial separation

sequences are generated according to the feed

transformed-composition. The practical conclusion or this

chapter is that by using RCM technique one can virtually

generate RD trains over the entire possible feed

composition space.

()pclmlZOllOll of Des>gn & 0ptra11on 111 ReaCllue Vi.s!dlallon 66

'5.0. INTRODUCTION

The art of process design involves finding process configurations,

operating conditions and ,.;zc of equipment that will allow an economical, safe

and environmental responsible operation, only by specifying the stale of the

feeds and the targets on the output streams of a system (Almeida-Rivera et

al., 2004a; Doherty and Buzad, 1992; Buzad and Doherty, 1995). This task is

not trivial and requires the development of specialized design tools (e.g.

computational methods, algorithms and procedures). At early stages of the

design activity (e.g. conceptual phase) the chemical process designer faces a

lru-gcly influencing question: is the processing route feasible? The impact of

preliminary design and feasibility analysis on the fate of the final plant is

enormous. Surprisingly, it is a common practice to allocate a relative small

fraction (< 2-3%) of total budget for the conceptual phase. Due to the coarse

level of detail, the design tools in the conceptual phase should be powerful

enough to screen feasible design alternatives within a huge design space

5.1. INPUT-OUTPUT INFORMATION FLOW

ln the case of the feasibility analysis stage , the n:quired input

information involves the process basis of design. In other words, feedstock and

product purities and operational boundaries should be defined. Additionally,

sustainability and safety constraints should be included as inputs. The

output information comprises the determination of distillation boundaries,

(non-) reactive azeotropic mixtures and feasible product compositions. The

domain knowledge at this design space belongs to the field of thermodynamics,

kinetics and overall mass balances. An extended overview of input and output

Information at this design level is given in table 5. 1.

The generation of separation sequences is composed of process

requirements related to the mode of operation, which is in turn determined

using operational skills. The temporal mode of operation mode is chosen based

on operational expertise in RD processing. For the sake of analysis

Optimizauon of Design & Operation m Reactwe Distillation 67

simplification and without restricting the validity of the approach to other

temporal modes, a

Table 5.1 • Input-output information ror the feasibility analysis phase.

Design specifications

PROCESS . - set of components - set of chemical reactions - feed composition - operational pressure PRODUCT - compositions target (s) - SHE constraints

Des.gn/operational variables

DISCRETE • distillation boundaries - (non-) reactive azeotropes CONTINOUS : - product feasible composit1ons

Domain knowledge

- thermodynamics - kineOcs - overall balances

Continuous operation is chosen. The output information at this design s pace

includes the type of task required for the design specifications (i.e. separation

and/ or reaction), the type of column internal (i.e. tray or packing), the number

of units and their connectivity. The relevant domain knowledge at this design

space corresponds to operational thumb-ruled skills and basic component

ba lances. An extended overview of input and output information a t this design

level is given in table 5.2.

Table 5.2 · Input-output information for the column sequwncing phase.

Design specifications Design/operational variables Domain knowledge

PROCESS : DISCRETE : • operational skill - mode of opeartlon • type of task • component balance

(separatK>n and I or reactive) - number of columns - columns connectrvity

CONTINOUS - feed ratio of reactant streams

Making each stage of the design approach opera tional requires the use of (novel)

design tools. The Residue Curve Mapping technique (RCM) is found to be of

particular effectiveness in the case of RD feasibility analysis and generation of

column sequencing.

5.2. RESIDUE CURVE MAPPING T ECHNIQUE

When an entertainer is added to a binary mixture, a ternary mixture

results, and it is then necessary to consider the phase equilibria for this new

mixture as well as to predict the ranges of possible overhead and bottoms

., ....... ,,,n & n...rnuon in H~c1d11H' O.sr./lallon 68 OplltNZGllOrl o, ~.., "I~

compositions. Triangular diagrams arc often used to describe the equilibrium

relationships for ternary mixtures. Especially important are the residue curves

on the diagrams. A residue curve represents the liquid residue composition

with time ns the result of a simple, one stage batch distillation. The results,

when plotted on a triangular graph, are called a residue curve because the

plot follows the liquid residue composition in the still. Different residue lines

result from different starting compositions. A collection or these curve.- for a

given ternary system is called a residue curve map.

A residue curve map has the following characteristics:

1. If we assign the direction of the residue curves as being from the starting

composition to the ending composition, then the arrow on each curve points

from a lower boiling component or azeotropc to a higher boiling compont:nt or

azeotrope.

2. The presence of azeotropes can create distillation boundaries which cannot

be crossed by a residue curve. These distillation boundaries represent the

residue curve on which the light or starting residue composition is a lower

boiling pure component or a7.eotrope and the heavy or ending composition

residue is a higher pure component or azeotrope. Any given pure component

point or azeotrope will be connected to some but not all other pure

component points and azcotropes on the graph. Those that are connected

form distillation

boundaries. These boundaries arc thermodynamic in nature.

3 These distillation boundaries partition the map into distillation regions.

The nature of these regions is such that two pure components which lie in

different regions cannot be separated using conventional distillation.

Some definitions are in order before we explain how residue curve maps apply

to enhanced distillation systems.

Node Residue curves begin and end at nodes.

Stable node The componenl or azcotropcs with the highest boiling point in

the region. ALI the residue curves in the region point to (terminate) at this

point.

Optimization of Design & Operalion in Reactiue Distillation 69

Uns table node The component or azeotropes with the lowest boiling point in

the region.

Saddle Residue curves move toward and then away from saddles. Pure

components and azeotropes which have a boiling point between the stable and

unstable nodes are saddles.

Vapor line The vapor line is the line formed by those vapor compositions

which are in equ ilibrium with the liquid compositions on the distillation

boundaries. In other words, if you take all the compositions on a distillation

boundary line as saturated liquid compositions; then determine the

composition of the vapors which are in equilibrium with all these points; and

plotted all these vapor comp ositions on the residue curve map, one line would

be form ed for each distillation boundary. That line is called the vapor line.

Vapor boil up curve The vapor boil up curve is the plot of the vapor

compositions in equilibrium with any given residue curve.

Binodal plot This is a constant temperature plot of all saturated liquid

compositions. It is used to identify the liquid-liquid region of the mixture.

Liquid boiling envelope This is the constant pressure binodal like plot of the

two liquid phase region of a ternary system.

In the residue map shown below,the residue curves are represented by the

light black curves,

The distillation boundaries are represented by the heavy curves,

The triangles indicate the azeotropes, and

The two phase regions (at constant pressure and at constant temperature)

are outlined by th e liquid boiling envelope and the binodal plot as indicated.

For this system you will notice that;

The binary azeotropes are all saddles. As a result the residue lines tend to

move toward them at first and then at some point they turn away from them

:o the stable node.

The ternary azeotrope, being a minimum boiling azeotrop e, is an unstable

:iode (in this case the only one in the system). This means that in any simple

::listillation, this azeotrope will always come out the top of the column.

Optimization of Design & Operation in Reactive Distillation 70

The pure components are all stable nodes. This means that in any simple

distillation they will always come out the bottom of the column. Which

component comes out the bottom depends on which region we are operating

in.

The liquid boiling envelope and the binodal plot are not the same. The

condenser will generally operate at fixed temperatures so it is a nalyzed using

the binodal plot. The top tray(s) of the column will b e at the column pressure,

so it must be analyzed using the liquidboiling envelope.

Please note the following properties of a residue curve map:

l.Based on experimental evidence, for ternary mixtures with very few

exceptions, there are at most three binary azeotropes and restrictions that

a pply to a ternary system

where, N1

N 1 + 81 = 3 N2 + 82 = B < 3 NJ + 8J = 1 or 0

the number of pure single component nodes.

N2 the number of binary nodes.

NJ the number of ternary nodes.

51 the number of pure component saddles.

52 the number of binary saddles.

5J the number of ternary saddles.

And 2N3-283 + 2N2-B+ 18T, = 2

2. For homogeneous azeotropes the nodes may be either s table (maximum

boiling) or unstable (minimum boiling). However, h eterogeneous azeotropes

can be either unstable n odes or saddles but not stable nodes (i.e. they cannot

be maximum boiling).

3. The liquid temperature always increases along residue curves in the

heterogeneous region.This property, coupled with fact that all singular points

on heterogeneous residue curve m aps are restricted to be either nodes or

saddles, means that the entire topological

methodology for analyzing and constructing homogeneous residue curve

maps from boiling temperature data alone extends verbatim to h e terogeneous

Op1tmtZC1t10n of Destgn & Operatton in React111c Dlst.Uation 71

nuxtures.

5.3. CONSTRUCTING A RESIDUE CURVE MAP

To help in understanding residue curve maps, you can use lhe

following procedure to construct a qualitatively accurate map by hand:

Step 1: Lab<'I the ternary diagram, (numcs and boiling points) with the low

boiling component at the top vertex; the high boiling component at the lower

right vertex; and the intermediate at the lower left vertex.Plot composit ions of

all o.zcotrope:; and label the points with their boiling points. This determines

the value of 8.

Step 2: Draw arrows on the edges of the triangle in the direction of increasing

temperature for each pair of adjacent species (components and/or

azeotropes).

Step 3: Determine the type of point (node or saddle) that each pure

component vertex is by using the arrows drawn in Step 2. This determines N 1

and Sl.

Step 4: (for ternary azeotropes, if present):

Determine lhe type of point of the ternary azcotrope (if one exists). The pomt is

a node if;

a) Nl + B < 4, and/or

b) Excluding the pure component saddles, the ternary azeotrope has the

highest, second highest, lowest, or second lowest boiling point of all the

species.

Otherwise, the point is a saddle. This determines N3 and S3.

Step 5: (for ternary saddle only, if prescntl:

Connect the ternary saddle, by straight lines, to all binary azcotropes and lo

all pure component nodes. Draw arrows on the lines in the direction of

increasing temperatures. Using these arrows determine the type of point of

each binary azcotropes. This determines N2 and S2. If N 1 + B • 6, special

checks should be made. If not, the sketch is complete.

.. ....

()pl1ml%llbon of Design & Operation in RMCfl"" °'511/lation 73

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Fig. 5 .1 Residue curve map

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Fig. 5.2 E.~rimenlal Residue curve map

5.4 . DISTILLATION CURVE MAPS

An alternative representation for distillation on a ternary diagram is a

distillation curve for continuous, rather than batch, distillation. The curve is

most readily determined for total renux. The sequence of liquid-phase

compositions, which corresponds to the operating line al total reflux, is

,, ~~n (\ '"'-rnhort '" RNICll\ .. D1$1tllOIJCm 74 ()JJtllOl.7..0rJon OJ ~-v ...,,__

ft tn' n~gular diaarom. Distillation curve maps can be arbitranly plotted on ~ -· .,.,. directed to increasmg or decreasing temperatures. In the former case, they

closely approximate residue curve maps.

S .4 . 1. PRODUCT COMPOSIT ION R EGIONS

Residue-curve maps and distillation-curve maps are used to ml:lkc

preliminary estimales of regions of feasible product compositions for

distillation of non-ideal ternary mixtures. The product regions are determined

by superimposing a column matenat balance line on the curve map diagram.

If a straight line is drawn that connects distillate and bottoms

compositions, that line must pass through the feed composition at some

intermediate point to satisfy overall and component material balances. For

such a material balance line, the distillate and bot toms composition s must

lie on the same distillation {residue) curve. Because of this, the feasible

product region can be established like so:

i. Find the limiling distillate composition point for the region. Draw a line

from this point, through the feed composition, to the opposite side of the

map. This point represents the bottoms composition w1th the lowest amount

of low boiler possible for the limiting distillate composition. Call this material

balance line M 1.

ii. Find the limiting bottoms composition point for the region. Draw a line

from this point, through the feed composition, to the opposite side of the

map. This point represents the distillate composition with the lowest amount

of high boiler possible for the limiting bolloms composition. Call this material

balance line M2.

iii. Locate and draw the distillate curve which contaln11 th e recd composition.

Coll this curve DP

iv. The areas on the convex side of DF, and lying between M 1 ruid DF and

between M2 and DP", are the feasible product regions.

For azeotropic systems, where disWlation boundaries arc present, a feasible

product region can be found for each distillation region.

Optimtzalion of /)eS'!}n & Operation on Reactive V.Slil1Wio11 75

5 .5 . C ONCLUDING REMARKS

Residue curve maps have shown to provide valuable insights and design

assistance for nonideal systems, particularly for reactive distillation.

Transforming the composition variables according to Doherty's approach allows

to define a reaction invariant space of lower dimension, formed by attainable

product compositions and where the conventional concepts for residue curves

can be applied.