low energy distillation schems

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Low Energy Distillation Schemes

G.T.PolleyPinchtechnology.com

Having looked at the thermodynamics of individual separations (Jan. 2002 and Feb.2002) we now turn our attention to how the more energy efficient schemes thatrequire more than one separation can be identified. We start by considering non-integrated systems.

Multi-Component Systems: Non-integrated Distillation Schemes

Consider the use of a distillation scheme for the separation of a four componentmixture into its constituent parts. The number of ways in which this can beaccomplished is shown in Figure 1. Each scheme requires three distillation columns.The terms in brackets indicate the components in the bottom product of eachdistillation column.

There are five possible schemes. It can be seen that these schemes consist ofdifferent combinations of nine individual separations.

Similar trees can be developed for other problem sizes.

The size of various problems are listed in Table 1.

These trees only have to be developed once for each problem size. They can thenbe used in the solution of any problem having that size.

Size Number ofSepns

Number ofPossibleSchemes

Number ofPossibleComponents

3 2 2 4 4 3 5 9 5 4 14 20 6 5 42 35 7 6 132 56

Table 1. Effect of Problem Size on Complexity

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Figure 1

The best non-integrated scheme can be identified by evaluating the cost of eachpossible separation and determining the overall cost of each scheme by summingcosts along each individual branch of the tree.

Given the speed and power of modern computers and the availability of shortcutdesign methods for distillation columns, such a procedure would not be excessivelytime consuming.

Thermodynamic Analysis: Heat Load Table

The information coming from the analysis of the possible separations includes valuesfor the dew and bubble point temperatures for both the reboilers and the condensers(i.e. the temperature spans over which heat is demanded and made available) andthe reboiler and condenser heat loads. This information can be displayed, reboilersside by side with condensers in a simple table (see Table 2).

If the information is listed in order of temperature, we see where heat from acondenser can be used to drive a reboiler. A load on the right hand side of the tableappears above one on the left hand side of the table.

We have a tool for determining the scope for energy saving through internalintegration of the columns. For instance, in the example shown in Table 3, we seethat only the reboiler used for the A/B split is at a low enough temperature to beintegrated with overheads from another column (opportunities associated withcolumn pressure manipulation will be considered later). The maximum heat savingfrom internal integration is 3.2 MW.

Figure 1. Schemes for Separating Four Components

ABCD

A[BCD]

AB[CD]

ABC[D]

B[CD]

BC[D]

C[D]

B[D]

A[B]

C[D]

A[BC]

AB[C]

B[D]

A[B]


Integration of Isobaric Multi-Component Systems

The derivation of a table for each separation scheme would be tedious and timeconsuming. What if we developed a table containing all of the possible separations.The result may be that given in Table 3.

Now, our objective is to identify opportunities for using condensers to drive reboilers.So, working down the table, the part that is of particular interest is that between thefirst appearance of a condenser load (on the right hand side of the table) and the lastappearance of reboiler load (on the left hand side).

There are two problems to be overcome before we can use this information. The first,is that not all condenser/reboiler matches are feasible. The second, is that we haven’tidentified opportunities for using one condenser load to drive more than one reboiler.

The first of these problems can be overcome by drawing up a list of mutual exclusivematches and a list of possible combinations. Like the separation trees, such lists onlyhave to be drawn up once (see Rajah & Polley, 1995). For problem involving up tofive components the lists presented in Table 4 apply. We can use these lists incombination with the Heat Load Table to identify feasible matches.

The second problem, that of identify multiple integration opportunities, can be solvedby using ‘cumulative heat loads’ rather than single operation heat loads. Forinstance, consider the reboiler on the B/C split, we see from the combination list thatsuch a split can occur in a scheme that also uses an A/BC split lower down thetemperature scale. The B/C reboiler has a load of 24.3 MW and the A/BC reboilerhas a load of 5.3 MW. This gives a cumulative load of 29.6 MW. A suitablecondenser placed higher on the temperature scale and having a heat load in excessof this value could be used to drive either or both of these reboilers.

Reboilers

Cond’rs

Split t - dew t - bub Load Split t – dew t – bub LoadABCD/E 106 106 63.3C/D 97.6 97.6 12.3AB/CD 75.5 81.2 27.2

ABCD/E 60.9 72.3 62.3C/D 62.7 62.7 12.6

A/B 49.9 49.9 3.2AB/CD 37.5 37.5 26.7A/B 12.4 12.4 3.2

Table 2. Simple Heat Load Table

Condensers can be treated in a similar manner. For instance, a condenser on aC/DE split could appear in the same scheme as one on a D/E split (which appearshigher in the temperature scale). So, the cumulative load available at thistemperature is the sum of these two loads.


Building these ‘cumulative’ loads into the heat load table the Cumulative Heat LoadTable is developed (Table 5).

Reboilers Cond’rsSplit t – dew t – bub Load Spli t – dew t – bub LoadABCD/E 106 106 63.3BCD/E 106 106 63.5CD/E 106 106 63.8D/E 106 106 63.5ABC/DE 103 103 21.5BC/DE 103 103 20.5C/DE 103 103 17.5ABC/D 97.6 97.6 16.1C/D 97.6 97.6 12.6

D/E 97.6 97.6 63.4AB/CDE 87.2 30.6B/CDE 87.2 30.5A/BCDE 79.4 7.6AB/CD 75.5 27.2B/CD 75.5 27.2

CD/E 75.5 81.2 63.5A/BCD 67.9 6.3

BCD/E 67.9 75.1 62.9AB/C 62.7 24.3B/C 62.7 24.3

C/DE 62.7 62.7 17.5C/D 62.7 62.7 12.6ABCD/E 60.9 72.3 62.3

A/BC 57.5 58.3 5.3BC/DE 57.5 58.3 19.7BC/D 57.5 58.3 16.1ABC/D 50.1 54.9 16.5ABC/DE 50.1 54.9 20.2

A/B 49.9 49.9 3.2B/CDE 49.9 49.9 30.1B/CD 49.9 49.9 26.9B/C 49.9 49.9 24.3AB/CDE 37.5 43.0 20.2AB/CD 37.5 43.0 26.7AB/C 12.4 12.4 24.2A/BCDE 12.4 12.4 7.6A/BCD 12.4 12.4 6.3A/BC 12.4 12.4 5.3A/B 12.4 12.4 3.2

Table 3. Heat Load Table Containing all Possible Components

Now let’s examine this table in more detail. The D/E condenser operates at thehighest temperature. So, we start by examining how this unit can be integrated.

Immediately below this condenser is the AB/CDE reboiler. The D/E split can occur inthe same scheme as the AB/CDE split. So, the match is feasible. We see from thecumulative load that the A/B reboiler can be included in the scheme.


Moving down the table, the next reboiler encountered is on the B/CDE spli. Again thisis a feasible match. The cumulative load indicates that are no other suitable reboilersexist in a scheme using these two splits.

The next reboiler encountered is that on the A/BCDE split. The cumulative loadinformation shows two options for a second integration (a B/CD or a B/C split).

Moving yet further down the table we encounter the AB/CD, B/CD and A/BCDreboilers. None of these splits can be present in a scheme having a D/E split.

Finally, we see opportunities to link the D/E condenser with either the AB/C reboileror the B/C reboiler.

Having identified schemes involving the D/E condenser, we can move to that havingthe next highest temperature. This is the one on the CD/E split. When we examinethe reboilers positioned below this unit we don not finad ant reboilers that can appearin the same scheme.

Moving on to the condenser on the BCD/E split. We find only the reboiler on the B/csplit to be a feasible candidate.

We also find that the other condensers present little opportunity for integration.

In summary, examination of the Table shows the following integration opportunities:1. Condenser D/E with reboilers on AB/CDE (30.6) and A/B (3.2) saving 33.8 MW2. Condenser D/E with reboiler on B/CDE saving 30.5 MW3. Condenser D/E with reboilers on A/BCDE (7.6) and/or B/C (24.3) with dual

integration yielding a saving of 31.9 MW4. Condenser BCD/E with B/C (24.3) and A/BC (5.3) saving 29.6 MW

Rather than examine fourteen possible schemes (the non-integrated case) we onlyneed examine four.

Procedure

In summary, the procedure developed above is:

1. Identify the possible schemes2. Analyse each possible individual separation3. Develop the Heat Load Table containing all options4. Identify thermal integration options5. Evaluate identified integrated schemes

General Observations

Consider the order in which the condenser and reboilers appear in there respectivelists. The letters used to designate the components are chosen in order ofcomponent volatility. A being the lightest component. E the heaviest. Since, it is thedew and bubble point temperatures which are important, this order can be expectedto be the same for all problems.

The divisions between the lists could change. For instance, the CD/E condenser inone problem could be at higher temperature than the B/CD reboiler. However,moving upwards, this condenser cannot occur in any scheme below the A/BCDEsplit.


In the case of the BCD/E condenser, this cannot be used in any additional schemebelow the B/CD reboiler.

The example being considered is that set by Heaven (1969) and used in manypapers on separation scheme synthesis. The details of the separation are given inTable 6. Examination of this table shows that streams D and E make up more thanhalf of the initial feed stock. It is not surprising that the D/E split should have such ahigh heat load. As the make-up of the feed shifts towards the lighter components theheat available from the D/E condenser falls and importance of other units willincrease.

Bringing these observations together, we can draw the following conclusions:

• the schemes identified for this problem are ‘general’• as the feed composition shifts towards the lighter components the

condenser on the BCD/E split becomes more important• another scheme that could possibly be important is one in which the

relative positions of the BCD/E condenser and the B/CD reboiler allowsintegration

So, for isobaric schemes, the designer does not need to evaluate all of the possiblecomponent separations and produce the Heat Load Table! The designer can start bydetermining the loads on the D/E and BCD/E condensers. If that on the BCD/Econdenser is the highest, the scheme involving that condenser should be evaluated.Opportunities of linking with B/CD reboiler should also be examined. If the load onthe D/E condenser is the highest, the schemes identified above should be evaluated.

Non-isobaric Schemes

The scope for integration, and hence energy needs, can be changed by operatingindividual separations at different pressures. Unfortunately, in addition to changingthe temperature levels of individual condensers and reboilers, changes in pressurealso result in changes in column heat loads. Andrecovich & Westerberg [1985] foundthat both column heat duty and the difference between reboiler and condensertemperatures increased with increasing pressure. Both quantities were found toexhibit approximately linear changes with respect to temperature level. Given thisfinding we do not need to re-evaluate the individual separations over a range ofpressures. We can determine loads and temperatures at the extremities ofpermissible pressure range and then use interpolation.

However, we do not to do this for each and every possible separation. We canidentify the significant potential changes using the Heat Load Table.

Pressure changes are only justified if they result in energy savings that are greaterthan those identified for isobaric operation. The first key point in the isobaric picture isthe temperature of the D/E condenser. We can improve upon the recovery from thisstream if we move a ‘valid’ reboiler below this level or shift the temperature of thiscondenser upwards.


Split Excluded Splits Valid Combinations

A/BA/BC A/BB/C A/B A/BCAB/C A/BC, B/C A/BA/BCD A/B, A/BC, AB/C B/CB/CD A/B, A/BC, AB/C, B/C A/BCDAB/CD A/BC, AB/C, B/C, A/BCD,

B/CDA/B

A/BCDE A/B, A/BC, A/BCD, AB/CD B/C, B/CDB/CDE A/B, A/BC, AB/C, B/C,

B/CD, AB/CDA/BCDE

AB/CDE A/BC, AB/C, B/C, A/BCD,B/CD, AB/CD, A/BCDE,B/CDE

A/B

C/D A/BC, AB/C, B/C A/B, A/BCD, B/CD,AB/CD, A/BCDE,B/CDE, AB/CDE

BC/D A/B, A/BC, AB/C, B/CD,AB/CD, B/CDE, AB/CDE,C/D

A/BCD, A/BCDE, B/C

ABC/D A/BCD, B/CD, AB/CD,A/BCDE, B/CDE, AB/CDE,C/D, BC/D

A/B, A/BC, AB/C,B/C

C/DE AB/C, A/BCD, B/CD,AB/CD, C/D, BC/D, ABC/D

A/B, A/BC,AB/C,A/BCDE,B/CDE,AB/CDE

BC/DE A/B, A/BC, AB/C, A/BCD,B/CD, AB/CD, B/CDE,AB/CDE, C/D, BC/D,ABC/D, C/DE

B/C, A/BCDE

ABC/DE A/BCD, B/CD, AB/CD,A/BCDE, B/CDE, AB/CDE,C/D, BC/D, ABC/D, C/DE,BC/DE

A/B, A/BC, AB/C, B/C

D/E A/BCD, B/CD, AB/CD,C/D, BC/D, ABC/D

A/B, A/BC, B/C,AB/C,A/BCDE, B/CDE, AB/CDE,C/DE, BC/DE, ABC/DE

CD/E A/BCD, B/CD, AB/CD,C/D, BC/D, ABC/D, C/DE,BC/DE

A/B, A/BC, B/C, AB/C,A/BCDE, B/CDE, AB/CDE

BCD/E A/B, A/BC, AB/C, A/BCD,AB/CD, B/CDE, AB/CDE,ABC/D, C/DE, BC/DE,ABC/DE, D/E,CD/E

B/C, B/CD, A/BCDE, C/D,BC/D

ABCD/E A/BCDE, B/CDE, AB/CDE,C/DE, BC/DE, ABC/DE,D/E, CD/E, BCD/E

A/B, A/BC, B/C, AB/C,A/BCD, B/CD, AB/CD,C/D, BC/D, ABC/D

Table 4. Valid Component Combinations


Split T – Bub Load C.Load C.Load Load T – dew Split63.4 63.4 97.6 D/E

AB/CDE 87.2 30.6 33.8 a/bB/CDE 87.2 30.5 30.5A/BCDE 79.4 7.6 34.8

b/cd31.9 b/c

AB/CD 75.5 27.2 30.5 a/bB/CD 75.5 27.2 33.5

a/bcd63.5 63.5 81.2 CD/E

A/BCD 67.9 6.3 30.6 b/c62.9 62.9 75.1 BCD/E

AB/C 62.7 24.3 27.5 a/bB/C 62.7 24.3 29.6

a/bc62.3 62.3 72.3 ABCD/E75.5cd/e

12.6 62.7 C/D

80.9 d/e 17.5 62.7 C/DEA/BC 57.5 5.3 5.3

Table 5. Cumulative Heat Load Table

Component Mole Fraction Molal Flow kmol/hr

A: Propane 0.05 45.36

B: iso-Butane 0.15 136.08

C: Butane 0.25 226.80

D: iso-Pentane 0.20 181.46

E: Pentane 0.35 317.52

Total 1.00 907.20

Table 6. Heaven’s Problem

Examination of the reboilers positioned above this point indicate that changes of theD/E condenser temperature relative to any of the following would be advantageous:ABC/DE, BC/DE, C/DE (with the C/DE option showing the largest savings).

Examination of the Heat Load Table indicate that modifications relative to the CD/Eand BCD/E condensers have no advantage over the D/E condenser and largerpressure changes would be required in order to bring these units into contention.These changes need not be considered.


Movement of the C/DE condenser relative to the D/E reboiler would be necessary toimprove savings above the isobaric systems. These savings would not exceed thoseachieved by manipulating the D/E condenser position. Very much higher pressurechanges would be required. This change does not need to be considered.

Similar analysis can be applied to other condensers in the table with similar results.

Analysis of pressure manipulation is only justified for the following separations:D/E with respect to ABC/DED/E with respect to BC/DED/E with respect to C/DE.

Integrating via feed vaporisers, intermediate condensers and reboilers

These options should only be evaluated after integration via overhead condenserslinked to base reboilers has been examined.

Advantages to be gained from cooling of column feed are generally secondary tothose to be gained through integrating phase changes. So, identification of theseshould be undertaken once the scheme has been identified.

In process synthesis we should always seek to reduce the size of a problem beforegoing into the detail of a problem. So, is there anything we can do before we start toexamine the thermodynamic profiles of individual separations?

Well, there is no point in using an intermediate condenser below the feed plate (i.e. ata higher temperature than the column feed) or an intermediate reboiler above thefeed plate. So, we could start by comparing overhead condenser with feedtemperatures.

This is done in Tables 7.

Examination of this table shows the opportunities that could be opened up by feedvaporisation and the use of intermediate reboilers. Using the table we can identifywhich thermodynamic profiles are worth examining.

We see that the new opportunities are:

* use of D/E condenser on the C/DE separation* use of D/E condenser to vaporise the ABCDE feed* use of D/E condenser to vaporise the BCDE feed

The engineer is therefore directed to examine the thermodynamic implications ofusing feed vaporisation on these streams. The thermodynamics of the followingseparations can then be examined:

• A/BCDE• AB/CDE• ABC/DE• B/CDE• BC/DE


This examination will show the scope for the use of both feed vaporisation and theuse of intermediate reboilers.

Possibilities of using intermediate condensers can be determined in a similarmanner. For instance, we see from Table 7 that heat from a D/E separation can beprovided over the range 97.6 (condenser temperature) to 103 (feed temperature). So,by examining a table which compares feed and reboiler temperatures we can identifywhich separations should be subjected to thermodynamic analysis.

Feed Stream T feed Condenser T dew

DE 103D/E 97.6

CDE 87.2ABCDE 80BCDE 79.4CD 75.5

CD/E 75.5BCD 67.9

BCD/E 67.9C/DE 62.7C/D 62.7

ABCD 60.9ABCD/E 60.9

BC 57.5 BC/DE 57.5BC/D 57.5

ABC 50.1 ABC/D 50.1ABC/DE 50.1B/CDE 49.9B/CD 49.9B/C 49.9

AB 37.5

Table 7. Comparison of feed and condenser temperatures

References

Rajah W. & Polley G.T. ‘Synthesis of Practical Distillation Schemes’, Trans.I.Chem.E. 1995, 73A,953-966

Heaven D.L. M.S. Thesis, University of California, Berkeley

Andrecovich M.J. & Westerberg A.W. ‘A simple synthesis method based on utilitybounding for heat integrated distillation sequences’, AIChEJ, 1985,31(3), 363-375

low energy distillation schems

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