Heat ingeration of distillation columns.pdf

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HEAT INTEGRATION OF

DISTILLATION COLUMNS

By Siti Shawalliah Idris, AMIChemE

CPE 633 PROCESS ENGINEERING II



Heat Integration Characteristics of Distillation

The dominant heating and cooling duties associated with a distillationcolumn are the reboiler and condenser

The sensible heat duties usually will be small in comparison with the latentheat changes in reboilers and condensers

Both the reboiling and condensing process normally take place over arange of temperature

Practical considerations usually dictate that the heat to the reboiler mustbe supplied at temperature above the dew point of the vapour leaving

the reboiler and that heat removed in the condenser must be removed ata temperature lower than the bubble point of the liquid

In preliminary design, both reboiling and condensing can be assumed totake place at constant temperature



Appropriate Placement of Distillation Column

Process T-H diagram NOT including distillation column

Total hot: QHmin + Qreb; Total cold: QCmin + Qcond




(Cont’d)

Distillation above the pinch

Total hot: QHmin + Qreb → QHmin + Qreb − Qcond;

Total cold: QCmin + Qcond → QCmin




(Cont’d)

Distillation below the pinch

Total hot: QHmin + Qreb → QHmin;

Total cold: QCmin + Qcond → Qcmin+ Qcond - Qreb



Appropriate Placement of Distillation Column (Cont’d)

The appropriate placement for separators is not across the pinch

If both the reboiler and condenser are integrated with the process, this canmake the column difficult to start up and control

Above the pinch, the reboiler can be serviced directly from the hot utilitywith the condenser integrated above the pinch. In this case, the overallutility consumption will be the same as that shown in previous figure

Below the pinch, the condenser can be serviced directly by the cold utilitywith the reboiler integrated below the pinch. In this case, the overall utility

consumption will be the same as that shown in previous figure



The Use of Grand Composite Curve for Integration

Given that the dominant heating and cooling duties associatedwith the distillation column are the reboiler and condenser duties

A convenient representation of the column is therefor a simplebox representing the reboiler and condenser loads

This box can be matched with the grand composition representingthe remainder of the process

The grand composite curve would inside all heating and coolingduties for the process, including those associated with separatorfeed and product heating and cooling but excluding reboiler andcondenser loads



Example:

The distillation column reboiler and

condenser duties are shown

separately and are matched

against it

The reboiler and condenser duties

are on opposite sides of the heat

recovery pinch and the columndoes not fit

The Use of Grand Composite Curve for Integration



The Use of GCC for Integration (Cont’d)

Although the reboiler andcondenser duties are both abovethe pinch, the heat duties prevent

a fit

Part of the duties can beaccommodated, and if heatintegrated that would be asaving, but less than the fullreboiler and condenser duties




A column appropriately placedabove the pinch

The reboiler duty can be suppliedby utility

The condenser duty must be

integrated with the rest of theprocess




A column appropriately placedbelow the pinch

The reboiler duty can be supplied

by integration with the process

Part of the condenser duty must beintegrated, while the remainder of

the condenser duty can be rejectedto cold utility




If a distillation column is inappropriately placed across the pinch, it may bepossible to change its pressure to achieve appropriate placement

If an inappropriately placed distillation column is shifted above the heat recoverypinch by changing its pressure, the condensing stream (a hot stream) is shifted frombelow to above the pinch. The reboiling stream (a cold stream) stays above thepinch.

If an inappropriately placed distillation column is shifted below the heat recoverypinch by changing its pressure, The reboiling stream (a cold stream) is shifted fromabove to below the pinch. The condensing stream (a hot stream) stays below thepinch.

The appropriate placement is a particular case of shifting streams across thepinch, which in turn is a particular case of the plus minus principle




Note: The relative volatility will also be affected,generally decreasing with increasing pressure

Note: Changes in pressure also affect heating andcooling duties for column feed and product

⇒ Both the height and the width of the box will change as

the pressure changes



Evolving Design of Simple Distillation

Columns to Improve Heat Integration

Now

Establish heat loads and temperature levels for such

modifications as:

Pressure

Reflux ratios

Feed preheating/cooling

Inter reboiling/condensing

And identify the best combined options.




Columns to Improve Heat Integration(Cont’d)

Column Across Pinch





Change Pressure





Change Distillation Reflux Ratio

RR Q

RR Q




Columns to Improve Heat Integration

Trade-off for stand alone column

Optimum RR/RRmin Around 1.1





If the column is inappropriateplaced with the process, thenincrease in reflux ratio cause acorresponding overall increase inenergy, and the trade-off rules

apply

If, however, the column isappropriately placed, then thereflux ratio can often be increased

without changing the overallenergy





Increasing heat flow through columndecreases requirement for distillationstages but increases vapor rate

In designs initialized by traditionalrules of thumb, this would have effect

of decreasing capital cost of column

Consequently, the optimum reflux ratiofor an appropriately integrateddistillation column will be problemspecific and is likely to be quite

different from that of a stand-alonecolumn operated from utilities

(Be prepared for unusual reflux ratios.)





Control

This Fit BUT This is easier

to control





Control – Conclusion

In principle, it is enough to integrate the reboiler OR the

condenser.



Double Effect

Stand – Alone Column Optimisation Double Effect

P

The classical application of

double-effect distillation is to

choose the relative pressures

of the columns such that the heat

from the condenserof the high-pressure column can

be used to provide the

reboiler heat to the low-pressure

column.



In isolation, we have saved energy



E l i D i f Si l Di till ti





Do not link the two column thermally

P

P

E l i D i f Si l Di till ti





Vapour Compression Distillation Column











For heat pumping to be economic on a stand-alone basis, it must operateacross a small temperature difference, which for distillation means closeboiling mixtures.

The use of the scheme is only going to make sense if the column is

constrained to operate either on a stand-alone basis or at a pressure thatwould mean it would be across the pinch.

Otherwise, heat integration with the process might be a much betteroption.

Vapor recompression schemes for distillation therefore only make sense forthe distillation of close boiling mixtures in constrained situations.





Summary

Columns must be considered and optimised in theprocess context

Appropriate placement of column is a fundamental

principle

Boxes used to represent the column against the grand

composite curve gives a simple pictur of opportunities.



Heat Integration of Distillation

Working Session





Data for the Process Grand Composite Curve

T* (oC) H (MW)

300 16

280 22

180 5

130 0

80 10

40 13

30 15

W k Sh f ( )



Working Sheet for part (i)

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

T * ( o

C )

Enthalpy (MW)

GCC (

Tmin = 10oC)

W ki Sh f P (ii)



Working Sheet for Part (ii)

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

T * ( o

C )

Enthalpy (MW)

GCC (

Tmin = 10oC)

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