chemical engineering progress (cep), design and operation

CEP November 2020 aiche.org/cep 39

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Glenn Graham ■ Don Bunning, P.E. ■ Jeremy Rader, P.E. ■ MATRIC

Despite great advances in process simulation, the need remains for real-life distillation testing. This practical guide gives a broad overview of how to set up experimental distillation projects.

Design and Operation Considerations for Distillation Experiments

Distillation is the most common separation technique

used in the chemical industry. Most of the world’s

commodity chemicals and nearly all liquid fuels are

produced by processes that include distillation steps. The

HI¿FLHQF\�DQG�HIIHFWLYHQHVV�RI�WKHVH�GLVWLOODWLRQ�VWHSV�LV�NH\�WR�WKH�RYHUDOO�HFRQRPLFV�RI�WKHVH�SURFHVVHV� Engineers use process simulation to model separations

in order to conceptualize, design, and optimize distillation

SURFHVVHV��+RZHYHU��LW�LV�RIWHQ�DOVR�QHFHVVDU\�WR�FRQGXFW�experiments with laboratory- or pilot-scale distillation col-

umns. Experiments may be required to demonstrate proof-

RI�FRQFHSW�IRU�D�VHSDUDWLRQ��YHULI\�FRPSXWHU�VLPXODWLRQV��address issues that cannot be predicted by simulations, or

generate samples.

� 6HYHUDO�W\SHV�RI�GLVWLOODWLRQ�FROXPQV�DUH�FRPPRQO\�XVHG�LQ�VXFK�H[SHULPHQWDO�ZRUN��EXW�WKH�VDPH�JHQHUDO�SULQ-

ciples apply and they all require some of the same types of

HTXLSPHQW��VXFK�DV�UHERLOHUV��FRQGHQVHUV��DQG�OLTXLG�YDSRU�contactors. The concepts presented here apply to most types

of experimental distillation columns.

� 7KLV�DUWLFOH�SURYLGHV�D�SUDFWLFDO�JXLGH�DQG�EURDG�RYHU-YLHZ�RI�KRZ�WR�VHW�XS�DQG�UXQ�H[SHULPHQWDO�GLVWLOODWLRQ�SURMHFWV��,W�GLVFXVVHV�WKH�YDULRXV�DVSHFWV�RI�ODERUDWRU\��DQG�pilot-scale distillation experiments, including equipment

VHOHFWLRQ�DQG�RSHUDWLRQ��%DWFK�DQG�FRQYHQWLRQDO�FRQWLQXRXV�FROXPQV�WKDW�RSHUDWH�XQGHU�YDFXXP�DUH�XVHG�DV�D�EDVLV�IRU�the discussion.

Why are experiments necessary? Distillation experiments may be necessary at any point

in the lifecycle of a process, from conceptualization to

ongoing operation. After a new separation has been pro-

posed, and possibly simulated by computer, demonstrating

it experimentally may be desirable. If adequate physical

SURSHUW\�GDWD�DUH�QRW�DYDLODEOH��FRPSXWHU�VLPXODWLRQ�PD\�not be possible, and experiments become the only means of

demonstrating the separation. In other cases, samples may

QHHG�WR�EH�SURGXFHG�E\�WKH�QHZO\�HQYLVLRQHG�SURFHVV�SHU�D�customer’s request.

In some processes, large numbers of compounds are

Reprinted with permission from Chemical Engineering Progress (CEP), 1RYHPEHU�� &RS\ULJKW��$PHULFDQ�,QVWLWXWH�RI�&KHPLFDO�(QJLQHHUV��$,&K(��

40 aiche.org/cep November 2020 CEP

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SUHVHQW��PDQ\�RI�WKHP�LGHQWL¿HG�RQO\�DV�D�UHWHQWLRQ�WLPH�RQ�D�JDV�FKURPDWRJUDSK��*&��VFDQ��&RPSXWHU�VLPXODWLRQ�RI�WKHVH�FRPSRXQGV�LV�GLI¿FXOW�RU�LPSRVVLEOH��7KH�PDLQ�FRPSRQHQWV�DQG�WKHLU�SK\VLFDO�SURSHUWLHV�PD\�EH�NQRZQ��and reliable computer simulation of the main separations

PD\�EH�SRVVLEOH��+RZHYHU��XQNQRZQ�FRPSRXQGV��ZKLFK�FDQQRW�EH�VLPXODWHG��PD\�EH�SUHVHQW�LQ�UHDO�IHHGVWRFNV��(YHQ�LI�FRQ¿GHQFH�LV�KLJK�LQ�WKH�SURFHVV�VLPXODWLRQ��H[SHULPHQWDO�YDOLGDWLRQ�RI�WKH�FRPSXWHU�PRGHO�PD\�EH�QHFHVVDU\�EHIRUH�proceeding with costly construction (1, 2). Phenomena such as foaming and fouling cannot be

VLPXODWHG��OHDYLQJ�H[SHULPHQWV�DV�WKH�RQO\�PHDQV�RI�GHWHU-PLQLQJ�WKHLU�SRWHQWLDO�VLJQL¿FDQFH��$OVR��LPSXULW\�JHQHUD-tion, which cannot generally be simulated, may occur in the

column, especially in the reboiler due to high temperatures

and/or long residence times. If the impurities can be identi-

¿HG��H[SHULPHQWDO�ZRUN�PD\�EH�QHFHVVDU\�WR�GHWHUPLQH�their rate of generation.

� (YHQ�IRU�D�SDUWLFXODU�GLVWLOODWLRQ�FROXPQ�WKDW�KDV�EHHQ�RSHUDWLQJ�IRU�VHYHUDO�\HDUV��VLWXDWLRQV�PD\�DULVH�WKDW�QHFHV-VLWDWH�H[SHULPHQWDO�ZRUN��$�QHZ�LPSXULW\�PD\�DSSHDU�DQG�LWV�VRXUFH�QHHGV�WR�EH�LGHQWL¿HG��,QFUHDVHV�WR�FROXPQ�FDSDFLW\�RU�QHZ�SURGXFW�TXDOLW\�VSHFL¿FDWLRQV�PLJKW�EH�GHVLUHG��UHTXLU-LQJ�H[SHULPHQWDWLRQ�WR�YHULI\�D�VXJJHVWHG�PRGL¿FDWLRQ�

Batch vs. continuous� &HUWDLQ�KLJK�OHYHO�FRQVLGHUDWLRQV�DUH�RIWHQ�DGGUHVVHG�HDUO\�LQ�WKH�GHYHORSPHQW�RI�DQ�H[SHULPHQWDO�GHVLJQ�EHIRUH�DOO�RI�WKH�GHWDLOV�DUH�VROLGL¿HG��6XFK�KLJK�OHYHO�FRQVLGHU-DWLRQV�LQFOXGH�EDWFK�YV��FRQWLQXRXV�RSHUDWLRQ��VFDOH��W\SH�RI�feed, safety precautions, and the performance metrics that

ZLOO�EH�XVHG�WR�HYDOXDWH�WKH�H[SHULPHQW�� 2QH�RI�WKH�¿UVW�FRQVLGHUDWLRQV�IRU�DQ�H[SHULPHQWDO�project is whether to use a batch or a continuous column

�)LJXUH��)RU�D�QHZ�SURFHVV��D�EDWFK�FROXPQ�PD\�EH�XVHG�

initially before progressing to a more complex continuous

FROXPQ��(YHQ�LI�WKH�HQYLVLRQHG�FRPPHUFLDO�SURFHVV�LV�FRQ-

WLQXRXV��H[SHULPHQWDO�EDWFK�GDWD�PD\�EH�VXI¿FLHQW�� An experimental batch setup is usually simpler than an

experimental continuous setup. A continuous column has the

additional requirements of feed and tails systems, as well as

a means of controlling its operation at steady-state condi-

WLRQV��,W�LV�PRUH�FRPPRQ�WR�KDYH�D�EDWFK�FROXPQ�DYDLODEOH�in a laboratory or pilot plant, and if a new installation is

UHTXLUHG��D�EDWFK�FROXPQ�ZLOO�EH�IDVWHU�DQG�OHVV�H[SHQVLYH�WR�set up than a continuous column.

In addition, experimental time requirements tend to be

on the order of days for a batch column, whereas it may

WDNH�ZHHNV�WR�H[HFXWH�DQ�H[SHULPHQW�ZLWK�D�FRQWLQXRXV�column. This time difference is due to two main distinc-

WLRQV�EHWZHHQ�WKH�FROXPQ�W\SHV��)LUVW��D�EDWFK�FROXPQ�LV�DEOH�WR�FRYHU�D�ZLGH�UDQJH�RI�FRPSRVLWLRQ�SUR¿OHV�GXULQJ�an experiment. Second, a continuous column requires a

VLJQL¿FDQW�DPRXQW�RI�WLPH�WR�UHDFK�VWHDG\�VWDWH�IRU�HDFK�VHW�of operating conditions.

� 6HYHUDO�LPSRUWDQW�DVSHFWV�RI�D�VHSDUDWLRQ�FDQ�EH�GHWHU-mined with a batch column, such as:

� ��VHSDUDWLRQ�GLI¿FXOW\�IRU�WKH�PDLQ�FRPSRQHQWV� ��WKH�PRVW�GHVLUDEOH�YDOXHV�WR�XVH�IRU�UHÀX[�UDWLR�DQG�pressure

� ��KRZ�WKH�PLQRU�LPSXULWLHV�VHSDUDWH�UHODWLYH�WR�WKH� main components

• whether impurities are generated at distillation

conditions

• whether foaming or fouling is a concern.

It may also be possible, in some cases, to generate

customer samples from a batch distillation column. If the

FRPPHUFLDO�FROXPQ�LV�DQWLFLSDWHG�WR�EH�EDWFK��WKHQ�REYL-ously an experimental batch column is appropriate. Batch

distillation is commonly used for small operations in the

pharmaceuticals or specialty chemicals industries, whereas

the commodity chemicals and fuels sectors generally use

continuous columns.

Sometimes a batch computer simulation program can be

used to model the batch separations. If the results from the

simulations agree well with the experimental batch data, this

DJUHHPHQW�SURYLGHV�FRQ¿GHQFH�WKDW�WKH�SK\VLFDO�SURSHUW\�predictions in the simulation model are accurate and the

H[SHULPHQWDO�GDWD�DUH�YDOLG��,I�WKH�HQJLQHHU�LV�FRQ¿GHQW�LQ�the simulation results, the system can then be modeled as a

continuous distillation, which may negate the need for an

experimental continuous distillation setup.

� +RZHYHU��LI�D�QHZ�FRQWLQXRXV�SURFHVV�LV�GHWHUPLQHG�WR�EH�YLDEOH�EDVHG�RQ�LQLWLDO�EDWFK�VWXGLHV��WKH�XVXDO�QH[W�VWHS�LQ�an experimental program is to operate a continuous column

to obtain experimental data that would better represent the

proposed commercial column.

(a) (b)Condenser

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S Figure 1. One of the first considerations in designing a distillation experiment is whether to use a batch (a) or a continuous (b) column.


A continuous laboratory- or pilot-scale column is able to:

� ��YDOLGDWH�FRPSXWHU�VLPXODWLRQV • test control schemes

• generate scale-up data

• produce customer samples.

Often, continuous separation schemes require two or

PRUH�FROXPQV��)RU�H[DPSOH��WKH�¿UVW�FROXPQ�PD\�UHPRYH�light components and the second column may generate the

product as a distillate stream. In these situations, one physi-

cal experimental continuous column can sometimes be used

for the entire separation scheme by processing the mate-

ULDO�LQ�WZR�SDVVHV��,W�LV�LPSRUWDQW�WKDW�WKH�FROXPQ�KDYH�WKH�correct pressure capability, number of stages, feed location,

reboiler heat source temperature, and condenser coolant

temperature for both passes. If these requirements cannot be

PHW��RU�LI�UHF\FOH�VWUHDPV�DUH�LQYROYHG��LW�PD\�EH�QHFHVVDU\�to use two physical columns in series.

Sampling considerations may differ between batch and

FRQWLQXRXV�FROXPQV��)RU�EDWFK�GLVWLOODWLRQ�FROXPQV��GLVWLOODWH�VDPSOHV�DUH�JHQHUDOO\�WDNHQ�HLWKHU�DW�UHJXODU�WLPH�LQWHUYDOV�RU�EDVHG�RQ�VSHFL¿HG�KHDG�WHPSHUDWXUHV�RU�TXDQWLWLHV�RI�GLVWLOODWH�FROOHFWHG��$�¿QDO�NHWWOH�VDPSOH�ZLOO�XVXDOO\�DOVR�EH�FROOHFWHG��DOWKRXJK�LQWHUPHGLDWH�NHWWOH�VDPSOHV�PD\�DOVR�EH�WDNHQ��)RU�D�FRQWLQXRXV�FROXPQ��VHYHUDO�VHWV�RI�VDPSOHV�RI�WKH�GLVWLOODWH�DQG�WDLOV�ZLOO�XVXDOO\�EH�WDNHQ�DW�VHW�WLPH�LQWHU-YDOV�WR�GHWHUPLQH�ZKHQ�WKH�FROXPQ�KDV�UHDFKHG�VWHDG\�VWDWH�

Scale of equipment The scale of the equipment required for the experiments

will depend on the purpose of the experiments. If the pur-

pose is to collect scale-up data, there will be a minimum col-

XPQ�GLDPHWHU��(DFK�W\SH�RI�YDSRU�OLTXLG�FRQWDFWLQJ�GHYLFH�will require a different minimum diameter. )RU�H[DPSOH��UDQGRP�SDFNLQJ�UHTXLUHV�D�PLQLPXP�FROXPQ�GLDPHWHU�WKDW�LV�EDVHG�RQ�WKH�VL]H�RI�WKH�SDFNLQJ�HOHPHQW��7KH�PLQLPXP�FROXPQ�GLDPHWHU�IRU�VWUXFWXUHG�SDFNLQJ�GHSHQGV�ODUJHO\�RQ�the crimp size of the corrugated sheets.

� ,I�PDUNHWLQJ�RU�FXVWRPHU�VDPSOHV�DUH�WR�EH�JHQHUDWHG��WKH�size of the required samples may dictate the scale. The deci-

sion regarding equipment size, in this case, will be based on

a trade-off between labor and capital costs, depending on the

quantity of sample material that must be produced.

� )RU�QHZ�SURFHVVHV��RQO\�D�OLPLWHG�DPRXQW�RI�DXWKHQWLF�IHHG�PDWHULDO�PD\�EH�DYDLODEOH�IRU�GLVWLOODWLRQ�H[SHULPHQWV��and the scale of the experimental column can be dictated by

DYDLODEOH�IHHG��,Q�RWKHU�FDVHV��HTXLSPHQW�RI�D�FHUWDLQ�VL]H�PD\�DOUHDG\�H[LVW��(LWKHU�RI�WKHVH�VFHQDULRV�PD\�LQÀXHQFH�the decision regarding the size of the distillation column to

be used.

� 2IWHQ��PP�GLD��FROXPQV�DUH�XVHG�IRU�JHQHUDO��SXUSRVH�ODERUDWRU\�WHVWLQJ��)LJXUH��+RZHYHU��ZDOO�HIIHFWV��e.g., OLTXLG�FKDQQHOLQJ�DW�WKH�ZDOO��KDYH�D�ODUJHU�HIIHFW�RQ�VPDOOHU�

diameter columns. Therefore, it is

FRPPRQ�WR�XVH��PP�GLD��FROXPQV�or larger to generate more-realistic

scale-up data.

Synthetic or real feed� )RU�LQLWLDO�GLVWLOODWLRQ�H[SHUL-ments, synthetic feed material is

often used so that the separation of

the main components can be studied

without any interference from

PLQRU�LPSXULWLHV��+RZHYHU��XVLQJ�DXWKHQWLF�IHHG�ZLOO�SURYLGH�D�PRUH�realistic representation of the process and should be studied

at a later point in the experimental program.

The impurities in the authentic feed may require addi-

tional analytical equipment or techniques. They may also

LQÀXHQFH�KDQGOLQJ�RU�FDXVH�RSHUDWLRQDO�FRQFHUQV��DQG�WKH\�ZLOO�OLNHO\�FRPSOLFDWH�FRPSXWHU�PRGHOLQJ�HIIRUWV�

Safety concerns� 6DIHW\�FRQVLGHUDWLRQV�FDQ�KDYH�D�ODUJH�LPSDFW�RQ�WKH�equipment design and the location in which the equipment

LV�LQVWDOOHG��$QG�WKH\�FDQ�LQÀXHQFH�WKH�QHHG�IRU�DX[LOLDU\�equipment.

� :KHQ�ZRUNLQJ�ZLWK�JODVV�FROXPQV�WKDW�KDYH�YDFXXP�MDFNHWV�RU�WKDW�ZLOO�EH�RSHUDWLQJ�XQGHU�YDFXXP��SODVWLF�QHW-ting or other means of shielding must be used to mitigate

the danger of an implosion. Pressurized distillations are

normally conducted in metal equipment rated for the design

FRQGLWLRQV��)RU�SUHVVXUL]HG�FROXPQV��FDUH�PXVW�EH�WDNHQ�WR�DYRLG�H[SRVXUH�WR�UDSLG�GLVFKDUJHV�IURP�YDOYHV��HVSHFLDOO\�GXULQJ�VDPSOLQJ��RU�IURP�¿WWLQJ�OHDNV� Certain operating conditions will necessitate further

safety precautions:

• High-temperature operations will require burn protec-

tion in the form of insulation or shielding.

� ��(PHUJHQF\�VKXWGRZQ��(6'��SURWHFWLRQ�PD\�EH�required, especially for unattended operation.

� ��9DFXXP�FROXPQV�PD\�UHTXLUH�OHDN�WHVWLQJ�SULRU�WR�HDFK�H[SHULPHQW��YDFXXP�OHDNV�PD\�EH�KD]DUGRXV�LI�R[\JHQ�reacts with the process chemicals.

• Reactions with the potential to run away and generate

H[FHVVLYH�KHDW�RU�JDVHV�ZLOO�QHHG�SURSHU�PLWLJDWLRQ�V\VWHPV� • Hazardous chemicals, or noxious chemicals with

GLVDJUHHDEOH�RGRUV��UHTXLUH�WKH�XVH�RI�D�YHQWLODWHG�HQFORVXUH�

X Figure 2. For general-purpose laboratory testing, a column with a diameter of 25 mm is often used, such as this distillation column with a vacuum jacket.


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for the safe operation of equipment.

� ��)ODPPDEOH�FKHPLFDOV�UHTXLUH�VSHFLDO�SUHFDXWLRQV�WR�SURWHFW�DJDLQVW�OHDNV�DQG�DGGUHVV�RWKHU�IDLOXUH�VFHQDULRV��$LU�PRQLWRULQJ�RU�YHQW�VFUXEEHUV�PD\�EH�QHFHVVDU\�IRU�ZRUN�ZLWK�KD]DUGRXV�RU�ÀDPPDEOH�FKHPLFDOV�

Performance metrics When designing the equipment and setting the conditions

IRU�GLVWLOODWLRQ�H[SHULPHQWV��FDUHIXO�DWWHQWLRQ�PXVW�EH�JLYHQ�WR�WKH�PHWULFV�WKDW�ZLOO�EH�XVHG�WR�HYDOXDWH�WKH�H[SHULPHQWV��These metrics might include:

� ��DELOLW\�WR�SURGXFH�UH¿QHG�PDWHULDO�WKDW�PHHWV�SURGXFW�VSHFL¿FDWLRQV� ��VHSDUDWLRQ�HI¿FLHQF\��QXPEHU�RI�VWDJHV� • column pressure drop and capacity

� ��SUDFWLFDO�RU�FRVW�HIIHFWLYH�FROXPQ�KHLJKW�DQG�GLDPHWHU��as well as heat duty

� ��KHDY\�RU�OLJKW�LPSXULW\�JHQHUDWLRQ • foaming or fouling problems.

Process simulation to aid in design After the major considerations are addressed, many

design details will need to be determined, especially for a

continuous column, which include:

• number of stages

� ��IHHG�ORFDWLRQ��IRU�FRQWLQXRXV�FROXPQV� • thermocouple locations

• sample point locations

� ��YDFXXP�SUHVVXUH�UHTXLUHPHQWV� ��ÀRZUDWHV • heat duties

• condenser and reboiler temperatures.

� 3URFHVV�VLPXODWLRQV�FDQ�EH�YHU\�XVHIXO�LQ�GHWHUPLQ-

LQJ�UHDVRQDEOH�LQLWLDO�YDOXHV�IRU�WKHVH�GHVLJQ�SDUDPHWHUV��Results of these simulations can also be used to determine

column diameter and height, as well as the sizes of feed

WDQNV��SURGXFW�UHFHLYHUV��IHHG�SXPSV��DQG�WKH�NHWWOH�UHERLOHU��6LPXODWLRQV�FDQ�SURYLGH�D�¿UVW�HVWLPDWH�RI�RSWLPXP�RSHUDW-LQJ�FRQGLWLRQV��*RRG�VLPXODWLRQV�FDQ�QDUURZ�WKH�UDQJH�RI�experimental conditions and reduce the number of experi-

ments that are required.

Pressure and temperature considerations Pressure and temperature need to be considered jointly,

VLQFH�WKHLU�HIIHFWV�DUH�UHODWHG��+RZHYHU��HDFK�KDV�LWV�RZQ�impact. The column should operate at atmospheric pressure,

if possible. Atmospheric pressure operation eliminates the

QHHG�IRU�SUHVVXUH�FRQWUROV��SURYLGHV�PRUH�RSWLRQV�IRU�WKH�HTXLSPHQW¶V�PDWHULDOV�RI�FRQVWUXFWLRQ��DQG�VLPSOL¿HV�RSHUD-tion of the column.

� )RU�FRPSRXQGV�ZLWK�KLJK�ERLOLQJ�WHPSHUDWXUHV��YDFXXP�operation may be required to reduce the column or reboiler

WHPSHUDWXUHV��([FHVVLYHO\�KLJK�WHPSHUDWXUHV�PD\�FDXVH� degradation reactions and/or generate impurities. Tempera-

ture limitations on column joints, sealants, and associated

WXELQJ�PD\�DOVR�QHFHVVLWDWH�YDFXXP�RSHUDWLRQ��,Q�VRPH�cases, limitations on the maximum temperature of the heat-

LQJ�PHGLXP��LI�\RX�DUH�QRW�XVLQJ�DQ�HOHFWULF�KHDWHU��ZLOO�LQÀXHQFH�WKH�SUHVVXUH�VHOHFWLRQ�� &RQYHUVHO\��IRU�FKHPLFDOV�ZLWK�ORZHU�ERLOLQJ�WHPSHUD-WXUHV��RSHUDWLRQ�RI�D�GLVWLOODWLRQ�FROXPQ�DERYH�DWPRVSKHULF�SUHVVXUH�PD\�EH�QHFHVVDU\�WR�DYRLG�WKH�QHHG�IRU�UHIULJHUD-tion in the condenser.

� ,I�WKH�H[SHULPHQW�LQYROYHV�UHDFWLYH�GLVWLOODWLRQ��KLJKHU�pressure may be necessary to raise the operating tempera-

ture enough to be in the correct range for the reaction.

+LJKHU�SUHVVXUH�DOVR�LPSURYHV�WKH�FROXPQ�FDSDFLW\��URXJKO\�E\�WKH�VTXDUH�URRW�RI�WKH�SUHVVXUH�LQFUHDVH��$�GLVDGYDQWDJH�RI�RSHUDWLQJ�DERYH�DWPRVSKHULF�SUHVVXUH�LV�WKDW�GRLQJ�VR�usually eliminates glass as a possible material of construc-

tion for the distillation column; the experiment, therefore,

ORVHV�WKH�WZR�PDLQ�DGYDQWDJHV�RI�JODVV�²�HDV\�YLVXDO�REVHUYDWLRQ�DQG�FKHPLFDO�LQHUWQHVV��,Q�DGGLWLRQ��FROXPQV�that operate at higher pressures may need to be housed in a

special location to allow safe operation.

� 9DFXXP�RSHUDWLRQ�KDV�WKH�SRWHQWLDO�WR�LPSURYH�WKH�YDSRU�OLTXLG�HTXLOLEULXP��9/(��FXUYH��2IWHQ��WKH�9/(�FXUYH�ZLGHQV�DW�UHGXFHG�SUHVVXUH��ZKLFK�LQFUHDVHV�WKH�UHODWLYH�YRODWLOLW\�DQG�LPSURYHV�WKH�VHSDUDWLRQ��)RU�VRPH�ELQDU\�V\V-WHPV��DQ�D]HRWURSH�FDQ�EH�HOLPLQDWHG��RU�VRPHWLPHV�PRYHG��which allows for the use of pressure swing distillation as a

ZRUNDURXQG�IRU�D]HRWURSHV�

S Figure 3. Glass columns with Oldershaw trays (4) are commonly used in laboratories. Here, an unjacketed 4-in.-dia. column is shown (a) without and (b) with liquid and vapor tra!ic. These column sections were custom-made with a smaller number of holes for high liquid-to-vapor ratio and with side ports for a thermocouple or sample apparatus. Glass allows visual observation of the tray activity (froth), foaming, and fouling issues.

(a) (b)


Column internals� 7KUHH�VWDQGDUG�RSWLRQV�DUH�DYDLODEOH�IRU�WKH�YDSRU�OLTXLG�contactors in a distillation column (3)��WUD\V��UDQGRP�SDFN-

LQJ��DQG�VWUXFWXUHG�SDFNLQJ��$�IXOO�GLVFXVVLRQ�RI�HDFK�FROXPQ�LQWHUQDO�W\SH�LV�EH\RQG�WKH�VFRSH�RI�WKLV�DUWLFOH��+RZHYHU��we will discuss some of the major considerations for each of

these options.

� *ODVV�FROXPQV�ZLWK�2OGHUVKDZ�WUD\V�(4) are commonly

XVHG�LQ�ODERUDWRULHV�DQG�SLORW�SODQWV��)LJXUH��7KHVH�DUH�QRUPDOO\�DYDLODEOH�LQ��PP��PP��PP��DQG��PP�diameters, as well as other sizes by special order. Tray

HI¿FLHQFLHV�DUH�LQ�WKH�UDQJH�RI��±��EXW�PD\�EH�RXWVLGH�of this range depending on the chemical system. The

HI¿FLHQF\�LV�XVXDOO\�VOLJKWO\�OHVV�WKDQ�WKDW�RI�D�SODQW�VFDOH�tray for the same operating conditions, so laboratory data

FROOHFWHG�ZLWK�WKHVH�FROXPQV�ZLOO�SURYLGH�D�FRQVHUYDWLYH�prediction for scaleup (5). � 7UD\HG�FROXPQ�VHFWLRQV�DUH�XVXDOO\�YDFXXP�MDFNHWHG�WR�UHGXFH�FRQGXFWLYH�DQG�FRQYHFWLYH�KHDW�ORVVHV�DQG�VLOYHUHG��i.e., VLOYHU�FRDWLQJ�LV�DSSOLHG�WR�WKH�LQVLGH�RI�WKH�YDFXXP�MDFNHW��WR�PLQLPL]H�UDGLDQW�KHDW�ORVVHV��7KH�FROXPQ�RIWHQ�KDV�WDSHUHG�JURXQG�JODVV�MRLQWV��PDNLQJ�LW�HDV\�WR�DVVHPEOH��The general inertness of glass allows for its use with most

FKHPLFDO�V\VWHPV��DQG�JODVV�SHUPLWV�HDV\�REVHUYDWLRQ�RI�WUD\�loading, foaming, fouling, and color formation.

� 5DQGRP�SDFNLQJ��)LJXUH��JHQHUDOO\�DOORZV�IRU�D�ORZHU�FROXPQ�SUHVVXUH�GURS�WKDQ�WUD\V��,W�LV�DYDLODEOH�LQ�D�YDULHW\�RI�PDWHULDOV�RI�FRQVWUXFWLRQ��LQFOXGLQJ�YDULRXV�W\SHV�RI�PHWDO��JODVV��FHUDPLF��DQG�SODVWLF��6RPH�SDFNLQJ�W\SHV�SUR-

YLGH�SURSHU�YDSRU�OLTXLG�FRQWDFWLQJ�LQ�FROXPQV�DV�VPDOO�DV��PP��*HQHUDOO\��WKH�FROXPQ�GLDPHWHU�PXVW�EH��±��WLPHV�WKH�VL]H�RI�WKH�SDFNLQJ�HOHPHQWV��DOWKRXJK�WKLV�UDWLR�FDQ�EH�DV�ORZ�DV�DERXW�VL[�IRU�FHUWDLQ�SDFNLQJ�W\SHV�� 5DQGRPO\�SDFNHG�FROXPQ�VHFWLRQV�FRQVLVW�RI�D�WXEH�RU�SLSH�ZLWK�D�SDFNLQJ�VXSSRUW�VFUHHQ�LQVWDOOHG�LQ�WKH�ERWWRP��ZKLFK�LV�XVXDOO\�D�FRQLFDO�VFUHHQ�IRU�VPDOOHU�GLDPHWHU�FROXPQV��/LTXLG�GLVWULEXWLRQ�DW�WKH�WRS�RI�WKH�SDFNLQJ�is important and is normally accomplished with a single

WXEH�ORFDWHG�LQ�WKH�FHQWHU�RI�WKH�SDFNLQJ�IRU�FROXPQV�XS�WR��±��PP�LQ�GLDPHWHU��$�GLVDGYDQWDJH�RI�UDQGRP�SDFNLQJ�LV�WKDW�WKH�VHSDUDWLRQ�HI¿FLHQF\�IRU�VPDOO�SDFNHG�FROXPQV�LV�PRUH�GLI¿FXOW�WR�VFDOH�XS�WKDQ�IRU�2OGHUVKDZ�WUD\V�� 6WUXFWXUHG�SDFNLQJ��)LJXUH��LV�DOVR�XVHG�IRU�ODERUDWRU\�FROXPQV��EXW�QRW�FRPPRQO\��7ZR�W\SHV�DUH�DYDLODEOH�� • corrugated wire mesh that is rolled to the diameter of

the column

� ��ÀDW�FRUUXJDWHG�VFUHHQV�RI�YDU\LQJ�ZLGWKV�WKDW�DUH�VWDFNHG�WRJHWKHU�WR�FUHDWH�HDFK�SDFNLQJ�HOHPHQW�� )ODW�FRUUXJDWHG�VFUHHQV�KDYH�D�ORZHU�SUHVVXUH�GURS�DQG�KLJKHU�FDSDFLW\�WKDQ�HLWKHU�UDQGRP�SDFNLQJ�RU�WUD\V�DQG�DUH�JHQHUDOO\�DYDLODEOH�IRU�FROXPQV�ZLWK�GLDPHWHUV�RI��±��PP��'HSHQGLQJ�RQ�WKH�FKHPLFDO�V\VWHP��WKH�GDWD�

S Figure 4. Random packing allows for a lower column pressure drop than columns with trays and is available in a variety of materials, including metal, glass, ceramic, and plastic.

W Figure 5. Structured packing is less commonly used in laboratory distillation columns than random packing. The packing on the left is made of corrugated wire mesh and is rolled up to the diameter of the column. The packing on the right is constructed of flat corrugated gauze sheets.


BACK TO BASICS

IURP�WHVWV�ZLWK�WKLV�W\SH�RI�VWUXFWXUHG�SDFNLQJ�VFDOH�ZHOO�� 6WUXFWXUHG�SDFNLQJ�LV�DYDLODEOH�LQ�YDULRXV�W\SHV�RI�PHWDO�DQG�RWKHU�PDWHULDOV�EXW�WHQGV�WR�EH�UHODWLYHO\�H[SHQVLYH��Proper liquid distribution is also required, as for random

SDFNLQJ��,W�LV�LPSRUWDQW�WKDW�WKH�GLDPHWHU�RI�WKH�SDFNLQJ�LV�sized correctly for the inside diameter of the column so that

LW�¿WV�SURSHUO\��$�SDFNLQJ�GLDPHWHU�WKDW�LV�WRR�VPDOO�FRXOG�allow liquid channeling down the wall, which could affect

the separation.

Reboiler and condenser� $�URXQG�ERWWRP�ÀDVN�ZLWK�D�KHDWLQJ�PDQWOH�LV�FRPPRQO\�used as the reboiler for laboratory columns with diameters of

��±��PP��$OWHUQDWLYH�UHERLOHU�W\SHV�PD\�EH�QHFHVVDU\�IRU�ODUJHU�FROXPQV�WR�SURYLGH�DGHTXDWH�KHDW�LQSXW��7KHVH�DOWHU-QDWLYH�UHERLOHUV�PD\�DOVR�EH�UHTXLUHG�LI�WKH�OLTXLG�UHVLGHQFH�WLPH�QHHGV�WR�EH�PLQLPL]HG�RU�LI�KLJK�UHERLOHU�VNLQ�WHPSHUD-WXUHV�DUH�D�FRQFHUQ��&RPPRQ�DOWHUQDWLYHV�LQFOXGH�SXPS�around loops with heat exchangers, thermosiphon reboilers,

DQG�ZLSHG�¿OP�HYDSRUDWRUV� Condensers for laboratory-scale columns are usually of

the updraft style and should be constructed with coils for

increased surface area, especially for chemical systems with

ORZ�ERLOLQJ�SRLQWV��8SGUDIW�FRQGHQVHUV�DYRLG�FRQGHQVDWH�VXEFRROLQJ��ZKLFK�FRXOG�DIIHFW�WKH�VHSDUDWLRQ�E\�PDNLQJ�

WKH�WUXH�UHÀX[�UDWLR�GLIIHUHQW�IURP�WKH�PHDVXUHG�UHÀX[�UDWLR��Water is commonly used as the coolant, but a chiller may be

required for compounds with low boiling points. A down-

draft condenser may be required if a decanter is needed for

D�FRQGHQVDWH�FRQVLVWLQJ�RI�WZR�OLTXLG�SKDVHV��e.g., aqueous

DQG�LQVROXEOH�RUJDQLF�SKDVHV��

Reflux control� ,Q�D�FRPPHUFLDO�FROXPQ��WKH�UHÀX[�UDWLR�LV�GHWHUPLQHG�E\�WKH�UDWLR�RI�WKH�UHÀX[�ÀRZUDWH�WR�WKH�GLVWLOODWH�ÀRZUDWH��+RZHYHU��LQ�D�VPDOO�JODVV�ODERUDWRU\�FROXPQ��WKH�PRVW�FRP-

PRQ�ZD\�WR�FRQWURO�WKH�UHÀX[�UDWLR�LV�E\�F\FOLFDOO\�GLUHFWLQJ�WKH�FRQGHQVDWH�EHWZHHQ�WKH�FROXPQ�UHÀX[�DQG�WKH�GLVWLOODWH�UHFHLYHU��7KH�UDWLR�RI�WLPH�DW�HDFK�SRVLWLRQ��UHÀX[�RU�GLVWLO-ODWH��LV�WKH�UHÀX[�UDWLR��7KH�DPRXQW�RI�WLPH�DW�HDFK�SRVLWLRQ�LV�FRPPRQO\�FRQWUROOHG�E\�D�OLTXLG�GLYLGLQJ�KHDG��/'+��HTXLSSHG�ZLWK�DQ�HOHFWURPDJQHW�DQG�WLPHU��)LJXUH��/'+V�DUH�JHQHUDOO\�DYDLODEOH�IRU��±��PP�FROXPQV��$Q�/'+�FDQ�DOVR�EH�XVHG�WR�FRQWURO�WKH�ÀRZUDWH�RI�WKH�VLGHVWUHDP�RI�a column with a sidestream line.

� )RU�PHWDO�FROXPQV�RU�ODUJHU�JODVV�FROXPQV��UHÀX[�FDQ�EH�FRQWUROOHG�E\�D�WKUHH�ZD\�YDOYH�FRQWUROOHG�E\�D�WLPHU��ZKLFK�IXQFWLRQV�XVLQJ�WKH�VDPH�FRQFHSW�DV�WKH�/'+��$OWHUQDWLYHO\��UHÀX[�FDQ�EH�FRQWUROOHG�E\�GXDO�SXPSV�DQG�ÀRZPHWHUV�RQ�WKH�UHÀX[�DQG�GLVWLOODWH�SURGXFW�OLQHV�

Heat losses Heat loss is a major concern for laboratory- and pilot-

VFDOH�FROXPQV��8QFRQWUROOHG�KHDW�ORVVHV�FDQ�FDXVH�VLJQL¿-

cant problems. Heat losses create internal condensation,

ZKLFK�FDXVHV�WKH�YDSRU�DQG�OLTXLG�ÀRZV�DQG�WKH�VHSDUDWLRQ�in the column to be different than what would be expected

EDVHG�RQ�WKH�PHDVXUHG�UHÀX[�UDWLR��,Q�DGGLWLRQ��KHDW�ORVVHV�FDXVH�WKH�FROXPQ�WR�EH�PRUH�KHDYLO\�ORDGHG�ZLWK�YDSRU�OLTXLG�WUDI¿F�DW�WKH�ERWWRP�WKDQ�DW�WKH�WRS��ZKLFK�QDUURZV�WKH�operating range of the column. In extreme cases, the column

PLJKW�EH�ÀRRGHG�DW�WKH�ERWWRP�DQG�UHODWLYHO\�GU\�DW�WKH�WRS�� 0XFK�RI�WKH�KHDW�ORVV�IRU�JODVV�YDFXXP�MDFNHWHG�FRO-umns occurs at the joints. Tall columns, or columns with

high process temperatures, are especially susceptible to

VLJQL¿FDQW�KHDW�ORVV��+HDW�WDSH�DQG�RU�LQVXODWLRQ�FDQ�EH� XVHG�WR�PLQLPL]H�KHDW�ORVVHV�DW�MRLQWV�DQG�QRQ�MDFNHWHG�column sections.

Heat tape and/or insulation can be an option for small

glass columns, but are often a requirement for larger glass

FROXPQV�DQG�IRU�DOO�PHWDO�FROXPQV��)RU�KHDW�ORVV�SURWHF-WLRQ��ZH�XVH�WKH�IROORZLQJ�V\VWHP��)LJXUH��(6): The

FROXPQ�LV�¿UVW�ZUDSSHG�ZLWK�D�OD\HU�RI�LQVXODWLRQ��WKHQ�ZLWK�KHDW�WDSH��DQG�WKHQ�D�VHFRQG�OD\HU�RI�LQVXODWLRQ�RYHU�WKH�heat tape. A thermocouple is inserted between the insula-

tion wraps, and another thermocouple is installed inside

the column to measure the process temperature. A process

Condensate Collection Cup

Vacuum Jacket

Cup Drain Tube

Swinger Hinge

Swinger

Electromagnet Contact Location

Distillate Tube

Reflux Tube

Tapered Joint (Connects to Condenser)

Rising Vapor Tube

Thermowell Port

Distillate Product Line

Rising Vapor Tube

Tapered Joint (Connects to Tray or

Packing Section)

S Figure 6. A liquid dividing head, shown here with a vacuum jacket, can be used to control the reflux ratio for laboratory distillation columns. The rising vapor passes through the dividing head and is condensed in a condenser located above the dividing head (not shown). The resulting liquid condensate runs back into the dividing head, where it is alternately channeled to the distillate tube and the reflux tube by the swinger (sometimes referred to as a swing arm, swing bucket, or tilting funnel). The position of the swinger is driven by an electromagnet that is controlled by a timer.


controller uses the difference between the temperatures

PHDVXUHG�E\�WKH�WZR�WKHUPRFRXSOHV�DV�WKH�SURFHVV�YDUL-DEOH�DQG�WKH�VHWSRLQW�LV�VHW�WR�]HUR��6HYHUDO�VHSDUDWH�KHDWLQJ�zones may be required, depending on the temperature pro-

¿OH�LQ�WKH�FROXPQ��7KLV�KHDW�ORVV�PLWLJDWLRQ�V\VWHP�DOORZV�the column to run adiabatically and eliminates internal

UHÀX[��,WV�PDLQ�GLVDGYDQWDJH�LV�WKDW�LW�LV�PRUH�FRPSOH[�WKDQ�D�YDFXXP�MDFNHW��2IWHQ��VLPSOHU�DSSOLFDWLRQV�RI�KHDW�WDSH�and/or insulation are adequate.

Process control and instrumentation� %DWFK�FROXPQV�XVXDOO\�UHTXLUH�RQO\�UHODWLYHO\�VLPSOH�FRQWUROV��2IWHQ��WKH�UHÀX[�UDWLR�DQG�KHDW�LQSXW�DUH�VHW�DW�¿[HG�YDOXHV��$�VSHFL¿F�WHPSHUDWXUH�LQ�WKH�KHDG��FROXPQ��or reboiler, or a real-time analysis, may be used to manu-

DOO\�FKDQJH�WKH�UHÀX[�UDWLR�RU�WDNH�D�GLVWLOODWH�FXW��)RU�VRPH�V\VWHPV��DQ�DXWRPDWLF�IHHGEDFN�ORRS��ZLWK�D�WHPSHUDWXUH�RU�FRPSRVLWLRQ�DQDO\VLV�DV�WKH�SURFHVV�YDULDEOH�DQG�WKH�UHÀX[�UDWLR�DV�WKH�FRQWURO�YDULDEOH��PD\�EH�XVHG�WR�RSWLPL]H�WKH�SURGXFW�SXULW\�YV��GLVWLOODWH�UDWH�� Continuous columns require more sophisticated control

schemes (7, 8), although for laboratory experiments, these

schemes are often implemented manually.

� ,I�WKH�FROXPQ�RSHUDWHV�XQGHU�YDFXXP�RU�DW�KLJK�SUHV-sures, then a pressure-control system will be required. The

VLPSOHVW�FRQWURO�V\VWHP�FRQVLVWV�RI�D�YDFXXP�JDXJH��YDFXXP�SXPS��DQG�EOHHG�YDOYH�WKDW�DOORZV�DLU�RU�QLWURJHQ�WR�EH�LQWUR-

duced at the inlet of the pump to control the pressure in the

FROXPQ�YHQW�OLQH��$Q�DXWRPDWLF�FRQWURO�ORRS�ZLWK�D�FRQWURO�YDOYH�LV�RIWHQ�XVHG�IRU�YDFXXP�FRQWURO�DQG�DOPRVW�DOZD\V�used for high-pressure control.

� ,W�LV�XVXDOO\�GHVLUDEOH�WR�KDYH�VHYHUDO�WHPSHUDWXUH�VHQ-

VRUV��QRUPDOO\�WKHUPRFRXSOHV��WKURXJKRXW�D�FRQWLQXRXV�FROXPQ��HVSHFLDOO\�LQ�ORFDWLRQV�ZKHUH�VLJQL¿FDQW�WHPSHUD-WXUH�FKDQJH�SHU�VWDJH�RFFXUV��+DYLQJ�WKHVH�WHPSHUDWXUH�PHDVXUHPHQW�SRLQWV�DYDLODEOH�LV�XVHIXO�IRU�SURFHVV�FRQWURO�as well as for troubleshooting and matching simulation and

experimental results.

If the column will be

operated unattended, then

some of the controls may

need to be automated

�)LJXUH��,Q�DGGLWLRQ��ESD controls may be

necessary for unattended operation, and might include auto-

matic shutdowns based on temperature, pressure, column

GLIIHUHQWLDO�SUHVVXUH��'3��OLTXLG�OHYHO��RU�ÀRZUDWH�

Analytical considerations� :KHQ�GHYHORSLQJ�DQ�H[SHULPHQWDO�SURJUDP��LW�LV�LPSRU-tant not only to arrange for the proper analytical support, but

to also plan for the column sampling schedule. Sometimes

VDPSOHV�DUH�DQDO\]HG�LQ�UHDO�WLPH��+RZHYHU��QH[W�GD\�RU�ODWHU�DQDO\WLFDO�UHVXOWV�PD\�EH�VXI¿FLHQW��(DUO\�LQ�DQ�H[SHUL-ment, frequent sampling will help to determine how much

WLPH�LV�QHHGHG�WR�UHDFK�VWHDG\�VWDWH�DQG�ZLOO�SURYLGH�UDSLG�IHHGEDFN�IRU�GHWHUPLQLQJ�LI�SURFHVV�FKDQJHV�DUH�UHTXLUHG�� 8VXDOO\��GLVWLOODWH�DQG�WDLOV�VDPSOHV�DUH�VXI¿FLHQW��+RZ-

HYHU��VDPSOLQJ�IURP�ZLWKLQ�WKH�FROXPQ�PD\�EH�UHTXLUHG�WR�determine if an intermediate compound is building up in the

FROXPQ�RU�WR�SURYLGH�FROXPQ�FRPSRVLWLRQ�SUR¿OH�GDWD�WR�YHULI\�FRPSXWHU�VLPXODWLRQV��)RU�DWPRVSKHULF�FROXPQV�ZLWK�trays, a syringe is often used to pull a liquid sample from

D�WUD\�¿WWHG�ZLWK�DQ�DSSURSULDWH�VDPSOH�WDS��,I�WKH�FROXPQ�RSHUDWHV�XQGHU�YDFXXP��PRUH�FRPSOH[�VDPSOLQJ�PHWKRGV�ZLOO�XVXDOO\�EH�UHTXLUHG��6DPSOLQJ�IURP�D�SDFNHG�FROXPQ�LV�H[WUHPHO\�GLI¿FXOW��VLQFH�WKHUH�LV�D�WHQGHQF\�WR�UHPRYH�D�PL[WXUH�RI�OLTXLG�DQG�YDSRU�ZLWKRXW�NQRZLQJ�WKHLU�UDWLR��$�special sampling trough in the column is often required to

JHW�D�WUXH�OLTXLG�VDPSOH�IURP�D�SDFNHG�FROXPQ�� 6RPHWLPHV�YDSRU�VDPSOHV�DUH�UHTXLUHG�IRU�V\VWHPV�ZLWK�FRQGHQVDEOH�YDSRU��&DUH�PXVW�EH�WDNHQ�WR�HQVXUH�WKDW�RQO\�YDSRU��ZLWKRXW�DQ\�OLTXLG��LV�FROOHFWHG�DQG�WKDW�FRQGHQVD-

TI

Thermocouple In Between Insulation

Layers

Column Inner Insulation

Layer

Heat Tape

Outer Insulation Layer

W Figure 7. Heat losses can be minimized by wrapping the column with insulation and heat tape. The heat tape and a thermocouple are wrapped between two layers of insulation. Source: R. Nunley.

Feed

Heat Exchanger

TC TT82 83

Reboiler

Steam

WaterBottom Product

LC 32

QT51

QC42

LC 22

PC 12

Reflux

Distillate

Cooling WaterCooler

Head Product

Plates

Reflux Drum

S Figure 8. This piping and instrumentation diagram (P&ID) shows a typical control system that would be necessary for an automated continuous distillation column at atmospheric pressure.


BACK TO BASICS

WLRQ�RI�WKH�YDSRU�KDV�QRW�RFFXUUHG�MXVW�SULRU�WR�VDPSOLQJ��(LWKHU�RI�WKHVH�VFHQDULRV�FDQ�VLJQL¿FDQWO\�FKDQJH�WKH�sample composition.

� 7KH�UHPRYDO�RI�VDPSOHV�IURP�D�FROXPQ�FDQ�XSVHW�WKH�FROXPQ�RSHUDWLRQ�LI�WKH�VL]H�RI�WKH�VDPSOH�LV�VLJQL¿FDQW�UHODWLYH�WR�WKH�ÀRZV�ZLWKLQ�WKH�FROXPQ��)RU�D�EDWFK�FROXPQ��WKH�UHPRYDO�RI�WRR�PDQ\�NHWWOH�VDPSOHV�FDQ�DIIHFW�WKH� base composition.

Other concerns Chemicals with high melting points may require the

KHDWLQJ�RI�WKH�HQWLUH�FROXPQ�DQG�DOO�IHHG�WDQNV��UHFHLYHUV��DQG�DVVRFLDWHG�OLQHV��)RU�FKHPLFDOV�ZLWK�PHOWLQJ�SRLQWV�XS�WR�DERXW��&��RQH�RSWLRQ�LV�WR�RSHUDWH�WKH�HQWLUH�FROXPQ�LQ�D�KRW�ER[��)RU�KLJKHU�PHOWLQJ�SRLQWV��WKH�HTXLSPHQW�PD\�QHHG�to be wrapped with heat tape and/or insulation.

Chemicals that tend to polymerize may require the addi-

tion of inhibitor at the top of the column. It is important to

KDYH�DQ�HVWLPDWH�RI�WKH�OLTXLG�ÀRZUDWH�LQ�WKH�FROXPQ��VLQFH�this will affect the inhibitor concentration. Also, polymeriza-

WLRQ�SUREOHPV�FDQ�RFFXU�ZKHQ�YDSRU�FRQGHQVHV�RQ�VXUIDFHV�within the column that are not exposed to inhibited liquid.

/DERUDWRU\�FROXPQV�ZLWK�2OGHUVKDZ�WUD\V�WKDW�RSHUDWH�DW�PRGHUDWH�WR�KLJK�ORDGLQJV�ZRUN�ZHOO�LQ�WKHVH�DSSOLFDWLRQV�EHFDXVH�WKH�YDSRU�ÀRZ�WHQGV�WR�VSUD\�WKH�LQKLELWHG�OLTXLG�onto most or all of the surfaces.

� )RU�FKHPLFDO�V\VWHPV�ZLWK�SRWHQWLDO�IRDPLQJ�LVVXHV�� WKH�DELOLW\�WR�REVHUYH�WKH�DFWLYLW\�LQ�WKH�FROXPQ�PD\�EH�QHFHVVDU\��9LVXDO�REVHUYDWLRQV�DUH�UHODWLYHO\�HDV\�ZLWK�glass columns. Metal columns may require sight glasses or

special cameras.

Column operation Leak testing. After a distillation column has been assem-

EOHG��EXW�EHIRUH�LW�LV�RSHUDWHG��D�OHDN�WHVW�VKRXOG�EH�SHU-IRUPHG��HVSHFLDOO\�LI�WKH�FROXPQ�ZLOO�RSHUDWH�XQGHU�YDFXXP��6XFK�OHDN�WHVWLQJ�LV�EHVW�GRQH�E\�HYDFXDWLQJ�WKH�FROXPQ�WR�WKH�ORZHVW�SUHVVXUH�SRVVLEOH��WKHQ�LVRODWLQJ�WKH�YDFXXP�pump from the column and determining the rate of pressure

ULVH��$Q�DSSUR[LPDWH�YROXPHWULF�OHDN�UDWH�FDQ�EH�GHWHUPLQHG�based on an estimate of the reboiler, column, condenser, and

GLVWLOODWH�UHFHLYHU�YROXPHV��7KH�OHDN�UDWH�VKRXOG�EH�QR�PRUH�WKDQ�DERXW��RI�WKH�H[SHFWHG�YROXPHWULF�YDSRU�ÀRZUDWH�IRU�V\VWHPV�WKDW�DUH�QRW�R[\JHQ�VHQVLWLYH��2[\JHQ�VHQVLWLYH�V\VWHPV�PXVW�EH�OHDNIUHH� Commissioning. Before running authentic materials,

columns are often commissioned to ensure that they gener-

ally operate correctly, and possibly also to determine reboiler

FDSDFLW\��FRQGHQVHU�FDSDFLW\��VHSDUDWLRQ�HI¿FLHQF\��DQG�RU�column capacity. Usually water, a minimally hazardous

RUJDQLF�FRPSRXQG��RU�D�NQRZQ�ELQDU\�V\VWHP��IRU�HI¿FLHQF\�GHWHUPLQDWLRQ��LV�XVHG�IRU�WKLV�SXUSRVH�

Start-up and operation. The procedure to start up a

EDWFK�FROXPQ�LV�VWUDLJKWIRUZDUG��1RUPDOO\��WKH�NHWWOH�LV�charged prior to start-up. Commonly, a batch column is

VWDUWHG�RQ�WRWDO�UHÀX[��$IWHU�OLJKW�FRPSRQHQWV�KDYH�DFFXPX-

ODWHG�LQ�WKH�WRS�RI�WKH�FROXPQ��WKH�UHÀX[�UDWLR�LV�XVXDOO\�VHW�WR�D�UHODWLYHO\�KLJK�YDOXH�WR�UHPRYH�WKH�OLJKW�FRPSRQHQWV�ZLWKRXW�ORVLQJ�D�VLJQL¿FDQW�DPRXQW�RI�SURGXFW��2QFH�WKH�EXON�RI�WKH�OLJKW�FRPSRQHQWV�KDYH�EHHQ�UHPRYHG��WKH�UHÀX[�UDWLR�FDQ�EH�UHGXFHG�WR�UHPRYH�WKH�SURGXFW�DV�TXLFNO\�DV�SRVVLEOH��$IWHU�PRVW�RI�WKH�SURGXFW�KDV�EHHQ�UHPRYHG��LW�LV�FRPPRQ�WR�LQFUHDVH�WKH�UHÀX[�UDWLR�WR�PDLQWDLQ�SURGXFW�purity as the last of the product is stripped out.

Columns are sometimes run in a semi-batch mode. A

semi-batch column is started up in the same manner as a

batch column, but after distillate withdrawal has started,

DGGLWLRQDO�PDWHULDO�LV�DGGHG�WR�WKH�NHWWOH�HLWKHU�DV�D�FRQWLQX-

ous feed or batch-wise.

The start-up and operating procedures for a continuous

FROXPQ�DUH�PRUH�GLI¿FXOW�WR�GH¿QH�WKDQ�IRU�EDWFK�FROXPQV�and therefore need to be carefully considered. Start-up of

a continuous column is similar to that of batch column in

WKDW�LQLWLDOO\�WKH�NHWWOH�LV�FKDUJHG�DQG�WKH�FROXPQ�LV�EURXJKW�WR�WRWDO�UHÀX[��$IWHU�WKH�FROXPQ�LV�KRW�DQG�ORDGHG�SURSHUO\��WKH�IHHG��GLVWLOODWH��DQG�WDLO�ÀRZV�DUH�VWDUWHG��7KHQ��WKH�RSHUDWLRQDO�YDULDEOHV�DUH�DGMXVWHG�WR�EULQJ�WKH�FROXPQ�WR�WKH�desired operating conditions.

Once a continuous column is operating at its control

setpoints, a period of time will be required to reach steady

VWDWH��8VXDOO\��WKH�FROXPQ�WHPSHUDWXUH�SUR¿OH��GLVWLOODWH�rate, and analyses of the distillate and tails streams are used

to determine when the column has reached steady state.

)OXFWXDWLRQV�LQ�WKH�SURFHVV�YDULDEOHV�DUH�FRPPRQ��VR�VHYHUDO�VDPSOHV�VKRXOG�EH�WDNHQ�RYHU�D�SHULRG�RI�WLPH�WR�HQVXUH�WKDW�WKH�SURFHVV�YDULDEOHV�DUH�QRW�WUHQGLQJ�XS�RU�GRZQ�� )RU�D�FRQWLQXRXV�FROXPQ��WKH�YDULDEOHV�WKDW�DUH�XVXDOO\�DGMXVWHG�WR�PRGLI\�WKH�VHSDUDWLRQ�DUH�WKH�UHÀX[�UDWLR�DQG�WKH�GLVWLOODWH�WR�IHHG�UDWLR��2WKHU�YDULDEOHV�WKDW�PD\�LPSDFW�WKH�separation are the number of stages, feed location, feed rate,

and pressure.

� )RU�D�VHSDUDWLRQ�V\VWHP�WKDW�KDV�WZR�RU�PRUH�FROXPQV�in series, the column operations need to be coordinated and

WKH�RSHUDWLRQ�EHFRPHV�PRUH�FRPSOH[��)RU�PRUH�FRPSOHWH�implementations of a process, recycle streams may be neces-

VDU\��ZKLFK�DGG�DQRWKHU�OHYHO�RI�FRPSOH[LW\�

Closing thoughts� $GYDQFHV�RYHU�WKH�\HDUV�LQ�SURFHVV�VLPXODWRUV��H[SDQ-

VLRQV�RI�SK\VLFDO�SURSHUW\�GDWDEDVHV��DQG�LPSURYHPHQWV�LQ�WKHUPRG\QDPLF�PRGHOLQJ�FDSDELOLWLHV�KDYH�JUHDWO\�LPSURYHG�our capability to accurately simulate separations by distilla-

WLRQ��1HYHUWKHOHVV��WKH�QHHG�UHPDLQV�IRU�ERWK�ODE�VFDOH�DQG�pilot-scale experimental distillation testing.


It is reasonable to expect that the capabilities of process

VLPXODWRUV�ZLOO�FRQWLQXH�WR�DGYDQFH��+RZHYHU��WKH�GHPDQGV�IRU�WKH�HI¿FLHQF\�DQG�HIIHFWLYHQHVV�RI�VHSDUDWLRQ�SURFHVVHV�are also expected to continue to grow. Many cases remain

in which distillation experiments are necessary and the

need for experimental testing is expected to continue for the

foreseeable future.

GLENN GRAHAM is a distillation subject matter expert and senior chemical engineer at MATRIC (Mid-Atlantic Technology, Research, and Innovation Center, P.O. Box 8396, South Charleston, WV 25303; Phone: 304-552-6554; Email: [email protected]; Website: www.matricinnovates.com). Previously, he worked for Union Carbide Corp. and the Dow Chemical Co. as a distillation specialist in their R&D separations groups. He has extensive experi-ence in the conceptual design of separation processes, design and operation of laboratory- and pilot-scale distillation equipment, and chemical process modeling. Graham holds BS and MS degrees in chemical engineering from Montana State Univ.

DON BUNNING, P.E., is a senior chemical engineer at MATRIC (Phone: 304-720-1049; Email: [email protected]). He has extensive experience in process development, new catalyst develop-ment, demonstration of new technologies in pilot units, commer-cialization, on-site startup support, and technology licensing. His more than 50 years of experience in the chemical industry includes positions at Union Carbide and Dow Chemical. He has served as technology manager for acrolein and derivatives, acrylic acid, acrylic esters, vinyl acetate, and glycol ethers. Since joining MATRIC, he has been heavily involved in process development and the design and construction of pilot plants. He has a BS in chemical engineering from the Univ. of Illinois and an MS in chemical engineering from West Virginia Univ. He is also a licensed Professional Engineer.

JEREMY RADER, P.E., is the pilot plant supervisor at MATRIC (Phone: 304-720-1058; Email: [email protected]). He has worked his entire career at MATRIC and has been involved in several di!erent pilot plant technologies involving distillation and various other separation technologies. Rader’s areas of specializa-tion include pilot plant construction, pilot plant processing, control systems, and implementation of distillation control philosophies. He has a BS in chemical engineering from West Virginia Univ. Institute of Technology. He also holds a professional engineering license in the state of West Virginia.

Literature Cited1. Graham, G. K., et al., “Experimental Validation of Column

Simulations,” Chemical Engineering, 125��2. Graham, G. K., et al., “Experimental Methods to Verify Distilla-

tion Simulations,” Chemical Engineering, 125��3. Krell, E.,�³+DQGERRN�RI�/DERUDWRU\�'LVWLOODWLRQ�´�(OVHYLHU�

$PVWHUGDP��1HWKHUODQGV��4. Oldershaw, C., “Perforated Plate Columns for Analytical Batch

Distillations,” Industrial and Engineering Chemistry Analytical Edition, 13 ��SS��±��$SU��

5. Kister, H. Z.,�³'LVWLOODWLRQ�'HVLJQ�´�0F*UDZ�+LOO��1HZ�<RUN�1<��

6. Nunley, R.,�³0DQDJLQJ�+HDW�/RVV�LQ�3LORW�3ODQW�2SHUDWLRQV�WR�6LPXODWH�)XOO�6FDOH�2SHUDWLRQ�´�SUHVHQWHG�DW�WKH�$,&K(�6SULQJ�0HHWLQJ��6DQ�$QWRQLR��7;��0DU��

7. Stichlmair, J. G., and J. R. Fair, “Distillation: Principles and

3UDFWLFH�´�:LOH\�9&+��:HLQKHLP��*HUPDQ\��8. Shinskey, F. G., ³'LVWLOODWLRQ�&RQWURO��)RU�3URGXFWLYLW\�DQG�

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chemical engineering progress (cep), design and operation

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