Note: Descriptions are shown in the official language in which they were submitted.
5726
APP~RAT~S FOR DISTI~LATIO~ COL~MN SIDE STREAM
WIT~DRAWAL
~ he present invention relates to an apparatus for
monitoring and controlling the operation of a distillation
column. More specifically, the invention relate~ to
measurement and control of internal liquid reflux from a
distillation column partial side ~tream draw tray.
Separation of a fluid mixture into VariGU~ component
fractions according to relative boiling points or boiling
point ran6es may be accomplished by fractional distillatio~
- generally carried out in a vertical column oontainin~ tray~
or bed~ of packing material which effect intimate contact
between the ~apour flowing up through the column and the
liquid flowing down. Such distillation columns yield an
overhead product con~isting of a lower boiling fraction and
a bottoms product of a higher boiling fraction. In
addition, it i~ often desired to remo~e one or more inter-
mediate boiling fraction~ as Eide streama from intermediate
locations in the column. When a liquid side stream with-
drawal i~ made, it usually comprises only a part of the
l~quid flowlng down the tower at the point of withdrawal.
In thle situation of partial withdrawal, the remainder of
the liquid pae~e~ to the fractionation stages of the column
below the ~lde etream draw as a reflux to maintain liquid-
~apour contact and effect efficient fractionation in this
portion of the column.
In order to take such a partial side stream draw, it
i8 nece~ary to accompli~h a di~i~ion of liquid flow, or
traffic, down the column lnto a side stream and a reflux.
Since the reflux liquid remain~ within the distillation
prooess and influenoe~ it~ operation, it is often
advantageous to make thio di~ision in a manner which allows
both measurement and control of reflux flowxate. Additional-
ly, it is mo~t desirable that the mea~urement- and control
funotion~ be carried out in a manner ~uch that no more of a
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57~6
hydraulic driving force is required than that which i8
normally available for gra~ity flow of liquid from a column
side stream draw tray to the next lower fract$onation stage.
Operating within the constraint~ of gravity flow obviate~
the need for the pumping of reflux flow through metering
and control devices, a common conventional practice.
At least one de~ice ha~ already been proposed for use
in dividing the liquid traffic into side ~tream and reflux
which also accomplishe~ metering of the reflus flow rate.
~his known device is an apparatus comprising a chamber for
collecting the total liquid traffic down the column at the
point of the desired partial side stream draw. Reflux and
side stream flow continuously out of the chamber, each
through one of two outlets. The reflux outlet consist~ of
a oalibrated orifice located on the bottom of the chamber.
Since flow through the orifice is dapendent in a known
manner upon liquid head above the orifice, monitoring of
liquid level in the chamber provides indirect measurement
of reflus flow rate.
Thl~ known concept of flow measurement can be adapted
to provide some degree of reflux flow control. For instance,
if a given constant flow of reflux through the chamber
orifice 1~ desired, the side ~tream flowrate could, in
theory, be ~aried as neces~ary to provide a given constant
liquid level in the chamber and thus a constant reflux flow.
~owever, ~uch indirect control logic ha~, in practice,
proven unsuccessful for providing stable control of the
reflus flowrate for the reason that liquid trafflc through
a distillation column i8 generally oharacterized by
continual fluctuationa in flowrate. Such fluctuation~ are
~ariable, both as to amplitude and frequency, in a manner
which doe~ not permit a response of side stream control
which i~ adequate to prevent these fluctuation~ from being
reflected ln the chamber liquid level and hence in the
reflux flowrate. In essen¢e, the particular control logic
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7~6
fails in thi~ application becau~e it is unable to reproduce
the variatione in liquid traffic with identical correspon-
ding ~ariations in side stream flow. As a con~equence,
minor up~ets and normal variatione in the distillation
process above the side draw tray are communicated to the
lower EectiOn~ of the column through variations in reflux
flow.
An additional ~hortcoming of many conventional metering
chamber devices is their inability to adapt to sub~tantial
changes in the operation of the column. ~oth metering and
control functions of prior art devices are accompli~hed
by the same orifice, calibrated for operation under a given
mode of column operation. Over the long term, ma~or changes
in the di~tillation process, involving, for instance,
variation in feed flowrate, feed composition, or side ~tream
flowrate, will neces~itate a shutdown of column operations
for replacement of the orifice in order that it will
function adequately in the new mode of operation.
It 1B an ob~ect of the present invention to offer a
solution to the afore-mentioned problems. ~he invention
provide~ an apparatus which a¢hieves division of liquid
traffic down a di~tillation oolumn into a side stream and a
metered and controlled reflux flow to lower ~ections of the
column in such a manner that the reflux flowrate is not
influenced by routine fluctuations in column operations
above the point of side stream withdrawal. ~he invention -
runctions ~pon any "liquid source" within the distillation
column including, but not limited to~ "liquid traffic"
flowing from a fractionation stage in the distillation
column above a aide draw outlet. Furthermoro, use of the
apparatus of the invention permits continued control of
reflux flowrate during periods when the column i~
operatin~ in a mode significantly different than that for
which it wa~ designed. ~hese advantages are realized
accordin~ to th~ invention without the need for pumping of
5 7 1~ 6
reflux flow.
The invention therefore provides an apparatus,
for dividing a liquid source in a multi-stage counter-
current distillation column having an upper fractionation
stage above a withdrawl outlet for use in withdrawing a
liquid side draw from the column and a lower fractionation
stage below said withdrawal outlet, said dividing taking
place into a reflux portion of measured and controlled
flowrate that passes by gravity flow to the lower
fractionation stage, and a sidestream portion that is
withdrawn from the distillation column through said with-
drawal outlet, said apparatus comprising a liquid reflux
reservoir adapted to collect said liquid source, said
apparatus further comprising a metering chamber, a control
valve and a liquid level measuring means, and said
reservoir positioned at an elevation intermediate to that
of the upper and the lower fractionation stages, said
reflux reservoir having in association therewith means
for maintaining a liquid level within the reflux
reservoir, said reflux reservoir being in liquid communi-
cation with said withdrawal outlet and being adapted to
direct the side stream portion of the liquid source to
the withdrawal outlet, and said reflux reservoir being
in liquid communication, through a first liquid conduit
with the said control valve means adapted to pass a cont-
rolled flowrate of said reflux portion of the liquid
source, via a second liquid conduit to the said metering
chamber positioned at an elevation below the reflux
reservoir, said chamber having at least one flow orifice
through which passes the reflux portion to the lower
fractionation stage, and said chamber having in
association therewith the said liquid level measuring
means for monitoring of liquid level in the chamber.
B
5736
4a
An advantageous embodiment of the invention
provides an apparatus for dividing liquid traffic down
a distillation column having a plurality of fractionation
stages through which pass liquid and vapour in
countercurrent flow, an
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~S736
outlet for a vapour product from the top of the column, an
outlet for a liquid product from the bottom of the column,
and an intermediate outlet for withdrawal of a side stream
liquid product, ~aid di~iding taking place into a side
stream product and a reflux flow to the fractionation ~tage
below the side stream product outlet and for the measure-
ment and control of reflux flowrate, said apparatu~
comprising a liquid reflux reservoir con~isting of a liquid
receiver being divided by a reflux weir into a first
re~ervoir and a second (side stream) reservoir~ a means for
¢ollecting all liquid traffic flowing down the distillation
column from the fractionation stage above the 6ide stream
outlet and for routing by gravity flow said liquid traffic
into the said first reservoir of the receiver; a control
valve positioned external to the column at a vertical
elevation below that of the top of the reflux weir, the
control valve being connected with the ~aid first reserYoir
through a liquid conduit1 a metering chamber positioned at
a vertical elevation below the top of the reflux weir,
said chamber having one or more calibrated flow orifices
in liquid communication with the next fractionation stage
below the side stream outlet~ the metering chamber being
connected with the oontrol valve through a liquid ¢onduit;
and a mean~ for measuring llquid level in the metering
chamber, said level ha~ing a proportional relation~hip to
reflux flow through said calibrated flow orifi¢es.
In it~ simplest aspects, the apparatus aocordin~ to
the invention provides for oollection Or liquid source, in
particular liquid traffio down a distillation ¢olumn, at
th0 fractionation stage from which side stream withdrawal
i8 deslred, into a liquid reflux reservoir or receiver. The
liquid reflux ressrvoir or receiver maintains an essential-
ly constant level sour¢e of reflux liquid by means of
suitable means for maintaining a liquid level within the
reflux reservoir. A reflux weir is an advantageous means
,
57~6
for this service. Reflux flow pagses fxom this receiver
through a conduit external to the column.
~he conduit contain~ a control valve which i~ used to
control flow of the reflux. Downstream of the control val~e,
- 5 reflux flow enter~ a metering chamber containing one or more
calibrated orifices, preferably on one or more of its
vertical walls, which orifices permit reflux flow onto the
fractionation ~tage below the point of side stream with-
drawal to be indirectly measured as a function of liquid
level in the ¢hamber. ~hat liquid source, in particular
liquid traffic which enter~ the liquid reflux reservoir
(or receiver) but which iB not taken as reflux through the
control and metering elements of the apparatu~ is conti-
nuously withdrawn from the distillation process as ~ide
stream. ~he apparatus of the invention, provides for a
more stable and preoise control of reflux than wa~ hereto-
fore practised. ~he control sy~tem does not attempt to
respond to the fluctuations in column liquid traffic 80 a~
to reproduce these ~ame fluctuation~ in side stream flow.
Instead, the control function provided by the invention
operates in a manner essentially independent of the influ3xe
of suoh fluctuations.
~ he apparatus according to the invention comprises
three elements, in combination with a distillation column
¢ontaining a plurality of fractionation stages. ~he three
elements must be interconnected with conduits for liquid
flow and positioned in such a manner that the liquid flow
passes, by action of gra~ity, sequentially to each of the
elements in a precise order. ~ppermost of tbe elements of
the apparatus according to the inYention is a liquid
reflux reservoir into whi¢h passes all liquid source, in
particular liquid traffic down the distillation column at
an intermediate location in the column from which a ~ide
ctxeam withdrawal is desired and at which an outlet from
the column ie pro~ided for said withdrawal. The liquid
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1~57~6 -
xeflux reservoir is provided with ~uitable means for
maintaining a liquid level within the reflux re~ervoir
and effects separation of the liquid source or traffic
into a side stream and a reflux. At a vertical elevation
below that of the liquid reflux reservoir i8 104ated the
seoond element of the invention, a control valve. ~ocated
downstream of the control valve, the third element is a
metering chamber which, in effect, translates a flowrate
parameter into a more readily determinable liquid level or,
equivalently, a differential pressure parameter by means of
the use of one or more flow orifice~. While the liquid
reflux re~ervoir and metering chamber may conceivably be,
at least in part, located external to the column, it i8
preferred that they are located within the column walls. As
will be illustrated below, through reference to the drawing8,
these element~ may be arranged within the column in a manner
which necessitQtee the use of no more internal column 6pace
than i8 normally provided for conventional means of side
stream wlthdrawal.
~he apparatus of the invention ie intended to be
applied in combination with a distillation column of any
oonventional design. As the invention generally provides
a means for withdrawing a side stream from a distillation
column, it is to be under~tood that the column comprises
fractionation stages both above and below the relative
vertical position of the side stream outlet. ~ractionation
stage~ as the term is used here, is intended to broadly
appl~ to any conventional means of effecting vapour-liquid
contact within a distillation column, common example~ of
which are valve trays, sieve trays, packed column sections,
and the like.
The liquid reflux reservoir provided with means for
maintaining a liquid level within the reflux reservoir
employed aB an element of the apparatus of the invention
may be of any convenient design, so long as it provides
l~s7~6
B
for collection of the entire liquid source or traffic
flowing from the fractionation ~tage above the 6ide strsam
draw location, for the maintenance of an es6entially
constant level liquid surge capacity from which reflux flow
may ~ontinually be drawn, and for the withdrawal of exce~s
liquid traffic, i.e., that not taken from the re~ervoir as
reflux, from the column as side stream product. In
addition the liquid reflux reservoir muæt not ~ub6tantially
interfere with the ~as~age of vapour traffic from the
fractionation 6tage below the draw location to that above.
In one of the possible embodiment6 of the in~ention,
the liquid reflux reservoir may take the general form of a
conventional chimney tray. ~he tray pan is provided with
two outlets, one for reflux and one for side 6tream, each
of which i6 connected to a conduit for pa~sage of the
respective liquid flows external to the column. ~he liquid
reflux reservoir, or draw tray pan, i8 equipped with a
vertical reflux weir running transversely acrosa the pan,
and between the two outlets, to divide the tray into two
liguid reservoirs. The reflux weir is an advantageou6 means
for maintaining liquid level within the reflux re~ervoir.
~iquid traffic entering thereceiver is directed to one side
of the weir and into a fir3t part of the reflux re6ervoir
which is in communication with the reflux outlet through
which reflux passes to the reflux control valve. That
portion Or the liquid traffic which enter~ the first part
of the reflux reservoir, but which iB not taken under
control through the reflux outlet, overflows the reflux
weir to a ~econd part, the side strsam reservoir. In oxder
to assure that an essentially constant reflux re~ervoir
liquid level iB always maintained by continuous flow over
the reflux weir, the liquid traffic directed to the first
part of the reflux reservoir must necessarily be of greater
flowrate than the reflux which is drawn from the fir3t part
of the reflux reservoir. While it is pos~ible to direct
114S7~6
only a portion of the liquid traffic to the first part of
the reflux reservoir~ it is generally desirable that all
liquid traffic enter the first part of the reflux reservoir
as there is then maximum assuranoe that flow requirements
for the reflux will at all times be met. Any entering liquid
traffic not directed to the fir~t part of the reflux
reservoir of the re¢eiver must neceesarily pa~s directly to
the side stream reservoir. Division of liquid traffic in
such a manner that a portion enters the first part of the
reflux re~ervoir and the remainder enters the side stream
reservoir would perhaps be desirable, for instance, when
multipass trays with multiple downcomers are employed as
column fractionation stages and when the requirements of
reflux flowrate can easily be met by that portion of the
total liquia traffic collected from fewer than all of the
multiple passes.
The side stream reservoir, in communication with the
side stream outlet, is maintained at a liquid level below
the top of the reflux weir under normal operation of the
apparatus. An essentially constant level source of liquid
is thus provided upstream of the weir, in the first part of
the reflux reservoir. If de6ired, liquid level in the side
etream reservoir may be monitored by means of conventional
liquid level instrumentation, and flowrate of side stream
withdrawal may be controlled to maintain a given level.
Alternatively, side stream liquid may be allowed to drain
freely from the side stream re6ervoir. A conduit is general~
ly provided for communication of the side stream liquid
from the side stream reservoir to the side stream outlet.
In some embodiments of the invention in which the side
stream reservoir is in dire¢t contact with the outlet, the
reservoir itself serves as a means to channel side stream
to the outlet and no other oonduit is required. It is
considered most significant that fluotu2tions in side
~5 stream reservoir level and in 6ide stream flowrate are of
ll~S7~6
no conoern in the operation of the apparatus of the inven-
tion as they have no influence upon the control of the
reflux flow.
It may al~o be desirable that the ~ide stream reservoir
of the liquid reflux re~ervoir, in an advantageou~
embodiment, i~ equipped with an outlet overflow weir and an
overflow downcomer to the fractionation ~tage below. ~he
overflow weir extend~ to a height above the normal liquid
level in the side ~tream reservoir. ~uring periods of
extreme upset in column operation or of instrument mal-
function, when more liquid traffic enter~ the liquid reflux
re~ervoir than i~ withdrawn a~ reflux and side stream, the
excess liquid overflows the overflow weir and i6 directed
to the lower fractionation stage.
The apparatus of the invention comprises a ~onduit to
route reflux from the liquid reflux reservoir to the control
valve9 located external to the column at a vertioal
elevation below the top of the reflux weir. ~he control
valve may be operated manually or automatically in response
to signal~ from appropriate instruments provided for main-
tenance of a desired distillation proce3a parameter, for
example, temperature on the fractionation stage below the
metering chamber, flowrate of reflux through the metering
chamber, etc. It is necessary that the control valve be
specified so as to operate under the relatively low
differential pressure provid0d by the gravity flow of the
reflux from the receiver to the metering chamber. Normal
tray spacing in a trayed distillation column, i.e., about
two feet, will generally provide sufficient driving force
for operation of a control valve. A~ a practical matter it
is advisable thatthe control valve be physically located
at a relative vertical elevation no higher than that of the
metering chamber liquid level, in order to prevent flashing
of reflux liquid to a vapour/liquid mixture downstream of
the valve. An advantageou~ embodiment of the invention
~Si7~6
involves the arrangement of the control function 80 that
the valve automatically provide~ conetant reflux flowrate
in respon~e to signals generated by the flow metering
chamber element.
A conduit i8 provided for communicating reflux liquid
from the control valve into the metering chamber. The
metering chamber element of the invention generally compri-
8e8 a box of any convenient shape. ~he chamber i8 equipped
with an inlet opening for interconnection with the conduit
from the control valve and with one or more outlet
- openings, in the form of calibrated flow orifices, for the
flow of reflux onto the fractionation stage below the
receiver. Since the flow into the chamber ie controlled and
the flow out of the chamber i~ metered, and further since
it is desired to meter and control the eame flow, it is
- critical that the chamber have ess2ntially no other
openings through which liquid may be communicated into or
out of the chamber during normal operation of the apparatu~.
~his, how~ver, does not preclude a chamber open at the top,
and located inside the column 80 long as the column and the
apparatus of the invention are of ~ch a de~ign that liquid
doe~ not normally pass into the chamber through eaid
opening. It i~ generally desirable that the chamber have
~ome opening at or near the top to permit vapour to freely
enter and exit the chamber vapour spaoe as the liquid level
in the chamber rises and falls.
~ he one or more calibrated outlet flow oririces may be
provided in one or more of the metering chamber walle, in
particular the side walls. While the apparatus will be
operable if some or all of the orificea are located on or
near the bottom of the chamber, a wider range of effective
flow mea~urement may generally be obtained if the orifice
or orifices extend substantially across the full vertical
dimenbion of one or more chamber ~ide walls. ~hrough
appropriate sizing and vertical poeitioning of the orifices,
~57:~6
it is possible to obtain es6entially any desired functional
relationship between liquid level in the chamber and flow-
rate of reflux liquid out of the chamber through the
orifices. Liquid level in the chamber is suitably monitored
by means of any one of the many types of conventional
devices known for such service, for instance, liqu~d float,
sight glass, or differential pre~sure instrumentation.
The present invention will now be described by way of
example in more detail with reference to the drawings, in
which: -
Figure 1 i~ a side view of an advantageous embodimentof the apparatus according to the invention in partial cross
~ection.
~ igure 2 ie a top sectional view of the liquid reflux
reservoir and metering chamber from line 2-2 of ~iguxe 1.
Figure 3 is a side sectional view of the metering
chamber from line 3-3 of Figure 2.
Figure 4 is a side sectional view of the metering
¢hamber from line 4-4 of Figure 2.
Referring now to figures 1-4, wherein the sQme referen-
ce numbers indicate the eame components, the cylindrical
wall of a distillation col~mn ie designated as 1. In
figuree 1-4, only a portion of the vertically extending
column, above and below the liquid reflux reservoir or
receiver, in this example a conventional draw tray
designated 31, ie pictured. One single-pas~ fractionation
tray, 32, above tha draw tray, iB shown equipped with a
weir 2 to maintain liquid levsl on the tray and a down¢omer
3 to direct liquid overflowing the weir to the draw tray.
~ere, the tray 32 fun¢tione to aid in collecting the liquid
traffic down the distillation column and in directing the
~low onto the receiver. ~he draw tray 31 iB supplied with
one or more chimneys 4 to bypase vapour flow upward through
the draw tray 31 without fractionation contact with liquid
thereon. In addition, the draw tray 31 i~ equipped with a
'
ll~S7 ~6
reflux weir 5 which divides the tray pan 80 a~ to provide
two distinct liquid reservoirs. ~he first part of the
liquid reflux reservoir upstream of the reflux weir 5, i.e.
on the side of the weir in direot communication with liquid
from the downcomer 3, is generally de~ignated 6, while the
second part, the side 6tream reservoir, on the other ~ide
of the weir i8 generally designated 7. In operation, liquid
flowing from the tray 32, through the downoomer 3, is
directed into the first part 6 of the reflux reservoir to
provide a constant-level ~ource of liquid from which reflux
may be drawn. ~hat portion of the liquid traffic entering
the draw tray 31 from the tray 32 which is not taken as
reflu~ overflows the weir into the reservoir 7 from which
it iB removed as side ~tream via a conduit 17 to a side
stream outlet 18. ~he draw tray is further provided with an
overflow weir B and an overflow downcomer 9, in order that
under ma~or upseta in column operation large quantities of
liquid traffio may overflow the receiver without the
constraint of reflux or side stream control.
A conduit 10, in liquid communioation with the
re~ervoir 6, runs external to the oolumn to a oontrol valve
11. A conduit 12 extends from the control valve 11 to an
inlet into a metering ohamber having an internal spaoe
generally designated 13. ~he metering ohamber floor may be
~5 coinoident with the deck of a tray 30, the fraotionation
stage below the reoeiver. ~his is shown in figures 1-4. The
chamber i8 further defined by at least three vertioal wall~,
two of whioh are~ in figures 1-4, the oolumn wall 1 and the
reoeiver overflow downoomer 9. One or more of the other
vertioal wall~, 14, oontain a flow orifioe in the form of a
rectangular vertioal ~lit (~lotted weir) 15 extending
across ~ubstantially the full vertioal dimension of the
wall 14. A liquid level monitoring and control means 16 is
affixed to the metering chamber and provides a signal which
direot~ the operation of the oontrol valve 11 to provide
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~57~6
14
a metered reflux of a controlled, and here constant~ flow-
rate.
~ he conduit 17 extends from the side stream reservoir 7
through the side ~tream outlet 18 to the destination of the
side stream draw external to the distillation column. Mean~
for optional control of 6ide stream flo~ through the conduit
~~ 17 and measurement of liquid level in the reeervoir 7 are
not shown.
Figure~ 2 and 3 particularly illustrate the disposition
of the metering chamber 13 within the ~pace d~fined by the
column wall 1 and the overflow downoomer 9. In the embodi-
ment of the invention depicted, a third vertical wall, 14,
defining the metering chamber ic shown a~ viewed from the
top and from the center of the column in ~igures 2 and 3,
respectively. Figure 4 indicates an orifice ~lit 15 in the
wall 14 (~lotted weir).
~ he drawing depicts an application of the invention to
a oonventional distillation ¢olumn containing single-pass
fraotionating trays. Appropriate arrangements for manifol-
ding of conduits and placement of metering chamber and¢hamber orifices wlll be apparent to those skilled in the
art and may for example permit application of the invention
to multipa~s trays.
Various modifications of the invention will become
apparent to thoee skilled in the art fxom the foregoing
description and accompanying drawing~.
Such modifications are intended to fall withln the
scope of the appended claims.
