GB2031746A - Distillation column - Google Patents

Abstract

A countercurrent liquid/vapour fractional distillation column is provided with a liquid reservoir 5,7 between upper and lower fractionation stages, from which reservoir a side stream is withdrawn through conduit 17 and from which reservoir liquid is passed to a lower fractionation stage via conduits 10, 12 and chamber 13, the flow through conduits 10, 12 being controlled by valve 11, the operation of which may be controlled by means 16 sensing the level of liquid in chamber 13. <IMAGE>

Description

SPECIFICATION Apparatus for distillation column side stream withdrawal The present invention relates to an apparatus for monitoring and controlling the operation of a distillation column. More specifically, the invention relates to measurement and control of internal liquid reflux from a distillation column partial side stream draw tray.

Separation of a fluid mixture into various component fractions according to relative boiling points or boiling point ranges may be accomplished by fractional distillation, generally carried out in a vertical column containing trays or beds of packing material which effect intimate contact between the vapour flowing up through the column and the liquid flowing down. Such distillation columns yield an overhead product consisting of a lower boiling fraction and a bottoms product of a higher boiling fraction. In addition, it is often desired to remove one or more intermediate boiling fractions as side streams from intermediate locations in the column. When a liquid side stream withdrawal is made, it usually comprises only a part of the liquid flowing down the tower at the point of withdrawal.In this situation of partial withdrawal, the remainder of the liquid passes to the fractionation stages of the column below the side stream draw as a reflux to maintain liquid-vapour contact and effect efficient fractionation in this portion of the column.

In order to take such a partial side stream draw, it is necessary to accomplish a division of liquid flow, or traffic, down the column into a side stream and a reflux. Since the reflux liquid remains within the distillation process and influences its operation, it is often advantageous to make this division in a manner which allows both measurement and control of reflux flowrate. Additionally, it is most desirable that the measurement and control functions be carried out in a manner such that no more of a hydraulic driving force is required than that which is normally available for gravity flow of liquid from a column side stream draw tray to the next lower fractionation stage. Operating within the constraints of gravity flow obviates the need for the pump ing of reflux flow through metering and control devices, a common conventional practice.

At least one device has already been pro posed for use in dividing the liquid traffic into side stream and reflux which also accom plishes metering of the reflux flow rate. This 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 consists of a calibrated orifice located on the bottom of the chamber. Since flow through the orifice is dependent in a known manner upon liquid head above the orifice, monitoring of liquid level in the chamber provides indirect measurement of reflux flow rate.

This 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 is desired, the side stream flowrate could, in theory, be varied as necessary to provide a given constant liquid level in the chamber and thus a constant reflux flow.

However, such indirect control logic has, in practice, proven unsuccessful for providing stable control of the reflux flowrate for the reason that liquid traffic through a distillation column is generally characterized by continual fluctuations in flowrate. Such fluctuations are variable, both as to amplitude and frequency, in a manner which does not permit a response of side stream control which is adequate to prevent these fluctuations from being reflected in the chamber liquid level and hence in the reflux flowrate. In essence, the particular control logic fails in this application because it is unable to reproduce the variations in liquid traffic with identical corresponding variations in side stream flow.As a consequence, minor upsets and normal variations in the distillation process above the side draw tray are communicated to the lower sections of the column through variations in reflux flow.

An additional shortcoming of many conventional metering chamber devices is their inability to adapt to substantial changes in the operation of the column. Both metering and control functions of prior art devices are accomplished by the same orifice, calibrated for operation under a given mode of column operation. Over the long term, major changes in the distillation process, involving, for instance, variation in feed flowrate, feed composition, or side stream flowrate, will necessitate a shutdown of column operations for replacement of the orifice in order that it will function adequately in the new mode of operation.

It is an object of the present invention to offer a solution to the afore-mentioned problems. The invention provides an apparatus which achieves division of liquid traffic down a distillation column into a side stream and a metered and controlled reflux flow to lower sections 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. The invention functions upon 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 side draw outlet. Furthermore, use of the apparatus of the invention permits continued control of reflux flowrate during periods when the column is operating in a mode significantly different than that for which it was designed.

These advantages are realized according to the invention without the need for pumping of reflux flow.

The invention therefore provides an apparatus, for dividing a liquid source in a multistage countercurrent distillation column having an upper fractionation stage above a withdrawal 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 withdrawal outlet, said apparatus comprising a liquid reflux reservoir adapted to collect said liquid source, 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 communication with said withdrawal outlet via flow passage means adapted to direct the side stream portion of the liquid source to the withdrawal outlet, and said reflux reservoir being in liquid communication, through first liquid conduit means with control valve means adapted to pass a controlled flowrate of said reflux portion of the liquid source, via second liquid conduit means, to a 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 liquid level measuring means for monitoring of liquid level in the chamber.

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 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, said dividing taking place into a side stream product and a reflux flow to the fractionation stage below the side stream product outlet and for the measurement and control of reflux flowrate, said apparatus comprising a liquid reflux reservoir consisting of a liquid receiver being divided by a reflux weir into a first reservoir and a second (side stream) reservoir; a means for collecting all liquid traffic flowing down the distillation column from the fractionation stabe above the side 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 said first reservoir through a liquid conduit; 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 control valve through a liquid conduit; and a means for measuring liquid level in the metering chamber, said level having a proportional relationship to reflux flow through said calibrated flow orifices.

In its simplest aspects, the apparatus according to the invention provides for collection of liquid source, in particular liquid traffic down a distillation column, at the fractionation stage from which side stream withdrawal is desired, into a liquid reflux reservoir or receiver. The liquid reflux reservoir or receiver maintains an essentially constant level source of reflux liquid by means of suitable means for maintaining a liquid level within the reflux reservoir. A reflux weir is an advantageous means for this service. Reflux flow passes from this receiver through a conduit external to the column. The conduit contains a control valve which is used to control flow of the reflux.Downstream of the control valve, reflux flow enters 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 stage below the point of side stream withdrawal to be indirectly measured as a function of liquid level in the chamber. That liquid source, in particular liquid traffic which enters the liquid reflux reservoir (or receiver) but which is not taken as reflux through the control and metering elements of the apparatus is continuously withdrawn from the distillation process as side stream. The apparatus of the invention, provides for a more stable and precise control of reflux than was heretofore practised. The control system does not attempt to respond to the fluctuations in column liquid traffic so as to reproduce these same fluctuations in side stream flow. Instead, the control function provided by the invention operates in a manner essentially independent of the influence of such fluctuations.

The apparatus according to the invention comprises three elements, in combination with a distillation column containing a plurality of fractionation stages. The three elements must be interconnected with conduits for liquid flow and positioned in such a manner that the liquid flow passes, by action of gravity, se quentially to each of the elements in a precise order. Uppermost of the elements of the appa ratus according to the invention is a liquid reflux reservoir into which passes all liquid source, in particular liquid traffic down the distillation column at an intermediate location in the column from which a side stream withdrawal is desired and at which an outlet from the column is provided for said withdrawal.The liquid reflux reservoir is provided with suitable means for maintaining a liquid level within the reflux reservoir 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 is located the second element of the invention, a control valve. Located 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 orifices. While the liquid reflux reservoir and metering chamber may conceivably be, at least in part, located external to the column, it is preferred that they are located within the column walls.As will be illustrated below, through reference to the drawings, these elements may be arranged within the column in a manner which necessitates the use of no more internal column space than is normally provided for conventional means of side stream withdrawal.

The apparatus of the invention is intended to be applied in combination with a distillation column of any conventional design. As the invention generally provides a means for withdrawing a side stream from a distillation column, it is to be understood that the column comprises fractionation stages both above and below the relative vertical position of the side stream outlet. Fractionation stage, as the term is used here, is intended to broadly apply to any conventional means of effecting vapourliquid contact within a distillation column, common examples 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 as an element of the apparatus of the invention may be of any convenient design, so long as it provides for collection of the entire liquid source or traffic flowing from the fractionation stage above the side stream draw location, for the maintenance of an essentially constant level liquid surge capacity from which reflux flow may continually be drawn, and for the withdrawal of excess liquid traffic, i.e., that not taken from the reservoir as reflux, from the column as side stream product. In addition the liquid reflux reservoir must not substantially interfere with the passage of vapour traffic from the fractionation stage below the draw location to that above.

In one of the possible embodiments of the invention, the liquid reflux reservoir may take the general form of a conventional chimney tray. The tray pan is provided with two outlets, one for reflux and one for side stream, each of which is connected to a conduit for passage of the respective liquid flows external to the column. The liquid reflux reservoir, or draw tray pan, is equipped with a vertical reflux weir running transversely across the pan, and between the two outlets, to divide the tray into two liquid reservoirs. The reflux weir is an advantageous means for maintaining liquid level within the reflux reservoir.

Liquid traffic entering the receiver is directed to one side of the weir and into a first part of the reflux reservoir which is in communication with the reflux outlet through which reflux passes to the reflux control valve. That portion of the liquid traffic which enters the first part of the reflux reservoir, but which is not taken under control through the reflux outlet, overflows the reflux weir to a second part, the side stream reservoir. In order to assure that an essentially constant reflux reservoir liquid level is 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 first part of the reflux reservoir.While it is possible to direct 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 assurance that flow requirements for the reflux will at all times be met. Any entering liquid traffic not directed to the first part of the reflux reservoir of the receiver must necessarily pass directly to the side stream reservoir.Division of liquid traffic in such a manner that a portion enters the first part of the reflux reservoir 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 liquid traffic collected from fewer than all of the multiple passes.

The side stream reservoir, in communication with the side stream outlet, is maintained as 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 desired, liquid level in the side stream 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 reservoir. A conduit is generally 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 direct contact with the outlet, the reservoir itself serves as a means to channel side stream to the outlet and no other conduit is required. It is considered most significant that fluctuations in side stream reservoir level and in side stream flowrate are of no concern in the operation of the apparatus of the invention as they have no influence upon the control of the reflux flow.

It may also be desirable that the side stream reservoir of the liquid reflux reservoir, in an advantageous embodiment, is equipped with an outlet overflow weir and an overflow downcomer to the fractionation stage below.

The overflow weir extends to a height above the normal liquid level in the side stream reservoir. During periods of extreme upset in column operation or of instrument malfunction, when more liquid traffic enters the liquid reflux reservoir than is withdrawn as reflux and side stream, the excess liquid overflows the overflow weir and is directed to the lower fractionation stage.

The apparatus of the invention comprises a conduit to route reflux from the liquid reflux reservoir to the control valve, located external to the column at a vertical elevation below the top of the reflux weir. The control valve may be operated manually or automatically in response to signals from appropriate instruments provided for maintenance of a desired distillation process 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 provided 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.As a practical matter it is advisable that the 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 advantageous embodiment of the invention involves the arrangement of the control function so that the valve automatically provides constant reflux flowrate in response to signals generated by the flow metering chamber element.

A conduit is provided for communicating reflux liquid from the control valve into the metering chamber. The metering chamber element of the invention generally comprises a box of any convenient shape. The chamber is 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 is controlled and the flow out of the chamber is metered, and further since it is desired to meter and control the same flow, it is critical that the chamber have essentially no other openings through which liquid may be communicated into or out of the chamber during normal operation of the apparatus.

This, however, does not preclude a chamber open at the top, and located inside the column so long as the column and the apparatus of the invention are of such a design that liquid does not normally pass into the chamber through said opening. It is generally desirable that the chamber have some opening at or near the top to permit vapour to freely enter and exit the chamber vapour space as the liquid level in the chamber rises and falls.

The one or more calibrated outlet flow orifices maybe provided in one or more of the metering chamber walls, in particular the side walls. While the apparatus will be operable if some or all of the orifices are located on or near the bottom of the chamber, a wider range of effective flow measurement may generally be obtained if the orifice or orifices extend substantially across the full vertical dimension of one or more chamber side walls.

Through appropriate sizing and vertical positioning of the orifices, it is possible to obtain essentially any desired functional relationship between liquid level in the chamber and flowrate 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, liquid float, sight glass, or differential pressure instrumentation.

The present invention will now be described by way of example in more detail with reference to the drawings, in which: Figure 1 is a side view of an advantageous embodiment of the apparatus according to the invention in partial cross section.

Figure 2 is a top sectional view of the liquid reflux reservoir and metering chamber from line 2-2 of Fig. 1.

Figure 3 is a side sectional view of the metering chamber from line 3-3 of Fig. 2.

Figure 4 is a side sectional view of the metering chamber from line 4-4 of Fig. 2.

Referring now to Figs. 1-4, wherein the same reference numbers indicate the same components, the cylindrical wall of a distillation column is designated as 1. In Figs. 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, is pictured.

One single-pass fractionation tray, 32, above the draw tray, is shown equipped with a weir 2 to maintain liquid level on the tray and a downcomer 3 to direct liquid overflowing the weir to the draw tray. Here, the tray 32 functions to aid in collecting the liquid traffic down the distillation column and in directing the flow onto the receiver. The draw tray 31 is supplied with one or more chimneys 4 to bypass vapour flow upward through the draw tray 31 without fractionation contact with liquid thereon. In addition, the draw tray 31 is equipped with a reflux weir 5 which divides the tray pan so as to provide two distinct liquid reservoirs.The first part of the liquid reflux reservoir upstream of the reflux weir 5, i.e. on the side of the weir in direct communication with liquid from the downcomer 3, is generally designated 6, while the second part, the side stream reservoir, on the other side of the weir is generally designated 7. In operation, liquid flowing from the tray 32, through the downcomer 3, is directed into the first part 6 of the reflux reservoir to provide a constant-level source of liquid from which reflux may be drawn. That portion of the liquid traffic entering the draw tray 31 from the tray 32 which is not taken as reflux overflows the weir into the reservoir 7 from which it is removed as side stream via a conduit 17 to a side stream outlet 18.The draw tray is further provided with an overflow weir 8 and an overflow downcomer 9, in order that under major upsets in column operation large quantities of liquid traffic may overflow the receiver without the constraint of reflux or side stream control.

A conduit 10, in liquid communication with the reservoir 6, runs external to the column to a control valve 11. A conduit 12 extends from the control valve 11 to an inlet into a metering chamber having an internal space generally designated 13. The metering chamber floor may be coincident with the deck of a tray 30, the fractionation stage below the receiver. This is shown in Figs 1-4. The chamber is further defined by at least three vertical walls, two of which are, in Figs. 1-4, the column wall 1 and the receiver overflow downcomer 9. One or more of the other vertical walls, 14, contain a flow orifice in the form of a rectangular vertical slit (slotted weir) 15 extending across substantially the full vertical dimension of the wall 14.A liquid level monitoring and control means 16 is affixed to the metering chamber and provides a signal which directs the operation of the control valve 11 to provide a metered reflux of a controlled, and here constant, flow-rate.

The conduit 17 extends from the side stream reservoir 7 through the side stream outlet 18 to the destination of the side stream draw external to the distillation column.

Means for optional control of side stream flow through the conduit 17 and measurement of liquid level in the reservoir 7 are not shown.

Figs. 2 and 3 particularly illustrate the disposition of the metering chamber 13 within the space defined by the column wall 1 and the overflow downcomer 9. In the embodiment of the invention depicted, a third vertical wall, 14, defining the metering chamber is shown as viewed from the top and from the center of the column in Figs. 2 and 3, respectively. Fig. 4 indicates an orifice slit 15 in the wall 14(slotted weir).

The drawing depicts an application of the invention to a conventional distillation column containing single-pass fractionating trays. Appropriate arrangements for manifolding of conduits and placement of metering chamber and chamber orifices will be apparent to those skilled in the art and may for example permit application of the invention to multipass trays.

Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings.

Such modifications are intended to fall within the scope of the appended claims.

Claims (14)

1. An apparatus, for dividing a liquid source in a multistage countercurrent distillation column having an upper fractionation stage above a withdrawal 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 withdrawal outlet, said apparatus comprising a liquid reflux reservoir adapted to collect said liquid source, 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 communication with said withdrawal outlet via flow passage means adapted to direct the side stream portion of the liquid source to the withdrawal outlet, and said reflux reservoir being in liquid communication, through first liquid conduit means with control valve means adapted to pass a controlled flowrate of said reflux portion of the liquid source, via second liquid conduit means, to a 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 liquid level measuring means for monitoring of liquid level in the chamber.

2. The apparatus as claimed in claim 1 for dividing liquid traffic down a distillation column having a plurality of fractionation stages through which pass liquid and vapour in countercurrent flow, an 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, said dividing taking place into a side stream product and a reflux flow to the fractionation stage below the side stream product outlet and for the measurement and control of reflux flowrate, said apparatus comprising a liquid reflux reservoir consisting of a liquid receiver being divided by a reflux weir into a first reservoir and a second (side stream) reservoir; a means for collecting all liquid traffic flowing down the distillation column from the fractionation stage above the side 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 said first reservoir through a liquid conduit; 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 control valve through a liquid conduit; and a means for measuring liquid level in the metering chamber, said level having a proportional relationship to reflux flow through said calibrated flow orifices.

3. The apparatus as claimed in claim 2, wherein the liquid receiver is transversely divided by a vertical reflux weir into a first reservoir and a second (side stream) reservoir.

4. The apparatus as claimed in any one of claims 2 or 3 comprising a means for collecting all liquid traffic flowing down the distillation column from the fractionation stage above the side stream outlet, for routing by gravity flow a portion of said liquid traffic into the said first reservoir of the receiver, said portion being of greater flowrate than the reflux flow, and for routing all liquid traffic with the exception of said portion by gravity flow into the side stream reservoir of the receiver.

5. The apparatus as claimed in any one of claims 2-4, wherein the receiver and the metering chamber are contained within the distillation column walls.

6. The apparatus as claimed in claim 5, wherein the calibrated flow orifices are located in one or more of the metering chamber's side walls.

7. The apparatus as claimed in any one of claims 2-5 wherein the side stream reservoir of the receiver is equipped with an outlet overflow weir and an overflow downcomer to the fractionation stage below.

8. The apparatus as claimed in claim 7, wherein the receiver is a draw tray equipped with a vertical transverse reflux weir, at least one vapour chimney, an overflow weir and an overflow downcomer.

9. The apparatus as claimed in claims 7 or 8, wherein the metering chamber is located in the overflow downcomer space of the draw tray.

10. The apparatus as claimed in claim 5, wherein the control valve responds automatically to a control signal generated by an operating parmeter of the distillation column.

11. The apparatus as claimed in claim 10, wherein the means for measurement of liquid level in the metering chamber also serves a control function by generating signals in response to which the control valve is automatically operated to maintain a constant liquid level in the metering chamber.

12. The apparatus as claimed in any one of claims 2-11, wherein the metering chamber is provided with at least one vertical wall, containing a flow orifice in the form of a rectangular slit (slotted weir) extending across substantially the full vertical dimension of said wall.

13. The apparatus as claimed in any one of claims 2-12, wherein said means for collecting all traffic flowing down the distillation column from the fractionation stage above the side stream outlet and for routing by gravity flow the liquid traffic into the first reservoir of the receiver consists of a liquid downcomer to the receiver.

14. The apparatus as claimed in any one of claims 2-13 wherein the metering chamber floor is coincident with the deck of the fractionation stage below the receiver.