WO2004071618A1

WO2004071618A1 - Dividing wall column control system

Info

Publication number: WO2004071618A1
Application number: PCT/US2003/004175
Authority: WO
Inventors: Charles P. Van Zile; James W. Harris
Original assignee: Uop Llc
Priority date: 2003-02-12
Filing date: 2003-02-12
Publication date: 2004-08-26
Also published as: AU2003210987A1

Abstract

The operation of the dividing wall section of a dividing wall column is controlled by a control apparatus comprising a ratio controller which divides the liquid flowing into the dividing wall section. The rate of return of overhead liquid to the column is set by monitoring the temperature in the top of the product dividing wall section and the sidecut product draw rate is set by monitoring a temperature in the bottom of the product dividing wall section.

Description

"DIVIDING WALL COLUMN CONTROL SYSTEM"

Background of the Invention

The invention relates to a control system for use in a dividing wall fractionation column. The operation of fractionation columns is regulated by control systems monitoring such variables as temperature, liquid levels and fluid flow rates. The control systems need to change operation of the column as by increasing the rate of heat input or decreasing the rate at which a stream is removed to compensate for occasional or periodic changes in the composition of the feed stream to the fractionation column or other factors which can affect the operation of the column. Control systems for fractionation columns have therefore reached a high state of development including the use of on-line analytical instruments and computerized optimization. They still .however, rely to a great extent on the measurement of temperatures and liquid levels in the fractionation column by equipment.

The dividing wall or Petlyuk configuration for fractionation columns was initially introduced some 50 years ago by Petlyuk et al. Recently the use of dividing wall columns has begun to expand because of the greater recognition that in certain situations dividing wall columns can provide benefits above those of conventional fractionation columns.

Control systems for dividing wall columns are not as mature or as commonly described as for conventional columns. An example of an actual control system for a dividing wall column is provided in US-A-4, 230,533 issued to V.A. Giroux. This reference is also relevant as liquid is collected in an upper portion of the column and divided between the two sides of the dividing wall by level control.

Brief Summary of the Invention

The invention is a method of controlling the operation of a dividing wall fractional distillation column. The liquid flowing downward in an upper portion of the column is collected and then divided by controlling the ratio between parallel feed and product dividing wall sections by use of suitable control equipment such as a ratio controller. Temperature measurements taken in upper and lower portions of the column are used to control respectively the rate at which overhead liquid is returned to the column and the rate at which the sidedraw product is removed from the product dividing wall section.

Thus in a broad form the invention is a control apparatus for controlling the operation of a dividing wall section of a dividing wall fractional distillation zone. The fractional distillation zone comprises a reboiler, an overhead liquid condensing and collection system, and a fractionation column containing vapor- liquid contacting devices. The fractionation column has a central, dividing wall section demarcated by a vertical dividing wall which divides the internal volume of the column into parallel feed and product sections having upper and lower ends. A first valve regulates the flow of a first liquid stream wherein the first liquid is stream is formed from an overhead product stream withdrawn from the overhead liquid condensing and collection system. The operation of the first valve is regulated in response to a temperature measurement made within the column at a point in an upper portion of the product section of the dividing wall section of the column. A second valve regulates the flow rate of a second liquid stream that flows to the feed section of the column and that flows from a liquid receiving tray that intercepts liquid flowing downward in the column at a point above the dividing section of the column and below an upper section of the column that contains vapor-liquid contacting devices. A third valve regulates the flow rate of a third liquid stream that flows to the product section of the column and from the liquid receiving tray. A fourth valve regulates the flow rate of a sidedraw liquid withdrawn from an intermediate point of the product section of the dividing wall section of the column. The operation of the fourth valve is regulated in response to a temperature measurement taken in a lower portion of the product section. A controller regulates the input of heat into said reboiler and a flow rate controller regulating the rate of withdrawal of a net bottoms stream from the column.

An specific apparatus embodiment of the invention may be described as a dividing wall fractional distillation column comprising a vertical outer column having a vertical central portion divided into a feed section and a parallel product section by a dividing wall, the parallel feed and product sections having upper and lower ends, with the dividing wall column also comprising an upper section containing vapor-liquid contacting devices; an overhead condensing system in which vapor rising from the upper section of the column is at least partially condensed and an overhead liquid is produced; a first flow control valve which controls the flow of a portion of said overhead liquid into the upper section of the column as reflux; a first temperature sensor, which monitors the temperature in the upper end of the product section of the column and generates a signal used in controlling the operation of the first flow control valve; a first liquid collection system located in an upper portion of the column below the upper section and above the dividing wall, the first liquid collection system blocking downward liquid flow through substantially all of the cross section of the column; a flow ratio controller which controls the division of liquid collected in the first liquid collection system into separate streams which flow via a first and second conduits into the feed and product sections of the column; a second liquid collection system, which is located in an intermediate portion of product section of the column, with the second liquid collection system having a liquid collection well from which a side product is removed from the column; a second flow control valve which controls the rate of side product removal in response to a second signal; a second temperature sensor, which monitors the temperature in the lower end of the product section of the column and generates a signal used in controlling the operation of the second flow control valve; a control system regulating the heat input to the column by a reboiler system located at the bottom of the column; and, a flow control system regulating the rate at which a net bottoms product is removed from the column.

Brief Description of the Drawing

The drawing is a simplified diagram of a dividing wall fractionation column employing a control system according to the subject invention.

Detailed Description and Preferred Embodiments The term divided wall fractionation column is intended to identify a fractional distillation column, of general applicability, which has as one of its basic components a substantially fluid tight vertical wall extending through a significant portion of its height and located in a central portion of the column to divide this central portion into at least two vertical, parallel vapor-liquid contacting sections. The top and bottom of the wall terminate in the column at a point distant from the respective end of the column such that there is open communication across the column interior at the top and bottom of the dividing wall and vapor leaving the top of each section and liquid leaving the bottom of each section flows in admixture into a common section. Components of the feed leaving the one section can then enter the other dividing wall section or continue toward the relevant end of the column. Each section will contain fractionation trays and/or packing intended to promote the separation. The feed stream to the column enters on a first receiving side of the dividing wall section of the column, typically in a middle portion of this receiving section of the column, but may enter near the top or bottom of the receiving section. As further distinguishing point the dividing wall column divides the feed stream into at least three different product streams. One of these is removed from the partial-column product section of the column opposite the receiving section. The other two product streams are removed near the top and bottom of the column in a manner similar to a conventional column. Dividing wall columns operate at conventional fractional distillation conditions. The pressure can range from subatmospheric to 500 psig and the bottom temperature can range from 10°C to 350°C. Cryogenic operations could also be performed. Pressure control can be performed by conventional means. For instance, many feeds amenable to separation in a dividing wall column often do not contain light or non condensable gases except as low volume impurities which can be removed via a vacuum system in the case of operation under a vacuum. An example of this is the fractionation of a kerosene boiling range hydrocarbon fraction to produce a limited carbon number range product as the feed to a downstream liquid-liquid extraction unit. Proper feed storage and processing normally greatly reduces the need for removal of light gases and hence pressure control is not a significant problem.

It is another objective to provide a control system to control the liquid flow rates through the two adjacent sections located in the middle portion (dividing wall section) of the column. This and other objectives are met by a control system which employs a liquid trap to collect descending liquid just above the dividing wall section of the column and a control system to then divide the liquid between the feed and product dividing wall sections of the column. This ratio is set initially based upon consideration of the calculated desired liquid flow (reflux) rates through each section of the dividing wall section. When the column is in operation the ratio may be adjusted, but it is normally fixed. Product purity is controlled by the reflux rate to the top of the column. The rate at which liquid collected in the overhead condensing system is returned to the column is controlled on the basis of a temperature measurement taken within or near the top of the upper portion of the dividing wall section of the column, preferably within the product section. The rate of liquid return is increased as the temperature at this point moves above a setpoint. As this rate of liquid return to the column is increased, liquid is collected at a faster rate in the liquid collection system, which is located in the upper section of the column just above the dividing wall section, raising the level in the liquid collection well. This system is referred to herein as the first liquid collection system. The level control in the well of this section will sense this increase in liquid level and signal for the manifold system to begin removing more liquid for passage downward into the dividing wall section of the column. The flow through one line of the manifold system will increase leading the ratio controller to increase the flow rate in the other line. This liquid flow will thus increase proportionately in both sides of the dividing wall column.

The rate at which the sidecut product stream is removed from a midpoint of the product section of the dividing wall section is set in response to a temperature measurement taken within a lower portion of the product section. Decreasing the net rate of sidecut product removal increases the liquid remaining to flow downward through the product section as lower reflux. The temperature in the lower section of the product section will therefore decrease. The rate of product withdrawal is therefore increased if the measured temperature is below a setpoint. Control may be affected through directly regulating the amount of totally trapped liquid which is returned as lower reflux or by only withdrawing a controlled amount of liquid, with the remainder overflowing a weir to form the lower reflux. The temperature setpoints may be set manually or be the result of a higher level control system. Preferably heat is added to the reboiling system at the bottom of the column at a substantially constant rate subject to only occasional, possibly manual adjustment.

The figure is a simplified illustration of a dividing wall fractionation column 1 employing the subject control system. The figure also illustrates the use of required supplemental control components at the top and bottom portions of the column. Referring now to the drawing, a feed stream comprising a kerosene boiling range fraction enters the fractionation column via line 2. The objective of the fractionation is to produce a middle fraction comprising substantially all of the C₉-C₁₅ hydrocarbons in the feed. Hydrocarbons with lower or higher carbon numbers are to be rejected in the overhead and bottoms respectively. The feed stream in this instance is fed near the midpoint of the dividing wall section of the column. The dividing wall section of the column 1 is that portion of the column which in this instance is to the left of the dividi ng wall 4 and which receives the feed stream 2. The opposite side of the dividi ng wall forms a section of the column referred to herein as the product secti on of the dividing wall section of the column. These two parallel fractionation sections form the central portion of the column. The entire fractionation column 1 contains fractionation trays 3 which extend horizontally across the cross section of the column. Larger full width or full diameter trays 3 are located in the upper and lower portions of the column which are located respectively above and below the divided wall section of the column. The feed and product sections of the dividing wall section of the column both also contain fractionation trays 3. The type, design, size and/or spacing of the fractionation trays may vary between the two sections of the dividing wall section of the column. Furthermore, any portion of the column may contain alternative vapor-liquid contacting devices, such as dumped or structured packing. Conventional trays, such as sieve trays arranged as multiple downcomer trays or cross-flow trays, can be employed. For example, the column designed for a commercial scale separation of the feed kerosene used in this description contained a total of 64 trays. Nineteen trays are located in the upper section of the column (above the dividing wall section) and eleven trays are located in the lower section of the column. Trays numbered 20 through 53 were located in the product section of the dividing wall section. A separate numbering system is used for 40 trays located in the feed section of the dividing wall section of the column.

The heavier components of the feed stream 2 will begin to move downward through the feed or receiving section of the dividing wall section of the column. They will eventually emerge in the liquid leaving the bottom of the dividing wall section and will flow into the bottom portion of the column 1. The least volatile, or heaviest, components of the kerosene will be concentrated into a liquid phase retained in the bottom of the column and withdrawn through line 5. The bottoms liquid stream of line 5 is divided into two portions. A first portion is removed through line 7 at a rate controlled by valve 18. Valve 18 is operated by an actuator 17 which receives a signal via the signal carrier 16. This signal is generated by the flow controller 15 which monitors the rate of flow through line 7. The flow controller in turn functions to reset the flow through valve 18 based upon a signal carried to it by the signal carrier 14. This signal is generated by the level controller 13 which monitors the level of liquid retained in the bottom of the fractionation column 1.

A second portion of the bottoms stream of line 5 is passed into line 6. This second portion flows through the reboiler 8 wherein it picks up heat energy prior to being returned to the bottom of column 1. This circuit provides the heat necessary for the fractional distillation which is performed within the column. A heating medium, such as hot oil or steam, flows through line 9 at a rate controlled by valve 10. The opening position of valve 10 is regulated in response to a signal carried by the signal carrier 12 from a flow controller means 11. Flow controller 11 monitors the flow of a heating medium which is leaving the reboiler 8. The location of this flow controller and valve are often reversed.

The lighter components of the feed stream of line 2 travel upward in the vapors rising through the feed or receiving section of the dividing wall section of the column and merge into the upper portion of the column. Rising vapors from the receiving section and the product section pass upward through the chimneys 46 into the undivided upper portion of the column 1. Fractional distillation continues to purify these vapors in the trays of the upper column portion. Rising overhead vapors eventually enter the overhead condenser 28. The figure illustrates the overhead condenser 28 as a contact condenser receiving a coolant from a line 36 at the top of the column. Condensation in the top portion of the column, together with the collection of the liquid from line 36, produces an inventory of liquid in a liquid collection system lo ated in the bottom of the contact condensing system. Vapor passageways pass vapor upward through the liquid collection system. Level controller 40 monitors the liquid inventory in the liquid collection system. Line 29 withdraws a stream of the liquid retained in the contact condenser 28. This stream should be substantially free of Cg-plus hydrocarbons. A first portion of this overhead stream passes into line 30 and a second portion passes through line 34. Fin fan (air) cooler 35 cools the portion of the overhead stream passed through line 34. Division of this stream forms the net overhead stream removed through line 37 and the coolant stream returned to the column through line 36. The rate of flow of the net overhead stream of line 37 is controlled by a valve 38 which receives a signal transmitted by the level controller 40 and carried by the signal carrier 39. The flow rate of the coolant through line 36 is set by a flow controller 41 which generates a signal carried by the signal carrier or conduit 42 to the flow control valve 43 which actually performs the regulation of the flow rate.

The first split off portion of the overhead liquid stream of line 29 is passed through line 30 at a rate controlled by a valve 33. Line 30 delivers this liquid to the top tray of the undivided upper section of the column 1. Valve 33 regulates the rate of flow of this liquid stream via a signal carried from the flow controller 31 by conduit 32. The flow controller 31 monitors the flow rate of the liquid stream of line 30. The flow rate of this stream is adjusted in response to a signal carried by the signal carrier 45 from the temperature controller 44. Temperature controller 44 monitors the temperature of the liquid or vapor present at the top of the product section of the divided wall section of the column 1 and preferably at the top tray. The exact location of this temperature sensing may vary and with minor adjustment may move downwardly within the product section of the dividing wall section. Other potential variation the invention implementation applies to the logic control apparatus that calculates the required adjustment in the valve 33 which may be located in either the temperature controller 44 or the flow controller 31 which receives a signal from the temperature controller. Thus the signal carried by the carrier 45 may only represent the temperature measured by the temperature controller 44 or an adjustment which is required in the flow through line 30.

Signals generated by the ratio controller 56 control the liquid supply to the top of both the feed section and the product section of the dividing wall section of the column. Although the supply of this liquid to each to feed and product section could use separate paths, preferably the sequence of starts by collecting the entire downward liquid flow through the column in the liquid collection well or trap out 47 of a tray which extends across the cross section of the column at a point preferably located just above the top of the dividing wall section of the column. A manifold system withdraws and divides this liquid into separate streams fed to each divided section of the column. Level controller 48 monitors the liquid level in liquid collection well 47. The level controller 48 generates a signal transmitted through the signal carrier 61 to a flow controller 58. The flow controller 58 in turn generates a signal transmitted through the signal carrier 59 to the flow control valve 60. Valve 60 regulates the flow of liquid through line 51. The liquid flowing through line 51 is delivered to the top tray of the product section of the dividing wall section. A pump (not shown) pressurizes liquid withdrawn from the collection well 47 before it enters line 51 and line 50. Optionally, a surge drum (not shown) may smooth the liquid flows through line 51 and 50. Valve 53 directly regulates the flow rate through line 50 in response to an actuating signal carried by signal carrier 52 from the flow control device 54. Flow controller 54 monitors the flow rate to the valve 53 and adjusts the signal being transmitted to valve 53 in response to an input signal carried to the flow controller 54 by the signal carrier 55. This signal is generated by the ratio controller 56 which adjusts the desired liquid split or division between the two sections or chambers of the dividing wall section of the column. In order to do this, the ratio controller must receive signals representative of the flows in lines 50 and 51 , which can be carried by signal carriers 55 and 57.

The relative flows in lines 50 and 51 can be switched between the two lines. That is, the level control signal from the level controller 48 can be used to regulate the flow in line 50 instead of line 51 , with the ratio controller 56 then acting on the flow in line 51.

Liquid enters the top of the product section or chamber of the dividing wall column through the open end of this section and vapor enters the bottom of the section. Both flows allow the introduction of kerosene boiling range hydrocarbons that exit as the sidecut product stream of line 23. Thus hydrocarbons vapors generated in the bottom of the column may enter the bottom of the product section and pass upward. Similarly hydrocarbons driven upward into the upper portion of the column 1 may enter as liquid from line 51. A substantially imperforate tray extending across a midsection of the product section of the column intercepts the liquid flowing downward through the product section and acts as a trap out tray. A liquid collection well 25 of this tray collects a quantity of this descending liquid. The level of the liquid in this well may be monitored by a controller not illustrated. A line 23 withdraws collected liquid from the well 25. Pump 24 pressurizes this liquid stream for discharging a controlled amount from the process as the middle product of the fractional distillation. In the illustrated embodiment, a valve 22 controls the flow rate of the product stream of line 23 in response to a signal delivered to it by the signal carrier 62. A signal controlling the actuator of this valve is generated by the flow controller 21 which in turn receives a signal from the signal carrier 20. Temperature controller 19 generated this signal on the basis of the temperature in a lower portion of the product section. The trap out tray also contains a vapor chimney 27 to allow the upward passage of vapor through the product section to fulfill the objective of the subject control system in not directly controlling the upward flow of vapor through the column. An overflow weir 26 on this trap out tray allows liquid in excess of that required for the generation of the side product stream of line 23 to overflow the weir and to continue downward to the remainder of the product section. The liquid overflowing the weir is the lower reflux liquid to the part of the product section located below the trap out tray. The flow rate of this lower reflux equals the liquid left after the net sidedraw product is removed. Alternatively, the division between the refluxed trap out liquid and the side product removed through line 23 can occur outside the column by not providing weir 26 and removing all liquid from the column by a total trap out. An optional (dotted) line 63 then returns a portion of this withdrawn liquid to the topmost tray in the lower portion of the product section as reflux. A valve receiving a signal from the temperature controller 19 may directly regulate the flow rate of this stream. Controlling the fraction of returned trap out liquid sets determines the amount of the trap out liquid removed as product. In this mode a level controller in the liquid collection well 25 can control the total rate at which liquid is removed from the process. There are many alternatives possible in both the equipment and the methods of performing the invention illustrated in the figure. For instance, when the control of the operation of a valve involves two or more controllers, certain logic functions could be located in either of the controllers or split between them. That is, the items referred to as a temperature controller or a level controller on the subject figure may generate signals which are only representative of a measurement or may generate signals representative of the actual change which should be performed. In this regard it is noted that the level controller 40, shown at the top of the column, directly sends a signal to the valve 38. In contrast, the level controller 13, shown at the bottom of the column, sends a signal to a second controller 15 with the second controller generating the signal which is transferred to the valve 18.

There are many types of appropriate sensors or controllers and the choice of the most appropriate sensors and the most appropriate valves, etc., will depend, for instance, on such variables as the composition of the material separated in the column and the conditions employed in the column. The various signal carriers or conduits may comprise electrical wires, cables, conduits, optical cables or even wireless transmission devices.

The internal condenser depicted in the drawing as a contact condenser is only one alternative means of providing the cooling and condensation required for the operation of the column. A more conventional external condenser and overhead receiver can be employed. The use of an internal condenser can reduce the capital cost of the overall fractionation zone.

A further possible variation is the integration of the depicted control system into a control system of the unit which generates the feed stream of line 2 or into the units or processes which receive the various product streams from the column or integration of the column operation into an advanced control system that regulates the associated processes. Optionally analytical instruments may measure the composition of one or more of the effluent streams to adjust the column operation for compositional divergence.

The subject control system and control method apply to any separation of volatile compounds suitable for a dividing wall fractional distillation column. Non- limiting examples include separation of a wide boiling range petroleum-derived fraction into fractions having narrower boiling point ranges, the separation of aromatic hydrocarbons such as benzene and toluene, the simultaneous stripping and rerunning of feed stream, the recovery of solvents or desorbents from the products of adsorptive or liquid-liquid separation processes, the separation of halogenated compounds, the separation of motor fuel blending components and the separation of product and recycle compounds in aromatic hydrocarbon alkylation, transalkylation and disproportionation processes.

Claims

Claims:

1. A control apparatus for controlling the operation of a dividing wall section of a dividing wall fractional distillation zone, the fractional distillation zone comprising a reboiler, an overhead liquid condensing and collection system, and a fractionation column containing vapor-liquid contacting devices, with the fractionation column having a central, dividing wall section demarcated by a vertical dividing wall which divides the internal volume of the column into parallel feed and product sections having upper and lower ends, the control apparatus comprising: a.) a first valve regulating the flow of a first liquid stream wherein the first liquid is stream is formed from an overhead product stream withdrawn from the overhead liquid condensing and collection system with the operation of the first valve regulated in response to a temperature measurement made within the column at a point in an upper portion of the product section of the dividing wall section of the column; b.) a second valve regulating the flow rate of a second liquid stream that flows to the feed section of the column and that flows from a liquid receiving tray that intercepts liquid flowing downward in the column at a point above the dividing section of the column and below an upper section of the column that contains vapor-liquid contacting devices; c.) a third valve regulating the flow rate of a third liquid stream that flows to the product section of the column and from the liquid receiving tray; d.) a fourth valve regulating the flow rate of a sidedraw liquid withdrawn from an intermediate point of the product section of the dividing wall section of the column, with the operation of the fourth valve regulated in response to a temperature measurement taken in a lower portion of the product section; e.) a controller regulating the input of heat into said reboiler; and f.) a flow rate controller regulating the rate of withdrawal of a net bottoms stream from the column.

2. A dividing wall fractional distillation column using the control apparatus of claim 1 wherein: the fractional distillation system has an overhead condensing system in which vapor rising from the upper section of the column is at least partially condensed and an overhead liquid is produced and the first flow control valve controls the flow of a portion of the first liquid stream into the upper section of the column as reflux in response to a first temperature sensor that monitors the temperature in the upper end of the product section of the column and generates a signal used in controlling the operation of the first flow control valve; the liquid receiving tray comprises a first liquid collection system that blocks downward liquid flow through substantially all of the cross section of the column; a flow ratio controller controls the division of liquid collected in the first liquid collection system between the second and third liquid streams; first and second conduits deliver the second and third liquid stream to the feed and product sections, respectively; a second liquid collection system is located in an intermediate portion of the product section of the column and a side product is removed from the second collection well; and, the fourth valve controls the removal of side products from the second liquid collection system via a liquid collection well in response to a signal from a second temperature sensor that monitors the temperature in the lower end of the product section of the column and generates a signal used in controlling the operation of the fourth flow control valve.

3. The control apparatus of claims 1 or 2 further characterized in that a liquid overflow weir is provided within the second liquid collection system.

4. The control apparatus of claims 1 or 2 further characterized in that the second liquid collection system comprises a total trap out tray and in that the fourth flow control valve controls the rate at which liquid withdrawn from the trap out tray flows through a transfer line back to the column at a point under the total trap out tray as lower reflux.

5. The control apparatus of claims 1 or 2 wherein a fifth valve is regulated in response to a measurement of the accumulation of overhead liquid in the overhead liquid condensing and collection system and the fifth valve regulates the flow of a net overhead product stream formed from the overhead product stream withdrawn from the overhead liquid condensing and collection system.

6. The control apparatus of claims 1 or 2 wherein a level measurement device measures the liquid level in a liquid collection well of the first liquid receiving tray.

7. The control apparatus of claims 1 or 2 wherein the first and second liquid streams flow together initially from the liquid receiving tray in a single conduit and a manifold system divides the initial single stream into first and second liquid streams.

8. The control apparatus of claim 6 wherein a ratio controller controls the division of the first and second liquid streams in response to a signal from the level measurement device measuring the liquid level of the liquid collection well of^'the first liquid receiving tray.

9. The control apparatus of claims 1or 2 further characterized in that the controller regulating the input of heat into the reboiler maintains the heat input at a substantially constant level.