US2725351A - Fractionation control system - Google Patents

Description

Nov. 29, 1955 w, GROTE 2,725,351

FRACTIONATION CONTROL SYSTEM Filed March 30, 1953 Feed lnlef Fracfionafor Bottoms Ou lef i INVENTOR: Henry W Grate A T TORNE Y5:

United States PatentO FRACTIONATION ONTROL SYSTEM Henry W. Grote, Hinsdale, lll., assignor to Universal Oil Products Company, Des Plaines, Ill., a eerporatien of Delaware Application March so, 1953, Serial No. 345,341

3 Claims. (Cl. 202160) In connection with an essentially binary fractional separating operation, where the light and heavy materials have widely different boiling points, there is normally a rather large temperature change Within a small portion of the column. Further, a temperature gradient or a temperature profile for the fractionating Zone, in such instances, would indicate a sharp temperature change or break at the zone of the interface between the invent ory of light material and the vapor of the heavier material. Thus in such instances, where only a single thermocouple or temperature sensitive indicator is in the upper control zone of a column, there can be a wide swing or change in the temperature reading in a matter of a few seconds as the temperature break passes the thermocouple. The variation ofcourse depends upon the difference in boiling points between the materials being fractionated, and in some operations may wellbe of the order of 200 F. or more. A wide swing inthe temperature indicator results in a radical change in a temperature controller and the rate of return of reflux to a column, so that as a net result there is a constant undesirable up and down level, or inventory, or light material within the upper portion of the column. Preferably there is a relatively constant quantity or depth of light material in the upper portion of the column'to maintain a good distillation separation and purity of overhead.

It is therefore a principal object of the present invention to provide an improved control arrangement for a fractionating system in a manner maintaining a relatively stable temperature profile and inventory of light material in the upper portion of the column.

It is also an object of the invention to provide control means operating automatically to regulate the return of reflux to a fractionating column and maintain a substantially stable inventory of light material in the upper portion of the fractionating zone.

Briefly, the control system for a fractionation zone, as provided by the present invention, comprises, maintaining a plurality of temperature indicating means spaced vertically within a control section in the upper portion of the fractionating zone, measuring temperature at each of the temperature indicating means and obtaining a substantially arithmetic average of the temperature gradient in the control section of the zone, and regulating the quantity of light material maintained within the upper portion of the zone responsive to the average temperature obtained for the control section whereby to maintain a desired inventory of light material therein.

Materials which have widely different boiling points may ordinarily be separated by flash distallation, however, where too much heavy material is obtained in the overhead stream and too much light material is obtained in the bottom stream it may be desirable to effect the separation of the materials by a relatively simple fractional distillation operation. .The present invention is directed to a'means for efiecting an improved operation in such instances. The separation .of isomers or other materials having boiling points of the order of something less than 100 F. difference are not necessarily separated advantageously in accordance with the present invention since a single temperature indicating'means connecting with a controller may provide an operation which is less sluggish and therefore preferable. In other words, a substantially arithmetic average of the varying temperature through a control section of a fractionating zone, provides an average effect upon a given controller and a resulting reduced and modified automatic effect regulating the return of reflux to the column, and such a modified result may provide a response .which is too slow and materials having relatively close boiling points are preferably separated by the more conventional control means. 7

One example of the desirable utility of the present invention is its use in connection with the separation of a water and butyl alcohol mixture, boiling at approximately 2002l0 F., from a bottoms stream comprising principally hydroxyanisol, having a boiling point of about 480 F. In this instance there is a difference in boiling points of the order of about 270280f F. and there is a very sharp temperature profile in the fractionating zonewhich makes it substantially impractical to effect regulation of the zone by a single thermocouple or other temperature sensing element therein connective with a temperature-controller, for the single thermocouple causes the control apparatus to change rapidly within a few seconds as the interface passes the thermocouple. In the present explanation and description the term interface relates to the sharp break in the temperature profile in the column and its effect is somewhat analogous to the effect of a liquid interface between liquids having widely different densities. The use of the multiple thermocouples connected in parallel and reading approximately an arithmetic average temperature over an appreciable zone in the fractionating column hence changes the ;temperature control apparatus somewhat more slowly with changes in inventory of light material-in the upper portion of the column. This improved arrangement thus makes it possible to operate on full automatic control in a very desirable manner.

Reference to the accompanying drawing and the following. description thereof will serve to more fully explain the. improved control arrangement in combination with a fractionating zone whereby to provide the desired advantage of maintaining a regulated inventory of light material in the upper portion of the fractionating zone.

Referring now specifically to the drawing, there is shown a fractionating column 1 being supplied with a mixed feed streamby way of line 2 and valve 3 at an intermediate portion of the column. The fractionating column 1 is also indicated diagrammatically as having a perforate plate 4 supporting a packed bed of granular material-5 throughout substantially the full height of the column, whereby the latter may serve as a packed fractionating zone effecting the fractional separation of the mixed feed stream. l

A substantially constant rate of heat input is shown as being supplied to the lower end of the fractionating col-. umn 1 by means of a reboiler 6. The latter connects with the column through a bottoms draw-off line 7, having valve 8, and by a return line 9. The bottoms stream draw-off is made from the lower portion of the reboiler shell by line 10 and valve 11, the latter being automatically regulated by level control 12.

The overhead product stream is passed from the upper end of fractionatorl by way of line 13 and valve 14, cooler 15, and line 16, which connects with receiver 17.

The lower portion of receiver 17 has an outlet line 18 with valve 19 connecting to a pump 20, which in turn is suitable to return reflux tocolumn 1 by way of line 21 and valve 22 and distributing header 25, or discharge overhead product by way of line 23 and control valve 24.

Although spaced temperature indicators may be used to provide readings for the manual calculation of an average temperature in the upper part of the column, in accordance with a preferred embodiment of the present invention, means is provided for effecting an improved automatic control for thefractionating column 1 by utilizing a plurality of vertically spaced thermocouples or temperature sensitive indicators 26 in an upper control section of the column and connected in parallel. The present embodiment indicates six thermocouples 26 being spaced a short distance apart in the upper portion of the fractionating zone, however, more or less indicators may be utilized in any installation, depending upon size, and

may be spaced throughout a control section which will maintain the desired inventory of light material in the upper portion of the fractionating zone. Each of the thermocouples 26 connect into a parallel type of electrical circuit, connecting with lines 27 and 28 which in turn connect with an automatically operating temperature controller 29. The latter connects through a control line 30 with the automatic control valve 24 in line 23 so as to vary the amount of overhead distillate being withdrawn from the system and in turn vary the amount of reflux return to the upper portion of fractionating column 1 by way of return line 21 and the spray heads or distributing means 25. The parallel connection of each of the thermocouples 26 with controller 29 effects an arithmetic average of the widely different temperatures in the entire control zone and thus provides an averaging effect on the controller, even though the actual temperature profile within the zone has a sharp break or variation at a given level corresponding to the aforementioned interface of temperature. The averaged temperature gradient permits the controller 29 to vary somewhat gradually or uniformly, even though there may be a wide temperature range, and thus vary in a gradual manner the amount of reflux being returned to the column whereby to maintain a more constant light material inventory in the upper portion of the zone.

In actual operation, an increase in the averaged temperature due to a shrinking of the light material inventory and by an increase in the heavy material vapor, is sensed by the parallel circuitof thermocouples 26 and indicated to the controller 29. The latter operates to close valve 24 whereby an increased amount of reflux is pumped by way .of-line 21 to the upper portion of thecolumn and there is a resultant increase in the light material inventory. Conversely, a decrease in the averaged temperature reading to controller 29 effects a decrease in the quantity of reflux being returned to the column byway of line 21, and a corresponding decrease in the inventory of light material.

It is not intended to limit the present invention to the use of any one type of control instrument or instruments. Any of the suitable conventional types of thermocouples or temperature indicators may be spaced within the zone, while the control instrument 29 may be a type of electrical, pneumatic, or combination instrument capable of varying a valve setting or other control means in accordance with changes in temperature from one or more sensing elements. It is also to be noted that the present improved control arrangement may be utilized in connection with a bubble tray type of fractionating column and is not limited to a packed column or to the use of any one type of packing material in a fractionating zone.

I claim as my invention:

1. In a fractionating column for the separation of a mixture of light and heavy materials of widely different boiling points, said column having a feed inlet for said mixture at an intermediate point in the height thereof, the combination of a plurality of temperature indicators electrically connected in parallel and disposed at vertically spaced points in an upper control section of the column above said inlet, a temperature controller in the parallel electric circuit of said indicators and adapted to read the arithmetic average of the temperatures sensed by said indicators in said control section, and means co-acting With said controller for regulating the quantity of light material maintained within the upper portion of the column in response to the average temperature reading obtained by the controller from said parallel connected temperature indicators.

2. In a fractionating column for the separation of a mixture of light and heavy materials of widely different boiling points, said column having a feed inlet tor sald mixture at an intermediate point in the height thereof and being provided with means for the return of light reflux material to the upper portion thereof, the combination of a plurality of temperature indicators electrically connected in parallel and'disposed at vertically spaced points in an upper control section of the column above said inlet, a temperature controller in the parallel electric circuit of said indicators and adapted to read the arithmetic average of the temperatures sensed by said indicators in said control section, and means co-acting with said controller for regulating the return of light reflux material to the upper portion of the column in response to the average temperature reading obtained by the controller from said parallel connected temperature indicators.

3. in a fractionating column for the separation of a mixture of light and heavy materials of Widely different boiling points, said column having a feed inlet for said mixture at an intermediate point in the height thereof and being provided With means for the return of light reflux material to the upper portion thereof, the combination of a plurality of thermocouples electrically connected in parallel and disposed at vertically spaced points in an upper control section of the column above said inlet, a temperature controller in the parallel electric circuit of said thermocouples, the parallel connection of each of said thermocouples with said controller providing a substantially arithmetic average of the temperature gradient in said control section, and means co-acting with said controller for regulating the return of light reflux material to the upper portion of the column in response to the average temperature of the control section provided by the parallel circuit of said thermocouples and controller.

References Cited in the file of this patent UNITED STATES PATENTS 1,775,540 Taylor Sept. 9, '1930 2,022,809 Kramer Dec. 3, 1935 2,456,398 Gerhold Dec. 14, 1948 2,580,651 Boyd Jan. 1, 1952

Claims (1)

1. IN A FRACTIONATING COLUMN FOR THE SEPARATION OF A MIXTURE OF LIGHT AND HEAVY MATERIALS OF WIDELY DIFFERENT BOILING POINTS, SAID COLUMN HAVING A FEED INLET FOR SAID MIXTURE AT AN INTERMEDIATE POINT IN THE HEIGHT THEREOF, THE COMBINATION OF A PLURALITY OF TEMPERATURE INDICATORS ELECTRICALLY CONNECTED IN PARALLEL AND DISPOSED AT VERTICALLY SPACED POINTS IN AN UPPER CONTROL SECTION OF THE COLUMN ABOVE SAID INLET, A TEMPERATURE CONTROLLER IN THE PARALLEL ELECTRIC CIRCUIT OF SAID INDICATORS AND ADAPTED TO READ THE ARITHMETIC AVERAGE OF THE TEMPERATURES SENSED BY SAID INDICATORS IN SAID CONTROL SECTION, AND MEANS CO-ACTING WITH SAID CONTROLLER FOR REGULATING THE QUANTITY OF LIGHT MATERIAL MAINTAINED WITHIN THE UPPER PORTION OF THE COLUMN IN RESPONSE TO THE AVERAGE TEMPERATURE READING OBTAINED BY THE CONTROLLER FROM SAID PARALLEL CONNECTED TEMPERATURE INDICATORS.

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