US2599133A

US2599133A - Method and apparatus for controlling fractionating columns

Info

Publication number: US2599133A
Application number: US94877A
Authority: US
Inventors: Clarence J Schilling
Original assignee: Air Products Inc
Current assignee: Air Products Inc
Priority date: 1949-05-23
Filing date: 1949-05-23
Publication date: 1952-06-03
Anticipated expiration: 1969-06-03

Description

June 3,

'c. J. SCHILLING METHOD AND APPARATUS FOR CONTROLLING FRACTIONATING COLUMNS Filed May 25, 1949 I N VEN TOR.

CLARENCE J. SCHILLING suchusefulness by high Patented June 3, 1952 METHOD AND APPARATUS Foe ooN'rRoL- LING FRACTIONATING COLUMNS Clarence J. Schilling, Allentown, Pa., assignor to Air, Products, Incorporated. a. corporation of Michigan; I

" Application May 23, 1949jS erialNo.9437? 7 Glaims.

This invention relates to automatic controls for fra'ctionating: columns ='andmore particularly to a method-of and apparatus for the automatic control of the purity of -a'product a iracti'onatingcolumn. I

produced in well known that the-- purity of-"a' product from a fractionating column'ma-y be controlled by varyingthe rate at which the product is withdrawn; '-'On-=increasing'- the rate of withdrawal,

the purity diminishes; on-decreasing the withdrawal-ra'te; the purity rises. This a commonly used "step fo'rcontrolling product purity. -.The

adjustment; of the product outlet valveis made manually by :the operator guided by observation 'overthe outletvalve:opening which-would be responsive directly to purity,- asf'or example by the use of a thermal-conductivity purity meter; :however; such' instruments are reduced in practical cost' and great delicacy of construction;

' This invention'has as an object the provision of a method-and apparatus for the automatic control of the purity of the bottom product produced in fractionating a mixture; bycontrolling the rate "at which the bottom product is withdrawn.

A further object is to control automatically the withdrawal of the bottom product by taking advantage or the wide changes in thetemperatu re differential between difierent-levels in fractionating a mixture.

"Another object of the invention is to provide in thefractionation of a mixture control of the piirity of the product at the bottom of the column by utilizing changes in the temperature differential between the bottom of the column and another point in the column at a higher level.

Ifhese and other objects are accomplished by the present invention wherein one means responsive to changes intemperature is provided at the bottom of a fractionating column where one component is at thema'ximum'purity, and another suchmeans'is provided in the neighborhood of a level higher inthe column where the concentration of a second component is beginning to drop from a maximum and the concentration of a third and more volatile component is negligible but is beginning'to increase. so that changes in the temperature differential between the two levels are utilized-to control' the rate of withdrawal of the first component and thus stabilize the purity of this component.

' While' this invention is applicable to the-control of the purity of a product obtained in the fractionation of any normally gaseous mixture,

it is found especiallyuseiul in connection with the' fractionation of air, wherein the -purity of volume, thusdisregardi-ng the minute quantities of the rarer elements which do not affect materiallythe composition of the products obtained in the separation of the air "into oxygen and nitrogen. I

Usually noattentionis given to the. argon content of the air.--'Argo'n"has a boiling point'about 3 C. below 'that of oxygen and about 10- C. above that-of nitrogen. Consequently, in the normal rectification of-' air, the argon tends to follow the'oxygen to the lower levels of the rectii'lertand thus tends to contaminate the oxygen product.

The concentration of the argon in both the liquid and Vaporsincreases materially until a point at a certain levelin the rectification column is reached'where the: argon concentrationis at a maximum. In practice, the percentage of argon accumulating at this point in the column is between 'Ia'n'd 12%. 1 v.

Since the boiling 'anddew-curves of argonoxygen mixtures lie close together,uthe mixture is. more difllcult to "separate than an oxygennitrogen mixture. Thus, on the lower-plates of a fractionating column for a tertiary mixture such as air, the separation is between the argon and the oxygen, and only a'negligible amount of nitrogen is present. At a level about one third of the way up from the bottom of the column, the percentage of argon on each plate begins to decrease from the maximum concentration, and the percentage of the nitrogen starts to increase. It is this level in the column that is utilized in the present invention for the location of the upper control means. 3 Y

At constant pressure, the boiling point of liquid oxygen-rich mixture riseswith increasing approach to purity in oxygen. If this change in temperature could be directly used to control the setting of the oxygen withdrawal valve, the purity of the oxygen could be directly controlled. This change in temperatureof the liquid oxygen in the "receiver is not,- however, available for directly actuating an'oxygen valve control mechanism; first, because it is of very small magnitude, and second, becauseit is impossible to avoid some small fluctuations in pressurewhich 'vary' the boiling point of oxygen of constant composition.

The effect of changes in column pressure may drop and a concentration of nitrogenisbeginning to appear.

In an air fractionating column, the tempera- .ture increases from plate to plate, downwardly;

and is at the maximum in the oxygen receiver. The temperature increments from plate to plate are small at the ends of the column where the change in composition from one plate to the next is slight, but at an intermediate point, par- .ticularly in the region of maximum argon concentration where the nitrogen concentration is negligible, the increment from plate to plate is relatively large.

At this level, spaced from the oxygen receiver by about one-third the total number of plates, the change in temperature following from a given change inpurityof the oxygen in the bottom of the column is from five to ten times as great as the changein temperature in the receiver itself. This temperature differential becomes sufficient to control accurately the width of opening of the oxygen valve, through temperature responsive means located within the column at the two levels, i. e-., at the receiver and in the neighborhood of a suitably selected level spaced from it where the concentration of the: argon is at a maximum but is just beginning to decrease and the concentration of the nitrogen is negligible but is. just beginning to increase.

Certainpractical applications of the invention will be described with reference to the attached drawings and the following descriptions thereof,

in which:

Fig. 1 is a diagrammatical view of the medial form of the invention in which the differential expansions and contractions of two fluids are caused to actuate the needle of an oxygen outlet valve through mechanical linkage;

Fig. 3 is a diagrammatical view illustrating a form of the invention in which fluid expansion is used to actuate the valve needle through an interposed solenoid;

Fig. 4 is a diagrammatical view illustrating a form of the invention in which the expansions of rigid metallic elements actuate the valve stem through mechanical linkage and,

Fig. 5 is a diagrammatical view illustrating a form of the invention showing a control device which does not depend on expansion to actuate the control elements.

Referring first to Fig. 1, reference numeral I indicates the medial portion of a two-stage fractionating column of conventional design, the high pressure section I I being separated from the low pressure section, of which a fragment is indivcated at I2, by a partition plate l3 and a nitrogen condenser I4. The stack of fractionating plates l separates oxygenin a desired approach to purity. This liquid collects in a pool l6 surrounding the condenserand is kept constantly are connected through capillary tubes I9 and 2t 4 boiling in condensing nitrogen vapor evolved at considerably higher pressure in the high pressure section II.

A fluid-filled bulb IT is located at some convenient level in or closely adjacent to oxygen pool l6, preferably a short distance below its normal surface level. A. similar fluid-filled bulb I8 is placed on or near'a plate higher up in the column, preferably in the region near the maximum argon concentration as described above. These bulbs with. appropriate parts of a mechanism 2! illustrated in Fig. 2, which will next be described. It will be understood that while the location of the lower bulb. in or very close to the liquid oxygen pool is essential, the other bulb may be located on any plate above it, the responsiveness of the apparatus increasing to a maximum at the plate where the argon concentration is at a maximum but is just beginning to decrease and the concentration of the nitrogen is negligible but is beginning toincrease, which is about one-third the way upthe-lengthof the low pressure column. 7

Above this level, the. responsiveness diminshes. Thus, in the specification and claims, the term is used in the neighborhood of the level at which the argon concentration is beginning to decrease from a maximum. and the nitrogen concentration is negligible butv is beginningto increase. This term is meant to include a range of a number of plates wherein the control apparatus will be operative. When originally designing a plant, the level for placing, the upper temperature responsive means is 'calculated for the particular purity of oxygen which. the plant is designed to produce.

If, however, in operation of the plant, thepurity ofthe. oxygen is to be varied, the same upper level can be used to control the rate of withdrawal of the oxygen product; For purities other than those for which the plant was designed, the upper level will still be in the neighborhood of the point where the argon concentration is beginning to drop from a maximum and the nitrogen concentration is beginning to increase from a negligible amount. Although the level will not be at the preferred point, the apparatus will still operate satisfactorily and incorporate the advantages of the invention.

In Fig. 2, 22' is a valve body having an orifice 23- in which a tapering needle 24' is reciprocated to vary the width of the annular passage through which oxygen fiows. This body is suitably mounted in oxygen withdrawal pipe 25. Oxygen withdrawal pipe 25 may be located either below the liquid oxygen level or above it, for removal of the oxygen as a liquid or as a gas respectively.

A rig-id yoke 26 is firmly attached to the oxygen pipe .or valve body or otherwise accurately positioned with regard to the latter. Within'this yoke are mounted two Sylphon bellows 21 and 28, opposing each other in action. Projections 29 and 30- from the free ends of the bellows engage opposite sides of a lever 3| which is pivoted in one arm of the yoke as at 3,2. The free end of the lever. is linked at slot 33 to a valve stem 34 to which valve needle 24 is attached. If this attachment is by a threaded connection as at 35 the needle may be adjusted manually to find the valve opening which gives the required oxygen purity under normal operating conditions. This is an initial adjustment only, not the operative control. 7

The capillary tubes l9 and 20 are so connected to the respective bellows that the expansion of fluid in thelower bulb l1 tends to move the needle "downwardly or in a closing direction while the expansion of fluid in upper bulb l8 tends to move the needle upwardly and thus increase the valve opening. Assuming the fluid in each bulb'to be a gas under compression, an increase in the purity of the oxygen in pool l6 causes its temperature to rise and: thus" causes the pressure inbulb H and bellows-2T:to:-increase. Atthe sametimega much greater increase in temperature occurs in the liquid surrounding bulb l8 and the pressure in this bulb and in bellows 28 increases to a greater degree. As these pressures act on-opposite sides of lever 3|, the .force tendingtolif-t the valve stem and increase the width-ofthe fiow opening is a function of .the temperature sj difference between the two levels in the column. With suitable proportioning' of bulb "capacities, initial gas pressures and linkages, all ofwhich'are subject to exact calculation in any given instance, the changing value of the temperature difference between the .two levels maybe. caused .tomove the valve stem and needle automatically tojnew positions a at which the changed rate of. flow through the valve offsets the change in purity of the oxygen. and thus causes it to return. to its original value.- y

When there is adecrease in the purity of the oxygen. in pool l6, its temperature drops .and thus causes the pressure in bulb l1 and bellows 21 to decrease. At the same time, a muchgreater, decrease in, temperature occurs in the liquid surrounding bulb l8. and the pressure in this bulb, and inbellows 28, decreases to a greater degree. As these pressures act on opposite sides of lever 3|, the force tending to lower the valve stem and decrease the width of the flow opening is a function of the increase in temperature difference between the two levels in the column. This changing value of the temperature differencebetween the two levels causes the .valve stem and needle to movedownwardly so that the new position of the needle. decreases the rate of flow 4 and thus the purityv of the product isincreased.

In thevuse of gasfilled bulbs, any gas which remains in the gaseous. phase at the lowest temperatures occurringinthis portion of the column and at thepressuretowhich the bulbs are filled may be utilized, as for example nitrogen, hydrogen orhelium. ...Or a liquid which does not freeze at eolumntemperatureland which has areasonably low vapor. pressure at atmospheric tempera ture,. ,as forexample propane or propylene, may be used. If the bulbs are charged with liquid, it is necessary to substitute a resilient element, such as a stiff, open-coil spring, for each of the rigidpins .29 and 30.

A form of indirect actuation of the valve stem by differential temperature changes is illustrated in Fig. 3, in .whichseveral of the parts may be identical with similarly numbered parts in Fig. 2. In this for m, thevalve stem 34 is an extension of the core 36 of a solenoid 31 and is urged downwardly by a spring 38 bearing on a collar 39, the solenoid being spaced from the valve by a rigid yoke 40. In this form, changes in the pressures in the

bellows

21 and 28 cause the arm 3! totravel across the contact points ofa'rheostat indicated at 4|, thus varying the amperage of an electrical current and the extent to which the core, and with it the valve stem, .are retracted against "the downward pressure exerted by the spring.

In the form illustrated in Fig. 4, the expansion and contraction of metallic rods is applied through mechanical linkage to raise and lower :Itwilhbeevident that the utilization of these 6 the valve stem. ,In this'figure, 22 indicates only the top of the stuffing box of the valve and l2-l2 are fragments of the shell of the column, separated to indicate a wide separation in the column length. Ayoke 42 of a metal having a relatively low coeflicient of expansion, such as irort or nickel, is fixedto the inner wall of the shellyand'arod 4.3 of a metal having a high coeificientotexpansion, such as copper, or aluminum,is attached to the inner end of the yoke and passed through the wall of the shell as through a smiling box or equivalent 44. A bracket 45 aflixed to the outer wall of the shell carries a bell --crank;4 and a projecting arm 41, the latter bearing on the upper end of a spring 48 of which the lower end bears on a collar 49 fixed to valve 48A and; urging valve stem 34 upwardly when rod 43A expands.

Iheaboye ,described structures have the common -iunctionsof producing expansion with rising temperature ofexpansible elements-gaseous, liquid orlsolid+located respectively at that level inthe column at which the oxygen is at the maximymmomentary purity and at a higher level at which ,thecolumnfiuids contain less oxygen; of utilizing the expansion and contraction of the lower lyirig element to urge the oxygen-with- .draw-al-controlvalve in a closing or opening direction, respectively, and of utilizing the expansionand eontraction. of the higher-lying element .to urge-the withdrawal-control valve in an opening or closing direction, respectively. This combination :ofsteps takes advantage of the greater degreeof temperature change at the higher level to add-tothesensitivity of the control and neutralizes any effect on temperature following from p fsssllrephanges of normal magnitude within the expansions'and contractions may be brought about inmany ways and by many devices of which H113 above described" structures are suggestive l a-3i;

Ingthepform. illustrated in Fig. 5, a control de- Y Ceis shown-whichdoes not depend on expansionlofla' gas,-=liquid or solid to actuate the contr'olelements. In-placeof the bulbs I! and I8 show'nsin. Fig. 1, resistance coils of a material whichachanges-tappreciably in resistivity with change .in. temperature are-used. The resistance coils.are-'.branched from a

current source

52, 53 th'ro'ughlines l 9A and ZOA-and return lines I93 and 2013:: Intrposed in lines Ian and 20B are opposed minute solenoids 54,- 55 which actuate rods. 50, 51-," which inturn actuate an arm 3| which.travels across-the contact points of a rheostat -4ll fI hus, changes-in resistance of the resistance coils changesthe amount of current flowing through minute solenoids 54, 55 which in turn acting througharm -3l vary the amperage of an electrical= current-to control a large solenoid linl zeditotheva-lveas ln'Fig. 3.

I-claim: -1'. -Appa a llS fO1 stabilizing the purity of 'oxy gn collecting- -inthe lower end of an air fractionating column having an oxygen withdrawal line comprising, means sensitive to temperature changes at a level in the column where the oxygen is at the maximum purity, means sensitive to temperature changes at another level in the column in the neighborhood of the point where the argon concentration is beginning to decrease from maximum concentration and the nitrogen concentration is low but is beginning to increase,

and means actuated by the first and second claimed means in response to changes in'the temperature differential between the, two'levels to control the rate of the oxygen withdrawal throughthe withdrawal line. 7

2. Apparatus for stabilizing the purity of a liquid product collecting in the lower end of a fractionating column for a tertiary mixture of difierent boiling points which may include inconsequential proportions of additional components, the column having a first component withdrawal valve, comprising, temperature responsive means located in the column at substantially v the level of maximum momentary purity of the necting each of the pressure chambers with one of the temperature responsive means, the conduit connections and operative connections being so arranged that an increase in the temperature difierential between the two levels results in the actuation of the valve in aclosing direction whereas a decrease in the temperature differential between the two levels results in the actuation of the valve in an opening direction.

3. Apparatus for stabilizing the purity of the highest boiling point'product of a fractionating column separating mixtures having at least three components of different boiling points, the col-- umn having a highest boiling point product withdrawal line, comprising means sensitive to temperature changes at one level in the column where the highest boiling? point product is at the maximum purity, means sensitive to temperature changes at a second level in thecolumn in the neighborhood where the concentration of a lower boiling point component is beginning to decrease from'maximum and the concentration of a still lower boiling point component is low but is beginning to increase; and means actuated by the first and second claimed means'in response to the changes in the temperature differential between the two levels to control the rate of highest boiling point product Withdrawal through the withdrawal line.

4. Apparatus for stabilizing the purity of the highest boiling point product of a fractionating column separating tertiary mixtures of different boiling point components, which mixture may include inconsequential proportions of additionalcomponents, the column having a highest boiling point product withdrawal line, commeans sensitive to temperature changes at a second level in the column in the neighborhood where the concentration: of the intermediate boiling point tertiary component is beginning to decrease from a maximum and the concentration of the lowest boiling component is low but is beginning to increase, and means actuated by the first and second claimed means in response to the changes in the temperature differential between the two levels to control the rate of the highest boiling point product withdrawal through the Withdrawal line. 7

5. The method of controlling the rate of Withdrawal of the highest boiling point product collecting in a fractionating column in which mixtures having at least three components of different boiling points are separated to stabilize the purity of the highest boiling point product, comprising utilizing the changes in the temperature difierential between a level in the column where the highest boiling point product is at the maximum purity and a second level in the column in the neighborhood where the concentration of a lower boiling component is beginnin to decrease from maximum and the concentration of a still lower boiling point component is low but is beginning to increase to control the rate of withdrawal of the highest boiling point product.

6. Themethod of controlling the rate of withdrawal of the highest boiling .point product collecting in a fractionatin column in which tertiary mixtures of different boiling points are separated, which mixture may include inconsequential proportions of additional components, to stabilize the purity of the highest boiling point product, comprising utilizing the changes in the temperature differential between a level in the column where the highest boiling point product is at the maximum purity and a second level in the column in the neighborhood where the, concentration of the intermediate boiling point tertiary component is beginning to decrease from a maximum and the concentration of the lowest boiling point component is low but is beginning to increase to control the rate of withdrawal of the highest boiling point product.

7. The method of controlling the rate of withdrawal of oxygenproduct'of an air fractionating column to stabilize the purity of the oxygen product withdrawn from the column, comprising utilizing the changes in the temperature differential between a level in the column where the oxygen product isat the maximum purity and a second level in the column in the neighborhood where the argon concentration is beginning to drop from a maximum and the concentration of the nitrogen is low but is beginning to increase to control the rate of withdrawal of the oxygen product.

' CLARENCE J. SCHILLING.

REFERENCES CITED 7 "The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,395,466 Barbet Nov. 1, 1921 1,492,063 Barbet Apr. 29, 1924 2,022,809 Kramer Dec. 3, 1935 2,316,056 De Baufre Apr. 6, 1943 2,380,417 De Baufre July 31, 1945

Claims

1. APPARATUS FOR STABILIZING THE PURITY OF OXYGEN COLLECTING IN THE LOWER END OF AN AIR FRACTIONATING COLUMN HAVING AN OXYGEN WITHDRAWAL LINE COMPRISING, MEANS SENSITIVE TO TEMPERATURE CHANGES AT A LEVEL IN THE COLUMN WHERE THE OXYGEN IS AT THE MAXIMUM PURITY, MEANS SENSITIVE TO TEMPERATURE CHANGES AT ANOTHER LEVEL IN THE COLUMN IN THE NEIGHBORHOOD OF THE POINT WHERE THE ARGON CONCENTRATION IS BEGINNING TO DECREASE FROM MAXIMUM CONCENTRATION AND THE NITROGEN CONCENTRATION IS LOW BUT IS BEGINNING TO INCREASE, AND MEANS ACTUATED BY THE FIRST AND SECOND CLAIMED MEANS IN RESPONSE TO CHANGES IN THE TEMPERATURE DIFFERENTIAL BETWEEN THE TWO LEVELS TO CONTROL THE RATE OF THE OXYGEN WITHDRAWAL THROUGH THE WITHDRAWAL LINE.