US2299899A - Method of analyzing volatile mixtures - Google Patents

Description

Patented Oct. 27, 1942 s PATENT OFFlC'E METHOD OF ANALYZING vom'rmr:

mx'rrmas Glen S. Houghland New York, N. Y., assigns! to a The M. W. Kellogg Company, Jersey City, N. J.,

Claims.

This invention relates generally to the analysis of mixtures of volatile substances, and particularly to a method for determining the amount of a constituent in such a mixture when that constituent is (al' eitherthehighest or the lowest boiling constituent ".of the mixture, neglecting mere traces of materials boiling respectively above oi below it, and (b) is present in small amounts, and (c) has a boiling point close to that of the constituent boiling next below or above it respectively.

Although not limited thereto, the method of my invention isespecially advantageous in analyzing mixtures of hydrocarbons, and may be of the greatest utility in providing control data for hydrocarbon fractionation systems. A casein point is the separation of iso-butane by fractionation from a mixture also containing normal butane and heavier hydrocarbons boiling'in the gasoline range. Such a mixture is produced in the catalytic alkylation. of iso-butane with butylene, it being desired to recycle unconverted iso-butane after separating it as completely as possible from the heavier alkylate and the non-reactive normal butane which is usually present. Due to the fact that there is only about 20 F. difference in a corporation of Delaware Application October 5, 1940, Serial No. 359,834

tion whereby the above method may be carried out comprises, in general terms, a fractionating column having an inlet at one end and at the other end a device for changing a stream of fluid into a diiferent fluid phaseby adding or subtracting heat. In the event that the apparatus 7 is to be used. to analyze for the lowest boiling con- "stituent in a mixture, the phaseand temperature-changing device will be a condenser; if the highest boiling. constituent is to be analyzed for, it will be a reboiler. Remaining parts of the ap-'- paratus include a'fiuid outlet at the same end of the column as-the inlet and pressure and temperature indicating devices connected to the interior of the enclosed space at various points.

The annexed drawing is a diagrammatic elevation of one form of the apparatus of the' invention whereby one embodiment of the analytical method of the invention may be practiced. Ref

erence will now be made to the drawing for a ents of mixtures.

boiling point between 150- and normal butane at the usual operating pressures, the control of a de-iso-butanizer" is quite critical. It is highly desirable to have frequent analyses of the bottoms for' iso-butane and of the overhead fornormal butane, but theusual laboratory methods for obtaining such information are, besides being.

diflicult to perform accurately, so time-consuming that results are available toolate to be of value.

It is an object of the'present invention to provide a method of analysis of the type described in the first paragraph of this specification which will give a rapid and, if desired, a continuous indication of composition. Anadditional object of the invention is'to provide an efllcient method of controlling a distillation column when one of the products of that column is a mixture of the type described in the first paragraph of this specification. Other objects will appear from the description of the invention hereinafter to be given.

In general terms the method. of analysis of the invention comprises subjecting the mixture to be analyzed to fractionation and the establishment of a zone wherein the constituent ana- ,ed for is present in a vapor and in aliquid in proportions greater than, but bearing a known relationship to, its proportion in the mixture under analysis and to the pressure and tempera ture in the zone. The apparatus of the invenmore detailed explanation of the invention. The apparatus shown in the drawing has been provided with all necessary means for carrying out analyses for both heaviest and lightest constitu- It is to be understood, therefore, that if only one type of mixture is to be analyzed, certain of the elements shown can be dispensed with, as they are then inoperative.

In the drawing, l is a fractionatin'g column provided with plates, baifles, packing or other means of insuring adequate vapor-liquid con- 'tact, packing being preferred in the small cplumn sizes which will normally be chosen. The diameter of the column will be'flxed bythe size of the sample-to be analyzed, and will seldom exceed four inches. The height of the column will depend upon that characteristic of the filling known as the height equivalent to one theoretically pertest plate," and upon the diilference between the 40 boiling point of the constituent analyzed for and that of the constituent boiling next adjacent thereto. Suillcient-height to' provide the eifect of about 25 theoretical plates will be adequate for most purposes.

, The base of the column lis provided with a The top of the column has a liquid inlet l0 regulated by a valve II and a'vapor 'outlet line l2 leading to a block valve l3 and a pressure control valve I4, whence it is vented. Branching from line I 2, line l5 leads to a block valve I6 and thence to a condenser H, to which a refrigerant is supplied via line |8 in-an amount regulated by valve IS. The outlet of the condenser I1 is connected to line 20 which is blocked by valve 2| and leads back to the top of the column. 8 and 22 and 9 and 23 are thermometer orthermocouple wells and pressure gauges respectively, the exact so that any desired portion 'of the tower may be used.

The apparatus shown in the drawing is adapted to manual-control, but in practice it will facilitate operation to provide automatic flow and liquid level controllers at various points which will be readily recognized from the hereinafter given description of the manner in which the apparatus is operated. A vapor trap on the liquid outlet 4 will. also be helpful.

The operability of the method and apparatus herein disclosed in giving information which will be indicative of the amount of a component in a mixture arises from the fact that if the percentage of one component in a volatile mixture is varied while the relative proportions of other components remain constant, the vapor pressure of the mixture at any given temperature will vary proportionately, or conversely, the temperature at which the mixture will have a given vapor pressure will vary proportionately. This fact is constantly made use of in conventional meth- Y ods of controlling distillation processes not involving the precise separation of closely boiling constituents, as the temperatures at the top and bottom of a column under a given pressure are then suiliciently sensitive indices of the composi-- tion of the top and bottom products. When an interval as small as 20 F. between the boiling points of constituents to be separated is encountered, however, and when as little as 1% of one constituent is to be left in the residue, this fact does not alone afiord sufiicient precision of control. For example, it was found in one instance that a change of only 1 F. at the bottom of a column corresponded to a 100% increase in the iso-butane content of the bottoms products, 1. e. from 1% to 2%. It is not commercially practicable to control bottom temperature within 1 F.

The manner in which theapparatus shown in the drawing is operated pursuant to my method of analysis will now be described with reference to the determination of the per cent iso-butane in a mixture of iso-butane, normal butane, and heavier hydrocarbons; During such a determination, valves 1, H, and I3 will remain closed, and the reboiler 6 will beinoperative.

and flows back down therethrough in countercurrent contact with the rising vapors, being finally withdrawn at the bottom through line 4.

As soon as liquid downflow is established, there will be a tendency for the low-boiling iso-butane to collect in the upper portion of the column. The vapor stream leaving the column will contain iso-butane which, after return to the column as liquid through line 20, is selectively revaporized by the heat of'the rising vapors and driven overhead again. No vapor is allowed to escape at the bottom through line 4, and after the air originally present has been purged from the apparatus by opening valve l3 for a short time, the pressure in the apparatus will be fixed by the temperature to which the liquid leaving condenser H has been cooled. Any pair of coexistent values of the temperature and pressure at the outlet of the condenser U will be indicative of the per cent iso-butane in the condensed liquid; of which the remaining portion will be normal butane. As a matter of fact, the temperature and pressure at any point will be indicative of the per cent iso-butane in the streams flowing through the column at that point. It will be understood by those skilled in the art of distillation that there will be a continuous change in the iso-butane content of the vapor and liquid streams traversing the tower, and that this content will be at a maximum at all points above the top of the packing in the tower, including the reflux circuit. Now, if the column is made very high, the liquid and vapor respectively entering and leaving the top of the column will be very high in iso-butane content, or even 100% iso-butane, while if the column is relatively short the iso-butane contentv at its top will not be much greater than that of the sample. The result desired, however, is that there be a point in the column where the pressure and temperature reflect a change in isobutane content with the greatest sensitivity, and this will be so at a point where the iso-butane content is nearest to 50 mol per cent, although any point where the iso-butane content is between about 30 and '70 mol per cent will afiprd sumcient sensitivity for a less precise determination. I therefore preferably make the column at least high enough for the iso-butane content to rise as high as 50 mol per cent at the top thereof. Additional height is of no advantage,

and as previously mentioned can be nullified by connecting the lines l2 and 2| to the column at the optimumheight. In the example under discussion, a height equivalent to about 16 theoretiduce a 50% concentration at the top. By providing a 25 or 30 theoretical plate column having a series of inlets and draw-ofls at various heights upon the size of the column, from 2 to 5 gallons (liquid) per hour being about right for a 3 inch column packed with tile saddles. vapors rise through the column and pass through The entering line |2- to line l5 andinto the condenser l1 where they are liquefied. The resultant liquid returns through line 20 tothe top of column I may accommodate any material likely to require analysis.

Having established a zone in the column where the pressure and temperature at any point accurately reflect iso-butane content at that point, the actual determination of iso-butane content at that point is relatively, simple. All

stituents. Such calculations are well known to those skilled in the art of distillation and need not-be enlarged upon herein.

Since the concentration ,of iso-butane in: the

upper portion of column I results from a fraciionation at constant (total) reflux, the relationship between the composition of the liquid at any point in the column and that of the sample is fixed by the degree of fractionation performed by the column packing between that point and the sampleinlet.

If the efliciency or height equivalent to one theoretical plate of the column packing istnown from-previous experiments then from the composition of the liquid or vapor at any point in the column it is possible'to calculate the composition of the feed, i. e. the sample under analysis.- The so-called tray to tray" calculation required is also well known to those skilled in the art and need not be given in detail.

In the event that the efficiency of the packing, plates or baffles desired to be used is not known, then the relationship between sample and column top compositions may be established by preliminary experiments, employing the usual laboratory method-of analysis to obtain the-necessary data on compositions. This will seldom be necesreaching it, suitably by means ,of liquid-level control of valve 1.- The vapors produced pass back up through the column in counter-current contact with the down-flowing liquid and leave the column via line 12. 1} suitable constant pres sure in the column is maintained by the pressure control valve ll.

As soon as vapor up-flow is established, there will be a tendency for the high-boiling normal butane to collect in the bottom'of the column to the exclusion of iso-butane. The reboiled vapors will contain normal butane which, on cooling by contact with the down-flowing liquid .will be preferentially condensed and returned to the bottom again. The pressure on the column having been fixed by adjustment of valve I4, the

.reboiler will supply heat enough to generate that pressure while vaporizing the down-flowing liquid. The temperature (and pressure) at all points in the column below the entrance of line M then become sensitive indices of the compositions at those points in proportion to the tappreach of the normal butane content at those sary, however, as a variety" of packing materials are available ofwhich the efllciencies have been determined over a wide range of conditions. The relationship between temperature, pressure and composition for any given apparatus and type of analysis is capable of being embodied in a chart or no inograpm'the preparation of which is well within the ability of one skilled in distillation processes. simplify the use of the method in controlling full-scale distilling operations.

Even when the eiliclency of a packing is known, somewhat greater accuracy is attainable by actual calibration of the apparatus. correcting the calculated results by comparing them with analyses made in the usual manner.

Obviously, the use of the method and apparatus will be made easier by fixingthe temperature at. the topo! the column and allowing only the pressure to vary. This may be accomplished if desiredby connecting a temperatureresponsive elementlocated in line 20 to position valve i9. Optionally, a pressure-responsive ele- Such a chart will greatly points to 50 mol percent.

, The distance, between the point at which thefeed is introduced -and' the point "at which the normal butane content approaches 50 mol per cent is againa function of the efliciency of the packing, and may be calculated or determined by experiment. It may be well at this point to mention that packing efliciencies depend to a oertain extent on the rateat which vaporand liquidrare passing, and that constancy of feed rate is important to the accuracy of the method herein disclosed. Calibration should obviously be done atthe same feed rate as is contemplated in: the actual analyses.

Asmight be expected, I find that approximately the same number of theoretical trays are necessary to get 50 mol per cent normal butane concentration at the bottom of the tower when analyzing for normal butane in iso-butaneas are necessary in the type oi'janalysis-flrst described.

The reason for this is that the difference between the boiling point of the constituent analyzed for and that of the next adjacent constituent is the controlling factor. I

Except for the fact that the temperature and pressure are preferably observednear the bottom instead of at the top of the column, the manner of obtaining the final analytical data in this second type of operation is exactly analogousito that described with reference to the first.

ment could be so connected, inwhich case only the temperature would wary.

A complementary procedure will now be described with reference to the drawing whereby an analysis maybe made when the highest boiling constituent of a mixture is to be analyzed for. A typical sample wouldbethe overhead product of the same distillation of which the previously "discussed'material was the bottoms,

that is, a mixture of a large amount of iso-butane with a small amount of normal butane. In analyz-ing such a sample in the apparatus shown, valves 3, 5,16, I9 and II remain closed and valve l3 remains open.

I introduce a liquid stream 'of the material It will be appreciated by those skilled in the art that if, in the first type of operation, the

temperature is observed at any point beyond the condenser in line 20, the effect of one more theoretical tray will be obtained as compared to an ,bbservation taken ahead of the condenser in the vapor stream traversing lines [2 and I5. This effect has to be taken into account in calibrating or calculating the column.

When analyzing the bottom product of a fullscale distillation as first described, I prefer to take a sample in vapor form directly from the vapor space belowthe bottom, tray of the fullscale column, passing it directly to line 2 and taking precautions to prevent condensation en under analysis into the tower through line lll,

and allow it to flow down through ,the'tower to the reboiler coil 6. Suflicient heat is supplied to the r'eboiler to completely vaporize a11liquid'75 route. If desired, however, it is entirely feasible to take a liquid sample of the full-scale'column bottoms and to pass this liquid through a vapor'- izer before introducing it into line 2. Similarly, in analyzing-a top product I prefer a liquid sample taken from the reflux accumulator or from the top tray of the full-scale column.

It is to be understood that my method is applicable to the analysis of bulk mixtures (taken from storage, for example) as well as of product streams issuing from processes. My method should not be confused with prior analytical methods wherein a miniature distillation column is used, and in which the procedure comprises exhaustively fractionating a sample of known weight or volume in order to separate and make possible the-volumetric or gravimetric measurement of 100% of the constituent analyzed for. In my method there is no separation of a pure component, in fact not over 50% concentration of any component is aimed at.

The presence in asample stream of traces of a constituent boiling outside (i. e., respectively above or below) a high or low boiling constituent to be analyzed for, while very undesirable, is not necessarily suflicient to defeat the method of my invention. For example, if very small traces of propane are present in a sample to be analyzed by the first procedure described, I vent the column periodically through line l2 to prevent the accumulation of propane in the top section.

Likewise I may bleed oil. accumulated pentane from the bottom when operating in accordance with the second type of procedure.

For obvious reasons, the method of my invention should be carried out as nearlyadiabatically as possible. To this end careful insulation and/0r jacketing of the apparatus are advisable.

I claim:

1. A method of determining the amount of a minor constituent in a mixture of distillable substances, said constituent boiling substantially at it completely into a diiferent fluid phase, causing the whole of the changed phase stream to flow at said point.

back through said zone in countercurrent contact with the original phase stream, and then withdrawing said changed phase stream from said zone, whereby the pressure and temperature of either of said flowing streams at any point in said zone are made joint indices with the efficiency of said zone of the amount of said minor constituent in said mixture and measuring the temperature and pressure at some point in said 4. In the column distillation of a volatile mixture the boiling point of one constituent ofwhich is close to the boiling point of a constituent boiling next adjacent thereto, it being desired that one of the products of said distillation .shall contain a maximum of one of said constituents and a minimum of the other, the improvement which comprises continuously withdrawing from the distillation column a stream of fluid of substantially the same composition as said product, introducing fluid thus withdrawn into one end of a separate iractionating zone of known inherent fractlonating characteristics, causing said fluid to filow to I he opposite end of said zone, altering the tempo ature 01 said fluid to convert it into a diflerent fluid'phase, and then causing the whole of-the changed phase fluid to flow back through said zone in countercurrent contact with original fluid as yet unchanged in phase, and then withdrawing said changed phase fluid from said zone, whereby the pressure and temperature at any point in said zone appreciably distant from the point of introduction of said original fluid are through the known fractionating characteristics of said zone made indices of the amount of the constituent desired to be excluded from said product contained in the original fluid, measuring the temperature and pressure at some point in said zone and regulating said column in accordance therewith.

5. A method of determining the amount of a minor constituent in a mixture of distillable substances, said constituent having a boiling point lower than any constituent present as more than a trace in said mixture but close to that of the constituent boiling next above, which comprises introducing a vapor phase stream of said mixture into the lower end of a fractionating column of known inherent vapor-liquid contacting efliciency, flowing said vapor stream up through said column, cooling said vapor stream to convert it into a liquid stream, passing the whole of said liquid stream downwardly through said column in countercurrent contact with said vapor stream, and withdrawing said liquid stream from said column,measuring the temperature and pressure at any point in said column appreciably distant from said lower end and using said measured temperature and pressure and said known column efliciency as joint indices from which to calculate the amount of said minor constituent in said mixture.

GLEN S. HOUGHLAND.

grate tn e 2,299,899.- v October- 27, 19b2,. f GLEN s. HOUGHLAND,

CERTIFICATE OF CORRECTION.

appears in the printed spe eifieation ng correction as follows: JPgeh, first "concurrent" read -coun'tercurrentpage -for 'fllo w" read "flowand that the d with this cor-fect ipn therein that the the Patent office.

l ef column, line 7-5 claim 2., for 14., second column ELine 21, cleimh; 'said Letters Patent should be rea 7 same may conform t'o the record of the base in Signed and sealed this 1st day of December,

the above numbered patent requiI-i e. Henryvafi Arsdale, (Seal) Acting Commise'ionep ofPetents.

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