US3338825A

US3338825A - Distillation of complex mixtures

Info

Publication number: US3338825A
Application number: US514002A
Authority: US
Inventors: Robert A Taggart
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1965-12-15
Filing date: 1965-12-15
Publication date: 1967-08-29
Anticipated expiration: 1984-08-29

Description

Aug. 29, 1967 R. A. TAGGART 3,3 DISTILLA'IION 0F COMPLEX MIXTURES Y Filed Dec. 15, 1965 2 Sheets-Sheet 1 v 2/ FEED 2 VAPOR FRACTION AIR-COOLED CONDENSER IYJ No.3

k i+m4 STEAM BOTTOMS FRACTION INVENTOR ROBERT A. TAGGART Aug. 29, 1967 R. A. TAGGART DISTILLATION OF COMPLEX MIXTURES 2 Sheets-Sheet Filed Dec. 15, 1965 OVERHEAD VAPOR TEMPERATURE CONTROLLED TEMPERATURE AT 12 M.

TOP REFLUX NO.1 SIDECUT No.5 SIDECUT No.3 SIDECUT OVERHEAD VAPOR I l 10A 12N 2P 4P TIME FIGZ

I 12M 2A 9. om Lo woz m mmakmmmz NO.1 SIDECUT TEMPERATURE CONTROLLED TEMPERATURE AT 8AM.

TOP REFLUX 147 230 NO.1SIDECUT NO. 3 SIDECUT w dash/512mb G 5 R E A 04 T 4 T N E O M V T T N R M T M ke A Q 0 R P Y m B WE m G T I w F P 4 P 2 N m A m A 8 7 3,338,825 Patented Aug. 29, 1967 3,338,825 DISTILLATION F COMPLEX MIXTURES Robert A. Taggart, Moss Point, Miss, assignor to Chevron Research Company, San Francisco, Calif., a corporation of Delaware Filed Dec. 15, 1965, Ser. No. 514,002 3 Claims. (Cl. 208-350) ABSTRACT OF THE DISCLOSURE In the distillation of a complex mixture such as crude oil in a distillation column from which sidecut streams are withdrawn, the reflux rate is regulated directly responsive to a temperature in the column remote from the reflux tray, indicative of the temperature where a sidecut is being withdrawn.

This invention relates to processes for continuously distilling multicomponent streams in fractionation columns from which one or more sidecut fractions are withdrawn in addition to overhead and bottoms fractions. More particularly, the invention relates to such distillation into multiple fractions of broad boiling range hydrocarbon oils, such as crude petroleum and fractions derived from petroleum and like hydroc arbonaceous materials including kerogens.

The invention is concerned with stabilizing column tem perature profile, and otherwise generally improving the operation, in a special class of fractionating columns which are used for distilling broad boiling range hydrocarbon streams to separate out an overhead vapor fraction, a bottoms liquid fraction, and at least one intermediate boiling range sidecut fraction. In the special class of columns, the overhead vapors are partially condensed, and at least a portion of the condensed fraction is returned to the column as liquid reflux. Thus, from the overhead there is withdrawn a net vapor fraction, and there may also be withdrawn a net liquid fraction having the composition of the reflux stream. The composition of the reflux will change if the temperature of partial condensation changes, and this temperature does vary for a number of reasons even though column pressure is maintained constant.

Heretofore, a primary control variable in operating such columns has been the overhead or top tray temperature, and this has been controlled or held constant by regulating the reflux return rate. Specifically, in the usual automated control system the temperature at the top tray is measured in the liquid or vapor, usually in the overhead vapor line, and a signal indicative of the temperature is generated and transmitted pneumatically or electrically and used to operate a mechanism regulating the opening and closing of a valve in the reflux return line. If the top tray temperature increases above the desired control point, the valve is opened to increase the flow of reflux until the top tray and vapor line temperatures are restored to the desired control point. Similarly, if the tem perature drops, the valve restricts flow.

In accordance with the present invention there is provided an improved method of operating such columns, comprising regulating the rate of returning the portion of condensed overhead fraction as reflux in response to a signal indicative of the temperature in the column where an intermediate boiling range sidecut fraction is withdrawn, in a manner adapted to thereby maintain said sidecut temperature substantially constant. The sidecut fraction is withdrawn from a point within the column between the feed inlet tray and the reflux inlet tray. Preferably the temperature being controlled constant is measured at the tray from which the sidecut is being withdrawn, but a temperature in the vapor or liquid space one or two trays above or below the tray from which the sidecut fraction is withdrawn will in some cases be found sufficiently indicative of the sidecut temperature. The temperature control point must be sutficiently below the top tray, where reflux is returned to the top tray, so as to be indicative of the sidecut temperature and not to be directly influenced by the top tray or overhead vapor temperature. However, the temperature control point cannot be so far down the column that too substantial a time lapse exists because of the liquid holdup in the column so as to make the controlled temperature unresponsive to changes in reflux rate. In general, therefore, the temperature control point will be between 3 and 15 trays below the top reflux tray, there being usually at least about 20 trays between the feed tray and the reflux tray in the class of columns under consideration. More desirably, the control tray is from 4 to 12 trays below the top. i

In the attached drawings, FIGURE 1 represents a schematic flow diagram for a fractionation column in an embodiment of the invention exemplified by the atmospheric distillation of crude petroleum;

FIGURE 2 shows graphically variations in the temperatures at various points in such a crude atmospheric column when controlled in accordance with the prior art method of regulating reflux rate to maintain a constant overhead temperature; and

FIGURE 3 shows graphically the improved stability of the temperatures at various points in the column when controlled in accordance with the invention.

Referring now to FIGURE 1 by way of example, in the distillation of crude petroleum at near-atmospheric pressure the crude feed in line 11 is preheated in heat exchanger 12 and furnace 13 to partially vaporize it, and the vapor-liquid mixture is fed to crude still 14 a few trays above the bottom. The liquid portion flows downward across bubble cap or perforated trays, or through similar vapor-liquid distributing-contacting means, and is further vaporized by live steam injected through line 15 near the bottom of the column. Alternately, a fired or steam heated reboiler, not shown, may be employed to generate additional upflowing vapors. An unvaporized portion is withdrawn as the net liquid bottoms fraction through line 16. The vapors, including any injected steam, flow upward through the trays above the feed tray in countercurrent contact with downflowing liquid provided by the liquid reflux returned to the top tray through line 28. Several liquid sidecut fractions representing fractions of intermediate and adjacent boiling ranges are withdrawn from trays between the feed tray and the reflux tray, as shown. Thus, a No. 1 sidecut is withdrawn through line 31 to recover a heavy naphtha boiling range fraction; a No. 2 sidecut is withdrawn through line 32 to recover a kerosene boiling range fraction; and heavier No. 3, No. 4, and No. 5 sidecuts are Withdrawn through

lines

33, 34, and 35 to recover respectively higher boiling distillate gas oil fractions.

As is well known, the principal factors determining the efliciency of separation of the crude feed into fractions having the desired boiling ranges in a properly designed column are the vapor-liquid traflic, i.e., the ratios of boil-up rate and reflux rate to feed rate, and column temperature profile, i.e., ture gradient from the bottom to the top of the column. In a crude petroleum atmospheric column, the maximum bottom temperature is limited to below the temperature at which thermal degradation by cracking of the oil could occur. Consequently, as mentioned, a primary control variable in operating such columns has been the overhead or top tray temperature, which was controlled constant by regulating the reflux return rate. In the illustrated embodiment of the invention, the temperature of the descending temperathe No. 1 sidecut in line 31 is instead determined by a thermometer or similar temperature sensing device 30, and the signal generated by TC is utilized to regulate the opening and closing of valve 29 in reflux line 28. If the temperature at 30 increases above the desired control temperature previously ascertained for the No. 1 sidecut, valve 29 is automatically opened to increase the flow of reflux to the top tray.

The reflux is obtained from the column overhead vapors in line 17, which are cooled in heat exchanger 12 and then further cooled and partially condensed in air cooled condenser 18, the mixture of uncondensed vapor and condensed fraction then passing through line 19 to reflux drum 20. In a large installation typical of crude distillation it is not practical to attempt to control the reflux drum temperature constant. Thus air cooled condensers and/ or water cooled condensers operated with fixed maximum water throughput rates are employed to obtain the maximum condensation at the prevailing conditions. The temperature of partial condensation will accordingly vary with atmospheric conditions and will also be aflected by the rate of flow of vapor through line 17 and the vapor temperature. From reflux drum 20 there will accordingly be withdrawn an uncondensed vapor fraction in line 21, at a rate which may be regulated by valve 22 controlled by PC so as to maintain a substantially constant pressure in drum 20, which will be reflected by a constant pressure in the distillation column. The rate at which condensate is collected in drum 20 will likewise vary depending on the condensation conditions, and a portion thereof is continuously withdrawn through line 24 by pump 25, any net amount to be recovered from the feed being withdrawn through line 26 containing valve 27 at a rate regulated by LC to maintain within limits the liquid level in reflux drum 20. The major portion, if not all, of the condensed fraction is returned by pump 25 through line 28 as reflux to the column at a rate regulated as previously described. Where steam is injected directly into the column, this will be withdrawn from drum 2% through line 23 as condensed water.

Referring now to FIGURE 2, there are shown plots of temperatures measured at various points in a column from which there were sidecuts withdrawn from distillation of crude petroleum, obtained during a 24-hour operating time when the column was operated according to the standard prior art technique of controlling the column overhead vapor temperature constant by using a signal indicative thereof to regulate the reflux flow rate. As shown, while the overhead vapor temperature was held quite constant, the temperature of the reflux stream varied substantially, and in addition the temperatures of the various sidecuts also varied substantially and in a manner which does not appear to be related to the reflux temperature. During the period between 8 am. and a.m., however, when the top reflux temperature was increasing rapidly, there was a substantially increased flow of reflux needed to keep the overhead vapor temperature from increasing above the set point. The greater flow of reflux appears to be related to the drop in the temperatures of the sidecuts even though the overhead vapor temperature seems unaffected.

Referring now to FIGURE 3, there are shown the same temperature traces for the same column when operated in accordance with the invention by regulating the reflux rate in response to the temperature of the No. 1 sidecut so as to maintain the temperature at that point in the column substantially constant. In this case, the No. 1 sidecut was withdrawn from a tray which was six trays below the top tray. As shown, the top reflux temperature still varied substantially during the day, the overhead vapor temperature varied considerably more than when it was controlled constant, but the temperatures of the sidecuts showed substantially less variation. In this case, the reflux rate varied only slightly, as the variations in overhead vapor temperature were ignored, whereas by the prior art control technique there would have been continuous adjustments to reflux made in an effort to maintain this temperature constant.

From the foregoing, it is seen that in the class of distillation columns under consideration, where there is both an overhead vapor fraction and an overhead liquid reflux obtained by partial condensation of column overhead vapors at variable temperature, it is much superior to permit the vapor overhead temperature to fluctuate and to control the reflux to the column responsive to a temperature remote from the reflux tray.

It should be noted that in most instances the net overhead vapor fraction and any net overhead liquid fraction, as in

lines

21 and 26 of FIGURE 1, will be further treated at least to adjust their compositions. Thus, the vapor fraction may be further cooled for partial condensation, this time at a fixed temperature, and the condensed portion may be combined with the liquid fraction. In this way it was possible to obtain ultimately net vapor and liquid overhead fractions of fairly constant composition when controlling the distillation column to maintain constant overhead temperature. This was not always the case, however, for it has been observed that when the top tray temperature is above the set point, the increased reflux would increase the overhead vapor rate, frequently causing a higher reflux temperature because of the condensing duty limitations and making the net overhead liquid fraction higher boiling than desired. Accordingly, the variations in overhead temperature permitted when operating in accordance with the present invention present no new or diflicult problem, because the variations are surprisingly only slightly more than occurred under the old method. Also, the overhead liquid fraction would ordinarily be redistilled anyway if a definite end boiling point were desired.

On the other hand, the more stable sidecut compositions provided by the invention represent a substantial improvement and advantage. Usually, at least one of the sidecuts will form the feed to another distillation column, where the importance of having a stable feed oomposition cannot be overemphasized. Also, a sidecut fraction may be fed to a refining operation such as reforming, hydrofining, or hydrocracking, where a stable feed composition permits closer control of operating variables for improved yield and product quality. The cumulative improvements can thus lead to substantial increases in profitability of the whole refinery.

FIGURES 2 and 3, as mentioned, represent operating data from a crude atmospheric column operated at different times in accordance with the prior art control method (FIGURE 2), and in accordance with this invention (FIGURE 3). Minor diiferences existed in the composition of the crude feed in the comparison cases, which accounts in part for the somewhat higher sidecut temperatures elected to be maintained in FIGURE 3 as compared to FIGURE 2. Continued operation in accordance with the invention with other crude blends, and adoption of the new method of operation in other installations, have demonstrated that improved temperature profile stability was always obtained and is attributable to the improved control procedure.

Undoubtedly, a satisfactory explanation or theory as to I why the new method of control is operative and superior to the previous method can be worked out, as any observed phenomenon must have a rational explanation. It is known that in certain types of distillation, such as the seperation of binary mixtures, it has been proposed to control the reflux rate in response to the temperature in the column a few trays below the reflux tray, rather than in response to the top tray or overhead vapor temperature. In these cases, however, a relatively pure component is being recovered as the sole overhead product; there is no sidecut stream; and the reason a temperature control point lower in the column sometimes appears advantageous is because the reflux temperature remains constant at constant pressure. It is only farther down in the column that the other component, .or an impurity, has suflicient concentration to cause a variable temperature reading indicative of the efliciency of the separaton and useful for control purposes.

The situation is completely different in the class of distillation columns to which the invention applies. If substantial variations occur in the composition of the material on the top tray even at constant pressure, these variations would be expected to cause unwanted changes in the temperatures at various points lower in the column. Accordingly it has always heretofore appeared advisable that an attempt should be made to control the overhead vapor temperature and composition constant so as to prevent these variations and stabilize column temperature profile. Thus, the control method of the invention wherein the overhead and reflux temperatures are permitted to fluctuate would not be a logical extension of the binary control system.

The present invention appears generally applicable to that class of fractionaton columns wherein there is withdrawn at least a net vapor overhead fraction obtained from partial condensation of the column overhead vapors to form liquid reflux, so that the composition of the reflux stream is sensitive to the condensation temperature, which varies, and there is withdrawn one or more sidecut fractions from a tray or trays between 3 and 15 trays below the top reflux tray, but above the feed tray. In utilizing the new method, the rate of withdrawing the sidecut whose temperature is maintained constant will desirably be held substantially constant. That is to say, the particular sidecut will represent a fixed fraction of the crude feed and hence will usually be withdrawn in a fixed constant ratio to the feed. The rate of withdrawal may however be modified in response to other variables. The sidecut used as a temperature control point need not necessarily be the uppermost sidecut fraction, but may be a second or third sidecut provided that is not too remote from the reflux tray.

The class of columns in which the invention can be used to advantage is not restricted to crude distillation. For example, a similar situation arises in the so-called main fractionator in a catalytic cracking process, which column is used to separate the products of catalytic cracking into a noncondensable vapor fraction, an overhead gasoline fraction, a bottoms fraction which may be contaminated with cracking catalyst fines, and one or more sidecut fractions containing gasoline and other distill-ate products of catalytic cracking including light and heavy recycle oils. These sidecut fractions may be further distilled in columns operating in conjunction with the main fractionator. It will be found that regulating the reflux rate to the main fractionator to maintain a constant temperature in one of the sidecut fractions will'provide improved stable operation as compared to the previous technique of attempting to control the overhead vapor temperature.

The main fractionator associated with catalytic cracking is typical of a type of separation applied to numerous streams derived from catalytic treatment of petroleum fractions such as hydrofining, reforming, and in particular hydrocracking where a substantial change in the boiling range of a reactant feed is accomplished. Columns of the class described are often utilized to separate the vapors and light products and a high boiling recycle stream from the main products, which are withdrawn as one or more sidecuts from a main splitting column.

In many such cases the sidecut is stripped of low boiling components, which are returned to the main fractionator at a higher tray. The amount of material so returned is usually small compared to the column vapor and liquid traflic, and does not interfere with column control by the present invention based on the temperature at a lower tray. In fact, in the examples herein as represented by FIGURES 1, 2, and 3 and the discussion thereof, the No. 1 side-cut was passed to such a sidecut stripper, and the vapors returned to the column between the reflux tray and the No. 1 sidecut tray.

From the foregoing it is apparent that the invention is not limited to the specific embodiments illustrated. FIG- URE 1 is schematic only, as those skilled in the art will recognize that numerous conventional auxiliary treatments and equipment comm-only associated with such distillation columns have been omitted. For example, additional liquid traflic may be provided in the column below sidecut withdrawal trays by withdrawing liquid, cooling it, and returning it at a higher point. Also, many variations in the methods of condensing the overhead vapors and controlling column pressure are known, and usable in conjunction with the invention. In particular, whether a net liquid overhead fraction is withdrawn in line 26 may be optional depending on the surge capacity in drum 20 and how much flexibility is designed into the overhead vapor condensing facilities. Thus, the true scope of the invention is intended to be defined by the appended claims.

I claim:

1. In a process for continuously distilling a broad boiling range hydrocarbon stream in a fractionation column to separate out an overhead vapor fraction, a higher boiling liquid bottoms fraction, and at least one intermediate boiling range sidecut fraction, wherein said overhead vapor fraction is partially condensed, the uncondensed fraction is withdrawn, and at least a portion of the condensed fraction is returned to the column as reflux, the improved method of operation which comprises regulating the rate of returning said portion of condensed overhead fraction as reflux in response to a signal indicative of the temperature in the column where a said intermediate boiling range sidecut fraction is withdrawn, between 3 and 15 trays below the reflux tray, in a manner adapted to thereby maintain said temperature substantially constant.

2. The improved method of claim 1 wherein the rate of returning reflux is regulated in response to the measured temperature of an intermediate boiling range sidecut fraction which is withdrawn from 4 to 12 trays below the reflux tray.

3. The improved method of claim 2 wherein said intermediate boiling range sidecut fraction is withdrawn in a substantially constant rate ratio to the rate of feeding said stream to the column.

References Cited UNITED STATES PATENTS 2,022,809 12/1935 Kramer 203-2 2,252,550 8/1941 Bragg 208350 2,684,326 7/1954 Boyd 203-2 2,725,351 11/1955 Grote 202 3,018,229 1/ 1962 Morgan 203-2 3,050,450 8/1962 Kleiss et al 203-2 DELBERT E. GANTZ, Primary Examiner. H. LEVINE, Assistant Examiner.

Claims

1.IN A PROCESS FOR CONTINUOUSLY DISTILLING A BROAD BOILING RANGE HYDROCARBON STREAM IN A FRACTIONATION COLUMN TO SEPARATE OUT AN OVERHEAD VAPOR FRACTION, A HIGHER BOILING LIQUID BOTTOMS, FRACTION, AND AT LEAST ONE INTERMEDIATE BOILING RANGE SIDECUT FRACTION, WHEREIN SAID OVERHEAD VAPOR FRACTION IS PARTIALLY CONDENSED, THE UNCONDENSED FRACTION IS WITHDRAWN, AND AT LEAST A PORTION OF THE CONDENSED FRACTION IS RETRUNED TO THE COLUMN AS REFLUX, THE IMPROVED METHOD OF OPERATION WHICH COMPRISES REGULATING THE RATE OF RETURNING SAID PORTION OF CONDENSED OVERHEAD FRACTION AS REFLUX IN RESPONSE TO A SIGNAL INDICATIVE OF THE TEMPERATURE IN THE COLUMN WHERE A SAID INTERMEDIATE BOLING RANGE SIDECUT FRACTION IS WITHDRAWN, BETWEEN 3 AND 15 TRAYS BELOW THE REFLUX TRAY, IN A MANNER ADAPTED TO THEREBY MAINTAIN SAID TEMPERATURE SUBSTANTIALLY CONSTANT.