Analytical Distillation Laboratory Design for Efficiency and Versatility C. 8. KINCANNON and MAURICE
0.BAKER
Houston Manufacturing Research laborofory, Shell Oil Co., Houston, Tex. F A laboratory arrangement is described which allows far efficient distillations. Each stall is designed to accommodate any one of a variety of types of columns and is provided with utilities for operation a t atmospheric or subatmospheric pressures. Selected operating pressures a r e maintained automatically. Constant boil-up rates a r e obtained b y semiautomatically controlling the calumn differential pressure. Central panels allow for rapid selection of reflux ratios and for reading of vapor temperatures. Manand column-hours per distillotion have been reduced abaut 25 and 35%, respectively. Distillation failures average about 1% as compared to 5% previously. Repeatability is about 25% better. Improved safety equipment is described.
P
analytical distillations constitute an essential operation in petroleum research and process control. Conventional distillation equipment and practices leave much to he desired as regards economy of operatom’ time, as well as precision and dependability of the resultant data. Taking temperature cut points from a batch distillation unit requires frequent or continuous attention of an operator. Manual control of heat input t o the kettle is a potential source of variation in the distillation data, because fractionating efficiency is affected by the load in the column. Furthermore, reduction of pressure is required for distillation of high boiling fractions and the reduction of pressure during a distillation hy manual procedures may be tedious and unsatisfactory. The present paper describes and discusses a distillation laboratory which was designed to increase consistency of data by providing dependable instrumentation to improve uniformity of operations, reduce man-hour requirement for distillation, reduce columnhour requirement for distillation hy automatically maintaining a fast distillation rate and by eliminating unnecessary down time for changing operating pressures, and increase versatility in selection of reflux ratio, in choice of operating pressure, in measurement or recording of temperature, REcssIoN
and in choice of type and size of fractionating equipment. COLUMN STALL DESIGN AND GENERAL ARRANGEMENT
The column stalls are designed to allow fractionating columns to be interchanged without altering the product receiving equipment. The arrangement of the product receiving equipment (receiver, glass valves, cold trap, etc.) and the control equipment is identical in all the stalls for convenience in operation and maintenance. An assembled stall unit is shown in Figure 1. Details of the column stall, and the arrangement of utility pipes, electrical conduits, vacuum lines, and instrument attachments in hack of the stall unit are shown in Figure 2. The fractionating section (Figure 1) is mounted on the back wall of the stall and separately from the product receiving equipment, which is mounted on aluminum rods fastened to the side i d l s of the stall. The receiver, cold trap, and glass manifold are designed to allow fractions to he removed during operation at either atmospheric or subatmospheric pressure. Sturdy support of this equipment reduces hreakage, and the entire assembly, being mounted separately, may he removed from the stall as a unit for repair or alteration. Adequate space is provided a t the lower section of the stall to accommodate a kettle and heater. A metal covered Glas-Col heating mantle which encloses the lower half of a spherical kettle is supported on aluminum rods with suitable spring shock absorber. Adjustable hinges mounted near the side wall of the stall allow the kettle support to be swung downward for removal of the kettle. The upper portion of the kettle is insulated and enclosed in a removable metal housigg which is hinged in the back and, when closed, fastens to the Glas-Col heating mantle with clips. Normally, Miter kettles are used, hut other sizes can be substituted, depending on the volume of the charge. To accommodate larger volumes, a few of the stalls are equipped with permanently mounted stainless steel kettles of 9-gallon capacity. The metal kettle is connected to the glass column with Dow-Coming type flanges using a Teflon gasket. A shelf of Transite, covered with a Monel metal tray, separates the kettle
section from the column section of the stall unit. This shelf provides working space, serves as a fire wall, and protects the column and kettle. As shown in Figure 2, space is provided behind the row of stalls for ready accessibility to auxiliary equipment and utilities. All pipe headers are placed above and to the rear of the
Figure :oIumn
1.
Asrembled
fractionating
VOL. 29, NO. 8, AUGUST 1957
1189
stalls. The condenser media which are normally used include water, steam, and refrigerated alcohol solution. Selection of medium is made through the indicated valve arrangement. The vacuum manifold is similarly arranged; any one of five different pressures (730, 180, 80, 20, and less than 20 mm. of mercury) may he obtained by opening the proper valve. The electrical service panel is placed apart from the condensing-medium manifold as a safety precaution.
ratios of 99, 49, 24, 9, 4, and 2 to 1 are usually desired and the instruments are normally set to operate at these reflux ratios. Thermometric System. The ther-
mocouple-instrument connector panel (Figure 6) provides a flexible system for making connections between thermocouples and instruments. The jacks are connected to iron-constantan
The arrangement of the laboratory is shown in Figure 3. Eighteen stalls are arranged along three walls of the room, with six stalls along each wall. An instrument panel board faces the entrance of the room and a 6 X 6 foot work bench with cabinets is located against the rear of the panel board. The laboratory sink is to the rear of the work bench space. The vacuum pumps are placed in a cabinet in the center of the hank of stalls along the back wall of the room. The reservoirs of the controlled-pressure systems are placed above and to the rear of the bank of stalls along the back wall. The utilities supplied to the panel board. work bench. and sink are laid 1mderthe floor from'the back wall of the I'oom. Part of the laboratory is showr,in 1Tigure 4. The uniformity of apparittus n A ; n m r.____ m l i i r in tho u _.-m.....tine i- nf m + d a"-..--. operation of several columns simultaneously, because manipulation is identical for all units. A wiring diagram of the electrical utility is shown in Figure 5 . An automatic voltage regulator is provided for the elcctric circuits supplying the kettle heating elements. As shown in the wirine diaeram. overload breaker switches are provided for protection of equipment. Additional breaker switches which are not shown in Figure 5 protect the electrical equipment in each column stall. As an added safety measure, a master switch which can be used in an emergency to disconnect the electrical supply to the room is located near the entrance of the laboratory. i.
y y y y
Y
Distillation column stall detail
A
___
~
I
Figure 2.
,
Figure 3. Arrangement of stalls equipment
DESCRIPTION OF INSTRUMENTS
Reflux Ratio Selection. Column heads which are operated by intervaltimer switches are used. The heads are the liquid-dividing and vapordividing types described by Collins and Lantz ( 1 ) . A central system for rapid selection of any desired reflux ratio is used. The reflux ratio selector panel and thermocoupleinstrument connector panel are shown in Figure 6. Each row of jacks on the selector panel is connected through a mercury-plunger relay switch t o a separately adjustable repeat-cycle timer, Any, or all, of the plugs supplying the respective fractionators may he connected to any of the timers through the panel. Reflux
1190
ANALYTICAL CHEMISTRY
Figure 4.
Analytical distillation laboratory
every 4 seconds and sixteen separate temperatures may be recorded. These instruments allow temperatures to be estimated to about lo F., and are used primarily for obtaining trends in temperatures for operational purposes. Distillate volume readings are marked on the recorded curves by the operator as required. The two self-balancing, temperature indicators are graduated at loF. intervals, and the temperature may be estimated to about 0.5" F. These indicators are used primarily as avtomatic temperature-signaling devices.
TO NORTH
WALL
TO EAST WALL
Automatic
Bm*D
Figure 5.
II
Electricol wiring for distillation laboratory
Figure 6. Upper.
Lower.
Panels ReRux ratio ~ e l e ~ t o r TheimOcOUple-inItrUmenf ~onnector
thermocouples located in the respective column stalls. The plugs are connected to the temperature-measuring instruments shown in Figure 7 ; they comprise one Leeds & Northrup Type K-2 potentiometer, two 16-point Leeds & Northrup Speedomax recorders, and two Brown Instrument Co. self-balancing indicating potentiometers. The K-2 potentiometer, which is capable of measurements to O.1° F., is used as a standard and for fraction cut points. One of the recorders covers the temperature range of 50" to 250' F., and the other 250' to 450" F. These instruments may be operated to m&e a continuous record of temperature from 50" to 450' F. Each instrument prints
Figure 7.
Instrument ponel
Temperature-Signaling
Device. I n distilling wide-boiling range petroleum materials, it is fre-. quently necessary to make cuts or take readings a t specified temperatures. The man-hour requircment for making temperature cut points is reduced by equipping each self-halancing potentiometer with a signaling device which warns the operator of a n approaching cut point.
Figure 8 shows the arrangement of the adjustable cam arm and microswitch which are essential parts of this device. A wing nut allows the attached arm to be easily adjusted and held rigid at any position of the scale. The microswitch may be used to energize a circuit to a buzzer or, by use of a relay, may he used to change from finite reflux ratio to total reflux. Each instrument handles one column a t a time. This special equipment is of outstanding value in the routine operation of analytical fractionators. Instruments with attachments which perform a similar function are now available commercially. Subatmospheric Pressure Control System. Operating pressures nor-
mallyused are 730,180, SO, and 20 mm. of mercury absolute and are automatically maintained in four independent pressure control systems. All four are available by manifolds and valves t o each column stall. A fifth system also available to each stall is held in reserve for utility
Figure 8. Automatic temperoture-signoling device VOL. 29, NO. 8. AUGUST 1957
1191
purposes and to supply the occasional need for pressures less than 20 mm. of mercury. Separate manostats are provided in each stall for use with this system. The reason for selecting 730 mm. of mercury in place of the usual atmospheric pressure is to eliminate loss of material through possible leaking joints during distillation. The normal pressure differential through 3 fractionating column causes the kettle pressure to exceed the overhead pressure. Khen the pressure in the kettle is greater than atmospheric pressure, any leakage through ground joints will be toward the outside. Slight reduction of the operating pressure from atmospheric eliminates such leaks. Correction of temperature readings to 760 mm. introduces no significant uncertainty. A sample of relatively !vide boiling range may be fractionated by starting at 730 mm. of mercury and subsequently reducing the operating pressure to 180, 80, and finally to 20 mm. of mercury as required t o prevent decomposition of the sample. A standard schedule of pressure reduction is followed on a given type of sample to obtain maximum consistency of results. Without instrumentation the reproduction of exact operating pressures is relatively difficult and the over-all consistency of results may suffer. Each of the pressure control systems consisti of control instruments, reservoirs, ,-inch copper pipe manifolded to all the stalls. and a vacuum pump I\ hich runs continuously. A *ketch of 3 controlled-pressure system is shown in Figure 9. The control instruments include a mercury manometer with electrical contacts of tungsten Tvire in each leg, an electronic relay to reduce the amperage a t the control contact of the mercury manometer, and 3 solenoid valve placed between the vacuum pump and the pressure surge reservoir. The controlling manometer is similar in design to those described by Willingham and associates (2). The reservoir has a capacity of 27 gallons. All metal-tometal connections and joints are sealed with solder. Metal-to-glass connections are made with rubber tubing. During normal operation, and with several columns operating a t the same pressure, variations in pressure a t the head of a column are so small that no change in the mercury level of 3 closedend manometer is noted. Instrument Control of Boil-up Rate. Because the separating pori-er of a distillation column is dependent, in part, upon the boil-up rate (load), maintenance of 3 constant boil-up rate improves the consistency of data. However, the load Trill vary with the selected operating pressure. The rate of boil up in the column determines the pressure gradient through the fractionating section. 1192
ANALYTICAL CHEMISTRY
Table 1.
Standard Deviations and Confidence Limits of Analytical Distillation Cut Points
1-01,
)sure.
Charge, 311.
113
730
3500 2000
430
80
450
80
480
80
3500 2000 1000 3500 2000 1000 3500 2000
560
20
1000
1000
650 a
3500 2000 1000
3500 2000 1000
20
Accumulated Overhead Product a t Cut Point Temp. Av., Wt. YG 2.5 2.6 2.5 29 4 30 6
Std. Dev. 0.08 0.07 0.26 0 28 0 34 0 23 0.50 0.62 0.45 0 47 0 44 0 75 0 49 0 59 0 29 0 48 0 42 0 29 0 42 0 74 0 65
30 7
31.7 32.7 32.9 35.5 36.5 36.6 49 9 50 8 51.1 62 1 62 6 63 3 70 0 70.0
71.0
Confidence Limit, 95% 0.23 0.20 0.72 0 78 0.95 0 64 1.38 1.72 1.25 1 33 1 23 2 08 1 36 1 65 0 80 1 34 1 16
0 81 1 17
2 06 1 82
Calculated to 760 mm. Hg.
- Ih-ET
Figure
9.
FOR AIR
Controlled subatmospheric pressure system
n I,
1-
HEITER
b
VALVE
110 v
Figure 10. Electronic relay circuit and wiring diagram for automatic reduction of pressure or control of heat input to kettle
Hence the boil-up rate may be kept constant by automatic control of the pressure drop through the column. For this application, the instrument varies the heat input, or alternatively reduces the operating pressure as required to maintain a constant pressure gradient through the fractionator. A wiring diagram for control by heat input or by pressure reduction is shown in Figure 10. The instrument is normally connected to regulate the heat input to the kettle. For this purpose the following items of equipment are used: a differential mercury manometer equipped with an electrical contact in each leg, an electronic relay, a mercury-plunger relay, and a variable resistor. The differential pressure manometer is inclined; one electrical conductor enters through a packing gland and is movable. Fritted-glass plates (coarse grade) are placed between the kettle and mercury reservoir and between the column condenser and the manometer leg. The fritted glass allows gases t o pass and prevents mercury from leaving the manometer. The electronic relay is employed to achieve a very low current and voltage across the manometer contacts, thus avoiding the possibility of igniting hydrocarbon vapor in the system. The current is about 3 pa. a t 6.3 volts, alternating current. .4n enclosed mercury-plunger relay load switch is used t o eliminate open arcing in the instrument case. A variable resistor is wired in series with the kettle heater element and is automatically cut into and out of the circuit as the electrical contact in the differential manometer is closed and opened. The autotransformer (Powerstat) is manually set a t a voltage which is more than adequate to maintain the desired load in the column, and the variable re-istor is manually set so that when it is in the circuit, the heat input is not adequate to maintain the desired load. To reduce surges within the
fractionator, the high and low heat inputs should be adjusted for a relatively small differential. The alternative method of controlling the column differential pressure is by regulating the rate of reduction in column pressure. This method is used during the period of change-orer from one standard pressure to another. The same instrument components are used, except that the electronic relay actuates a solenoid valve which is placed b e h e e n the column and vacuum source, instead of switching a resistor into the heater circuit. The kettle heater is usually switched off during this period. A three-pole, double-throw switch (Figure 10) permits selection of either of the two methods of control. This semiautomatic control of distillation rate saves man-hours and column time. Bring-up time is reduced and down time required for pressure reduction is eliminated. Furthermore, the quality of data is improved by maintaining a constant boil-up rate throughout the distillation. SAFETY FEATURES
Vent System. All gases vented from columns and vacuum pumps are introduced into a 3-inch metal duct and exhausted from the laboratory. An auxiliary exhaust fan is placed in the outside wall of the laboratory t o remove vapors Tvhich may accumulate in the room. Fire Protection System. Conventional fire protection equipment, consisting of hand fire extinguishers and fire blankets, is readily available. Metal pans are placed beneath kettles to confine spilled materials. I n addition, the laboratory is protected with a permanently installed carbon dioxide system. A valve manifold is arranged so carbon dioxide can be directed to blanket simultaneously all of the stalls on any one of three sides of the room. Nozzle outlets are located
in each kettle zone as well as behind each bank of stalls. The valve manifold and carbon dioxide cylinders for the fire protection system are placed in a hallway near the entrance of the laboratory. EXPERIMENTAL
The new column room arrangement has been in operation for about 5 years. At the present time perforated glass plate, metal packed, and rotary ribbon columns are being used. With this variety of columns, boil-up rates varying from about 100 ml. per hour to 4 gallons per hour may be obtained. Repeatability varies, depending on column types, operating pressures, reflux ratios, and volumes charged. Examples of the repeatability obtained for a perforated glass plate column (30plate, 1 inch in inside diameter) in the fractionation of a hydrocarbon charge are shown in Table I. The reflux ratio was 4 to 1 and five determinations of each volume were made. These values are estimated to be a t least 25y0 better than those obtained by manual techniques. Electrical maintenance has increased, owing to the various automatic innovations introduced. However, this increase is offset by the reduced amount of time required in making column type changes, since less breakage is experienced and little or no manifold variation is required. LITERATURE CITED
(1) Collins, F. C., Lantz, \-ernon, IND. ENG.CHEM., A N A L . ED. 18, 673
(1946).
( 2 ) Willingham, C. B., Taylor, W. J., Pignocco, J. M., Rossini, F. D., J . Research h-atl. Bur. Standards 35, 219 (1945).
RECEIVEDfor review May 17, 1956. Accepted March 12, 1957. Division of Analytical Chemistry, 129th Aleeting, ACS, Dallas, Tex., April 1956.
Determination of Tyrosine and Tryptophan in Proteins W . L. BENCZE and K A R L SCHMID Department of Medicine, Harvard Medical School, and Massachusetts General Hospital, Boston 7 4,Mass.
b A new spectrophotometric method for the determination of tyrosine and tryptophan in proteins is based upon measuring the absorbance in the range between 278 and 293 mp. The slope of the line drawn tangent to the two characteristic maxima of the absorption curve is indicative of the content and ratio of these two amino acids. The error introduced in these determinations b y the influence of the
bathochromic shift in the absorption spectra of tyrosine and tryptophan is diminished as compared with that of earlier methods. This method was further applied to the determination in certain proteins of the hydrogenbonded nonionizable tyrosine.
S
determination of tyrosine and tryptophan in proteins (1) offers an advantage over chemiPECTROPHOTOMETRIC
cal methods because it does not require hydrolysis, which often leads to partial decomposition of these two amino acids (6, 11, IS,16,16). The contentof tyrosine and tryptophan established by the spectrophotometric methods of Holiday (7, 8 ) and Goodwin and Morton ( 5 ) is calculated from absorbances measured a t two definite wave lengths. A large error is introduced in these results by the hathochromic shift in the absorption VOL. 2 9 , NO. 8, A U G U S T 1957
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