ANALYTICAL EDITION
MARCH 15, 1938
outlet instead of a tapping through the pipe wall can be used. Figure 1 shows specifications for a standard 2-inch piping ring, but the same order of magnitude of clearances is obtained with other pipe tees and piping. The static flow connection from the flow pipe to the manometer consists of a n outlet reducing tee. T h e tee is of the same size as the flow pipe with a nominal 0.25- or 0.125-inch pipe outlet. One end of the flow pipe is screwed into the tee, so that it just emerges into the inner chamber of the tee. The other end is threaded to a length t h a t will allow it to be screwed onto the tee, and t o approach the other pipe to within 0.05inch. T h e threads on the longer threaded pipe are removed b y machining or filing,
169
leaving only those threads that are engaged b y the female threads of the tee. Thus a static fluid chamber is provided between the interior aTalls of the tee and the outer walls of the penetrating pipe. This annular chamber connects b y means of the slot to the flow pipe and b y means of the reducing outlet to the manometer.
Literature Cited (1) Raker, C. P., and Komich, A. J., 9, 533 (1937).
ISD. ESG. (:HEM., Anal. Ed
RECEIVSDSovember 30, 1937.
High-Vacuum Gas-Analysis Apparatus EDWARD C. WARD, Alco Products Incorporated, New York, N. Y.
A simple method is described for the accurate fractional analysis of light hydrocarbons, using high vacuum in the order of 1 m m . of mercury absolute. The separation is accomplished i n a condenser train with temperatures controlled by liquid air. Unusual accuracy is obtained because of the high vapor pressure
T
HE conventional type of apparatus for the separation of
a low-boiling hydrocarbon mixture into its individual components employs a fractionating column usually operating at pressures ranging from atmospheric down to 100 mm. mercury absolute. This type of apparatus has found wide acceptance i n the oil and gas industries and is adaptable to the analysis of a wide range of gaseous and low-boiling hydrocarbon mixtures. Another type of apparatus has been used by this laboratory for several years and is believed to offer certain advantages in the analysis of hydrocarbon gases and vapors. Fractionation is effected in this apparatus at low pressures by means of a series of simultaneous partial distillations and condensations through a series of tubes, maintaining a temperature gradient from tube to tube.
TABLEI. VAPORPRESSURE RATIOS (Working under normal pressures) High-Vacuum Standard Separation Apparatus Column Methane-ethane 2000 480 Ethane-pro ane 100 13 P r opane-isot ut ane 12 4.5 Isobutane-n-butane 5 2 n-Butane-isopentane 15 3.5
Some of the advantages claimed for this procedure are the following: 1. A small gaseous sample, nominally 150 cc., is sufficient for a complete analysis, thereby eliminating the use of expensive high-pressure bombs or large bulky low-pressure containers. 2. Traces of material at the ends of the distillation are determined with unusual precision-for example, small amounts of heavy hydrocarbons in absorber residue gases may be determined to 0.02 per cent and light hydrocarbons in stabilizer residues may be detected within 0.1 per cent based on the original sample. 3. Supplementary tests such as oxygen determination, bromination of unsaturates, slow combustions, etc., may be made on gas fractions without removing the sample from the apparatus. 4. The time required for an analysis is only about 2.5 hours.
ratio of the components at the point of separation. Individual components are determined within 0.02 to 0.1 per cent, depending on their concentration i n the original sample. Results obtained by this method have been used for equipment design and plant control over a period of years. The ease of separation of two hydrocarbons varies directly with the ratio of their vapor pressures a t the particular temperature employed. This ratio increases materially as the temperature is reduced (Table I); therefore, it is highly desirable to effect a separation a t the low temperatures attainable under high vacuums. For several reasons it is difficult to operate the usual form of low-temperature column a t pressures in the order of 1 mm. of mercury. The vapor capacity of small diameter columns a t 1 mm. of mercury or less is markedly reduced, so that the time required to complete an analysis is greatly extended. Small fluctuations in pressures a t these high vacuums greatly affect the fractionation and accuracy of the results. Shepard and Porter ( I ) employed a high-vacuum apparatus for the analytical separation of gaseous hydrocarbons by a series of fractional condensations and distillations through a condenser train without the use of reflux. The apparatus described in this article depends essentially upon the same principle as the Shepard and Porter method, but is of different design, requires less time per analysis, and gives results which are thought to be of the same precision.
Apparatus T h e separations are effected a t a pressure of less t h a n 1mm. absolute in a train of four condenser tubes held accurately t o temperature by the procedure described below, using liquid air as the cooling agent. The apparatus is constructed entirely of Pyrex glass with all fused joints except in the Orsat section where substantially atmospheric pressures are used and danger of leaks is small. It consists (Figure 1) essentially of a condenser train for the lowtemperature separation of the hydrocarbons, an internal highvacuum pumping system for transferring sample and fractions, a McLeod gage for determining the pressure in the system, a constant-volume buret for measuring both sample and fractions, drying tubes, and an Orsat system for running sup lementary tests. As the apparatus does not work well with iquid samples, a
170
INDUSTRIAL AND ENGINEERIXG CHEhlISTRY
VOL. 10, NO. 3
'
CONDENSERS W I T H TRIPLE JUNCTION COPPER CONSTANTAN THERMOCOUPLES
-
h
\ TEMPERATURE CONTROL COIL
rr-T
1I: ; '
SMALL
DEWAR
DEWAR FLASK FOR LIQUID AIR TOEPLER
PUMP
--
'I
______ MERCUiiY RESERVOIR
/ MANOMETER CONTROL
R u a a E R DIAPHRAGM
FIGURE 1.
,
J
DI.IGRbhf OF ilPPAR.4TCS
fractionating column assembly is incorporated into the system for the analysis of liquids. This is similar to the conventional gasanalysis apparatus. The column itself (Figure 1) is of the usual type, vacuum-jacketed, silvered, and provided with a spiral wire packing. A 2-liter flask immersed in a water bath serves as a gas buret to measure the overhead vapor. .4n open-end and a closedend manometer measure the column and buret pressure, respectively. A topping still (not shown) is sometimes used to top heavy samples such as crude oils, absorption oils, heavy gasolines, etc., the tops then being analyzed in the column. A connection to the diffusion pump makes it possible to transfer fractions from the column apparatus to the high-vacuum apparatus for further analysis, or to the Orsat for determination of unsaturates. Small amounts of propane in stabilizer bottoms may be very accurately determined by removing the propane and a small part of the butane in the column and reanalyzing this mixture in the high-vacuum apparatus. The apparatus is made entirely of Pyrex glass with all joints fused; consequently leaks are rare even when operating at low pressures. An efficient and trouble-free device for the automatic control of column pressure is shown. A U-tube with a very small hole in the bottom is partially immersed in liquid air, which slowly
runs into the tube until the ifiaide level reaches the outside level. When a puff of air is blown through the U-tube by the solenoidoperated air pump, the liquid air in the tube is blown into the reflux condenser of the column, the amount of liquid depending upon the depth of immersion of the tube. The air pump is operated by the 110-volt circuit from a relay, which in turn IS actuated by a dry-cell circuit. The dry-cell circuit is interrupted by contact wires in the open-end column manometer and a clockoperated circuit breaker in series. As long as the mercury is high enough in the open end of the manometer to close the circuit across the contact wires, a shot of liquid air will be delivered every 10 seconds until the mercury has dropped sufficiently to break the contact. The operation of a liquid fractionating column is familiar to most gas chemists. The procedure for the analysis of liquids is therefore not described.
Analysis of Gases The apparatus is evacuated t o a pressure of 0.001 mm. of mercury or lower and all cocks are closed. A clamp-top bottle containing the gas sample is inverted in the sampling funnel which
MARCH 15, 1938
is filled with Tvater, and the bottle is unstoppered and slipped over the rubber stopper at the bottom of the funnel. Any water in the bottle is drawn off through the water drain. As the constantvolume buret manometer is of the open-end type, it is necessary to take a blank reading before starting the analysis. This is done by setting the mercury on the buret side of the manometer at the scratch mark and then reading the level on the other side. Gas is admitted to the pumping system through the drying tubes and thence t o the constant-volume buret, using the Toepler pump t o bring the pressure up to atmospheric. The sample cock is closed and the residual gas in the system pumped out to the Hyvac pump and discarded. 4 pressure and temperature rcading, along Tvith the blank preswre reading previously taken, evaluates the quantity of gas in the buret. REMOVAL O F CARBOS DIOXIDE. The sample is pumped into the Orsat leveling bottle and allowed to pass slowly through the Ascarite tube, through one drying tube, and to the diffusion pump inlet ahere it is transferred back into the buret and measured. The loss represents carbon dioxide or other acidic gases. One pass through the ilscarite tube is sufficient t o remove all the carbon dioxide. REXOVAL O F XIETHASE A S D LIGHTERCOSSTITUESTS . The temperaturr-control coils, which are copper cylinders closed on one end and Tvound with resistance wire, are slipped over the condenser tubes and fastened to the binding posts on the rheostat board which form a support as vel1 as an electrical contact for the coils. They are filled n-ith light gasoline to afford contact and are cooled by immersing in liquid air. (Holders for liquid air flasks are not shown.) The condenser tubes are provided with triple-junction copper-constantan thermocouples having an ice and water cold junction and connected through a multipoint and 4 arc immersed switch to a potentiometer. Condensers .i in liquid air throughout the methane removal. When condenser 3 reaches 8 temperature of -160" C., the small Dewar flask is slipped over the condenser coil and then immersed in liquid air. KO. 3 rheostat is then adjusted so that the heat input through the resistance coil just balances the heat loss through thc small Dewar flask to the liquid air and the temperature of the condenser is held constant a t -160" C. I n a similar manner eondenser 2 is held a t a temperature of -142" C., and KO.1 a t -128" C. The sample is transferred to condenser 1 and then admitted slowly to Nos. 2, 3, 4, and 5 in succession. The sample has noTv distributed itself along the condenser train. The heavier constituents are condensed in No. 1, and of the lighter constituents, only a part of the methane fraction has been able t,o reach S o . 5. The McLeod gage is opened to S o . 5, which is then opened slightly t o the diffusion pump inlet. As the methane fraction slowly escapes to the pumping system, it is pumped into the buret by running the mercury up into the Toepler pump occasionally. As the pressure is reduced in No. 5, the condensed material begins to redistribute itself in the train, moving from right to left. There is a fractional distillation from each condenser, followed by a fractional condensation in the next. When the pressure in No. D has been reduced to 0.25 mm. of mercury, the methane separation is considered complete. All the gas in the pumping system is pumped into the buret and measured and all the cocks are closed.
TABLE 11. SEPARATION TEMPERATURES AND CUTTISG PRESSERES EMPLOYED WITH HIGH-\-.4CEUM APPARATIX Separation
Pressure
KO.5
Mm. Methane-ethane
0.25
Ethane-propane
0.1
Propane-butane
0.1
Butane-pentane
0.1
171
ANALYTICAL EDITION
Condenser No. 4 No. 3 a
Liquid air temperature Liquid air temperature Liquid air temperature Liquid air temperature
c.
OC.
So. 2
So. 1
c.
c.
-175
-160
-142
-128
-160
-143
-128
-113
-142
-128
-113
-100
-117
-103
-90
-7i
ANALYSISOF FIXEDGASES. The methane fraction is transferred to the Orsat section and tested for oxygen, using an alkaline pyrogallol solution. If it is desirable to test for carbon monoxide, hydrogen, or nitrogen, a slow combustion analysis is run on a portion of the methane fraction or a cuprous chloride pipet may be inserted in the Orsat train for the absorption of carbon monoxide. The copper oxide tube may also be iised for the determination of carbon monoxide and hydrogen. When all desired tests have been made on this fraction, it is pumped out by the Hyvac pump and discarded. DETERMINATION OF ETHANE AND ETHYLENE.For the removal of the ethane fraction, condenser 5 is used merely as part of the pumping system and the final separation takes place in No. 4. No. 4 is brought up t o a temperature of -160" C., No. 3 to
-143" C., etc. (Table 11). The LlcLeod gage is opened to No. 4 and ethane-ethylen? is removed like methane until the pressure in No. 4 has been reduced to 0.1 mm. of mercury. The cocks on the condenser train are closed, No. 5 is allomd to come to room temperature, and the ethane-ethylene fraction is pumped into the buret and measured. The fraction is then transferred to the Orsat section, treated Tvith saturated bromine water to remove unsaturates, scrubbed Kith caustic, and transferred back to the buret, and the ethane is measured. The loss represents ethylene. RE.\%OV.kL O F I I E A V I E R I$YDROC.4RBOSs. The heavier fractions are removed and tested for unsaturates in the same manner, except that a different set of temperatures is used for each fraction (Table 11). MEASCREMEST O F RESIDCE. As the pentme and heavier residue fraction is easily condensed, it is not desirable to put it through the pumping system into the buret. For this reason the capacity of the condenser system has been calibrated xvith respect to the burrt. This permits the residue to be measured in two different ways. When the residue is small, as in the case of a dry gas, it is measured directly in the eondenser system by mean5 of the McLeod gage. This affords a very accurate measurement. When the residue is too large to be measured in this manncr, the condenser system is opened to the buret and the pressure on this combined system measured by the buret m:inometer.
Discussion SPEEDOF AXALYSISASD LIQCID AIR REQUIRED. The fractional analysis consumes about 2 . 5 hours in the hancls of a n experienced operator. If absorption analysis or the ot'her supplementary tests are necessary, more time is required. Liquid air consumption is from 1.5 t o 2 liters per analgeie. PRECISIOX OF AXALSSIS. On two check analyses, methane will ordinarily check t o about 0.1 per cent (basis original sample), and ethane, propane, and butane t o less t h a n 0.1 per cent. In the case of very d r y gases t h e pentane and heavier fraction nil1 usually check within 0.02 per cent, b u t on heavy gases containing considerable pentane and heavier, a check within 0.1 per cent is normal (Table 111).
TABLE 111. CHECKANALYSES Methane Ethane Propane Butanes Pentanes
Lean Gas Original Check 88.17 88.08 6.20 6.15 3.53 3.56 1.53 1.50 0.65 0.63 100 00
100 00
Rich Gas Original Check 15.70 15.75 17.32 17.42 33.72 33.74 27.76 27.66 5.50 5.44 100 00
100 00
Hundreds of analyses b y this method have been succeskfully used in t h e preparation of material balances, in the design and testing of equipment, in checking the composition of gases and liquids in equilibrium, and for general plant control purposes. GASESANALYZED KITH THE APPARATCS. T h e high-vacuum fractionation train, as such, is used for the separation of hydrocarbon gases only. Fixed gases are removed along with methane and are thus separated from the heavier hydrocarbons. This makes supplementary tests, such as slow combustion, Orsat, etc., more easily accomplished. B y use of proper pipets and reagents in the Orsat system, i n conjunction with the low-temperature fractional analysis, almost a n y gaseous mixture that will not attack mercury m a y be analyzed. T h e gases most commonly determined include hydrogen, carbon monoxide, oxygen, nitrogen, helium (charcoal absorption tubes, not shown, are provided for the determination of helium), carbon dioxide, hydrogen sulfide, methane, ethylene, ethane, propylene, propane, total butylenes, butanes, and pentane and heavier residue All these determinations are made in the one integral apparatus with none of the danger of loss or contamination of sample which would be encountered in transferring from one apparatus t o another.
Literature Cited (1) Shepard and Porter, IND. ESG. CHEM., 15, 1143 (1923).
RECEIVED November
1, 1937.
