An
Improved
Gas Absorber
A p p l i c a t i o n to Determination of Butadiene PHILIP J. ELVING AND THEODORE
L. STEIN
Publicker Industries, inc., Philadelphia, Pa. A n absorber of the contact gas type is described, which can b e used to determine butadiene in gaseous mixtures that are suitable for analysis b y the Tropsch-Mattox or similar absorbers and is applicable to other methods of gas analysis where a minimum amount of reagent i s to b e used with efficient absorption.
B
ECAUSE of the increasing demand for butadiene for the manufacture of synthetic rubber, considerable work has been done on spectrophotometric and other methods for determining butadiene in mixtures and assaying the purity of product butadiene. However, the method described by Tropsch and Mattox (6) based on the selective absorption of butadiene by maleic anhydride is still in use. The original U-shaped absorber described by Tropsch and Mattox has not been entirely satisfactory for rapid work and a number of modifications of this absorber and of other absorbers for the same purpose have been made (1, 2, 4). The principal objections to the Tropsch and Mattox absorber are the need for careful pressure control during the absorption step to avoid desorption of dissolved gases from the maleic anhydride, the large number of passes usually required for complete absorption, and the danger of clogging due to condensation of volatilized maleic anhydride. The absorber described in this paper can be used t o determine butadiene in gaseous mixtures which are suitable for analysis by means of the Tropsch-Mattox or similar absorbers. It is also applicable to other methods of gas analysis where a minimum amount of reagent is to be used with efficient absorption. It is especially advantageous for the use of absorbents which are solids or react too slowly at room temperature and which therefore have to be used a t an elevated temperature. The absorber is of the contact gas type and has the usual advantages of that type of absorber, including a large area of contact between gas and absorbent. The essential principle of the method, involving the use of a small amount of liquid or solid reagent supported by a suitable confining liquid such as oil or mercury, is not new and has been used by the Phillips Petroleum Company and the Dow Chemical Company ( 2 ) in butadiene absorbers.
marked on the vertical tube 1 cm. above the contact chamber, serves as a reference point. The horizontal portion is 15 cm. long, ending in a 12/1 ball joint. At a point 4 em. from the ball joint, a 2-mm. inside diameter capillary stopcock with an offcenter bore is attached to the line. The neck of the leveling bulb is provided with a lip to allow the passage of mercury in and out of the bulb. If the absorbent seems t o volatilize into the tubing, the apparatus can be modified by extending the tubing from the contact chamber vertically for 3 cm., then obliquely at a 30" angle for 6 cm., and horizontally for 6 cm. The contact chamber is held in a 600-ml. beaker by a suitable arrangement such as the following: The 600-ml. beaker is heated to redness about 2.5 cm. (1 inch) above the point corresponding to the position of the widest part of the leveling bulb when the apparatus is assembled. The glass is pushed inward a t this point to form a projection. This operation is repeated once or twice to form two or three symmetrically arranged projections. The leveling bulb is heated, and grooves are pressed on the widest part, so that the bulb can be inserted within the beaker despite the projections. After insertion of the bulb, a turn of the beaker locks it in place.
.D.
.ARY
L
A
1
ELECTRICHOT
1
GAS AMPOULE
Figure
PLA~E
I
The 600-ml. beaker contains mercury which is the confining fluid for the maleic anhydride; 100 ml. of mercury in excess of the amount to fill the packed contact chamber are used. The 600-ml. beaker rests in a 1500-ml. beaker, being supported by a triangle of glass rod covered with rubber tubing. The larger beaker, which contains distilled water rests om an electric hot plate. The water level, which should be kept at or above the reference line, is above the top of the inner, 600-ml. beaker and provides a seal to prevent the evaporation of mercury with the resulting mercury hazard. The water lost by evaporation must be replaced. This can be readily done by adjusting the dropping rate of a separatory funnel which is used as a water reservoir. An oil bath kept at any desired temperature may be more convenient.
APPARATUS
The apparatus consists of a sampling unit, a gas-measuring unit, and an absorption unit as shown in Figure 1. The measuring unit consists of a water-jacketed 100-ml. gas buret, approximately 75 cm. long, equipped with an off-center bore stopcock, and a leveling bulb. The gas is measured over mercury. The buret is equipped with 12/1 spherical socket joints by means of which the buret is connected to the absorption and sampling units. The sampling system consists of a short manifold having a capillary bore three-way stopcock and connected to a 2-ml. liquid sample ampoule and a 500-ml. gas ampoule by 12/5 socket joints. The gas ampoule which uses mercury as a displacement liquid has stopcocks a t both ends and is connected a t the lower end to a 500-ml. leveling bulb by a length of rubber tubing. The stopcocks are preferably of the pressure type. The absorption unit consists of a contact chamber made from an inverted 250-ml. Pyrex leveling bulb which is packed with pieces of Pyrex rod, 6 to 10 mm. long and 6 mm. in diameter. The upper end of the contact chamber is attached to capillary tubing, 1.5 mm. in inside diameter, extending vertically for about 3 cm., a t which point it is bent horizontally. A line,
INTRODUCTION AND REMOVAL OF REAGENT
Five grams of C.P. maleic anhydride are used in the absorber and are replaced when 300 ml. of butadiene have been absorbed. If isobutylene is present in the sample, 2% of diamylamine is added to the maleic anhydride to prevent polymerization of the isobutylene. REMOVAL OF MALEIC ANHYDRIDE FROM THE ABSORBER. Heat the apparatus until the bath is a t least 90" C. Raise the liquid in the contact chamber to the reference line of the capillary tube. Discard the gas which is in the buret. Raise the mercury from the buret to the stopcock of the absorption unit and close the buret stopcock. 722
November, 1945
Attach t o the stopcock of the absorption unit by means of a short length of rubber tubing provided with a Hoffman clamp a trap consisting of an evacuated 200-ml. round-bottomed flasli fitted with a one-hole rubber stopper and a glass tube. Heat the capillary line and stopcock of the absorption unit with a small Bunsen burner until they are just too hot to touch. Open both the Hoffman clamp and the absorption unit stopcock, and allow the maleic anhydride and about 10 ml. of mercury to flow into the trap. Close the absorption unit stopcock. Replace in the 600-ml. beaker the approximate amount of mercury withdrawn. INTRODUCTION OF MALEICASHYDRIDE. Perform the steps in the first paragraph of the directions for the removal of maleic anhydride before introducing fresh maleic anhydride unless these conditions have already been obtained during removal of used maleic anhydride. Carefully melt about 5 grams of maleic anhydride in a crucible in a hood. Heat the capillary line and stopcock of the absorption unit with a small Bunsen burner until they are just too hot to touch. While the tube below the stopcock of the absorption unit dips into the molten maleic anhydride, open the stopcock and allow the anhydride to flow into the contact chamber. PROCEDURE F O R D E T E R M I N A T I O N O F BUTADIENE
Liquid samples are completely vaporized from the small ampoule into the gas reservoir ampoule. The vaporized gas sample in the gas reservoir should be mixed by alternately decreasing and increasing the pressure by lowering and raising the mercurycontaining leveling bulb attached to the gas reservoir ampoule. Ampoules containing gas samples may be attached in place of the gas reservoir ampoule. Dilution of samples with an inert gas, saturation of the reagent with residual gas, or similar experimental devices may be used if necessitated by the nature of the sample. Low AND INTERMEDIATE COSCENTRATIONS. After the bath has begun to boil, raise the maleic anhydride to the reference line of the capillary tube. Measure to the nearest 0.1 ml. about 50 ml. of the gas to be analyzed and then pass the gas into the absorber. After 2 minutes, withdraw most of the residual gas from the contact chamber and then return it to the contact chamber. After 2 minutes return the residual gas to the buret, raising the maleic anhydride to the reference line; close the buret stopcock and measure the residual gas volume. Repeat the absorption process until constant volume is obtained, and then discard the residue. Analyze additional samples from the gas ampoule until the per cent absorbed in consecutive trials is constant.
% ’ butadiene
72 3
ANALYTICAL EDITION
= 100
(sample volume sample volume
If desired, larger gas samples can be used. The packed 250ml. bulb will hold safely 75 ml. of gas; larger samples can be analyzed by using an absorber constructed from a larger leveling bulb. If the sample contains a relatively high percentage of butadiene--e.g., 4001,or over-a 100-ml. sample can be used with the 250-ml. absorber, owing to the rapid absorption of the butadiene; care should be taken that the volume of sample in the absorber a t any one time does not exceed 60 or 70 ml. PUREBUTADIENE.This procedure is suitable for the assay of nearly pure butadiene (97 t o 100%) containing only Cs and C, hydrocarbons as impurities. After the bath has begun to boil, raise the maleic anhydride level to the reference line of the capillary tube. If the maleic anhydride has been used previously, carefully measure about 40 ml. of dry nitrogen or air and introduce it into the absorber. After 2 minutes, withdraw the nitrogen into the buret, raising the anhydride to the same reference line; close the buret stopcock, and measure the volume. If the volume has changed by more than 0.1 ml., repeat the process with another sample of nitrogen until the volume remains constant. Finally,
raise the maleic anhydride to the reference line and discard the residual gas. Carefully measure about 10 ml. of dry nitrogen and introduce it into the absorber. The nitrogen is intended to dilute the impurities of the sample, so that the error due to their solubility in maleic anhydride is reduced, and to facilitate the measurement of the residual gas. Carefully measure about 100 ml. of gas sample in the buret. Introduce the sample a t such a rate that no more than 50 ml. of gas is in the pipet a t any time. The rapid absorption of butadiene makes this possible. Expose the residual gas to the reagent for 2-minute periods until the volume is constant. Analyze additional samples from the gas ampoule until the per cent absorbed in consecutive trials is constant.
yo bu;;:re
=
sample volume $. nitrogen volume - final volume sample volume
)
The accuracy is usually 0.2% 519 compared to the gravimetric method. If it is desired to convert the volume per cent of butadiene to weight per cent, the corrected volume per cent which equals the mole per cent is multiplied by the ratio of the density or specific gravity of butadiene to that of the gas sample a t the temperature and pressure concerned. The volume per cent found experimentally may be corrected for deviations from the ideal gas l a w as described by Robey and Morrell ( 6 ) . Nitrogen or n-butane is preferred to air by some for flushing of the system and dilution of the sample, since oxygen apparently contributes to the production of a viscous condition in the reagent ( 3 ) . The gas sample may be added to a measured volume of diluent gas in the buret and the absorption performed after the two gases have mixed. This practice seems more desirable for maintaining equilibrium saturation of the maleic anhydride with physically dissolved compounds. DISCUSSION
The apparatus is readily made, rarely broken, and easily operated. The dead space is small. The whole sample is exposed to the reagent a t the same time, obviating the need of repeated passage through the absorbent as is necessary with the usual Ushaped absorber. An added advantage of this absorber is the almost complete absence of clogging due to condensation of volatilized or entrained maleic anhydride. The time required for the evaporation of a liquid sample of pure butadiene and the analysis of the resulting gas is 35 to 40 minutes; a check analysis can be made in 15 minutes’ additional time. A gas sample can be analyzed in 20 minutes. The working time is about 10 minutes less than the over-all time. The absorber described has been used both in the analysis of research samples and in plant control for the past two years and has proved entirely satisfactory. ACKNOWLEDGMENT
The authors would like to thank E. Shoemaker and G. Volz of this company for suggestions in the design of this absorber. LITERATURE CITED
(1) Elving, P. J., and Breger, I. A., private communication. (2) Gregg, C. L., IND. ENQ.CHEM., ANAL.ED., 17, 728 (1945). (3) Gregg, C. L., private communication. (4) Hinckley, J. A.. private communication: Rubber Reserve Co.,
Butadiene Laboratory Manual.
(5) Robey, R. F., and Morrell, C. E., IND.ENO.CAE%,ANAL.ED., 14, 880-3 (1942). (6) Tropsch, H., and Mattox, W. J., Ibid., 6,104-6 (1934). PUBLICATIOK of this paper delayed at the request of the Office of Censorship.
