Analysis of the Light Constitutents in Crude Petroleum by Low -Temperature Fractional Distillation R. J. ASKEVOLD
AND
M. S. AGRUSS, The Pure O i l Company, Chicago, I l l .
I
X T H E calculation of material balances on distillation and cracking units in oil refineries, it is often desirable to know the composition of the light gases occurring in the original crude oil charge. This is particularly important a t present because of the increasing use of these materials in the production of aviation gasoline. The low-temperature fractional distillation columns commonly used for gas analyses are not entirely suitable for determination of the light constituents in crude oil because of the small quantities of these gases present. Difficulty is also caused by the fact t,hat most crudes contain some water, which tends to collect and freeze in cold portions of the column, causing partial plugging and consequent flooding. The method here described is particularly suited for laboratories where such analyses are but seldom required and where regular debutanizing equipment is not available. Considerable time is saved, compared to operation of a pilot still. The apparatus permits the use of a crude oil sample sufficiently large to provide measurable quantities of butane and lighter gases for distillation in a n ordinary low-temperature fractional distillation column, and a t the same time removes water from the portion of the sample which enters this column.
CONVENTIONAL LOW T E M R M T U R E FRACTIONATING COLUIW.(
Methane Ethane Propane Isobutane n-Butane Pentanes+ Total
2x0J
i
CfC
~ - -
SCREEN-
Figure 1 illustrates the auxiliary still which may be used in conjunction with any low-temperature fractional distillation column having a still body of 20-ml. capacity o r more. Before a n analysis, the column of the auxiliary still is filled with sodium hydroxide pellets to form the packing, and pellets are placed in the Bask to act, as boiling stones. The entire system is evacuated to a pressure of less than 1 mm. of mercury, by means of the pump regularly used with the gas-analysis equipment. The reflux condenser of the low-temperature column is cooled below the boiling point of methane, and a Dewar flask containing acetone and dry ice is placed around the inlet tube of this column. The crude oil sample bomb is then connected to the inlet stopcock of the auxiliary still, and sample is sloivly introduced to the flask by means of this stopcock. Immediately after sampling is finished, the electric heating is started to strip light constituents from the crude oil. During distillation through the pellet-packed column, water and hydrogen sulfide will be removed; heavier hydrocarbons will condense in the still kettle of the low-temperature column, and lighter ones will be refluxed by the cold reflux condenser. Heating of the auxiliarv still should be slow enough for good fractionation, but should be continuously increased, to ensure a slightly higher pressure in the auxiliary still than in the low-temperature fractional distillation column a t all times; reversal of this pressure differential will cause some of the cold condensate t o flow back into the auxiliary column, with rapid vaporization and a
Table
THERMOMETER ro i w * c
JOINT
Figure 1 . Auxiliary
Still
DIAGONAL BORE STOPCOCK ~ M an M TUBING
/
violent pressure surge. If the pressure ir! the lowtemperature fractional distillation column reaches atmospheric during the period of crude-stripping, methane is tlistilled off a t a rate suflicient to maintain atmospheric pre.s;sure. The crude may tie safely assumed to be free of butane and lighter gases when the temperature indicated by the thermometer a t the top of the auxiliary column reaches 55' C. .it this point the electric heater is lowered and the connection closed betxveen the two columns. The condensate in the column is fractionated in the usual manner to separate methane, ethane, propane, isobutane, and n-butane, after which the residue of pentanes and heavier is distilled back into the stripped crude oil hy cooling the auxiliary still with dry ice. The weight of each light hydrocarbon may he calculated from the amount diatilled, and the weight percentage of each obtained on the basis of the water- slid hydrogen sulfide-free crude oil, by weighing the crude residue. If it is desired to fractionate the stripped re.qidue to obtain further data, the flask containing this residue may be transferred io any suitable crude assay column, and distillation continued.
I. Typical Analyses on Crude Oils
Weight Percentage, Water-Free and Hydrogen Sulfide-Free Van Zandt Cabin Creek Eureka B A B A B A B 0.00 0.00 0.00 0 . 0 0 Trace Trace Trace Trace 0.00 0.01 0.01 0.02 0.04 0.06 0.02 0.02 0.16 0.14 0.16 0.16 0.39 0.49 0.25 0.25 0.09 0.11 0.33 0.36 0.44 17.50 0.35 0.22 0.48 0.46 0.53 0.43 1.08 ~ . 1 4 0.68 0.66 99.25 99.30 98.97 99.03 98.05 97.81 98.72 98.86 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
Anfichigan
*Corning Mid-Continent Cumberland B A B A B A B Methane Trace Trace 0.00 0.00 0.01 Trace Trace Trace Ethane 0.04 0.03 0.02 Trace 0.04 0.01 0.03 0.04 Propane 0.22 0.22 0.22 0.25 0.09 0.07 0.48 0.45 Isobutane 0.25 0.28 0.18 0.25 0.05 0.04 0.25 0.29 n-Butane 0.51 0.49 1.18 1.13 0.08 0.04 1.15 1.09 Pentaneaf 98.98 99.08 98.30 98.37 99.73 99.84 98.09 98.13 Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 A. Propdsed method. B. Debutanizer method.
24 1
Using the auxiliary still described in this paper, a number of analyses have been made on batches of crude oil, which were also distilled in a pilobsize batch debutanizing still, with a 18.9-liter (5-gallon) stainless steel kettle, 180-cm. (6-foot) saddle-packed glass column, and refrigerated total condenser capable of refluxing propane. Suchdistillations yielded completelydebutanized straightrun gasoline, and the gases were measured and collected in a gas holder. -4nalytical results on these relatively large volumes of gas, when calculated t o the basis of the crude, show good agreement with analyses described here (Table I). PREBENTED before the Division of Petroleum Chemistry at the 105th Meeting of the AXERICAH CKEXICA SOCIETY, L Detroit, Llich.
