Hydrocarbon Traces In
By P.K. Rao
When the reactors of Hydroprocessing Units and Reformers
and other plant equipment are given for maintenance or for
regeneration of the catalyst, it becomes necessary to remove all
residual hydrocarbons from the system. Use of explosimeters is not
satisfactory for this purpose, because of its qualitative nature and
absence of oxygen. A more accurate method is the use of a chromatograph.
RTOL Lab's editor, P.K. Rao provides the procedure below.
When the reactors of Hydroprocessing Units and Reformers and other
plant equipment are given for maintenance or for regeneration of the
catalyst, it becomes necessary to remove all residual hydrocarbons from
the system, since these residual hydrocarbons are injurious to the health
of the men entering these equipment for maintenance jobs or they may lead
to explosion in the subsequent regeneration operations. These equipments
are usually purged with nitrogen to remove all hydrocarbons from the
system. For ensuring that the equipment is free from hydrocarbons, the
nitrogen coming out of the system is tested for them. The level of
hydrocarbons that need to be detected in the nitrogen is usually below
0.1% by volume.
Using an MSA explosimeter for testing the presence of hydrocarbons is
not satisfactory because:
- explosimeter readings are not quantitative. They give only an
indication of the explosivity of the gas
- explosimeter is designed for use at pressures near atmospheric
whereas, the nitrogen coming out of the system is at a pressure much
higher than atmospheric
- enough oxygen should be present in the gas for getting a meaningful
value because explosimeter works on the principle of change in the
electrical resistance due to change in the temperature of the gas.
This change in temperature of the gas is accomplished by burning the
hydrocarbons or other combustible gases with oxygen over the filament
of the explosimeter.
Nitrogen used for purging would be a higher pressure and is free from
oxygen. The absence of oxygen necessitates dilution with air in known
ratios which is not always practicable at the site. Moreover since the
reading on the explosimeter gives only qualitative results, when the gas
is diluted with air, the results become more qualitative and become highly
undependable. The method described here gives very accurate values of
hydrocarbons even at very low concentrations of the order of a hundred
parts per million.
The method uses a gas chromatographic technique. A gas chromatograph
with a flame ionization detector is necessary.
Since only total hydrocarbon content in nitrogen is required, it is not
necessary to use any separation column on the gas chromatograph. A short
length of empty column connecting the injection port to the detector
serves the purpose of transporting the sample to the detector.
|2. Preparation of The
Connect the injection port with an empty SS, preferably glass, tubing
of approximately 1 M length and 3 mm dia to the detector. If the gas
chromatograph is equipped with a gas injection valve, connect a gas
sampling loop of 2 ml or of a higher capacity. In the absence of gas
injection valve , use a hypodermic syringe for introducing the sample
through the injection port septum. Maintain the oven at room temperature.
Adjust the carrier gas flow to about 2 ml per minute. Pure nitrogen from a
cylinder can be used as the carrier gas.
Adjust the hydrogen and air flow for the flame to give maximum response
and minimum noise. Use the attenuation to give a deflection of half to
three-fouths of the chart width with the standard gas mixture on the
Use a certified standard gas mixture containing 2 %v butane in nitrogen
or argon. In the absence of standard certified gas mixture, prepare the
standard as described below.
Establish the volume of a gas sampling tube of about 1 litre capacity
made of borosilicate glass (sometimes referred to as "sausage
tube") by filling it with water and measuring the volume of water
filled. Drain off the water and dry the tube in an air oven. Pass nitrogen
from a cylinder while it is still hot to expel the water vapor and then
cool to room temperature. Close the stop cocks at both the ends.
Based on the established volume of the sampling tube, calculate the
volume of butane required to give 2% v of butane on the volume of the
tube. For example, say, the volume of the tube is 920 ml. The volume of
butane required to give 2% on the volume of the tube will be,
x/(920+x) = 2/100
x = 18.8 ml
where x is the volume of butane required.
Remove the plunger from the barrel of a hypodermic syringe of suitable
capacity which is accurately calibrated with water previously and purge
the barrel with butane gas.(Butane from the LPG section of the refinery
may be used for this purpose). Insert the plunger into the barrel and
adjust the volume to the calculated volume after bringing the syringe to
Introduce the butane in the syringe into the sampling tube through the
septum. Let it remain for 10 to 15 minutes for complete mixing of the
gases. This gives 2%v butane in nitrogen.
|4. Collection of Sample
of Purging Nitrogen
The nitrogen from the unit at the exit point would initially contain
light hydrocarbon vapors which may include butanes and pentanes but as
purging is continued, only heavy hydrocarbons such as nonanes or decanes
would be present due to their lower vapor pressure. Rubber absorbs these
heavier hydrocarbons and if samples are collected in rubber bladders, they
are likely to give large errors, particularly when the nitrogen contains
very small quantities of these hydrocarbons. A gas sampling tube is well
suited for collecting the sample of nitrogen and should, therefore, be
preferred. A teflon bladder may be used if available.
|5. Measurement of Peak
An electronic integrator which records peak areas also along with
percentages is best for accuracy. In the absence of electronic integrator,
use any mechanical instrument such as a planimeter.
Bring the chromatograph to the operating conditions and stabilize for
30 minutes. A steady baseline indicates that the chromatograph is
stabilized. Adjust hydrogen-air ratio of the FID so that that the flame
noise is least. Select an attenuation where noise level is least,
(baseline is stable) and peak height for the standard is about
three-fourths of the chart width.
Withdraw 1 ml of standard through the septum of gas sampling tube with
a hypodermic syringe and inject it into the chromatograph through the
injection port. If a standard certified gas mixture is available, the
barrel of the syringe may be purged with it and 1 ml of standard may be
injected through the septum. Alternately, if a gas injection valve is
available in the chromatograph, attach a sampling loop of 1 ml capacity
and inject the sample through it using a leveling bottle to displace the
sample from the sampling tube. Standard gas mixture may be directly
injected through the loop.
After obtaining the chromatogram for the standard, inject the sample in
a similar way and obtain the chromatogram for the sample also.
Record the areas of sample and the standard as exhibited by the
integrator or measured by the planimeter.
||Area of the peak for the
sample X 2
|Hydrocarbons %v =
||Area of the peak for the
Note 1: The author or RTOL do not undertake any responsibility for any
accidents arising or any infringements of patents or copy rights by using
the procedure or method described in this article. Users are expected to
be knowledgeable about them. The author and RTOL disclaim any disputes
arising by using the procedure/method described in this article.
Note 2: The procedure and/or method described in this article are for
directional guidance of the users. It should be understood that no inter
laboratory co-relations have been done on the precision limits of the
Note 3: The author invites comments, improvements, or mistakes that may
have been committed, from the readers and users. They are requested to
send them to:
The author has 37 years experience in the laboratories of four
refineries, one fertilizer plant and one steel plant. He retired as Senior
Manager Quality Control of Gujarat Refinery of M/S Indian Oil Corporation
Ltd, India. He authored a book on ISO 9000 and was a faculty in many
seminars on ISO 9000. He was a nominated member of many technical
committees on Bureau of Indian Standards and several contributions to the
formulation of specifications and test methods for the petroleum products.
During his service period with oil refineries and fertilizer plants, he
came across many situations in which test methods have to be devised
because no standard test methods nationally or internationally were
available to suit such situations. The present article is one of them.
To read more of P.K. Rao's inovative test methods and applications
download his book titled ...
Chemical Test Methods"
"Assisting the production and
process engineering groups for controlling the unit operations, process
design, and problem solving."
Also see Guide
to Refinery Process Technologies (Second Edition)