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Hydrocarbon Traces In Purging Nitrogen

(by Chromatography)

By P.K. Rao

hydrocarbons - chromatograph

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.

 

1. Introduction:

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:

  1. explosimeter readings are not quantitative. They give only an indication of the explosivity of the gas
  2. 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
  3. 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 Chromatograph

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 recorder.

 

3. Standard Gas Mixture

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
or
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 room temperature.

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 Areas

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.

 

6. Testing

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.

 

7. Calculation

 

Area of the peak for the sample X 2
Hydrocarbons %v =
Area of the peak for the standard

 


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 results.

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:

 

About the author:

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 hi book titled ...

"RTOL Chemical Test Methods"

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