Measurement of H₂S in Crude Oil and Crude Oil Headspace Using Multidimensional Gas Chromatography, Deans Switching and Sulfur-selective Detection

The overall goal of this analytical method is to measure the amount of hydrogen sulfide present in the headspace above crude oil samples or the amount dissolved in the samples themselves using gas chromatography. This method can help improve the upgrading and handling of heavy crude oil as hydrogen sulfide is toxic and when dissolved in crude oil can deactivate catalysts and be corrosive. The main advantage of this technique is that hydrogen sulfide is determined in a single step with no interference from other hydrocarbons present in the sample.

To begin open a dean Switch calculator program on a computer and input the column dimensions, carrier gas, temperature, desired flow rates and detectors used. The calculator will define the pressures needed for the inlet and pressure control module or PCM and the length of restrictor tubing needed for installation between the dean switch and the flame ionization detector or FID. Note these pressures and input them into the method file file to install the column, ensure that the instrument oven and inlet are at room temperature.

Turn off the gas flow to the inlet and PCM supplying flow to the dean switch following the manufacturer's instructions. Install the dimethyl polys laane column between the split split list inlet and the dean switch. Then install the plot column between the dean switch and the sulfur chemiluminescence detector.

Finally, install the restrictor tubing between the dean switch and the FID. The dean switch is controlled by a valve in the gas chromatograph. Locate the valve timing point section in the method parameters in the software.

Set the switch to on and create time events for the valve that controls the dean. Switch, beginning with the valve turning off at 0.5 minutes and a second event that turns the valve on again at 3.0 minutes. Next, place a gas bag containing the calibration gas in the fume hood and place a rubber nipple or equivalent penetrable cover on the nozzle of the gas bag.

Then open the nozzle on the gas bag using a 250 microliter gas tight glass syringe. Puncture the rubber nipple on the top of the bag and withdraw 250 microliters of calibration gas. Cap the syringe with an inlet SEPTA before transporting it to the gas chromatograph.

Remove the SEPTA cap from the syringe. Then manually and swiftly inject the calibration gas into the gas chromatograph. Simultaneously starting the software acquisition, the hydrogen sulfide will appear as a peak on the sulfur chem luminescence detector signal trace.

Record the retention time of this peak. Begin decreasing the time of the heart cut window one side at a time. Continue in this manner until the hydrogen sulfide peak disappears from the chromatogram.

Add 0.2 minutes to this time and not it as the upper limit of the heart cut window. It is critical that the dean switch be timed properly in order for this method to be successful. If the switching window is too narrow or at the wrong times, hydrogen sulfide will not be quantified accurately.

Perform the same procedure on the lower end of the time window. Gradually increasing the time of the valve off event for sequential injections until the peaks are no longer visible. Subtract 0.2 minutes from this time and not it as the lower limit of the heart cut window.

Finally, save the valve on off commands into the method file for liquid sample calibration. Ensure that a liquid autos sampler is installed on the split split list inlet of the gas chromatograph. Place the four previously prepared calibration vials in vial positions one through four on the autos sampler tray.

Use the gas chromatograph software to perform one injection per vial according to the method. Integrate the carbon diss sulfide peak in each chromatogram using the software provided with a gas chromatograph to perform liquid sample analysis. Transfer approximately one milliliter of neat crude into a gas chromatograph autos sampler vial, and cap the vial.

Place the filled vials in the autos sampler tray and load the liquid analysis method on the gas chromatograph software. Using the gas chromatograph software and the previously configured method, use the automatic sampler to perform three replicate injections per vial to calibrate the instrument for a gas sample. Inject the calibration gases before.

Repeat the manual injection of calibration gas a minimum of three times. Then integrate the hydrogen sulfide peaks in the three injections with the gas chromatograph data analysis software. Using a spreadsheet program, calculate an average response factor for hydrogen sulfide by dividing the average area of the hydrogen sulfide peaks by the concentration of hydrogen sulfide in the gas bag.

Next, fill a 500 milliliter Amber Glass bottle with 450 milliliters of the crude to be analyzed. Attach a septum topped cap to the top of the bottle. Place the bottle to be analyzed in a temperature controlled environment using a one milliliter glass gas tight syringe, puncture the septum top and leave the syringe in the top of the bottle to provide an avenue for pressure release.

If the gas in the head space should build up, leave the bottle in the temperature controlled environment for 24 hours, gently lifting and shaking the bottle once an hour to equilibrate the hydrogen sulfide between the liquid and head space. To analyze the Headspace gas puncture the septum top and withdraw 250 microliters of gas into a gas tight glass syringe cap the syringe end with a piece of inlet septa and transport the syringe to the gas chromatograph manually and swiftly inject the gas into the inlet simultaneously starting the software to begin data collection. This chromatogram of a liquid crude sample is representative of the results obtained through this procedure.

The hydrogen sulfide peak is resolved from other peaks and easily quantified chromatograms of gas samples show similar resolution of hydrogen sulfide with the additional peaks remaining unidentified. For the purpose of this method, if the concentration of hydrogen sulfide in the gas sample is too high for the detector to quantify accurately, the chromatogram will show a flat topped peak. While attempting this procedure, it is important to remember that the pressures and timing points are individual to each system and will likely vary from instrument to instrument.

After watching this video, you should have a good understanding of how to sample both liquids and gases for hydrogen sulfide analysis and how to program a dean switch for heart cutting GC analysis. Following this procedure, additional methods may be developed to analyze other light sulfur gases present in crude oil such as dimethyl sulfide or methylome captan. Don't forget that hydrogen sulfide gas can be highly toxic and should not be handled outside of a vented fume hood.

Additionally, crude oil can be toxic and flammable and should only be handled while wearing proper protective gear.