The overall goal of this method is to study the phase behavior of oil samples under refining process conditions by in situ visualization of liquid samples at high temperature and pressure using cross polarized reflection microscopy. This method provides a means of investigating the following behavior of oil samples under visbreaking and high conversion conditions and studying how stability is related to conversion. We can investigate severe process conditions since the reactor was designed to withstand 160 bars of pressure at 450 Celsius.
In this technique, a stainless steel reactor with a sapphire window at the bottom is illuminated by cross polarized light which gives high contrast images of both isotropic and anisotropic media. First, clamp the microreactor vertically upside down with the bottom face seal open. Ensure that the gas line fittings are closed to prevent the sample from spilling when it is loaded.
Weigh a subsample of oil and a thin spatula. Then use the spatula to load approximately 0.6 grams of the oil sample into the reactor. Weigh the subsample container and spatula again to determine the amount of sample loaded.
Then slide a custom-machined magnet onto the thermocouple. Slide a 1/16 inch front ferrule over the thermocouple into the fitting cavity with the wider side facing up. Dip a cotton swab in toluene ensuring that the swab is not dripping wet.
Wipe the fitting groove and other sealing surfaces to remove contamination from sample loading being careful not to drip toluene into the reactor cavity. Once wiping the sealing surfaces leaves no sample traces on the swab, allow the sealing surfaces to air dry. Place a seal ring into the fitting groove.
Place a clean dry sapphire window on top of the sealing ring followed by a brass pad. Apply pinhead-sized drops of lubricant to the brass pad. Then tread the bottom knot onto the bottom face seal fitting finger tighten the knot so that it covers the brass pad and securely holds the window in place.
Transfer the reactor still upside down to a vice. Use a wrench to tighten the bottom knot by 90 degrees. Visually inspect the reactor for defects in the seal or sapphire window and replace any defective parts.
Connect the loaded sealed microreactor to the gas lines. Then pressurize the system with nitrogen gas at five megapascals and close the appropriate valves to isolate the microreactor from the gas cylinder. If the microreactor pressure remains stable for more than 30 minutes, perform additional leak tests as needed at higher pressures.
Then if using a non-inert gas in the experiment, repeat all previous leak checks with the target gas with the help of a leak detector. Once the integrity of the seal has been verified, depressurize the microreactor setup. Place the microreactor in a stainless steel heating block within a coil heater.
Place the heating assembly on a platform over the inverted microscope objective. Enclose the heating assembly in a casing filled with ceramic wool and secured with a hose clip. Turn on the microscope using cross polarized light.
Set the objective to the lowest magnification. Adjust the vertical position of the objective lens to focus on the inside surface of the sapphire window. Guided by binoculars, position the microreactor so the ferrule edge is the inner radial boundary of the field of view of the binoculars.
Check that the ferrule is not visible in the camera field of view. Rotate the microscope nose piece to move the objective lens out from under the reactor. Connect the thermocouple to a temperature controller.
Set the external magnet motor to 120 RPM to begin stirring the sample. Pressurize the microreactor setup with the target gas. Use a back pressure regulator to control the pressure as needed.
To begin the experiment, ensure that the microscope objective is clear of the heating block whenever it is not in use. Then turn on the temperature controller and set the reactor temperature to 200 degrees Celsius. Once the sample comes up to heat, check the setup pressure and temperature.
Move the objective lens into position under the microreactor and adjust the reactor position and lens focus as needed. Check for movement of the ferrule or the flickering of small mineral solids in the sample indicating that stirring is ongoing. Rotate the objective lens away from the heating block.
Heat the sample to 300 degrees Celsius and perform the same checks and adjustments. Then heat the sample to 350 degrees Celsius and check the setup again. Set the reactor to the desired reaction temperature which is typically between 400 and 450 degrees Celsius for heavy oil cracking reactions.
Move the objective lens under the reactor, focus the lens and take a snapshot of the sample. Rotate the lens away from under the reactor and note the sample temperature. Repeat this measurement at one-minute intervals until reaction completion using the same magnification, lighting and camera settings throughout.
Leaving the objective underneath the reactor after taking a snapshot would make it overheat and not only this may deteriorate the objective but also taking a snapshot with an overheated objective would artificially enhance image brightness and lead to inconsistent results. Once the reaction has finished as determined by estimated reaction time or changes in the sample appearance, turn off the temperature controller and magnetic stirrer and depressurize the microreactor. Remove the microreactor from the heating assembly and cool it to room temperature under a stream of cool air.
Use custom computer code to extract the mean values of the red, green and blue channels from the micrographs along with the corresponding information from the hue, saturation and intensity color space in order to obtain quantitative image analysis. After the microreactor has cooled, disconnect the gas lines. Secure the reactor upside down in a vice and unseal the reactor by loosening the bottom knot with a wrench.
Transfer the reactor to a fume hood and unscrew the bottom knot. Remove the brass pad and sapphire window. Use tweezers to remove the ferrule and magnet being careful not to scratch the sealing surfaces.
Lever the sealing ring from its groove with a small spatula likewise taking care not to scratch the groove. Soak pieces of paper towels in toluene or dichloromethane and scrub the inside cavity of the microreactor to remove the bulk of the material adhering to the cavity walls. Then clean the interior walls with coarse grit Emery cloth without allowing the Emery cloth to contact the sealing surfaces.
Clean the magnet surface with coarse grit Emery cloth. Clean the center hole of the magnet with a 1/16 inch wire dipped in solvent. Clean the sapphire window with cotton swabs soaked in toluene, DCM or acetone.
Scrub the microreactor walls and sealing surfaces with cotton swabs soaked in toluene or DCM until after completely scrubbing the reactor surfaces the cotton swab shows only negligible traces of the oil sample. Allow the microreactor to air dry. Using this method, a thermal cracking experiment on a sample of Athabasca vacuum residue was monitored and visualized.
The experiment was performed under nitrogen gas at atmospheric pressure with a final temperature set point of 435 degrees Celsius. Image analysis provides information about the progress of thermal cracking reactions. The recommended method is to describe image information in the hue, saturation, intensity color space.
The evolution of the sample brightness can be related to the evolution of the refractive index while changes in color correspond to the evolution of spectral properties. The upcoming video will describe the evolution of the Athabasca vacuum residue sample during this experiment as the temperature is increased from 350 to 435 degrees Celsius using brightness and color analysis as shown in the present image. Initially, many small mineral solids are observed.
Before significant reaction occurs, little color change is observed and brightness increases with temperature. The chemical transformation of cracking reactions is first shown as a decrease in brightness. As the cracking reactions progress, a blue color shift is also observed.
Eventually, the anisotropic mesophase forms as bright stationary heterogeneities as indicated by an increase in brightness intensity. While attempting this procedure, it's important to produce a consistent set of images with the same illumination and the same image requisition settings. Following this procedure, other types of oil samples can be tested in various ranges of temperature and pressure conditions in order to answer additional questions relevant to other refining processes.
After its development, this technique paved the way for researchers in refineries to investigate following problems in visbreaking and high conversion. After watching this video, you should have a good understanding of how to perform in situ visualization of oil samples under refining process conditions including reactor loading, lick testing, observing the sample under reaction, and finally post reaction cleanup. Don't forget that emulating high conversion conditions requires about 150 bars of hydrogen pressure which can be dangerous.
Having the setup equipped with fail-safe features and conducting very thorough lick testing are very strong requirements in this case.
