Impact of Asphaltene-Rich Aggregate Size on Coke Deposition on a Commercial Hydroprocessing Catalyst

Abstract

Diverse coke deposition mechanisms and models, all supported by experimental data, have been proposed for catalytic hydrogenation processes related to heavy oil and bitumen refining. In this contribution, nanofiltration is used to partition Athabasca bitumen and Maya crude oil so that the potential impact of asphaltene-rich nano-aggregate size on coke deposition on a commercial NiMo/γ-Al 2 O 3 hydrotreating catalyst could be investigated without introducing artifacts related to the use of solvents. Crude samples were filtered at 473 K through 5, 10, 20, and 50 nm ceramic filters. The feeds, permeates, and retentates are characterized in detail in a companion contribution (Zhao and Shaw, Energy Fuels 2007, 21 (5), 2795-2804). Batch coking experiments were conducted at 653 K for 2 h at a 30:1 feed to catalyst ratio (weight basis), with feed stocks, permeates, retentates, and control samples. Surface area, pore volume, and pore size distribution of coked catalysts were measured. The results were found to be consistent with partial filling of catalyst pores for all cases. Pore mouth plugging was not observed. Elemental analyses performed on cross sections of coked catalyst pellets show that coke deposition within pellets was diffusion limited for all cases evaluated while vanadium deposition, arising primarily from the maltene fraction, was not diffusion limited. Asphaltene-rich nano-aggregate size and maltene composition are shown to play secondary roles in coke deposition within catalyst pellets.

Heavy Petroleum Composition. 3. Asphaltene Aggregation

“…Vapor pressure osmometry (VPO), diffusion measurement, − gel permeation chromatography (GPC), and size-exclusion chromatography (SEC) applied to asphaltenes have produced a wide range of average molecular weights, thought to be influenced by concentration-dependent self-association. ,,− Asphaltene molecular weight measurements are inconclusive because of asphaltene self-association at sub-parts per billion (ppb) concentrations. − Thus, VPO and GPC measurements typically reflect the mass of asphaltene aggregates as well as asphaltene monomers. , Fluorescence depolarization (FD) − yields an average asphaltene molecular weight (MW) of 450–850 g/mol but remains controversial. − Light and small-angle X-ray scattering, ,− ultracentrifugation, , ultrasonic relaxation, , ultrafiltration, − calorimetry, low-frequency dielectric relaxation spectroscopy, and direct-current (DC) conductivity ,,, measurements report asphaltene aggregate formation at sub-ppm concentrations. Asphaltene aggregates have been detected at high temperature in reservoir, and the results suggest in situ stable aggregate formation. , …”

Characterization of Acid-Soluble Oxidized Asphaltenes by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Insights on Oxycracking Processes and Asphaltene Structural Features

“…Traditionally, asphaltenes have been the villains of the oil industry for causing flow assurance problems by aggregation and precipitation upon changes of the temperature, pressure, and oil composition during oil production. − Asphaltenes are also reported to strengthen oil and water emulsions, poison refining catalysts, and increase coke formation during processing. − As a consequence of asphaltene definition based on solubility regime (insoluble in light n -alkanes, such as heptane and pentane, but soluble in aromatic solvents, such as toluene, benzene, or pyridine), the chemical variability among asphaltenes extracted from different oils by different methods is large, − because of their different behaviors of solution. − Scientists have tackled the problem of asphaltene aggregation and precipitation by means of phase behavior modeling, − where a detailed molecular characterization of asphaltene building blocks is fundamental to provide structural insights needed for accurate models. , Asphaltenes are considered the most refractory fraction of a refinery feedstock, and as the oil industry shifts toward heavy oil inventories as a result of the shortage of light oil reserves, production of vast quantities of asphaltenes is expected worldwide. For instance, asphaltenes are produced within the province of Alberta, Canada, as a consequence of oil sands production and upgrading activities involving carbon rejection in solvent deasphalting units; one Alberta facility was designed for processing up to 2500 tons/day of asphaltenes .…”

Membrane Diffusion Measurements Do Not Detect Exchange between Asphaltene Aggregates and Solution Phase

“…One of the most important features of petroleum asphaltenes is the tendency to self-associate and adopt a colloidal character in solution based on the observations from various analytical methods, including small-angle neutron scattering (SANS), − small-angle X-ray scattering (SAXS), − vapor phase osmometry (VPO), − ultracentrifugation, , ultrasonic, ultrafiltration, − and to a lesser extent, electron paramagnetic resonance spectroscopy (EPR) . The molecular aggregation of asphaltenes has been detected at high temperatures, suggesting that the aggregates may be remarkably stable in the heavy oil or bitumen and significant in both production and upgrading and refining. , In a complex mixture, a variety of sizes of aggregates are expected, ranging in size from dimers to colloidal particles, depending upon the solution conditions.…”

“…The most frequently used methods, such as X-ray and neutron scattering, − ,− ,, and vapor pressure osmometry, give no indication of whether the aggregates are in dynamic exchange with species in solution or whether they are static colloidal particles. Experiments with ultracentrifugation and nanofiltration ,,− suggest a very low rate of exchange with solution species, because rapid exchange would not provide separation of aggregates by these techniques. Similarly, the gradual changes in gel-permeation chromatograms with time for asphaltenes in dilute solution observed by Strausz et al suggested very slow rates of dissociation.…”