Although incompatible crude oils have not been previously discussed in the scientific literature, they are discovered to be relatively common. Consequently, with the need of a predictive method, the Oil Compatibility Model was derived from two hypotheses. One hypothesis is that asphaltenes precipitate from the oil at the same mixture solubility parameter, no matter that the oil is blended with noncomplexing liquids or another oil. The other hypothesis is that the solubility parameter of a mixture is the volumetric average solubility parameter. As a result, the solubility parameter of a crude oil and its flocculation solubility parameter on a toluene−n-heptane scale can be determined based upon mixing the crude oil with toluene and n-heptane and determining if each mixture dissolves or precipitates asphaltenes. Thereafter, the correct proportions and correct order of blending oils to ensure compatibility can be specified. A refinery example is given where determining the correct order of blending potentially incompatible crude oils mitigated the coking of vacuum pipestill furnace tubes.

The plugging of a fixed bed hydrotreater was determined to be caused by an incompatibility of the mixture of hydrotreater feeds. Insoluble asphaltenes coked a heat exchanger upstream of the hydrotreater, flaked off the heat exchanger, and plugged the catalyst bed. The Oil Compatibility Model and tests were expanded to include oils that contain no asphaltenes. As a result, the most insoluble asphaltenes were determined to be in the fluid catalytic cracker bottoms and the poorest solvent oils were the virgin gas oils. By maintaining the feed composition within the compatibility limits predicted by the Oil Compatibility Model, subsequent plugging of the hydrotreater was avoided.

The fouling of heat exchangers by asphaltenes in petroleum crude oils is found to be caused not only by incompatible crude oil mixtures but also by mixtures that are nearly incompatible. It is reasoned that the tendency for asphaltenes to adsorb on heated metal surfaces increases as the oil mixture approaches compositions at which asphaltenes precipitate. Based upon data for mixtures of Forties and Souedie crude oils, this region of near-incompatibility occurs for ratios of the solubility blending number to the insolubility number that are between 1.0 and 1.4. These numbers can be measured on individual oils using the oil compatibility model and tests and calculated for mixtures.

Air oxidation of vacuum resid delayed coker feeds promotes the formation of anisotropic shot coke. The combination of the microcarbon residue test (MCRT) on a feed to the delayed coker followed by crosspolarized light optical microscopy on the coke produced in the MCRT is a predictive test for the morphology of the coke formed in delayed cokers. Sponge-coke-forming feeds produce cokes with highly anisotropic (ordered) 10-60 µm flow domains, whereas shot-coke-forming feeds produce cokes with a less anisotropic mosaic structure of 1-10 µm. Air oxidation increases both the asphaltene content and the polarity of the asphaltenes by increasing the organic oxygen heteroatom content of the asphaltenes in vacuum resid feeds. The higher solubility parameter of the oxidized asphaltenes favors phase separation from the hydrocarbon matrix and leads to shot coke formation. Another possible explanation is that the oxygen incorporated in the asphaltene structures leads to more rapid coke formation, thus favoring shot coke. Our experiments do not allow a distinction to be made between these two possible explanations for the increased shot coke formation tendency following oxidation of the resid.

The present study was carried out using the microcarbon residue (MCR) test to investigate the mechanism by which vanadium-and nickel-containing additives cause a dramatic reduction in mosaic size in coke from delayed coker feeds. Since a fine mosaic microtexture is one of the key characteristics of shot coke, these additives have the potential to steer the morphology of the coke produced in a delayed coker drum to freeflowing shot coke. Midcontinent U.S. (MCUS) vacuum resid was selected because of its low metals content and its tendency to produce exclusively sponge coke in delayed coking. This allows us to easily observe changes in its shot-coke-forming tendency by monitoring the reduction in the microscopic domain size of the MCR coke using polarized light optical microscopy. The concentrations and chemical states of vanadium porphyrin, acetylacetonate and naphthenate, and nickel porphyrin were quantified using X-ray photoelectron spectroscopy before and after coking. The surface concentration is depressed for vanadium and nickel porphyrins added from 1000 to 10 000 atomic parts per million carbon atoms to MCUS vacuum resids. This observation is consistent with the behavior of native porphyrins in resids. The surface concentration of native vanadium and nickel porphyrins is depressed relative to the bulk in both petroleum residua and asphaltenes to the same degree as in feeds with additives. Following the coking of MCUS with vanadium and nickel porphyrin additives in the MCR test, the surface concentration gets closer to the bulk average. No evidence was found for the decomposition of any of the added vanadium and nickel porphyrins following coking. For resids, the surface concentration of vanadium acetylacetonate was severely depressed relative to the bulk; however, the surface concentration of vanadium naphthenate was enhanced relative to the bulk. Both vanadium acetylacetonate and vanadium naphthenate transformed into V 2 O 5 following coking, and the surface concentration of vanadium was comparable to the predicted bulk average. All of the above-mentioned vanadium and nickel additives produced a coke having a fine mosaic domain structure characteristic of shot coke. The observation of similar coke morphology for soluble vanadium additives with very different chemical end products in the coke argues that additives have similar effects on the major hydrocarbon decomposition pathways. The disparity of vanadium and nickel concentrations at the surface is a new observation that confirms that petroleum resid is inhomogeneous at the microscopic level and asphaltenes self-associate via a mechanism involving secondary bonding interactions.