Ind. Eng. Chem. Res. 1993,32, 955-959
955
Measurement of Asphaltene Particle Size Distributions in Crude Oils Diluted with n-Heptane Kevin A. Ferworn, William Y. Svrcek, and Ani1 K. Mehrotra' Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, Alberta, Canada T2N IN4
The formation and growth of asphaltene particles from heavy crude oils diluted with n-heptane at 22 "C and atmospheric pressure was studied using a laser particle analyzer. The results obtained with six crude oil samples indicate that the asphaltene precipitation is an instantaneous process leading to a unimodal, log-normal distribution. At typical laboratory conditions, the particles remained essentially unaltered in size and population density. A vast majority of the particles were noted to be far from round in shape, with the mean particle size ranging from 4.5 to 291 pm. It was found that the oil-to-diluent ratio is an important parameter in determining the size of the generated asphaltene particles; higher dilution ratios yielded larger particles. The mean asphaltene particle size was also found to increase with the average molar mass and the asphaltene content of crude oils.
Introduction Projected shortages of liquid fuels have led refineries to investigate new sources of liquid hydrocarbons such as petroleum residua, heavy oils, and oil sand bitumens. However, these heavier feeds are rich in the asphaltene portion of petroleum, which is not very amenable to refinery processes and is usually responsible for coke laydown and catalyst inefficiency (Speight et al., 1984). In order to more effectively deal with the problem of asphaltene deposition and be able to control with some confidence, we need to better understand the mechanism of asphaltene flocculation and precipitation in petroleum fluids. Secondary and tertiary recovery programs often encounter asphaltene deposition problems (Moore et al., 1965). An example of such a recovery program is the miscible flooding of petroleum reservoirs by carbon dioxide, natural gas, and other injection fluids. The introduction of a miscible solvent in a reservoir will, in general, result in alterationsin thermodynamic conditions and reservoir rock characteristics. One major alteration is asphaltene precipitation, which affects productivity by causing plugging or a reversal of wettability in the reservoir (Hirschberg et al., 1984). Asphaltene precipitation problems are not, however, limited to petroleum reservoirs. Deposition may occur on the well site, in wells, tubing, or piping, or in any of the refinery vessels used to upgrade the crude oil.
Asphaltene and Resin Characteristics Asphaltenes are commonly defined as the n-pentaneinsoluble, benzene-soluble fraction of an oil or another carbonaceous source (Speight and Moschopedis, 1981). Not surprisingly, this somewhat informal definition has led to an effort to describe asphaltenes in terms of chemical structure or elemental analysis. It has been demonstrated, however, that the asphaltenes precipitated by different solvents and from various petroleum sources differ appreciably, making a purely compositional definition impossible. There is a close relationship between asphaltenes, resins, and the high molecular weight aromatic hydrocarbons which exist in petroleum (Nellensteyn, 1938).The heavy aromatics, on oxidation, gradually form neutral
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resins which on further oxidation become asphaltenes. As neutral resins are convertedto asphaltenes in the presence of air or oxygen at somewhat elevated temperatures, both are very similar in chemical structure. Asphaltenes are highly polydisperse molecules containing a broad distribution of polar groups in their structure, and published molar mass data for petroleum asphaltenes range from 500 g/mol to a high of 500 000 g/mol (Long, 1981). Vapor pressure osmometry (VPO) is a common method of determining the molar mass of asphaltenes, but the measured value is dependent on the type of solvent used to precipitate the asphaltenes. Asphaltenes are not crystallized and cannot be separated into individual components, making the final chemical analysis essentially unimportant. On heating, asphaltenes decompose, forming carbon and volatile products above 400 "C. The color of dissolved asphaltenes in benzene is deep red at low concentrations, although near 3 mg/L the solution is distinctly yellow (Kawanaka et al., 1991). Resins are highly polar, with molar mass in the range of 250-1000 g/mol, and exist in oils in true solution (Koots and Speight, 1975). While little research has been undertaken purely with resins, it is generally agreed that resins act as peptizing agents for asphaltenes in crude oils. Since asphaltene deposition may take place in reservoirs, the injection of peptizing agents (i.e., resins) in the correct amounts and places could control or even eliminate the deposition problem (Leontaritis, 1988). Primary asphaltene molecules (or micelles) have characteristic sizes of the order of 2-5 nn. Asphaltene micelles are lyophobic with respect to low molecular weight paraffin hydrocarbons and lyophilic with respect to aromatic hydrocarbons and resins (Sachanen, 1945). Both aromatic hydrocarbons and resins are readily absorbed by asphaltenes. Having absorbed the aromatic hydrocarbons and resins present in the crude oil, the asphaltenes are well dispersed in the crude. Both resins and aromatic hydrocarbons act as peptizing agents for the asphaltene micelles, that is, inhibiting flocculation between the molecules. Therefore, any action of a chemical or electrical nature (such as the addition of low molecular weight paraffin hydrocarbons) which depeptizes these particles will lead to flocculation and precipitation of the asphaltenes. These molecules must flocculate or agglomerate to form larger particles (larger than 1pm) before buoyancy forces can 0 1993 American Chemical Society
956 Ind. Eng. Chem. Res., Vol. 32,No. 5, 1993
overcome Brownian forces and permit the agglomerates to settle out of solution or suspension. The present study was undertaken to better understand the mechanism and results of asphaltene flocculation and precipitation. The objective of this work is to gain knowledge into the chemical properties of asphaltene particles generated from petroleum oils by the injection of a paraffin solvent. Experiments employing a laser particle analyzer provide new data on the effect of varying diluent concentrations for different crude oils, as well as the effect of agitation and shearing on asphaltene particles.
Experimental Section Apparatus: Laser Particle Analyzer. The heart of the experimental apparatus for this study of asphaltene deposition is a Brinkmann 2010 laser particle analyzer. The analyzer is driven by a personal computer that both executes software and controls collection of data. The basic concepts behind the operation of the analyzer are as follows. A fine, low-energy red beam is emitted by a helium/ neon laser and focused through a series of objectives to a rotating wedge prism. The prism spins the laser into a circular, optically defined path and is focused thereafter to a spot smaller than 1pm inside the analytical cell. The beam diverges after the cell onto a photodiode, registering a potential (measured in millivolts) (LeBlanc, 1988). The basis of measurement is to monitor the potential in the photodiode versus time. As the beam hits the first interface of a particle, a discreet, sharp decay of the potential occurs and a lower level is maintained during the transition period. At the second interface, the potential recovers upward to the initial value, producing a time-based square pulse. The length of this pulse multiplied by the angular velocity of the beam gives the diameter of the measured particle. The laser beam may pass through the diameter of the particle or through a chord. While the former would produce a perfect square pulse, the latter would have a less discrete decay and recovery of the background potential. If this rise and fall time is greater than a criterion set in the apparatus, the pulse is edited and the particle ignored from the total sample. The instrument collects the "good pulses" until a preset number has been obtained or the desired statistical confidence of the distribution has been met. The laser particle analysis produces a mean spherical diameter regardless of shape. To better quantify the shape of the measure particles, image analysis is used to enhance the laser function. Image analysis allows the particles to be viewed and studied during counting through the use of a microvideo camera. A strobe provides background light (transmitted light only) for the microvideo camera with a resolution of 1pm and a magnification factor that can be calibrated by means of a standardized grid. Scanned particles are located on a monitor where they may be viewed optically or in turn analyzed with the image analysis software. Experimental Setup and Procedure. Figure 1 is a schematic of the experimental setup. The bitumendiluent mixture is stored in the reservoir while being circulated through the flow loop by a peristaltic pump. Tygon tubing was used to transport the mixture of fluids and was replaced at the end of each test to ensure no asphaltenes from previous experiments could influence a new sample. The fluids were pumped out of a sealed reservoir at approximately 3 mL/min. The reservoir is kept on a stirring plate, and therefore, the generated asphaltenes are well suspended for the duration of the
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Figure 1. Schematic of flow apparatus. Table I. Physical Properties of Six Crude Oil Samples from Oil Fields in Alberta, Canada average asphaltene density petroleum molar mass content at 22 OC sample (dmol) (mass %) (a/cm3) Judy Creek 187 0.50 0.812 Norman Wells 197 0.85 0.856 354 8.09 0.911 countess Eyehill 489 12.18 0.980 Cold Lake 585 16.71 0.995 Primrose 552 18.09 0.998
test. The final feature of the apparatus is a vortex breaker located in the reservoir, which disrupts the flow of asphaltene particles down the vortex created by the magnetic stir bar. As is evident from these precautions, the main goal in the movement and storage of the particles is to keep them suspended in the oil-diluent mixture and to prevent their settling out into sludges. The reservoir is initially water wet to greatly reduce the amount of asphaltene particles adhering to the beaker. Ignoring this step could result in a nonrepresentative sample with a noticeable amount of asphaltenes depositing on the sides and bottom of the reservoir. The reservoir is sealed from the atmosphere in order to reduce the amount of oxygen with access to the oil-diluent mixture and to prevent possible evaporation of the solvent. The mixture with suspended asphaltene particles is pumped through the apparatus for 30 s in order to produce a representative sample, and at this point the particle analysis is begun. Statistics are taken at even intervals to determine asphaltene formation rates and for comparison to measurements taken on other samples. All of the experiments reported here were performed under atmospheric pressure at a temperature of 22 OC. The solvent used in these experiments was n-heptane, of a minimum purity of 99,5% ,supplied by BDH Chemicals. Crude Oil Samples. Table I gives the data on each of the six petroleum oils, all of which are from commercial oil fields in Alberta (Canada), considered in this study. The procedure used to clean the oil samples was dependent on the characteristics of the oil. The two light oils (Judy Creek and Norman Wells) were centrifuged to remove water, sand, and any other impurities. The four heavy oils were diluted with toluene to reduce viscosity and help break possible water emulsions. After centrifuging, the bitumen-toluene mixture was introduced to a roto-vap to distill and remove toluene from the sample. It is important to note that the residual concentration of toluene in the sample is small enough (
