Energy & Fuels 2001, 15, 1057-1058
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Fouling of Nearly Incompatible Oils Irwin A. Wiehe* Soluble Solutions, 3 Louise Lane, Gladstone, New Jersey 07934
Raymond J. Kennedy ExxonMobil Corporate Strategic Research, Route 22 East, Annandale, New Jersey 08801
G. Dickakian F.A.C.T., 1911 Pleasant Creek Drive, Kingwood, Texas 77345 Received March 19, 2001. Revised Manuscript Received June 5, 2001
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.
Introduction The two largest direct costs in petroleum refining are for crude oil and for energy. One way to reduce both costs is by the mitigation of fouling in the crude preheat train, from the desalter through the vacuum pipestill furnace. Energy is lost because the buildup of thermal insulating foulant in preheat exchangers and in pipestill furnace tubes and the effect is large because of the combination of high volume and large temperature increase. While fouling in the crude preheat train has been considered the expected price of doing refining, we now realize that not only can such fouling be controlled, it can even be eliminated. This can be accomplished by heat exchanger design1 and by eliminating the fouling agents based upon the diagnosis of the cause. This paper discusses the mitigation of the fouling of asphaltenes after the blending of crude oils, one common cause of preheat fouling. This is achieved by restricting the proportions and order of blending. For some time, there has been a need to relate crude oil properties to preheat train fouling for the purchase and scheduling of crude oils. This would be particularly useful in the purchase of low cost, opportunity crudes to reduce the largest refinery direct cost. However, such a crude oil purchase decision carries the risk that it could also increase fouling and result in a net economic loss. Previously,2-4 it was shown that crude oil incom* Author to whom correspondence should be addresed. Tel.: 908470-0847. Fax: 908-470-0939. E-mail:
[email protected]. (1) Polley, G. T.; Wilson, D. I.; Pugh, S. Proceedings of the 3rd International Conference on Refining Procedures; AIChE: New York, 2000; pp 519-523. (2) Dickakian, G. U.S. Patent 4,853,337, 1989. (3) Wiehe, I. A. Proceedings of the 1st International Conference on Petroleum Phase Behavior and Fouling; AIChE: New York, 1999; pp 82-87.
patibility, the precipitation of asphaltenes on blending of crude oils, can cause catastrophic fouling and coking in the preheat train. For this reason, the oil compatibility model and tests3-5 were developed to predict the proportions and order of blending of oils that would avoid incompatibility. However, it is known that even compatible oils can undergo asphaltene fouling, albeit at a slower rate. Therefore, this paper introduces the concept of fouling by the adsorption of asphaltenes at surfaces that depends on the distance from incompatibility, a quantity that can also be measured with the oil compatibility model and tests. The Oil Compatibility Model. The oil compatibility model3-5 is a solubility parameter based model that enables one to predict the compatibility or incompatibility of any mixture of any number of oils. This is based upon testing the compatibility of the individual oils with different proportions of a model solvent, such as toluene, and a model nonsolvent, such as n-heptane. These tests enable measuring the solubility parameter of the mixture at which asphaltenes just begin to precipitate. This solubility parameter on a reduced n-heptane-toluene scale is called the insolubility number, IN. In addition, the tests measure the solubility parameter of the oil that on a reduced n-heptane-toluene scale is called the solubility blending number, SBN. Once these compatibility numbers are measured for the oil components, the criterion for compatibility of any blend is that the volume average solubility blending number is greater than the insolubility number of any component in the blend. Previously,4 the crude oil, Forties, was measured to have a solubility blending number of 27 and an insolu(4) Wiehe, I. A.; Kennedy, R. J. Energy Fuels 2000, 14, 56-59. (5) Wiehe, I. A.; Kennedy, R. J. U.S. Patent 5,871,634, 1999.
10.1021/ef010063i CCC: $20.00 © 2001 American Chemical Society Published on Web 07/12/2001
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Energy & Fuels, Vol. 15, No. 5, 2001
Figure 1. Schematic diagram of the Thermal Fouling/Coking Test Unit.
bility number of 11 while the crude oil, Souedie, has a solubility blending number of 63 and an insolubility number of 39. On the basis of the compatibility criterion, mixtures of these two crude oils above 67 vol % Forties are incompatible, precipitating asphaltenes and causing rapid fouling. However, even when these two crude oils were blended to 50 vol % Forties, they caused rapid refinery fouling and coking because they were blended in the wrong order.4 Since the rate of dissolution of precipitated asphaltenes is slow compared to the rate of refinery processing, Forties needs to be blended into Souedie so that no asphaltenes are ever precipitated in the process. Distance from Incompatibility. The ratio of the solubility blending number of a mixture of oils to the maximum insolubility number in the mixture is a measure of the distance from incompatibility. When this ratio is one or less than one, incompatibility is predicted. A value slightly greater than one predicts compatibility but nearly incompatible and a value much greater than one is very compatible. This ratio may be directly measured by the heptane dilution test4 where one determines the maximum volume (mL) of n-heptane, VH, that can be mixed with 5 mL of oil without precipitating asphaltenes:
VH SBN )1+ IN 5 Alternatively, this ratio can be calculated for a blend of oils based upon measurements of the compatibility numbers of the component oils. Nevertheless, when one considers the adsorption of asphaltenes on a metal surface of process equipment, like a heat exchanger, it makes sense that the more soluble the asphaltenes are in the oil, the less tendency the asphaltenes will have to adsorb at a surface. Experimental Section The fouling by Souedie and Forties crude oils as well as several blends was measured in the laboratory with accelerated fouling tests. These tests were conducted using a Thermal Fouling/Coking Test Unit, now called a Hot Liquid Process Simulator and manufactured by Alcor Petroleum Instruments, Inc., as diagrammed in Figure 1. Oil (600 mL), at room temperature and under 700 psig nitrogen pressure to prevent volatilization, was pumped at 3 mL/min. through an annulus in which a 318 carbon steel heater tube in the center was
Wiehe et al.
Figure 2. Thermal fouling data on Forties-Souedie blends show incompatible blends foul more but nearly incompatible blends still cause moderate fouling. heated at a constant temperature of 760 °F. As foulant built up on the heater tube surface, the insulating effect of the foulant reduced the ability to heat the flowing oil and caused the temperature at the outlet of the annulus to decrease. Therefore, the decrease in temperature of the flowing oil at the annulus outlet over a 3-hour period was used as a measure of the fouling rate of the oil.
Results The variation in the fouling rate with volume percent Forties crude in blends with Souedie crude is shown in Figure 2. The fouling rate at 25% Forties is only slightly higher than expected from a line drawn through the points at 0% Forties (only Souedie) and 100% Forties. Since at 75% Forties the blend is incompatible, it is not surprising that the fouling rate is the highest measured for this set. The surprise is that the rate of fouling at 50% Forties is higher than the rate of fouling of either pure component of this blend, even though this blend is compatible. Based upon this limited data, blending oils such that the solubility blending number of the mixture is greater than 1.3 times the maximum insolubility number ensures that the fouling rate is not greater than the fouling rate of either of the pure components. On the other hand, by maintaining the solubility blending number of the mixture to be greater than 1.4 times the maximum insolubility number keeps the fouling rate within the experimental error of the expected fouling rate. Therefore, nearly incompatible oils are in the range of SBN/IN between 1.0 and 1.4 but especially between 1.0 and 1.3. A process for blending petroleum oils to avoid fouling by such nearly incompatible oils is the subject of a recent patent.6 Conclusions This paper demonstrates that fouling of heated metal surfaces by asphaltenes does not require the asphaltenes to be insoluble. Instead, the asphaltenes can adsorb on the surface from compatible oils, especially if the asphaltenes are in the region of nearly incompatible as measured by the ratio of the solubility blending number to the insolubility number being between 1.0 and 1.4. Since the data that supports this is based upon small laboratory tests, larger laboratory and commercial tests are encouraged. EF010063I (6) Wiehe, I. A.; Kennedy, R. J. U.S. Patent 5,997,723, 1999.
