The 5th International Conference on Stability and Handling of Liquid Fuels

Technical Program

Technical Presentations

The conference was officially opened by Dr. C.W.M. Dessens, Director General of Energy for the Dutch Ministry of Economic Affairs, who provided an overview of the strategic importance of Rotterdam as a port and terminal, and the "oil gateway" to Europe. During the conference, 55 papers were presented in 12 technical sessions, with an additional ten posters included in a separate session.

Jet Fuels - Sessions I and II

The keynote paper in these sessions addressed quality control at both the refinery and the airport and provided several examples of past and current problems that have been encountered. The next paper discussed several new techniques that are proving useful for assessing stability of jet fuel in storage, and addressed two case studies in which these new techniques have been used. In another paper, results were presented on the thermal stability of a number of fuels obtained using several test procedures. These showed that thermal stability is not as dependent on test conditions as previously thought, but that either a measure of the number of phenols or the total polar material present in jet fuels is strongly indicative of thermal stability. Two papers were given related to the use of StadisÂ conductivity improver additive. In the first of these, the effects of fuel contaminants on the performance of StadisÂ in jet fuels was reported. Problems encountered in StadisÂ 450 treatment of merox sweetened jet fuels were discussed in a second paper. The possible causes for sporadic nonconformance of merox-treated jet fuel to the silver corrosion test and some refinery process changes that have been made to alleviate the problem were addressed in another paper. Then, some of the limitations inherent in current methods of modeling jet fuel thermal stability were discussed.

The effects of copper, metal deactivator, and aging on jet fuel thermal stability as measured by gravimetric JFTOT were also discussed. Following this, data were presented that suggest the mechanism of deposit formation on fuel-wetted hot metal surfaces involves thermal-oxidation reactions that are limited by the available oxygen. The speaker also reported that metal surface composition affects the quantity of surface deposition. In the subsequent paper, it was reported that activated carbons had been found to be very effective in improving jet fuel stability during stressing at high temperatures, apparently as a result of termination of free radical chain reactions on the carbon surfaces. This was followed by a presentation on work in progress to develop an additive for in-flight oxygen scavenging to improve thermal stability. Another speaker discussed other work in progress focusing on the development of thermal stability additive packages for JP-8. Then, data were presented that demonstrated the usefulness of the quartz crystal microbalance and pressure monitoring for evaluating performance of jet fuel additives. The final paper of the session reported on application of dynamic near isothermal methods to evaluate jet fuel additives.

Microbiology

The keynote paper in this session provided an overview on anti-microbial strategies for storage and use of distillate fuels to satisfy health, safety, and environmental regulations. This was followed by a presentation of case studies on the use of a two-phase program for diagnosis and control of microbial contamination in diesel and gasoline storage and distribution systems. Another speaker discussed attempts to harmonize microbial sampling and testing methods for distillate fuels, including simple tests suitable for laboratory assessment of microbiological quality. Factors affecting the selection of a fuel biocide were discussed in a subsequent paper.

A new field procedure was described involving catalase measurement for rapidly estimating microbial loads in fuels and water bottoms. Data demonstrating an excellent correlation between traditional tests and this new procedure were also presented. The finding of viable bacteria in a tank used for storage of gasoline was discussed, and test results that were presented suggested that the presence of oxygenates in the fuel may be contributing to microbial growth. In the closing presentation, a biocide that purportedly conforms to some of the strict environmental restrictions in effect in Europe was described and examples of its use for treatment and preservation of liquid fuels discussed.

Long-term and Strategic Storage

The session opened with a presentation on experience gained in the storage of motor gasolines in large caverns in salt. With suitable precautions, it was shown that this fuel can be maintained in usable condition for as long as nine years. Another paper discussed how developing countries can organize strategic petroleum stockpiles, and options for financing these reserves within budget constraints were presented. Assessing the storage stability of diesel fuel using a recently published standard method was discussed in another paper. Finally, the metal-deactivating and anti-oxidant properties of several additives that appear to be potentially suitable for use in long-term storage of distillate fuels were reported on.

Predictive Systems and Methods

This session included three presentations on computerized expert quality prediction systems. The first of these discussed the numerous aging parameters that must be considered in developing a system. The second presentation discussed the mathematical approach, logic, and programming tools used in developing an expert system. The final presentation on computerized expert systems pertained to diagnosis of microbial-related stability and operative problems in jet and diesel fuel storage facilities.

Despite the proliferation and increasingly widespread use of expert systems, interest continues in laboratory methods for predicting storage stability of fuels. In the final paper in this session, a calorimetric method previously shown to be useful for rapid prediction of the storage stability of refinery-fresh fuel was reported to have been extended to older fuels. The test results exhibited very good correlation with those of another widely used accelerated aging technique.

Test Rigs and Simulators

The thermal stability characteristics and filterability of jet fuels containing pipeline drag reducer (PDR) additives were investigated in small scale and realistic fuel simulation tests. These experiments confirmed the results of other studies that indicated that the PDR additive can reduce a fuel's thermal stability. In another study, new test devices (rigs) and systems were used to evaluate jet fuel thermal stability and additive performance. The relationships of oxygen consumption, residence time, and bulk fuel and wetted wall temperatures on coking deposits that formed were also investigated in this study. Finally, the stability of a diesel fuel following four years of storage was investigated using a single cylinder engine operated under conditions simulating those in the field. In evaluating fuel stability, test results were assessed using a combination of subjective and objective methods.

Deposit and Insolubles Measurements

In the opening paper in this session, ellipsometry was shown to be a suitable technique for measuring the thickness of JFI'OT tube deposits. This has long been a goal in characterizing thermal stability of jet fuels. The relationship between visual rating and deposit thickness/volume for a number of fuels was demonstrated, and the potential of ellipsometry for quality assurance purposes was discussed. In another paper, it was demonstrated that viscosity is the major variable that must be considered in interpreting the filter blocking tendency of fuels when measuring solids formed during typical accelerated tests. Light scattering photometry was evaluated as a means of monitoring the formation of soluble deposit precursors formed in fuels. Monitoring the formation of these precursors may constitute a useful predictive measurement.

Trimethylpyrrole in dodecane has been shown to be an acceptable reference fuel for a widely used accelerated stability test. In evaluating stability using this mixture, the quantity of both adherent and filterable insolubles must be considered, not just the total insolubles formed. In another paper, the accuracy of particulate measurements of aviation turbine fuel using the contaminated fuel detector was reported to be improved by converting light transmission data to light absorbance data. This conversion yields results that can be better correlated with data obtained from a commonly used accelerated aging test.

Gasolines

In a study on gasoline containing 50 percent cat-cracker stock, it was found that rust from the walls of a storage tank had no effect on oxidation stability. Sediment, comprised mostly of oxygenated polymers, taken from the tank bottom reduced the storage period significantly. Data derived from the study were successfully used to predict storage period. In a study of two types of anti-oxidants, it was found that the storage stability of gasoline could be improved considerably by using elevated concentrations of an alkylphenolic-type additive. Elevated concentration of an aromatic diamine additive resulted in a reduction in storage stability. Another paper addressed the effect of some metals on the oxidation stability of gasoline. On the basis of kinetic data obtained in the study, a correction coefficient (km) evaluating the influence of metals on the oxidation process was determined. In the final paper in the session, it was reported that cyclic olefins and cyclic diolefins were the most prolific gum formers. Thiols enhanced gum formation from all olefinic types. Sulfides and disulfides exhibited both accelerating and inhibiting effects on gum formation, depending on the structure of the olefins involved.

Heavy Oils and Refinery Processing

In a study on the compatibility of residual fuel oils, it was found that many blends were stable even when the component fuels came from different refineries. Flocculation of asphaltenes from incompatible mixtures occurred virtually instantaneously. Density, pour point, sulfur content, and xylene number were shown to be useful tools for predicting thermal stability of blends. In a second paper on compatibility and stability of residual fuels, a rapid analysis procedure for determining toluene equivalence and a simple algorithm for calculating the Bureau of Nfines Correlation Index were presented. These two parameters were found to be valuable in evaluating stability of residual fuel oil blends.

In a departure from technical presentations, a strategy was presented to ensure the continuing viability of the U.S. refining industry. This strategy is designed to stimulate the yield of environmentally acceptable transportation fuels and minimize the output of low value products and waste streams.

Middle Distillate Fuels - Sessions I and II

The keynote speaker for these sessions presented the findings of a search of the world literature on aging of middle distillate fuels covering more than 50 years. The presentation provided details on fuel components that individually may not have a deleterious effect on fuel quality, but when present with other components could be harmful. The kinetic models found in the literature that seemed most promising for explaining instability reactions were also reviewed. In an evaluation of ten commercial stability additives, it was found that all but one resulted in a decrease in sediment formation. All the additives significantly improved Nalco Pad Test results, inhibited formation of suspected sediment precursors, and promoted an increase in the concentration of phenalenones. In another study, it was found that aliphatic olefins accelerated the degradation of light cycle oil. In light waxy gas oils, the aliphatic olefins inhibited the degradation process, contrary to general assumptions. Three possible mechanisms were proposed to explain these effects.

Another speaker presented results on the characterization of sediment precursors extracted from cracked middle distillates with methanol. The effect of some commercial additives on the stability of these cracked fuels was determined before and after methanol extraction using both ambient and accelerated storage techniques. Fuel composition, additive structure, and storage conditions influenced performance. A study to determine the safety of testing a hydrocarbon distillate fuel under elevated oxygen pressures and temperatures was reported. A literature search failed to reveal any experimental results on the effects of oxygen pressure on the autoignition temperature. No instance of autoignition occurred in laboratory tests at temperatures up to 120'C and oxygen pressures to 1000 kPa. There is concern that the low-sulfur diesel fuel now being produced in the U.S. may be prone to formation of hydroperoxides during storage.

In a study of a number of high and low sulfur diesel fuel samples, hydroperoxide susceptibility of both were found to be equivalent and acceptable. However, under progressively severe thermal and oxidative stress, low sulfur fuels appeared increasingly less stable than high sulfur fuels. In another study, it was reported that solid phase material extracted from middle distillate fuels is the product of reactive fuel constituents originally present in the fuel and intermediate to the formation of fuel-insoluble sludge. The composition of this material was also shown to change during fuel blending processes and to become more oxygen rich.

New Fuels and Environmental Mandates

The increased refinery processing required to produce fuels meeting new environmental regulations with respect to sulfur and, in some areas, aromatics content apparently is resulting in a loss of intrinsic lubricity and modifications of oxidation susceptibility. In the keynote paper in this session, results from the evaluation of several thousand samples of fuel confirm a general decrease in fuel lubricity. A relatively high additive concentration was found necessary in some instances to restore fuel lubricity to its pre-1993 levels. In another study, the performance of a number of different additives was evaluated with respect to their antioxidant performance. An-tine stabilizers were found to be the least effective, while a phenylenediamine was most effective as an antioxidant. Results of this study also suggest that some of the methods currently used to test for fuel stability may not be suitable for the new generation of low sulfur fuels.

Detergency, conductivity, and corrosion protection have also been affected by increased hydrotreating of fuels. Additive technology available to effectively restore the required performance was discussed in another paper. Fuels not meeting the new on-highway, low sulfur requirements were previously required to be dyed blue for easy identification. This raised many concerns among users about possible detrimental effects of the dye. No storage stability problems were found on addition of the blue dye in laboratory studies, although unacceptable levels of immediate particulate contamination resulted. The results of this study are made trivial by the fact that a red dye is now required. The study does, however, raise important considerations for future work on mid-distillate fuel storage and handling. The U.S. Navy has initiated a program to determine the effects of low-sulfur diesel fuels on durability of shipboard fuel handling and injection equipment. Tests commonly used in evaluating fuel lubricity may need to be modified to account for the uniquely high salt and water concentration present in shipboard fuels.

Poster Presentations

Gravimetric JFTOT was shown to be useful for ranking diesel fuels for their thermal stability in one poster. In another study, results indicate that light, straight-run diesel fuel, when treated with biocide and a stabilizer additive package, can be stored in underground tanks for up to five years without any significant microbial or chemical damage, provided the tank is in good condition and no product turnover takes place. In a study of the storage stability of a light cycle oil, four of five procedures used to treat the oil resulted in a decrease in the sediment formed during accelerated aging. Composition of the fuel before and after aging and of the sediments formed were characterized.

An automatic on-line stability analyzer has been developed for heavy fuel oils. Measurement of asphaltene content of the stream should be useful in process quality control in refineries and blending operations. This is especially important when heavy fuel oils are produced from a variety of different crude oils. The storage stability of naphtha, light gas oil, and heavy gas oil obtained from coprocessing 5 and 30 wt. percent coal with crude oil was studied using accelerated ageing procedures. The high coal fractions generally exhibited poorer stability and other characteristics than comparable low coal fractions.

In another poster, it was shown that spent caustic solutions from treatment of distillate fractions for removal of H2S, phenols, and mercaptans can be recycled through crude oil desalting units to neutralize inorganic acids that may form in atmospheric towers. This process reduces the volume of fresh caustic solution needed and helps minimize disposal of potentially harmful waste streams. In an investigation of a health and environmental problem associated with jet fuel handling, a chromatographic sniffing technique coupled with gas chromatographic identification of compounds was developed. This technique can be useful for characterizing sulfur-organic compounds and can better control the caustic wash treatment of the fuel.

An automated deposit-measuring device (ADMD) has been shown to be useful for evaluating lacquer-type fuel and lubricant deposits on JFTOT heater test tubes. Good correlation exists between the volume of deposit as determined by the ADMD and carbon burnoff. Another poster investigated the susceptibility of refinery-fresh fuel to microbial contamination. Samples of the refinery fuel, free of additives, were inoculated with contaminated fuel taken from ship's tanks. In the absence of water and nutrients, the inoculum did not appear to survive. A predictive test for fuel stability that might be used at the refinery and which might serve as a basis for recommending storage terms for fuel stocks was suggested in another poster. Stressing of the fuel followed by separation and quantification of the hexane insolubles from the filtered sample serves as the basis of the test.