GB2300264A

GB2300264A - Determining metal dissolution rates in fuels

Info

Publication number: GB2300264A
Application number: GB9608385A
Authority: GB
Inventors: Spencer Edwin Taylor
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1995-04-24
Filing date: 1996-04-23
Publication date: 1996-10-30
Also published as: GB9608385D0; GB9508235D0

Abstract

A method for determining the rate of metal dissolution in fuels, particularly hydrocarbon fuels such as aviation fuel involves determining small mass changes in a metal electrode in contact with fuels and comprises: (a) immersing a metal coated quartz crystal working electrode 8 in the fuel 4 (b) connecting the crystal to an oscillating circuit (c) detecting and measuring the change in frequency of the crystal; and (d) analysing the results obtained in step (c). A reference electrode may be coated in an inert metal and immersed in the fuel.

Description

METHOD FOR DETERMINING METAL DISSOLUTION RATES IN FIJELS The present invention relates to a method for determining the rate of metal dissolution in fuels, particularly aviation fuels.

As a result of necessary handling and transportation, metal contamination of fuels can sometimes occur, rendering the fuel off-specification with respect to thermal stability. Such problems occur typically during sea transportation.

Recently, the effects have been traced to on-board copper dissolution from heating coils.

The presence of trace levels of dissolved copper, typically in the range of from 10 to 100ppb, has a detrimental effect on the thermal stability of aviation fusels, as reflected for example, in the measured breakpoint in Jet Fuel Thermal Oxidation Tests (JFTOT) which can be reduced by as much as 40"C as a result of xcopper contamination.

Verification of the metal leaching propensity of fuels has been based on the results of "copper strip" tests and tests involving the controlled storage of fuels over specimens of representative metallurgies followed by trace metal analysis to determine the levels of contamination. Often these experiments are lengthy and by the time the results of the extended tests are known failures in the JFTOT or other indicators of contamination are also known. There is, therefore, a requirement for a method which will rapidly determine a fuel's propensity for copper leaching, especially in cases where it is known that the likelihood of the fuel coming into contact with copper-containing metallurgies is high. Such a method must be sensitive to either the detection in solution of parts per billion (ppb) levels of copper or to the dissolution of very small masses (micro grams) of copper.

We have developed a method capable of fulfilling the aforementioned conditions and according to the present invention there is provided a method for determining the rate of metal dissolution in fuels which comprises (a) immersing a metal coated quartz crystal working electrode, and optionally a reference metal coated quartz crystal reference electrode, in the fuel; (b) connecting the crystal to an oscillating circuit (c) detecting and measuring the change in frequency of the crystal; and (d) analysing the results obtained in step (c).

The technique of the present invention provides an accurate method for determining small mass changes in a metal electrode in contact with fuels. Such data can then be related to the dissolution of the metal in the particular fuel.

The method of the present invention can be used for any fuel, either heavy or light hydrocarbon fuels such as crude oil, diesel, gasoline, and aviation fuel. The method of the present invention employs the piezoelectric resonant properties of a quartz crystal which changes proportionately with small masses of material deposited on the electrode surface. Quartz crystals suitable for use in the present invention are commercially available and may be cut at specific angles to the principal optical axis. Suitably, the crystal is cut at an angle of+35"15'.

The method requires the use of at least one metal coated quartz crystal.

The metal acts as an electrode. The first crystal, the working electrode, comprises the metal known to dissolve in fuel. Suitably, the metal coating is copper, iron or silver. The second crystal, the reference electrode, has a metal coating of an inert metal. Suitable metals are gold, and platinium. The crystal is coated on both sides with the appropriate metal. Suitably the coating on the working electrode has a thickness of from 1500 to 3000A. Suitably the coating on the reference electrode has a thickness of from 1800 to 2200A. The preferred coating for the working electrode is copper. The preferred coating for the reference electrode is gold.

Where the method of the present invention employs the working and the reference, the two crystals are preferably in juxtaposition and are crystals, connected to an oscillating circuit which drives separate trigger inverters. One suitable system is based on S.Bruckenstein and M.Shay, Electrochimica Acta, Vol.30, pages 1295-1300 (1985). The frequency changes in the quartz crystals are suitably detected by a means which accepts the output from the trigger inverters and measures the frequency differences between the two oscillators. An optical isolator which isolates the frequency measuring circuitry may also be used. The output from the optical isolator is sent to a frequency meter which interfaces to a computer which utilises a data logging routine for frequency change and time.

The process of the present invention detects the frequency changes between the crystals. The frequency changes are computated using appropriate computer software and the following equations. The crystals oscillate in the thickness shear mode and the fundamental frequency (fq) is related to the thickness of the quartz plate (t) by equation (I.) fo = Vt,=N (I) 2t where Vtr is the transverse wave velocity (cms-l) normal to the plane of the quartz plate and N is the frequency constant of the quartz crystal. The thickness of the quartz plate sandwiched between two electrodes of overlapping area A is related to its mass mq by equation (11) t=rnq (11) PqA where pq is the density of the quartz.

Thus the fundamental frequency can be calculated through equation (III) fo = pqN (III) mq which indicates that for a crystal of uniform thickness the frequency is influenced solely by the mass of quartz per unit area of electrode. If a change in mass (Am) occurs on one electrode. If a change in mass (Am) occurs on one electrode the corresponding frequency change is given by equation (IV) f0 + f = QqNA (IV) mq+ Am Thus the relationship between change in frequency and change in mass can be represented by equation (V) f = f02 Am (V) pqNA The invention will now be described by way of example with reference to Figures 1 and 2 the following example, wherein Figure 1 is a schematic diagram of a metal coated quartz crystal.Figure 2 is a schematic diagram of the crystal immersed in the fuel.

Figure 1 shows a quartz crystal ( I ) having a metal coating (2) on its upper and lower faces. There is a central area of overlap (3). In Figure 2, a sample of fuel (4) is incubated in ajacketed all-glass vessel (5), fitted with a thermocouple (6) and a condenser (7). The fuel is maintained at a temperature of 105+1 C by circulating silicone oil from a Contraves thennostat bath. The quartz crystals (8) are immersed in the fuel (4). The quartz crystals (8) are connected to the oscillating circuitry (not shown) which is in turn connected to the computer (not shown).

Example 1 110my of an aviation grade fuel as detailed in Table I was placed in the glass vessel. The fuel was heated to 105"C. At a temperature of approximately 100"C, a copper quartz electrode was immersed in the fuel. Prior to this, it was ensured that the resonator would remain "in-tune" after immersion in the fuel by determining the oscillating frequencies in both air and heptane and recording the appropriate shift in frequency. Following immersion of the resonator, frequency differences were monitored and computer logged over a period of two hours using suitable electronic circuitry. The results are given in Figure 3. It can be seen that with time, there is an increase in frequency, thus reflecting a decrease in mass of the copper electrode and thus dissolution of copper in the fuel.

TABLE I

FUEL DESCRIPTION JFTOT H:M Break oint 0C 1 100:0 295 2 0:100 280 3 80:20 ND 4 60:40 ND 5 50.50 ND 6 40:60 ND 7 20:80 ND H = HYDROFINED FUEL M = MEROX FUEL ND = Not determined

Claims

Claims: 1. A method for determining the rate of metal dissolution in fuels which comprises: (a) immersing a metal coated quartz crystal working electrode in the fuel (b) connecting the crystal to an oscillating circuit (c) detecting and measuring the change in frequency of the crystal; and (d) analysing the results obtained in step (c).
2. A method according to Claim 1 which also comprises immersing a reference metal coated quartz crystal reference electrode in the fuel in juxtaposition to the metal coated quartz working electrode.
3. A method according to Claim 1 or Claim 2 in which the fuel is a heavy or light hydrocarbon.
4. A method according to Claim 3 in which the hydrocarbon is a crude oil, diesel, gasoline, or aviation fuel.
5. A method according to any one of the preceding claims in which the metal coating of the working electrode is copper, iron or silver.
6. A method according to any one of the preceding claims in which the metal coating of the reference electrode is an inert metal.
7. A method according to Claim 6 in which the inert metal is gold or platinum
8. A method according to Claim 4 or 5 in which the metal coating of the working electrode is copper and the fuel is aviation fuel.