METHOD AND APPARATUS FOR CALCULATING DERIVED CETANE NUMBERS

Abstract

A method and apparatus for calculating the derived cetane number of a liquid hydrocarbon sample is disclosed. The method comprises combusting (19) the sample in a constant volume combustion chamber (45). The method comprises obtaining (23) a pressure versus time combustion profile (69) of the sample wherein the profile comprises a first region (81) and a second region (83), the first region (81) including the start of combustion, and the second region (83) relating to a later time than the first region. The method comprises selecting a single data point from the second region (83) of the combustion profile (69), said data point representing a combustion delay (CD) of the combustion profile; and calculating a derived cetane number for the sample using the time value associated with said single data point.

Claims

1. A method of calculating the derived cetane number of a liquid hydrocarbon sample, the method comprising: (a) injecting a sample into a combustion chamber, the combustion chamber being held at a constant volume; (b) combusting the sample in the combustion chamber; (c) measuring the pressure in the constant volume combustion chamber as a function of time after the injection until combustion is completed; (d) obtaining a pressure versus time combustion profile of the sample injected into said constant volume combustion chamber, wherein the profile comprises a first region and a second region, the first region including the start of combustion, and the second region relating to a later time than the first region; (e) selecting a single data point from the second region of the combustion profile, said data point representing a combustion delay of the combustion profile; and (f) calculating a derived cetane number for the sample using the time value associated with said single data point.

2. The method of claim 1 further comprising the step of bringing the combustion chamber pressure to a pre-determined pre-injection pressure, prior to injection of the sample.

3. The method of claim 1, further comprising the step of monitoring the chamber pressure to check for leaks in the chamber prior to injection of the sample.

4. The method of claim 1, wherein the derived cetane number, CN, is calculated using an equation of the form $\mathrm{CN}=x_1+\frac{x_2}{{\mathrm{CD}}^{1.5}}-x_3\mathrm{CD}$ where x.sub.1, x.sub.2 and x.sub.3 are constant coefficients and CD is the data point representing the combustion delay.

5. The method of claim 4 wherein the coefficients x.sub.1, x.sub.2 and x.sub.3 are empirically determined.

6. The method of claim 1, further comprising the step of calibrating the combustion chamber by measuring the combustion profile of a reference sample.

7. The method of claim 4, further comprising the step of calibrating the combustion chamber by measuring the combustion profile of a reference sample, wherein the coefficients x.sub.1, x.sub.2 and x.sub.3 are determined using the calibration of the combustion chamber, and wherein the coefficients x.sub.1, x.sub.2 and x.sub.3 are empirically determined.

8. The method of claim 1, further comprising the step of performing at least one additional injection of the sample into the combustion chamber after measuring a combustion profile of a sample.

9. The method of claim 1, wherein the volume of the combustion chamber is adjusted after the combustion profile of a sample has been measured.

10. The method of claim 1, further comprising the step of performing a blockage check, the blockage check comprising: (i) opening the combustion chamber; (ii) measuring the pressure in the chamber over time until a pre-determined pressure is reached; (iii) determining the time, t1, between opening the chamber and the chamber reaching the predetermined pressure (iv) comparing t1 to a pre-specified threshold time, tt; and (v) providing an indication if t1>tt.

11. An apparatus for measuring the derived cetane number of a liquid hydrocarbon sample, the apparatus comprising: a combustion chamber; a fuel injector arranged to inject the sample into the combustion chamber; a combustion pressure sensor for measuring the pressure in the chamber over time; and a control system arranged to receive and analyse data from the pressure sensor; wherein the control system is configured to calculate a derived cetane number for the sample using a single data point of a pressure-time profile produced using the data received from the pressure sensor, said single data point representing a combustion delay of the combustion profile.

12. An apparatus of claim 11, further comprising a removable flange, wherein the flange is arranged such that when it is removed, the interior of the chamber can be inspected.

13. An apparatus of claim 11, wherein any of the combustion chamber, the fuel injector, the combustion pressure sensor and the control system are housed in an explosion proof case.

14. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0054] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0055] FIG. 1 shows a flow chart illustrating the steps of a method for calculating a cetane number according to an example embodiment of the invention.

[0056] FIG. 2 shows an apparatus for measuring cetane number in accordance with an example embodiment of the invention.

[0057] FIG. 3 shows a close up view of part of the interior of the apparatus of FIG. 2.

[0058] FIG. 4 shows a close up view of part of the interior of the apparatus of FIG. 2.

[0059] FIG. 5 shows an example combustion pressure versus time profile measured using the method of the present disclosure.

TEXT OF FIG. 1

[0060] With reference to the numbered boxes of FIG. 1 the text of FIG. 1 is as follows:

[0061] 5: Calibrate the combustion chamber using a reference fuel.

[0062] 7: Measure chamber pressure using a pressure sensor and depressurize if required.

[0063] 9: Bring chamber to a pre-determined, pre-injection pressure and temperature.

[0064] 11: Allow the chamber pressure and temperature to stabilise and perform a leak check.

[0065] 13: Prepare a liquid hydrocarbon sample for injection for injection.

[0066] 17: Inject the sample into the combustion chamber.

[0067] 19: Combust the sample in the combustion chamber.

[0068] 21: Measure the pressure in the combustion chamber as a function of time.

[0069] 23: Obtain a pressure versus time combustion profile for the sample.

[0070] 27: Select a single data point representing the combustion delay from the combustion profile.

[0071] 29: Calculate a derived cetane number for the sample using the time value associated with this single data point.

[0072] 33: Perform at least one additional injection of sample into the combustion chamber.

[0073] 35: Perform a blockage check of the chamber.

[0074] 37: Perform a calibration check.

[0075] 39: Adjust the volume of the chamber if necessary.

DETAILED DESCRIPTION

[0076] An example of the apparatus of the present disclosure and the method of the present disclosure are described below.

[0077] FIG. 1 shows a flow chart (1) illustrating the steps of a method for calculating a cetane number according to an example embodiment of the invention.

[0078] Block 1 (3) of the flowchart shows method steps that are performed prior to the injection of a sample into the combustion chamber.

[0079] Prior to injection of a sample into the combustion chamber, the chamber is calibrated using a reference fuel (5). The calibration is performed using a primary reference fuel with a known cetane value.

[0080] The chamber pressure is measured prior to injection (7). If the chamber pressure is not at ambient pressure or at an accepted pre-specified pressure the chamber is de-pressurized.

[0081] The chamber pressure is brought to a pre-determined pre-injection pressure (for example 100 kPa) prior to injection of the sample (9). The pre-injection pressure is reached by increasing the chamber pressure to, 90 kPa, and then topping up the chamber pressure in small increments of 10 kPa.

[0082] Once the pre-injection pressure has been reached the chamber temperature and pressure may be allowed to stabilise for a fixed period of time before fuel injection (11). The chamber pressure is monitored to check for leaks in the chamber during this time period. If a leak is evident, for example, if a pressure drop is detected, an indicator or alarm may be activated.

[0083] A liquid hydrocarbon sample is prepared prior to injection (13), by increasing the pressure to a pre-determined pre-injection pressure. The temperature of the sample is increased also to a pre-determined value.

[0084] Block 2 (15) of FIG. 1 shows steps of injecting and combusting the sample and measuring the combustion profile.

[0085] The liquid hydrocarbon sample is injected into the combustion chamber (17), and after injection of a sample, the sample is combusted in the combustion chamber (19). Pressure in the chamber is measured as a function of time (21) to obtain a combustion profile for the sample (23). The profile will comprise a first region and a second region, with the first region including the start of combustion, and the second region relating to a later time than the first region.

[0086] Block 3 (25) of FIG. 1 shows the steps that are taken to calculate a derived cetane number for the sample from the combustion profile. A single data point representing the combustion delay is selected from the second region of the combustion profile (27).

[0087] A derived cetane number for the sample is calculated using the time value associated with this single data point (29). The derived cetane number, CN, is calculated using an equation of the form

[00002]$\mathrm{CN}=x_1+\frac{x_2}{{\mathrm{CD}}^{1.5}}-x_3\mathrm{CD}$

[0088] where x.sub.1, x.sub.2 and x.sub.3 are constant coefficients determined from the calibration of the apparatus using a fuel sample having a known cetane value and CD is the time value representing the combustion delay. The coefficients may be determined using standard curve fitting techniques.

[0089] Block 4 (31) of FIG. 1 shows the steps that are taken after measurement of the combustion profile. After measurement of the combustion profile for a sample, at least one additional injection of the sample into the combustion chamber is performed (33), and preferably at least two further injections of the sample are be made at timed intervals. Sample that is injected during these additional injections is not combusted.

[0090] A blockage check of the chamber is also performed (35), by opening the combustion chamber and measuring the pressure in the chamber over time until a pre-determined pressure is reached. The time, t1, between opening the chamber and the chamber reaching the pre-determined pressure is determined, and is compared to a pre-specified threshold time, tt. An indication is provided if t1>tt.

[0091] A calibration check of the chamber is performed (37) using a secondary fuel, and the volume of the combustion chamber may be adjusted after measuring the combustion profile of a sample (39).

[0092] Any or all of these steps may be repeated for the same or for different liquid hydrocarbon samples. Not all of these steps may be carried out in each testing cycle, for example, a blockage check and/or calibration step may not be carried out in each cycle.

[0093] FIG. 2 shows an analyser (42) for measuring cetane number in accordance with an example embodiment of the invention. The analyser (42) comprises two explosion proof enclosures (43a, 43b) mounted one on top the other on a base plate (90). The lower of the two enclosures (43b) houses an analysis cell which is shown in more detail in FIG. 3. The higher of the two enclosures (43a) houses the electronics system of the analyser (42) and is shown in more detail in FIG. 4. A touch screen (91) is mounted on the front of the upper enclosure (43a) to allow user input to control the analyser (42). In use, the analyser (42) is connected to a production line producing liquid hydrocarbon fuels.

[0094] The lower explosion proof box (43b) is shown in the open configuration in FIG. 3. A combustion chamber (45), a fuel injector (47) arranged to inject the sample into the combustion chamber (45), and a combustion pressure sensor (49) for measuring the pressure in the chamber over time are located within the explosion proof box (43a). The apparatus further comprises a fuel injector controller (51) that is used to control the fuel injector (47) and fuel pressure amplifier (53) that is used to increase the fuel pressure prior to injection. The apparatus comprises a further pressure controller (57) for measuring and controlling the pressure in the chamber prior to injection of the sample and/or in between measurements of combustion pressure curves. The apparatus comprises a temperature sensor (not shown in FIG. 3) for measuring the temperature on the interior of combustion chamber (45). The apparatus comprises a bypass (not shown in FIG. 3) which diverts fuel around the combustion chamber. In use, when a sample is not being tested, fluid may flow through the bypass before being returned to the production line.

[0095] The upper explosion proof box (43a) is shown in the open configuration in FIG. 4. A power supply (61), input/output cards (62) including a sound card, terminal connections (63) via which electrical connections (not shown) are provided between the upper and lower enclosures (43a, 43b), and USB hub (64) are located within explosion proof box (43a) and connected to a computer (65) which is mounted to the rear of the touch screen (91). Computer (65) forms part of the control system of the analyser and includes software for recording and analysing the pressure readings received from combustion pressure sensor (49) as well as receiving input from a user and providing outputs to the user.

[0096] FIG. 5 shows an example combustion pressure profile (69) obtained using methods of the present invention. The profile shows the pressure measured in the combustion chamber as a function of time during combustion of a sample.

[0097] At time=0 ms, sample is injected into the combustion chamber. Prior to injection of the sample, the combustion chamber pressure may have been set to an initial value (P.sub.0) (71). This pre-injection pressure may be, for example 1000 kPa. When plotting the combustion pressure profile of a sample, measured and recorded pressure values may be offset in a linear manner such that Po corresponds to a pressure of P=0 kPa.

[0098] A drop in the pressure of the combustion chamber, indicated by a dip (73) in the pressure-time curve for the sample may be measured as, or shortly after the sample is injected. The ignition delay (ID) (75) is the elapsed time between injection of the sample and the time at which combustion of the sample begins. In this case, the time at which combustion begins may be taken to be the time at which the pressure in the combustion chamber reaches a predetermined value above the pre-injection chamber pressure. In FIG. 1, the time at which combustion begins is indicated by the point ID on the curve, and is the point at which the measured pressure is 20 kPa above the pre-injection chamber pressure. Therefore the ignition delay time is the time at which the pressure in the combustion chamber reaches a value P.sub.ID=P.sub.0+20 kPa. In other embodiments, however, the time at which combustion begins may be taken as the time at which the pressure in the combustion chamber returns to the pre-injection pressure (P.sub.0) following the initial drop in pressure.

[0099] As the sample is combusted, the pressure in the combustion chamber increases up to a maximum value (P.sub.max) (77). Once combustion of the sample is complete, the pressure in the combustion chamber may begin to decrease. The combustion delay (CD) (79) is defined as the elapsed time between injection of the sample, and the time at which a pressure representing the midpoint of the net pressure increase of the combustion pressure curve is reached, therefore the combustion delay is indicated by the time at which the pressure in the combustion chamber reaches P.sub.CD=(P.sub.0+P.sub.max)/2.

[0100] The combustion profile comprises a first region (81) and a second region (83), the first region (81) including the start of combustion, and the second region (83) relating to a later time than the first region (81). In FIG. 5, the first region (81) encompasses the elapsed time between injection of a sample, and the time at which combustion of the sample begins. In FIG. 5 the second region encompasses the time between the end of the ID time period and the time at which combustion of the sample is complete.

[0101] In order to calculate the derived cetane value for the sample, a data point from the second region (43) that represents the combustion delay (CD), i.e. the point at which the P.sub.CD=(P.sub.0+P.sub.max)/2 is extracted from the pressure-time combustion profile and is used to calculate the derived cetane number using an equation in the form:

[00003]$\mathrm{CN}=x_1+\frac{x_2}{{\mathrm{CD}}^{1.5}}-x_3\mathrm{CD}$

[0102] In one embodiment, the following equation is used to calculate the derived cetane number:

[00004]$\mathrm{CN}=44.770+\frac{406.925}{{\mathrm{CD}}^{1.5}}-0.252\mathrm{CD}$

[0103] This equation has been found to provide accurate calculation of derived cetane number for cetane numbers in the range 35 to 65.

[0104] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0105] For example, in the above apparatus various elements of the analyser are located within explosion proof boxes. Such elements may alternatively be located in one or more purged cells.