3 replies to this topic

[anitaasuquo]

Hi, I am a rising sophomore engineering student at Purdue. This summer I am doing a case study. It requires that I find the density of sweet crude oil at -40 degrees Celsius. I have been steered towards looking at ASTM standards and API standards. A lot of the information provided is over my head. Is there anywhere I can find such information that would be easy for me to understand? Thanks.

[srfish]

Hi,

The answers I have are older and my be hard to find. They are:
Nelson W.L. "Petroleum Refinery Engineering",McGraw-Hill
Maxwell J.B. "Data Book on Hydroarbons", D. Van Nostrand Co.

[ELEMAN]

Hello everybody:

You should pay a visit to Jon Watson in the site www.viscoanalyser.com, he is a fellow member of this forum and has been working in R & D on this subject; designing spreadsheets for such calculations.

Good luck!

[JMW]

The spreadsheets at www.viscoanalyser.com are designed to replicate the calculations, which are now the standard rather than the tables, and have been optimised for use by bunker operators and ship engineers.

I would be loathe to suggest anyone use them as a convenient means to obtain answers without a proper understanding.
All such tools are for convenience and they should only be used by those who understand them and can and have validated their use for their own purposes.
I would suggest reading the Manual Of Petroleum Measurement Standards Chapter 11.

Fundamentally, the density of a hydrocarbon changes with temperature.
This much is easy enough to understand.
But different oils exhibit a different rate of change of density with temperature and therefore hydrocarbons are divided into different commodity groups where the oils within a group can be characterised by a common set of tables or calculation. It is a working relationship and thus may not be exact for all the members of the commodities groups. The extent to which error between the calculation and table and between either and the actual results should be understood.
It is important to understand that originally, oil density was measured at different temperatures using hydrometers, corrections applied for using the hydrometer at temperatures other than the calibrated conditions and the tables compiled.
Naturally enough the tables are correct for the range of values over which they were tabulated and the permitted interpolations defined.
Later on, various tables were expanded using a digital density meter (a vibrating glass U tube sensor - see Anton Paar).
Then the various tables were expanded by extrapolation. and calculation methods introduced.
Some tables or data in the tables have been "infilled" i.e. interpolation by calculation or adjustment of the actual data to regularise the results in the tables.
Hence, the use of calculations must be done with knowledge of the limits of applicability and a knowledge of thee extrapolation limits for a particular commodity group.

Over the years refinements have been added. Amongst then the introduction of new commodity groups - and recommendations as to which tables (or K0/K1 values to use in calculations) for some of the more exotic refined products.
Today there is a need to investigate and advise on how these tables and calculations may be adapted to deal with bio-diesel and bio-diesel/mineral oil blends.
Other factors that have had to be considered are the changes in temperature measurement standards. We tend to assume that the temperature scales we use are unchanging but this is not true. 0degC and 100degC today are not what they were 50 years ago. (see International Practical Temperature Scale).
All these and other factors go into producing a means to predict the density of a hydrocarbon at some temperature based on the measurement of density at some other temperature.
It is then important to understand how the usage of the tables or calculations differs from one industry to another.
For example, some people could not reconcile the results from the ViscoAnalyser spreadsheets with results from other "industry standard" programs used in the marine industry where it transpires that densities are automatically rounded up to the nearest 0.5kg/m3 and where it can be expected that individual hydrometer corrections are not applied.

Hydrometers are commonly used and often misused.
For hydrocarbons it is generally calibrated to be used at 15degC and to report the density at 15degC. But in different industries and with different fluids other types of hydrometer may be used.
A 15/4 hydrometer used in brewing is intended to measure at 15deg and report the density at 4degC. Wrongly used or understood, serious errors can result, especially in process control.
In hydrocarbons it is common to use the hydrometer at temperatures other than the calibration temperature. Therefore there is a correction factor that is applied to the hydrometer reading that corrects for this use.
For example, the density of a bunker fuel may be measured at 50degC. This is then referred to as the observed temperature. If a hydrometer is used a correction is applied to the reading to allow for the change in buoyancy and calibration due to expansion. This reading is then used to determine thee base density, the density at 15degC. What the operator really wants to know is the density at 62degC because that isthe temperature of thee fuel which is being supplied by volume. So what is required is a volume correction factor that allows the operator to calculate the volume of fuel at 15degC from the volume of fuel at 62degC from a measurement of density at 50degC. The calculation of density at 62degC is necessarily in two steps, not one. First find the density at 15degC and then find the "alternative density" at 62degC from the density at 15degC.

Individual hydrometers may also have an unique calibration correction which compensates the graduated scale for the actual value. This must be applied before using the spreadsheets or other calculation programs.
It should be noted also that pressure has an effect on density. The versions of the spreadsheets on www.viscoanalyser.com do not yet have a pressure correction.

The value of any "tools" such as spreadsheets must be that they are simply a tool to save valuable time and hopefully avoid human error in routine calculations. They should ideally be used by those who fully understand the underlying calculations, methods and theory and who, before relying on such tools, have taken the care to validate them for the conditions under which they will use them. The spreadsheet may be much more convenient that tables, for example, but periodically and in cases where the use is outside normal conditions, the results should be cross checked by conventional methods. that may, in come cases require laborious calculations or in this case reverting to the look-up tables. Of course, in the real world, such tools will be given to operators to use who do not understand and perform their operations according to precise directions and in which case the responsibility to understand lies with the supervisor.