15 replies to this topic

Hello

I am doin a project at the moment and i have hit a snag - is it possible to measure the density mass of a liquid mix (methanol and water) by calculation,there is a density meter but it only reads % -
Its a 60:40% of water to MeOH.


Any help would be great
Cheers
Nukem

[gvdlans]

It would help if you would tell us what type of density meter you are using. If you have a manual, you may also tell what the % scale means. Maybe you can tell what typical values you get from the meter (e.g. readings for 40% methanol and 60% methanol).

Apologies for not explaining properly. The Density meter is set to read and measure the percentage content of the meOH and thats all - the type is unknown as it is old and well out of the way.

It reads the density of the MeOH 38-42% usually and the rest is made up of water. What im asking is there a way to measure the density of the combined liquid (MeOH & Water) in kg/m^3.

I need to know the density for the pumping charteristics; pipe flow and the affects that changing the overall density would do to the pumping dynamics in terms of delivery in the pipes .....etc........

MeOH 791.8 kg/m3 (40%)
Water 1000 kg/m3 (60%)

Added together they give.....?

Cheers
Nukem

[gvdlans]

There is a table in Perry's Handbook Chemical Engineers' Handbook (table 2-109 in 1999 edition) that shows density for methanol/water mixture at 5 different temperatures (0, 10, 15, 15.56 and 20°C) and for 0, 1, 2,...,100% methanol concentrations.

When you put this data (for the temperature in your lab, e.g. 20°C) in a spreadsheet you can fit a 2nd order polynom through the datapoints. With this equation you can now find the density given a certain MeOH concentration.

Of course you can also skip the curve fitting part and read directly from the table. For example, a 40% MeOH mixture has a density of 934.5 kg/m3 at 20°C.

Since this is a student forum I think you have to find the table and the equation yourself (I am not going to do your homework).

Excellent!!!!
Thanks a million;in the lib now so il go checking and report back.

Thanks
Nukem

Truly that is brilliant i found the 7th edition wth the CD and it gives a rake of info - now all i have to o is try work it out for -19 degrees . Thanks a million.

Nukem

[gvdlans]

You're welcome. Using the data for 0°C is probably good enough (within the accuracy limits of your measurement).

Another suggestion is that you should have mentioned the -19°C (°F?) temperature in your original post. Don't leave us guessing at the details. The density for MeOH you gave in an earlier post (791.8 kg/m3) is at 20°C, not at -19°C.

Yes i was a little vague again sorry!
Its °C i am measuring in - i think i will just go with the 0°C as you said or is there any point in try to extrapolate the -°C using excel?

[gvdlans]

Well, you have to assess for yourself whether that's worth the effort. I don't know how accurate the measurement is or how accurate the results have to be. Again, it is my guess that the temperature effect will be minor.

There is another table in Perry's called "Densities of Inorganic and Organic Liquids", table 2-30 in 1999 edition. It gives an equation to calculate density of methanol as a function of temperature in the range 175K<T<513K. This may be useful to estimate the temperature effect

Is it possible to say if you have 1000m^3 of the mix and 40% is MeOH @ density 792 kg/m^3 and 60 % water 1000 kg/m^3

So that
400*792=316800
600*1000=6000000
Add them
and divide by 1000 to get the mixed density 916.8 kg/m^3

If so how do you relate the mix to temp?

Nukem

[luciferonus]

Hi i am wondering does methanol form azeotrope with water?

[gvdlans]

To Nukem,

As a reply to your post on Nov 15: No, as you could have seen yourself in the data from Perry, density of a 40% methanol/60% water mixture is higher than 0.4*density methanol + 0.6*density of water.

To Aljage,

You have density data at 25°C, 20°C (as well as 0, 10, 15, 15.56°C, from Perry's). You can easily fit a curve through these points and see whether a linear relation is good enough or whether you need to go for a second order equation (e.g. use curve fitting option in Excel). This is no rocket science and I am not going to do your homework...

[JMW]

Your density meter is old, unknown and out of the way?
Then how do you know it is still accurate?
Also, does it only report % or does it report that as %wt.wt or % vol.vol? Be sure it reports the % methanol in water and not the other way round.

To measure the %, it already measures the density of the solution and is probably programmed with the densities of the pure methanol and pure (100%) water.
The usual approach is to measure the density and temperature of the solution and then calculate the density at 15degC (or equivalent). This value is then used with the densities of the pure compoenents, at that temperature, to determine the ratio.

The way it determines the density at the reference temperature also needs to be understood and the data used for the determination validated.
In the simplified approach the density at 15degC is simply scaled in % concentration terms. The relationship between density and concentrtion isn't always linear so where the variation is signifcant. this may lead to inaccuracies unless that relationship has been characterised into the density meter calculations.

I would suggest that when using any instrument it is important to understand exactly how the instrument works, how it processes the raw data and what any output or display really means.

Note also that "accuracy" can mean many things. It could be thta the density meter is calibrated to report the result against a national standard or can be traced back to national standards.
However, it is also not uncommon for the calibration to be adjusted such that the reference standard it is calibrated to agree with is a local standard e.g. the orginal laboratory test equipment.
In many cases a process will have been characterised against a lab instrument and the density meter introduced to provide an online measurement.
Rather than debate which is the more "accurate" (online can be as acurate as 0.1kg/m3) it is often desirable to retain the lab instrument as the reference and not have to re-characterise the process being monitored.
It is also important to ensure that it is properly maintained and calibrated and replaced/upgraded when necessary.