6 replies to this topic

[Pires]

Hello,

 

So, i'am currently working on a refinery with propylene dryers, and the aim is to obtain propylene/propane (78/22)wt% with less than 70 ppm of water. At the moment that's not always the case.

 

The catalyst used is Alumina and after i read Ankur's blog post "Zeolite Based Molecular Sieve Adsorbents in The Chemical Process Industry" i realized that maybe Molecular Sieve 3A would be more proper (but not sure?). We also have a coalescer before the dryers. 

 

Also, when a sample is analyzed in the laboratory (using a Karl Fischer titration), we get strange results e.g. 35ppm, then 10 minutes later it can measure like 75 ppm, so i'am really confused about this.

 

Thank you,

Pires

[Bobby Strain]

Could be lots of causes. But, before you can determine the cause, you must have a reliable analytical method.

 

Bobby

[Pires]

Bobby,

 

What do you mean with a "reliable analytical method"?

 

Thanks for the reply

[Bobby Strain]

You must be able to determine the water content of the propylene exiting the dryer. Otherwise you are searching in the dark. The method you describe does not seem to be reliable. There are meters on the market that are electronic and are widely used. If it were my facility, I would install an online analyzer. They are not very expensive.

 

Bobby

[P.K.Rao]

Karl Fischer method can give higher results due to interference by atmospheric moisture. The equipment you are using should be completely protected from atmospheric moisture. Please check for any gaps. Karl Fischer reagent also needs careful standardization. Detecting end point is another point. Various persons have various degrees of color sensitivity. .Using an electronic .end point detection is less prone to this problem. Sodium acetate trihydrate is good for standardization. It retains moisture well.

 

Another point is, whether you are measuring volume ppm or weight ppm? If you are determining volume ppm, what type  of gas meter you are using? Use a dry gas meter. Wet gas meter can give errors. If you are determining weight ppm, please see that the weight is taken in the shortest time because propylene evaporates fast due to which errors can be there. Errors can come from other sources as well. You have to identify them and eliminate.

 

I believe Molecular sieve 5A is better for moisture removal.

[Pires]

Bobby,

 

The water content of the propylene exiting the dryer is determined by a dew point transmitter (not working at the moment), so the only way to do it for now is by the KF method.

 

The method we're using is the ASTM E700 (withdrawn already) but is the only one that "works" for this cases i guess. Unfortunately there is almost no information available about this method to find any reasons for the "strange" results.

 

 

P.K. Rao,

 

We are measuring weight ppm (the propylene is in liquid phase). To take the sample from the dryers we use stainless steel bottles so i don't think the propylene will evaporate. It may have interferences due to the porosity of the bottle maybe?

With the Molecular Sieve 5A the propylene will not be adsorbed with the water?

 

Thanks for your answers,

Pires

[P.K.Rao]

That is fine. Please ensure that the steel bottle is dry by passing hot air/nitrogen through it for sufficient time.