4 replies to this topic

[saeed.s]

Dear friends
As you can see in attached file, PDT 2005 is a eccentric orifice that show vapor / liquid ratio in stripper re-boiler outlet and also controlling the fuel gas to re-boiler. My question is how we can calculate vapor / liquid ratio based on differential pressure?
Thank you

Attached Files

•  DP.bmp   1.88MB   37 downloads

[djack77494]

I cannot see the things you claim that I should be able to see in the attached picture and have very little idea about how your system works. I am gathering that the circled orifice, which you say is eccentric, is used to control firing of fuel gas into the reboiler (which I assume is the fired heater to the right of the reboiler return line). I can't tell which line flows to the reboiler, but that might not be very pertinent.

Sorry to disappoint you further, but I really cannot answer your question. Measuring a pressure drop across an orifice plate will not ordinarily give you any information about vapor/liquid ratios. However, your control scheme is very likely effective despite this. If you feed the reboiler from a pump, which I assume is the case, then your flowrate is likely nearly constant. The amount of vaporization that occurs will result in very large variations of your volumetric flowrate since the vapor volume will greatly exceed the liquid volume. If fact, for any significant degree of vaporization, the contribution due to liquid flow will be negligable. Thus the dP you measure will correlate to the vapor flowrate under the simplistic system I have described. Again, with the total (mass) flowrate fixed, the vapor flowrate is used to infer the degree of vaporization. Not enough? Fire the heater harder. Too much and you should reduce the heat input.

[Root]

Hi,
PDT, is UOP Temperature control scheem and UOP is employing this where temp outlet across the heater is very less(few degrees, may be 3~5). Indeed,rate of vaporization is controlling in this scheem and when reflux is more on that time this scheem not work because of PDT will control only vapourization not the temperature. This is the main draw back of this temperature control scheem.
The refinery, i'm working, have same temperature control scheem.
Toor

[engg]

I cannot see the things you claim that I should be able to see in the attached picture and have very little idea about how your system works. I am gathering that the circled orifice, which you say is eccentric, is used to control firing of fuel gas into the reboiler (which I assume is the fired heater to the right of the reboiler return line). I can't tell which line flows to the reboiler, but that might not be very pertinent.

Sorry to disappoint you further, but I really cannot answer your question. Measuring a pressure drop across an orifice plate will not ordinarily give you any information about vapor/liquid ratios. However, your control scheme is very likely effective despite this. If you feed the reboiler from a pump, which I assume is the case, then your flowrate is likely nearly constant. The amount of vaporization that occurs will result in very large variations of your volumetric flowrate since the vapor volume will greatly exceed the liquid volume. If fact, for any significant degree of vaporization, the contribution due to liquid flow will be negligable. Thus the dP you measure will correlate to the vapor flowrate under the simplistic system I have described. Again, with the total (mass) flowrate fixed, the vapor flowrate is used to infer the degree of vaporization. Not enough? Fire the heater harder. Too much and you should reduce the heat input.

Djack

Your post is interesting but i cannot understand it?

Can you explain why there will be vaporization? as the stream is going back from reboiler i suppose it to be fully vapor.

secondly, why would heat input reduce while you are increasing the heat input by firing the reboiler?

[djack77494]

Can you explain why there will be vaporization? as the stream is going back from reboiler i suppose it to be fully vapor.

engg, Now it's my turn to say I don't understand what you are saying. You're asking me to explain why there is vaporization? Typically in a multicomponent distillation system such as (presumably) yours, you draw off heavy liquid from the bottom of the column, add heat to partially vaporize the drawn liquid, and return either the vapor part (for a kettle reboiler) or the entire vapor + liquid for most other reboilers. As (incompletely) shown in your drawing it appears to me that the stream going from the reboiler back to the distillation column is not fully vapor but is partially vaporized.

secondly, why would heat input reduce while you are increasing the heat input by firing the reboiler?

Based on this comment, I think you misunderstood my response. Firing the heater more (harder) results in more heat input = more vaporization = more vapor flow and more pressure differential across the orifice.

Best wishes, Doug