5 replies to this topic

[Ghasem.Bashiri]

Dear experts

within some olefin, NGL or LNG plants we need vaporization of dense fluid (dense phase).

I means we heat a fluid in liquid state but above critical conditions.

With such concept, it is possible to transfer a stream from liquid to vapor phase in single exchanger.

My first point is that during such phase change there is not reliable heat curve (physical properties).

Then different thermal rating software will have problem for heat transfer calculation and area prediction.

What is advice for such design.

Also, why when we work in under-critical condition, we should heat up fluid up to saturated vapor in single exchanger and then make it superheat in another exchanger. why it is not possible to assume such two exchanger in single shell and make superheat in single exchanger. when we use two exchanger one for saturated vapor and second for super heating, we have a system with different equilibrium in sub-section without separation and isolating.

 

Ghasem Bashiri

 

[srfish]

I take exception to  "there is not reliable heat curve(physical properties)" statement. Many software packages have reliable physical property data.

[Ghasem.Bashiri]

The issue that we have a transition state from liquid to gas via dense phase. in dense phase we expected to have regular heat curve as input to simulation software to thermal rating. However, simulation software are not able to provide such curve as there is not usual phase change. That is it.

[serra]

I agree with srfish,

given T, P  you should be able to calculate density, enthalpy and transport properties in dense phase,

you can use a std. or extended EOS or a multi parameter model

[Pilesar]

  In dense phase fluids, the simulation software will still assign 'vapor' or 'liquid' for calculations. Keep in mind that this designated phase is somewhat arbitrary. At some point within the dense phase region, the designated phase and the calculation methods will change. This may result in a discontinuity in your calculated heat curve at the phase change point. You may wish to develop two different heat curves -- one for the 'liquid' and one for the 'vapor' portion of your stream within the dense phase region. Put the calculated data on the same graph and they should show a seamless transition for your properties. If you see small discontinuity, interpolate the transition region yourself. Since there is not a real phase change, either the liquid heat curve or the vapor heat curve can usually be extrapolated across the artificial transition point without too much loss of accuracy. It is just a matter of smoothing the data from both heat curves into one seamless whole.

  As to your second question of a separate exchanger for boiling and superheating, the great difference in heat transfer coefficient and fluid velocity make it difficult to provide optimized equipment for both services. The single-exchanger design can get quite complicated and tricky to predict the resulting performance. A lot depends on the amount of superheating, the control scheme, the heating mediums available, etc. For real process heat transfer services, robustness matters. Can the process be controlled for all different expected conditions? This may be difficult in an all-in-one compromise exchanger. You can develop a design both ways and choose the best solution. If you don't have much design experience, sometimes the design problems in a compromise exchanger are not that obvious. So have an experienced designer review your work for best results. The two-exchanger solution is much simpler for the designer.

[Pilesar]

If you are an HTRI member, you may be interested in the recorded webinar presentation posted just last week:

Is My Design OK? Modeling LNG Vaporizers Near Critical Point

You will need to log in to your HTRI account to view the webinar.