How to fix poor VLE data when sizing S&T exchanger?

[knapee]

Dear Sir/Madam

I have a project given poor S&T thermosiphon & VLE data and to be required to find optimum size exchanger through HTRI.

The TEMA type : NEM ( vertical), shell per unit : 1, in series : 1
Tube dimension: OD 19.05 mm, length:3048 mm, pitch: 23.81, flow angle:30 degree

The Shell side
Fluid name : steam
Inlet Temp : 270.5C
Inlet Pressure: 40kg/cm2g
Fouling Factor: 0.0001 m2-hr-C/kcal

The tube side
Fluid Name : phenol
Fluid quantity : 336610kg/hr
Lquid in : 336610 kg/hr (density:889.8kg/m3)
Liquid out : 249176kg/hr (density:890.9kg/m3, viscosity : 0.217CP, MW=94.12, specific heat=0.633kcal/kg-C,thermal conductivity=0.1081kcal/hr-M-C)
Vapor out : 87435kg/hr (density:5.49kg/m3, 0.012CP, MW=94.12, specific heat=0.406kcal/kg-C, thermal conductivity=0.0223kcal/hr-M-C)
Inlet Temp: 214C
Inlet Pressure :1.29kg/cm2g
Pressure drop allowable:0.265kg/cm2
Fouling Facotr :0.0002 m2-hr-C/kcal

Heat duty : 9.615 MMKcal/hr

Here is my question :
1.The VLE data(pls see attached file) seems not cover the tube side out pressure so the HTRI has to
Extrapolate

The running message is following:

The pressure profiles given for the cold fluid do not cover the operating pressure range of the heat exchanger. The vaporization profile inside of the exchanger for this run may not be accurate since heat release and fluid properties have been extrapolated. It is recommended that the maximum and minimum system pressures be used as reference pressures.

2.The steam velocity =0.99 m/s in shell seems to slow, beside that I can’t see any problem of this thermosiphon case.

Is any special input requirement for thermsiphon design in HTRI, such as recirculation ratio or pressure drop ?

I would look forward your any comments and really appreciated for your time and effort.Thank you so much.

 

[sshep]

Hey Knapee,

It seems reasonable at a quick look through. 26% vaporization is on the high side for a vertical, but not unreasonable.

I was curious about the heat of vaporization, and also the piping geometry and column liquid level data- surely you entered something for those parameters.

best wishes,
sshep

 

[knapee]

I notice I have to input the reboiler type, either thermosiphon reboiler, or force reboiler.( In this case, it is thermosiphon type). Also, the required liquid static head, and inlet pressure location (either at inlet nozzle or at column bottom) have to be inputted. But, I don't quite know how to clarify the physical meaning between the pressure location at inlet nozzle and column bottom.

Besides that, the main inlet/outlet pipe length has to be inputted. (Can anyone suggest typical value for this?)

However, if the total driving force of the reboiler less than the total resistance, it will seriously impact the operation of the reboiler and distillation column.

And, the low recirculation will make the hold time of reboiler increasing so that the the unwanted heavies will be overheated to increase the vaporization rate.

Another question is someone suggests me to increase the inlet/outlet spacing for avoiding the acoustic vibration, and indeed it work well. But, can anyone please suggest a reference for me how to choose a proper spacing physical intuitively.

Thank you very much for your all kindly reply.

 

[sshep]

Hey Knapee,

The outlet pipe is usually pretty short (say 2m)unless you have a cone head with some vertical run which is best avoided in my experience. The inlet pipe may be much longer depending on what is going on under the skirt. In many cases the inlet pipe will provide much of the resistance, and if made oversized to reduce pressure drop, a butterfly valve is installed just to give some additional resitsance if needed, but usually not used.

Per Liebermann the main risk of excess circulation is foaming. On the otherhand the risk of under circulation is high vapor fraction and instability- which can only be mitigated after the fact by running at high liquid level. I favor higher circulation (lower vapor fraction) since it is easier to mitigate. The ciculation rate (assuming minimal pipe resistance) is mainly a function of the exchanger length vs the liquid level. The return to the column is fixed above the liquid level, and the hydrostatic head balance achieved by the length below that point (either by column length of piping below the column bottom). For a new installation you have total flexibility for how to do it, but for a retrofit it must be done by extending the bottom column nozzle and new skirt penetration if needed; or by putting a baffle inside the tower to hold a constant liquid level (level contol then moved to the baffle overflow side).

With respect to setting the pressure at the exchanger inlet, the normal input is the column bottom pressure (above the liquid) and then the liquid level vs exchanger bottom tubesheet elevation.

I don't really know guidence about accoustic vibration. The warnings seem most common at the inlet due to superheat and it being the point of highest vapor flow. Designers have often asked me (as a plant engineer) in renewals and retrofits if I hear any vibration, which is usually "no", and only occassionally "yes" so I think the software is conservative in most cases, but this is out of my area of expertise.

best wishes,
sshep

 

[rmw]

Do you really have 19.05 mm tubes on a 23.8 mm pitch? I don't know, but that seems tight to me for adequate steam penetration into the bundle, notwithstanding the ligament thickness.

rmw