13 replies to this topic

[Santosh Bhavirisetti]

Hello,

 

Can any one suggest to me how to size an optimum re-circulation rate in a non-agitated vessel that is heated by an external heat exchanger in a continuous process?  What is the impact of the fluid's viscosity in deciding the re-circulation rate and how does one ensure heat transfer dissipation inside the tank without fluid agitation?

 

I believe the re-circulation rate is very important in this case in order to avoid over heating the material and sizing the re-circulation pump.

 

Thanks,

Santosh

[PingPong]

This is too vague for the rest of the world. You should give more details, such as reactor inlet (feed) temperature, reactor outlet (product) temperature, inlet and outlet temperature of heating medium, type of reaction (endothermic or exothermic?), where is the feed introduced (in reactor or at exchanger inlet?), et cetera.

 

Purpose of circulation rate is to transfer heat from heat exchanger to reactor, and to agitate the fluid, as your reactor does not have an agitator.

 

Without any details of the reactor system it seems to me that a high circulation rate is best in this case, especially because the reactor has no agitator.

[srfish]

PingPong is right about we needing more details. You will need to know more about the tank(vessel). The size is important. There will need to be an evaluation of the best vessel jacket. There needs to be details about the heat exchanger.

 

There is an article that was published in July 1967 by Hydrocarbon Processing that you might find interesting. The title is "What is Optimum Batch Heat Transfer".

[Santosh Bhavirisetti]

Hi Pong,

 

Here is the more detailed explanation about problem:

 

We have a rawmaterial called surfactant is one of the major ingredient in detergent making. RM is stored outside building and open to atmosphere. The raw material need to be maintained b/w 70 to 120 F.  if temp is out of these limits material will be not useful to process.  So, it is directed from business unit 72 F is best to the process.

 

Due to some business reasons, we are not allowed to work on tank modification or agitator installation over the tank. There is no further discussion on providing agitator and any other system.

 

We have got only option that provide a plate type heat exchanger and maintain proper re-circulation to ensure uniform and required temp across the tank. But, material is bit viscous to achieve uniform temp with out agitator.

 

SO, the challenge is to find optimum re-circulation rate.

 

I have listed below material properties, operating conditions, ambient conditions, tank geometry.

 

Material off spec limit: 70 to 120 F

Operating temp: 72F to 105F

 

Material properties:

spec gravity @105 F: 1.0125

specific heat at 105 F: 0.433 btu/lb.F

thermal conductivity @105 F: 0.091 btu/hr.ft.F

Viscosity data:

@60F-310 cp

@70F-248cP

@90F- 158 CP

@105F-90Cp

 

Material inlet temp: 60F

Outlet temp needed: 72F

 

Tank total working vol:8000 USgallons

material unload from truck and truck vol: 5000 us gal

 

Ambient temp: 0F

wind speed: 11 mph

 

Tank dia: 15 ft

shell ht: 16 ft

Total heat transfer area: 1143 ft2

Tank is flat bottom and resting on concrete pad (there is no legs)

 

MOC of tank: tank has two layers: inner 3/16 inch thick ss316L, outer layer 1/4 inch CS

Tank got insulation: its thick 120 mils

thermal conductivity: 0.23 btu.in/hr.ft2 or 0.033 W/m.K

 

Tank has diptube and spiral ring at bottom. Ring has 6 distributing holes.

 

No reaction takes place inside tank.

 

 

I hope above information is enough to find re-circulation rate.

 

 

A new heat exchanger may select as per re-circulation rate and heat load. that will not be a big deal. Hot water is going to be heating media in heat exchanger.

 

Thanks,

Santosh 

[Santosh Bhavirisetti]

Srfish,

 

Can you Please share suggested article.

 

Thanks,

[Bobby Strain]

You can provide eductor mixers inside the tank. But you will need access to install them. Maybe a drawing or sketch will help. With a bit of ingenuity, you might design some system of nozzles that you can drop into the tank from a manway on the top. And you have not indicated what the heating medium is. And you might ask for suggestions from the surfactant supplier.

 

Bobby

[Santosh Bhavirisetti]

Bobby,  I agree with your answer. I mentioned above, we got some distribution nozzles. and I'm not sure whether those work like educator.  It improves better mixing. However, do we have any guide lines to select re-circulation pump rate?? based on viscosity factor & volume of tank.

 

I got some answers, as below

1) we should size re-circulation pump for 1 tank volume turn around time for 1 hr.

2) Re-circulation is not matter, only its matter of energy carry over through re-circulation which will be equal to heat loss from tank.

 

But, I am not satisfied with two answers which I have received. Please share your thoughts..

[Santosh Bhavirisetti]

All,

 

Any update further on my question. Please share at least some related works or literature.

 

Thanks,

[breizh]

http://www.flygt.com...ges/Mixing.aspx

 

Hi ,

Consider to install a submersible equipment . This can improve the homogeneity and "maintain" the temperature  . You should consider to contact their representative .

 

Hope this helps

 

Breizh

Edited by breizh, 11 July 2014 - 02:09 AM.

[Chris Haslego]

From what I'm seeing here, I think you may be focused on the wrong problem.  You mention that you have to use a plate heat exchanger to maintain the tank temperature / perform mild heat up during truck unloading.  

 

The first piece of information that you need (in my opinion) is this:

 

"How much flow can I push through my existing or a reasonably sized plate heat exchanger?"

 

For example, if you can pump 300-500 GPM through the heat exchanger with a reasonable pressure loss, then you'll turn your tank over quickly and mixing won't be a issue (plate heat exchangers are excellent mixers as well as heat exchangers).  So long as you can draw and return the flow from reasonably different points within the tank, this scenario would work out well.

 

On the other hand, if you have an existing plate heat exchanger that is to be repurposed and it is relatively small, then you may need to revisit the mixing problem.  Or, you may be able to add plates or change the type of plate in the heat exchanger.

 

 

[ankur2061]

Hello,

 

Can any one suggest to me how to size an optimum re-circulation rate in a non-agitated vessel that is heated by an external heat exchanger in a continuous process?  What is the impact of the fluid's viscosity in deciding the re-circulation rate and how does one ensure heat transfer dissipation inside the tank without fluid agitation?

 

I believe the re-circulation rate is very important in this case in order to avoid over heating the material and sizing the re-circulation pump.

 

Thanks,

Santosh

Santosh,

 

You cannot continuously recirculate a liquid and maintain a constant temperature. A certain amount of heat due to dynamic friction gets added to the continuously re-circulating liquid thereby gradually (very slowly) raising its temperature. This means that you cannot have a fixed or optimum re-circulation rate in order to maintain a constant temperature. Even an external heat exchanger transferrring heat to the re-circulating liquid with a constant re-circulation rate will not prevent temperature fluctuations (increase or decrease) of the bulk liquid.

 

If your aim is to accurately control the temperature of the bulk liquid in the non-agitated vessel, then you need to have provision for both heating and cooling the re-circulating liquid. Heating when the bulk fluid temperature falls below the desired temperature and cooling when the bulk fluid temperature rises above the desired value.

 

A scheme depicting both heating and cooling with a constant re-circualtion flow rate is attached. In my opinion, this scheme should be able to provide a fairly constant temperature of the vessel contents.

 

Hope this helps.

 

Regards,

Ankur

Attached Files

•  Ext_Recircn_Loop_f_Temp_maintenance.xlsx   14.87KB   341 downloads

[Padmakar Katre]

Hi,

Just an input from my end, viscosity change with temperature much higher and definitely affect pump capacity. Do you have a facility to heat up the truck material while unloading into tank to a certain temperature. I have seen facilities such as stub-in heat exchangers with U bundle inside tank for heating and maintain the fluid outlet temperature i.e. pump suction. Please clarify the requirement of temperature is at the processing end and not inside tank if later is also required addition of steam coil inside the tank is also an option.

Hope it helps.

[Santosh Bhavirisetti]

I agree with Chris's answer, but existing pump and piping need to be changed due to high flow rates(300-500 gpm). I have gone through the mixing problems but not found any reference with respect to viscosity. If any one have good reference to select re-circulation rate which gives a uniform mixing inside the tank (instead of 300-500gpm).  

 

Chris, thanks for your answer.

[Santosh Bhavirisetti]

Padmakar,

 

Truck does not have any heating system. but, material's vendor ensure to supply the material with required temperature i.e, b/w 70 to 80 deg F.

 

Please share the information you have.

 

For your information, not only delivery temperature, inside tank temperature is important to keep material good to process. If the material below 70 deg.F, that will be wasted and not good for the process.