3 replies to this topic

[Padmakar Katre]

Dear All Forum Members,
I am seeking your opinion aboout the coil steam flow to vacuum unit (VDU) fired heaters. I am currently involved in revamp of a CDU/VDU where I need your comments on the steam flow to coils(turbulizing steam). Somewhere it is referred that the coil steam in heater should 0.5 wt% of the HC feed. Whereas somewhere I had read about the distribution of steam in VDU could be like 50% of steam in heater inlet as coil steam and the balance as stripping steam to column. I just need to know is there any thumb's rule or guideline for to calculate the steam to heater coils. In case of this heater it is recommended to change the existing coils of larger size to lower size to get rid-of an increased heat-flux in heater due to lower velocity. So the cost involved is almost 40-50 % of the heater total cost. I am putting around 1000 Kg/hr of coil steam in heater where as 3000 kg/hr as column stripping steam. If I will make it reverse and a little more addition to coil steam to keep the velocity in heater as per requirement I could save a lot. Every one here is welcome to give is comment and suggestion. If further details are needed please let me know I will definately provide for your analysis. Thanks in advance.

[Zauberberg]

There are no universal answers on your questions. The key thing is to:

- Avoid peak oil film temperatures exceeding cracking temperatures, which are different for various vacuum feeds
- Minimize oil residence time for achieving target COT
- Provide adequate pass balance control

Since your system employs stripping steam, there must be a precondenser in VDU tower overheads. Keep its condensing capacity in mind when calculating the maximum amount of steam that can be injected into the heater coils.

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[Padmakar Katre]

There are no universal answers on your questions. The key thing is to:

- Avoid peak oil film temperatures exceeding cracking temperatures, which are different for various vacuum feeds

Dear Zauberberg,
Thanks for your prompt reply. My peak oil film temperature is within the limit as COT we have fixed and lower the vacuum column pressure (deep vacuum) to the extent the existing vacuum column dia is adequate.

- Minimize oil residence time for achieving target COT
- Provide adequate pass balance control

The oil velocity through the coils is my main issue as the net feed to the heater is reduced so to compensate the velocity I need to put an extra steam. Hope this gets clear.

Since your system employs stripping steam, there must be a precondenser in VDU tower overheads. Keep its condensing capacity in mind when calculating the maximum amount of steam that can be injected into the heater coils.

I am increasing coils steam and reducing the stripping steam so the net steam consumption is almost same. I just need to know the advantages and dis-advantages of putiing much higher coils steam/turbulizing steam in coils.
Waiting for your comment.

[Zauberberg]

I wouldn't agree with the observation that more velocity steam will compensate for lower stripping steam rate - at least not in the stripping section of the tower, where major effect of stripping steam is expected. Those hydrocarbons will not see the presence of steam, and their partial pressure will not go down to the extent as in the case of full stripping steam rate. I would try to simulate these conditions in a common distillation system (e.g. components up to C16) where this effect may be observed more accurately than in cases of vacuum residue.

A trade-off when having higher velocities inside the heater coils is to have higher degree of vapor superheating i.e. having the vacuum tower feed more far away from vaporization equilibrium conditions. The higher the velocity, the higher is the deviation from equilibrium conditions inside the heater coils and inside the heater transfer line.

As a general conclusion, if you are absolutely sure that peak oil film temperature is below the cracking limit, there's no need to increase the velocity steam rate - why would you do so? It will only put the additional load on overhead condenser and affect the system pressure.

Best regards,