3 replies to this topic

[jaivanthrao]

Dear All,

 

Greetings!

 

Background:

I am performing the simulation in HYSYS for Crude CDU and VDU unit.

 

To enhance the crude processing capacity of these units, we want to incorporate the Pre flash column (PFC). The purpose of this column is to reduce the load of CDU overhead section.

This will be done by flashing the crude of 173 oC at nearly atm pressure and the flash vapour will be stripped off to produce the un-stabilized Naphtha. This Naphtha will be further treated in stabilizer along with Un-stabilized Naphtha produce in CDU.

 

Due to introduction of PFC in Crude PHT, the feed to existing CDU heater is reduced by 5 wt% and Un-stabilized Naphtha production from CDU is 62 % instead of original 100%. This means that the 5% crude vapour of PFC is equivalent to 38% of un-stabilized Naphtha. The yield of un-stabilized Naphtha w.r.t crude inlet flow rate is approx. 13%.

 

 

Problem Description:

 

The main objective of this task is to avoid the modifications of existing units such as CDU heater, CDU column, CDU overhead system, VDU heater, VDU column, VDU vacuum system etc.

 

While performing the simulation our aim is keep the performance of the CDU as it is for the middle and heavy distillate so that the feed characteristics for the present VDU remain unchanged.

 

Below are some of our findings for your reviews and suggestions.

 

 

·         The crude flow to CDU heater is reduced due to introduction of  PFC. However the heater load is not considerably reduced. This is due to higher requirement of heater outlet temperature.

 

·         The heater outlet temp is 375 C instead of 360 C of present case even though there is no heavy distillate.

 

·         To reduce the flash vapour requirement and hence the heater outlet temperature, the heavy distillate draws are not considered. As a result the CDU yield is reduce by 4 mass% compare to present case.

 

 

Please provide your guidance over the possible impacts of PFC on overall crude distillation including CDU and VDU.

 

Kind Regards,

Jaivanth Rao Jadhav

[PingPong]

As you already found out: removing the lightest part of the crude in a PFC makes it more difficult to vaporize the rest of the distillates in the crude heater. That was to be expected.

 

If I understand you correctly: even though you increase the heater outlet temperature from 360 to 375 ^oC you still produce 4 % less distillates when installing a PFC. If you do not want to increase the heater outlet temperature further (to avoid oil cracking), then you should consider reducing the crude column pressure, and thereby also the heater outlet pressure. That requires detailed check of the crude column hydraulics and condenser capacity at lower pressure.

You may also need to reduce the naphtha yield from the PFC, for example by increasing PFC pressure.

 

Installing a PFC in an existing crude unit has an impact on the whole heat balance of the unit. It takes heat from the preheat train to vaporize the PFC naphtha + reflux. Heater inlet temperature is likely to drop, while heater outlet temperature needs to increase, hence very little (if any) reduction in heater duty. Drawoff temperatures of products and pumparounds in the crude column are also changing, thereby impacting the crude preheat train performance.

 

What kind of crude are you processing?

What is the yield (v% or wt%) of each product in the existing CDU (without PFC)?

Edited by PingPong, 29 October 2015 - 10:44 AM.

[jaivanthrao]

Crude is Arab mix and kuwait oil

[PingPong]

What is the yield (v% or wt%) of each product in the existing CDU (without PFC)?

 

 

To enhance the crude processing capacity of these units, we want to incorporate the Pre flash column (PFC).

The purpose of this column is to reduce the load of CDU overhead section.

If the only objective is to unload the CDU overhead system, then I suggest you focus on debottlenecking that system.

 

I don't think adding a PFC in an existing CDU operating on fairly heavy crude mix is cost effective.

 

One could design a new CDU for Arab mix with a PFC but then one has to design the crude column for only a small overpressure and a fairly low atm residue cutpoint, design the transfer line such as to minimise heater outlet pressure, and design the VDU vacuum column such that a vacuum diesel is produced as the top product.

 

In any case: adding a PFC to an existing CDU has so many implications that you should involve a qualified company to do a detailed feasibility study. This is not something for amateurs. Having access to Hysys is not enough, experience and insight is what really matters.

Edited by PingPong, 30 October 2015 - 12:07 PM.