13 replies to this topic

[Fatema1987]

Hi All,

 

 

We were trying to implement APC at Naphtha Hydrotreating Unit (NHT) at Stabilizer column. However, during Step-Test for MVs we have seen inconsistent behavior at column profile. Every Step test has a different gain which makes it hard to finalize the model. We tried to run many APC post models resulted of those step tests in a trial to control Stabilizer column but we failed to achieve stabilization as a minimum requirement. 

 

 

Process Description: unstabilized hydrotreated Naphtha enters Stabilizer C-8131 it is heated in Stabilizer Feed/ Bottoms Exchanger E-8132 through E-8137 by heat exchange with the stabilized Naphtha from C-8131. The Stabilizer column bottoms are splitted into two streams. The bottom product stream is routed to Naphtha Splitter unti (NSU) through the feed/bottom exchangers under flow control. This flow control is cascaded to C-8131 bottom level control. The other stream goes for reboiling. This reboiling stream is further splitted into two. One goes to the stabilizer fired reboiler heater F-8131(approximately 62%) and the other to the Splitter reboiler heater F-8240(approximately 38%) convection section under heater pass flow control. The outlet from both heaters are combined and fed back to the stabilizer column. 81-PC-2100A is Furnace fuel gas pressure controller for F-8131 where this controller will regulate the fuel gas valve 81-PCV-2100A based on final temp required by TRC 81TC2075.

 

Overhead vapours from C-8131 are condensed in E-8138, further cooled in E-8139 and the condensate is collected in V-8133.A part of the overhead liquid is pumped back to C-8131 as reflux and the product is pumped to the Gas Tail Unit (GTU). 

 

The NHTU Stabilizer is having a very large reflux to distillate ratio (more than 10 times). The reflux drum (V-8133) level controller (81LC2023) is manipulating the distillate flow (81FC2027), and the top tray temperature controller (81TC2035) is manipulating the reflux flow (81FC2025). A small percentage adjustment of reflux flow by the temperature controller will result in a large percentage change in the distillate flow, since the level control will need to adjust the distillate to compensate the change in reflux to control the level. This kind of control scheme is normally called the (direct) energy balance control scheme.

 

 

Examples of the issues; 

- If I keep RefluX controller 81FC2025 in Auto and Reboiler Fuel gas controller 81PC2100A in Auto control without changing any set point, I will expect the column profile won't change majorly. Unfortunately, after few minutes the tray temp 81TC2035 will start ramping up without a clear reason!

- During 22nd Sept step test, STAB reboiler FG controller was taken on Auto “81PC2100A” and SV was increased by 0.05 barg & fixed. After this step, pressure increased in column “81PC2052” as well as BTM temp increased “81TC2075” and reached a temporary steady state. After few minutes of steady state, again BTM temp 81TC2035 increased drastically and pressure too without any change in reboiler FG opening valve!

- On 18th Sept, we tried to test reflux controller on MANUAL where opening was reduced by 1%. The tray temp 81TC2035 increased from 106 C to reach about 118 C until Panel Operator (PO) intervened to stop this increasing.

- Keep in mind, if we increase reboiler temp, the OVHD pressure will drop in this column while reflux flow will increase.

 

We have plenty of unclear behavior for this column where it supposes to be simple and direct model ! Please advise.

 

 

 

 

Attached Files

•  C-8131.pdf   357.72KB   20 downloads
•  Stabilizer PFD.JPG   73.48KB   6 downloads

Edited by Fatema1987, 01 November 2022 - 12:10 AM.

[breizh]

Hi,

Difficult to comments from Internet! Do you have Trends to share with us for better understanding of the problems? 

What are your expectations in term of performance of the unit?

From the drawing of the column this is a quite old unit, did you perform regular maintenance/inspection?

Any possible issues with the interns of the column? 

Is it a new problem?

Etc, 

My 2 cents

Breizh 

[Bobby Strain]

This doesn't look like APC. I'm not surprised the control is unstable. You need to consult with one who understands fractionator controls.

 

Bobby

[SilverShaded]

Is it a problem with C5's?  Odd stabiliser behaviour is sometimes associated with them bubbling up the column.  Light Naphthas have large gaps in boiling points between species.

 

A simulation of this column might answer some questions.

[Fatema1987]

Hi,

Difficult to comments from Internet! Do you have Trends to share with us for better understanding of the problems? 

What are your expectations in term of performance of the unit?

From the drawing of the column this is a quite old unit, did you perform regular maintenance/inspection?

Any possible issues with the interns of the column? 

Is it a new problem?

Etc, 

My 2 cents

Breizh 

Hi Breizh,

 

Yes it is an old column. Last TA was in 2017 yr where the column was inspected and found in a good shape in terms of internal parts.

The column is controlled well with this basic layer of controllers and stabilized. We thought earlier having such zero-control freedom at NHT stabilizer, will keep the column unstabilized and in cycling. You can clearly see that OVHD and BTM are controlled strictly. In fact, the column is very well control with this strict layer of control.

 

I dont know if we can call it as an issue but to implement APC we have seen taking any slave-controllers in Auto or manual and conduct a step test will always show different Gains and ramping behavior for tray temp. It is really challenging. Sometimes, Keeping the controllers at Auto instead of CAS without changing set-value, will disturb the column profile in few minutes!

 

We have a different NHT unit in our second refinery where this behavior is not observed there.

 

I have attached the Gain matrix resulted of first step test. If I am dropping tray temp Set-point of TC2035, then the pressure at OVHD PC2052 will increase. In many times, a second order behavior was seen at PC2052 but again it is not consistant at every step test outcomes. In addition, the opening of PC2052 is saturated at 100% opening most of time.

Attached Files

•  NHT Matrix.JPG   55.82KB   4 downloads

Edited by Fatema1987, 31 October 2022 - 11:30 PM.

[Fatema1987]

This doesn't look like APC. I'm not surprised the control is unstable. You need to consult with one who understands fractionator controls.

 

Bobby

 

For a trial purpose, we changed this basic layer of control from energy control into Mass control. The reflux was cascaded with the OVHD level controller. The distiallate controller was kept on Auto. Controllers were tuned. This scheme failed to stabilize the column. Again, we tried to cascade the distillate with the tray temp while keeping Reflux with level controller. We failed again. Last trial, we cascaded reflux controller with Feed/Reflux Ratio controller and keep Distillate controller cascaded with the OVHD Level controller. Again it is failed !

[Fatema1987]

Is it a problem with C5's?  Odd stabiliser behaviour is sometimes associated with them bubbling up the column.  Light Naphthas have large gaps in boiling points between species.

 

A simulation of this column might answer some questions.

Hi Silver,

 

Yes it is an odd stabilizer behavior! I am interested to know ur experience with such behavior.

Attached Files

•  LPG & OVHD Off Gas.png   310.45KB   5 downloads
•  BTM Composition.png   385.44KB   1 downloads
•  Feed Composition.png   381.79KB   2 downloads

[breizh]

Hi Fatema,

You may be interested with the document attached (Distillation basic controls) 

I understand you have a second unit operating under the same control philosophy, without facing problem.

Again, make sure your hardware is in good condition, this is based on experience with distillation columns.

Next TA double check!

Breizh

[SilverShaded]

 

Is it a problem with C5's?  Odd stabiliser behaviour is sometimes associated with them bubbling up the column.  Light Naphthas have large gaps in boiling points between species.

 

A simulation of this column might answer some questions.

Hi Silver,

 

Yes it is an odd stabilizer behavior! I am interested to know ur experience with such behavior.

 

The model appears to be a Naphtha Splitter rather than a stabiliser?  Splitters are generally better behaved.  Also you appear to have modelled it with offgas whereas I might have thought it should be sub-cooled?

Edited by SilverShaded, 01 November 2022 - 09:20 AM.

[Bobby Strain]

The most likely reason that your control doesn't work is because you are controlling reboiler outlet temperature. To make it work, you need to move the control to a sensitive place within the column itself. A simulation will show the best location.

 

Bobby

[Fatema1987]

 

 

Is it a problem with C5's?  Odd stabiliser behaviour is sometimes associated with them bubbling up the column.  Light Naphthas have large gaps in boiling points between species.

 

A simulation of this column might answer some questions.

Hi Silver,

 

Yes it is an odd stabilizer behavior! I am interested to know ur experience with such behavior.

 

The model appears to be a Naphtha Splitter rather than a stabiliser?  Splitters are generally better behaved.  Also you appear to have modelled it with offgas whereas I might have thought it should be sub-cooled?

 

Yes we have three outlet streams from Stabilizer: Off gas production, LPG production and Bottom mix Naphtha. The bottom mix Naphtha will be sent to a Splitter where it will be separated into Light Naphtha to PENEX unit and Heavy Naphtha to Platofrmer (CCR).

[Fatema1987]

The most likely reason that your control doesn't work is because you are controlling reboiler outlet temperature. To make it work, you need to move the control to a sensitive place within the column itself. A simulation will show the best location.

 

Bobby

 

When we changed the control into mass control where Reflux controller is cascaded to OVHD Level Controller, we were able to maintain stable LPG product and assuming more recovering for C3 & C4 from Off-Gas. However, the RVP of Stabilizer Bottom Naphtha is no longer controlled and this stream will affect on PENEX feed in next unit. Also, with this control we tried to break cascade control at Reboiler to have a better stability. Unfortunately, the Naphtha feed from CDU was varying too much and there is no intermediate tank before feeding naphtha into NHT unit. Thus, whatever is coming from CDU, NHT has to process it. So reboiler should act based on this feed disturbance.

Edited by Fatema1987, 02 November 2022 - 09:48 PM.

[Fatema1987]

Hi Fatema,

You may be interested with the document attached (Distillation basic controls) 

I understand you have a second unit operating under the same control philosophy, without facing problem.

Again, make sure your hardware is in good condition, this is based on experience with distillation columns.

Next TA double check!

Breizh

 

Thanks Breizh. Based on given document, our configuration is FIG. 8.19s where If overhead composition is to be controlled, the reflux flow to the column is throttled by a temperature controller. However, Bottom RVP should be maintained as well. Currently, Bottom Flow to reboiler is constant and not throttled but Reboiler FOT 81TC2075 is controlled through throttling FG valve.

[SilverShaded]

 

 

 

Is it a problem with C5's?  Odd stabiliser behaviour is sometimes associated with them bubbling up the column.  Light Naphthas have large gaps in boiling points between species.

 

A simulation of this column might answer some questions.

Hi Silver,

 

Yes it is an odd stabilizer behavior! I am interested to know ur experience with such behavior.

 

The model appears to be a Naphtha Splitter rather than a stabiliser?  Splitters are generally better behaved.  Also you appear to have modelled it with offgas whereas I might have thought it should be sub-cooled?

 

Yes we have three outlet streams from Stabilizer: Off gas production, LPG production and Bottom mix Naphtha. The bottom mix Naphtha will be sent to a Splitter where it will be separated into Light Naphtha to PENEX unit and Heavy Naphtha to Platofrmer (CCR).

 

Ok but you havent modelled the stabiliser, youve just used a component splitter whch is not a real model.

BTW I Totally agree with Bobby's comment above, stabilisers (like many columns) are usually controlled by the tray temperature well above the reboiler.  In the current scenario i'm going to guess that if the naphtha end point changes the reboiler outlet temperature also changes accordingly. (as will bottom temperature).  Therefore reboil duty may be floating dependant on naphtha end point, not controlling RVP of the naphtha.

Edited by SilverShaded, 03 November 2022 - 03:10 AM.