Asro Pun’ Blog

Outline of Process Control

Posted by asro pada 15 Maret 2010

For my Sensei /Kyoshi : Teruhiko Sasaki San, Yasuki Tazawa San and  Kazuhiro Suzuki San.  And my friends on TR-21-09:  Mu Xiaohong San (China),  Javad Shahmohammadi  San (Iran), Abbas Hamzah Abbas San (Iraq),  Nurlan Kenzheakhmetov San (Kazakhstan), Bader Battal Al-Shammari San (Kuwait), Abdelhamed I. A. Abdelrhman San (Libya), Inyamah Desmon Ugochukwu San (Nigeria), Imtiyaz Alam San (Pakistan), Alaeldin Hassan Abuagla Abusin San (Sudan),  Nguyen Xuan Thinh San (Vietnam), Le Duy Ba San (Vietnam) and  Khaled Abdulla Mohammed Alebbi San (Yamen).

 

PROCESS CONTROL HIERARCHY

Implementation of a control system in process industries (process control) is categorized into several levels/hierarchy, as shown below.

The lowest level is a Basic Regulatory Control (BRC) directly connected to the process through field devices (transmitters & control valves).  The next level is an Advanced Regulatory Control (ARC), and  the highest level is a Modern Control.  [Note: There are literature categorize  process control into 5 levels hierarchy : A Basic Regulatory Control (BRC), an Enhanced Regulatory Control (ERC), a Model Predictive Control (MPC), a Process Optimization Control (POC) and a Macro LP.]

Both levels, the ARC and the Modern Control is known as an Advanced Process Control or abbreviated as APC.  Most of the BRC and ARC implemented in the DCS, while the Modern Control implemented in Server or PC.  [Note: There are DCS vendors state that the  MPC can be implemented in their DCS such as Foxboro I/A Series and Emerson DeltaV].

The BRC is a control system of the most commonly used, and known as:

  • Single input single output (SISO) system.
  • Feedback control.
  • Using conventional PID algorithm.
  • Suitable for linear system.

The BRC can not handle  process with the following characteristics:

  • Non linearity.
  • Large dead-time.
  • Inverse response.
  • Interaction.
  • Large disturbance.

The  advanced regulatory control (ARC) or sometimes is known as an Enhanced Regulatory Control (ERC) is control schemes  designed to overcome some disadvantage of the BRC.  The following are examples of the ARC schemes:

  • Dead-time compensation is used to improve  the single loop process with large dead-time.
  • Feed forward control  is used to minimize  a deviation  caused by load disturbance.
  • Non-interactive control is design to eliminate an interaction between loops.

However, besides these advantages, the ARC also has a limitations, as follows:

  • It can not handle complex non-linear processes.
  • It can not handle process constraint.
  • It can not provide integration comprehensive solution for all control related problems.

The  modern control comes to minimize the weakness of the BRC and ARC, with provide a comprehensive solution.  A model predictive control also known as MPC is one of the modern control most widely used. What is MPC? The following are features of MPC:

  • Model/Multivariable.  The MPC uses mathematical models of the process to calculate control action (MVs moving).  The MPC can handle more then one input and output (multi variable) simultantly.
  • Predictive.  Using process model and previous measurement data, the MPC is  able to predict future beahavior  and deviation of CVs.
  • Control.  The MPC minimize deviation of CVs from desired operating condition (SP), not only current deviation, but also future deviation by introducing sequense of MVs movements. The MPC also anticipates and prevents the process constraint by using optimization methode.

Example of MPC software:

  • DMCplus : Aspen Tech.
  • RMPCT : Honeywell.
  • SMOC-II : SGS, Yokogawa.
  • Connoisseur : Invensys.
  • MVAC : Axens.
  • STAR : Honeywell
  • Process Perfector : Pavilion

PROCESS CONTROL ON DEPROPANIZER UNIT.

To facilitate the understanding of the process control hierarchy, the following will be shown example of its application on a Depropanizer unit.  The Depropanizer unit chosen because it is quite simple, so easy to explain.

Depropanizer, Process Description & Basic Regulatory Control.

The Depropanizer separates a mixture of hydrocarbon into C3 (propane) on the top/overhead and heavier componens on the bottom.   The Depropanizer unit consists of  a Main Column (C11), Overhead Receiver (V12), Condenser (E12), Reboiler (E11) , Feed Tank (V11)  and associated equipment, as shown below.

A feed withdrawn from Feed Tank (V11), flows into the side of  Main Column (C11) under control FC11 cascaded from LC11.  In the C11 column,  the propane (C3) separated from the heavier components. A vapor portion ascends to the top of the Column, while the liquid descends to the bottom.  The vapor then flows  into the Overhead Receiver (V12) through two separate lines,  the first directly to the V12, while  others passed on  Condenser (E12). So, Hydrocarbon enter to the V12 are a mixture of liquid and vapor. In the V12, vapor is separated from liquid.  Vapor leave the V12 to Flare under control PC11.  Liquid from the V12 bottom is pumped by P11.  Some of the bottom liquid is returned to the C11 column as a reflux under control FC12 which cascaded from AC12, and others is sent to the another process unit as a overhead product under control FC13 which cascaded from LC12. AC12 is analyzer control that controls  the C4 content in the overhead product. The C4 content is overhead product specification.

Some of the vapor that ascends to the top of the Column produced by Reboiler E4, by recirculating some liquid from C11 bottom back to the column.  The E4 is steam reboiler, where steam flow is controlled by FC15 which cascaded from AC13. By using P12,  liquid from the C11 bottom is sent to another process unit as bottom product under control FC14 which cascaded from LC11.  The C3 content in the bottom product is controlled by AC13.

ARC on the Debutanizer.

According to the operator experience during they operated the unit, performance of the  control especially AC12 and AC13 were very slow in response to setpoint changes or disturbances. This is because both analyzer AT12 and AT13 have long dead time.  Therefore,  they issued the work order for control schemme evaluation to Engingeering Departement.  Fortunately, the engineer who perform evaluation just graduated from the TR-21-09 course. He remember what Suzuki San  explained during  dead-time compensation sesion. He try to implement the dead-time compensation scheme to minimize the existing (BRC) control problem, as follow.

The result, performance of AC12 and AC13  become better then before.

MPC on Depropanizer.

However, several weeks later any change in operating condition, the feed often fluctuates which causes control performance decrease.  At the same time, interaction between overhead loop and bottom loop became stronger.  New work order issued to Engineering Departement. Again,  TR-21-09 inagurate engineer was assined as evaluator.   Theory of MPC explained by Sasaki San still clear in his mind, also all thinks about MPC that were discussed with Tazawa San in front of the free vending machine during coffee break time still in memorized. So, he applied the MPC technology to overcome  the existing BRC & ARC control problem, as follow

The MPC used 3 CVs (controlled variables), 2 MVs (Manipulated variables) dan 1 DV (disturbaced variable).  The CVs are AC12.PV, AC13.PV and PDI13.PV, in this case PDI13 is constraint variable. The MVs are FC12.SP and  FC15.SP. The DV is FC11.PV.  Finally, the results are  excellent.

Satu Tanggapan to “Outline of Process Control”

  1. I really speculate the reasons why you titled this specific blog,
    “Outline of Process Control Asro Pun Blog”. In any case I personally enjoyed the article!

    Thanks,Ludie

Tinggalkan Balasan

Isikan data di bawah atau klik salah satu ikon untuk log in:

Logo WordPress.com

You are commenting using your WordPress.com account. Logout / Ubah )

Gambar Twitter

You are commenting using your Twitter account. Logout / Ubah )

Foto Facebook

You are commenting using your Facebook account. Logout / Ubah )

Foto Google+

You are commenting using your Google+ account. Logout / Ubah )

Connecting to %s

 
%d blogger menyukai ini: