CHAPTER-11 ADVANCED PROCESS CONTROL Chapter No 11 Page No 1
INTRODUCTION The traditional control philosophy, what is called instrumentation in the chemical industries, is based on single loop control (sometimes called SISO- single input, single output). Each process operation has a number of independent or single loops for feedback control of temperatures, pressures, flows, liquid levels, and sometimes compositions. The term single loop means there is one measurement, one controller and one final control element, usually a valve. A process plant has thousands of such control loops. The controllers usually have little or no logic circuitry to tie the many loops together. As a consequence the operators must perform some of the operations with the control valves switched to manual, and has to implement process logic by switching in and out of automatic mode. A multivariable control or APC, is one that has the built in intelligence to look simultaneously at two or more process variables and to choose, in a given situation, the best of several programmed strategies (algorithms) for manipulating one or more control valves (or other final control elements). APC can be defined as Use of logic, predictive algorithms, thermodynamics, calculations, real-time control models and other control techniques to achieve economically related plant operating targets. APC technology is based on bottom up control approach, which means even if the top layer fails, the next lower level control continues to operate. Chapter No 11 Page No 2
LEVEL 0: Control of basic parameters viz; pressure, temperature, level & Flow using PID type regulatory control. LEVEL 1: Dynamic control such as feed forward, adaptive & calculated Control. (Pump around duty and cut point controllers etc.) LEVEL 2: Optimization of set points - constraint control, multi variable Controls (APC). LEVEL 3: Planning and scheduling model-using techniques of OR & RTO. FUNCTIONS OF MULTI-VARIABLE PREDICTIVE CONTROLLERS (MVPC) Multiple Mvs are manipulated to keep multiple PVs at their Set Point Uses linear dynamic process models and past input data to predict future behavior of both controlled variables and process constraints. The dynamic models may be expressed as parametric (transfer functions) models or step response (time domain) models The predictions of these process values are updated from actual measurements at each control execution to account for unmeasured disturbances and modeling error The controller calculates an optimum set of future control moves to minimize errors between predicted and desired process behavior using a least squares optimization solution The controller solution protects both present and future constraints for both controlled variables and manipulated variables It takes care of dynamic interactions, long dead times. Inverse response and disturbance variables feedback control. Chapter No 11 Page No 3
Practical Example of Advanced Process Control: When crude switch takes place in the crude unit from one type to another, the heater and the column operations get upset depending upon the severity of the switch. During the process of changeover, it is common to encounter loss of level in some sections of the column while offloading occurs in other. The transient period throws the products off spec, with consequent loss of yield in more valuable products and the time taken to stabilize the operation is significantly high. The Multi-variable controller (MVPC) enables handling of crude switches smoothly, reduction of transient time and at the same time keeping the product qualities within the specifications (constraints). Instead of having operators manually adjust control units for specific variables, APC provides generalized models that automate regulatory and constraint control. These models are dynamic models of the process that can predict how the process will respond over time to changes in basic operating conditions. They allow operators to prepare in advance for possible violations of operating limits, to take advantage of constraint relaxation to maintain process conditions as close as possible to their optimum. Hardware permits us to automate the process control with speed, precision, and reliability that are completely beyond the capabilities of human operators. Chapter No 11 Page No 4
ADVANTAGES OF APC The advanced control techniques can better coordinate the interactions that frequently occur between material and energy flows in single-loop control systems, where changing one variable requires adjusting others, to compensate for side effects. By reducing process variability, advanced control allows plants to run closer to their operating constraints. This, in turn, cuts energy use, as well as raw material and processing costs. It also improves product yield and quality, safety and productivity while lowering pollution. The product quality is the feedback, which the process operators use to correct the operating conditions. Looking at the product quality, the relevant process parameters can be manipulated so as to control the product quality in question. Advanced control implementation reduces stabilization time during feed change, which in turn results in minimum product quality give away (slop generation). The feed forward action of the advance control algorithms helps to operate the plant steadily by adjusting the operating parameters before the disturbance actually reaches downstream. Advanced control algorithms allow applying dead time compensation techniques to compensate for long delays in process response, permitting tighter control. Chapter No 11 Page No 5
ADVANCED CONTROLS IN PRACTICE APC is a powerful tool with a short pay back period in optimizing plant operations resulting in improved margins. Refineries world wide have implemented APC in almost all the major process units viz Crude distillation unit, Hydrocracker unit, Fluid catalytic cracking unit, Catalytic reforming unit, Vis breaking unit, Delayed coker unit Normally APC is implemented in step by step approach, first in the primary distillation unit and then in the secondary units. APC in the crude distillation unit ensures a consistent feed quality to the secondary units with respect to the cut points. For example, Hydrogen unit feed FBP, CRU feed IBP, Hydrocracker feed FBP etc. In refineries where allied infrastructure is adequate, APC can be simultaneously implemented in the primary and secondary units. UNIT AVU Control Objectives: There are eight objective of the control system implemented in AVU, which are necessary to design the control architecture. To maximize unit throughputs subject to unit limitations To maintain product qualities within specifications To maximize LPG subject to weathering Maximize Diesel subject to recovery /pour point in winter Maximize feed stock generation in secondary units Minimize utility requirements like steam, cooling water Minimize energy consumption Provide smooth and quick crude switch capabilities Chapter No 11 Page No 6
To achieve these objectives, the following seven RMPCT controllers are provided: Pre-heat train controller CDU Heater Controller CDU column controller VDU column and Heater controller MTO splitter controller Naphtha stabilizer Controller Naphtha splitter controller The following list shows the number of Controlled Variables (CV), Manipulated Variables (MV) and Disturbance Variables (DV) for individual controllers: S. No Controller CVs MVs DVs 1 Pre-Heat train RMPCT controller 4 5 3 2 CDU Heater RMPCT Controller 13 10 3 3 CDU column RMPCT controller 17 12 7 4 VDU column and Heater RMPCT controller 22 15 4 5 MTO splitter RMPCT controller 5 4 6 6 Naphtha Stabilizer RMPCT controller 4 2 4 7 Naphtha splitter RMPCT controller 5 4 2 Chapter No 11 Page No 7
Using the inferential Property Prediction Package (IPPP) software/ Fractinator Tool Kit provide the following are the inferred properties being predicted at an interval of 5 minutes. i. LPG weathering ii. C5-90, cut (light naphtha) 95% and FBP iii. 90-120. C cut 5% point and IBP iv. HN flash v. ATF/SKO flash vi. ATF/SKO 95% and FBP vii. MTO flash viii. MTO FBP ix. HGO pour point x. HGO recovery @ 370. C or FBP xi. Vacuum diesel 95% xii. HVGO 95%. a) Preheat train Here main control variables are the desalter pressure and crude flow in the individual streams of Preheat Train I and III. Desalter pressure is controlled by manipulating 03 PC 1201 opening and crude flow in individual streams of a particular preheat train is determined by the temperature difference between the two streams. Chapter No 11 Page No 8
b) CDU furnace In the CDU furnace main control variables are furnace COT, total furnace flow and temperature difference between two consecutive passes. To maintain these parameters in their desired values or ranges individual pass flows and fuel oil pressure is manipulated. Under normal circumstances, furnace COT should be a manipulated variable for the CDU column but here that is not the case. This is because keeping COT as an MV will keep increasing FO pressure without any limit. FO pressure above 4.2 creates problem in the furnace and hence it needs to be maintained below this point. Consequently, COT is a CV and not MV. c) CDU column The chief parameters to be controlled in CDU column are top temperature, individual cuts draw temperatures, Kero FBP, Kero Flash, Heavy Naphtha Flash and HGO recovery. Individual cuts draw temperatures, their FBP can be controlled by manipulating the corresponding rundowns or corresponding CRs or CRs of cuts below or above. Flash point of a cut can be controlled by manipulating reboiling in the corresponding stripper or draw of the cut above it. HGO recovery can be controlled by its draw and draws of cuts above it, by manipulating the stripping steam or by manipulating the CRs (esp. HGO CR). d) Vacuum section Here main control variables are VDU column top temperature (which is normally in auto remote with VD CR to maintain the temperature at the desired value), VR penetration (controlled by manipulating chiefly HVGO IR, VDU furnace COT, column stripping Chapter No 11 Page No 9
stream) and temperature difference between consecutive passes of VDU furnace (controlled by manipulating the individual pass flows of the furnace). e) Naphtha stabilizer Here main parameter to be controlled is LPG weathering. This can be done by manipulating the stabilizer top temperature and reboiling in the column. f) Naphtha splitter Important parameters to be controlled are NSU furnace COT (done by manipulating FG pressure in the furnace), C5 90 cut naphtha FBP and 90 120 cut naphtha IBP (done by manipulating splitter top temperature, top pressure and also maintaining appropriate NSU furnace COT). g) MTO splitter Here main control variables are splitter top temperature (maintained by manipulating top reflux), MTO flash (chiefly controlled by top reflux) and MTO FBP (maintained by manipulating MTO IR and reboiling in the column done by HVGO CR). Chapter No 11 Page No 10
OPERATING PROCEDURE AND EMERGENCY HANDLING WHILE APC IS IN LINE 1. For implementing APC in the unit, a numbers of controllers have been designed. An overview page for APC has been made with the name Honeywell RMPCT overview. The page contains controllers name and their status. A particular controller can be switched ON or OFF from the overview page itself. 2. Provision has been made for switching on and off the individual controller from the overview page or from the individual controller graphic page. The controller can be switched ON/OFF by clicking in the request column for that controller. For switching ON enter 1 and for switching OFF enter 0. 3. For each controller, MV (Manipulated Variable) and CV (Controlled Variable) page has been prepared and has been linked through the overview page. The controller MV and CV pages are also linked through their individual process graphic page. 4. The controlled variables are the parameters, which the APC is trying to control in the ranges provided by the Panel Engineer. The set point range can be given in the column set point HI and set point LO. 5. The manipulated variables are the variables, which the APC is trying to move (in the range provided by the Panel Engineer) to keep the CV in the range specified by the Panel Engineer. The MV range can be given in the column set point HI and set point LO. In no case, the APC is going to write the set point to any of the MVs outside the range specified by the Panel Engineer. 6. The Disturbance variables (DV) are the variables, which are beyond the control perspective like ambient temperature, feed to the column etc. In that case, the controller will take feed forward action to nullify the effect of DVs. Chapter No 11 Page No 11
7. Any particular CV/MV can be taken in line or dropped by putting ON/OFF status of that CV/MV to 1 or 0 respectively. 8. The controller can also be switched on/off from individual Controller s MV or CV page by clicking on the top right corner of the page. For switching ON enter 1 and for switching OFF enter 0. 9. For all the manipulated variables (MVs), COMP mode has been enabled. When the APC controller is in line, the respective MVs will run in COMP mode and will be receiving set point from the APC computer. When the panel engineer feels that the particular MV is not behaving the way he wants, he can click on the particular MV and switch back to LOC to get the control back to DCS. The panel engineer can than control that MV the way he wants and no set point will be passed from the APC computer. 10. A computer (APC Client) has been provided on the panel itself, which will aid the panel engineer in viewing the trends of all the DCS related tags (SP, MV, OP). The computer screen can also be used to view the controllers online. The controllers can be individually switched off from the computer. 11. Emergency Handling: On each controller graphic page, APC ESD (top left corner) symbol is shown which can be used to switch off all the APC controllers at one time. Just click it and enter 0 to turn off all the controllers during emergency. Chapter No 11 Page No 12