Advanced Process Control
EXECUTIVE SUMMARY
Process control entails different types of process tools that are used to solve multivariable and/or discrete control problems. Therefore, advanced control is the practice of using different elements from a wide range of disciplines that often includes signal processing, artificial intelligence, control engineering, decision theory and statistics. Some of the tools found in advanced control of processes are; sensor controls, fault detection control and classification, model predictive tools and system feedback. Process advanced control systems have been successfully used in processes to curb challenges such as;
Processing feed-stocks of different qualities
High energy requirements
Strict safety margins to carter for differences in the feed quality
Product purity specifications that often need tough quality controls
This report expounds on the need of advanced control in today’s industries and the numerous benefits that an industry is bound to reap from by installing the latest control technology in the market. A lot of emphasis has been put on control systems in refinery industries, especially the distillation units because of the intensive energy that is utilized in the process to achieve high purity products. Three related articles have been covered in this report. The three articles are:
A. Goodyear (http://www.controlglobal.com/articles/2010/PrcessControls1011.html)
B. Practical Advance Process Control (http://www.controlglobal.com/articles/2006/031.html)
C. Distillation process control (http://www.controlglobal.com/articles/2006/209.html)
INTRODUCTION
The advanced process control (APC) technology has evolved over the recent years and it is now almost a mandatory requirement for any processing industry. APC combines different control tools that are geared toward greater process efficiency and proper management of operations within plants. Some of the industries that heavily rely on APC to achieve higher efficiency of process operations include;
Semi-conductor industry: such an industry is characterized by hundreds of multiple re-entrant steps that take place. Therefore, to effectively control the overall production process advanced control is used.
Petrochemical industries: it is used to control multivariable problems by utilizing dynamic relationship that exists between the multiple variables to easily tell their future behavior. Corrective actions are then executed based on the predictions made to keep the disturbance variable at the acceptable limit.
Mining industries.
Advanced control of processes has opened new opportunities that were not possible by use of PID controller only. The use of advanced control has enabled complex process situations to be solved by mathematically by use of process variables. The mathematical expressions generated are then employed in the automatic and flexible operation of the plant. Consequently, the application of APC in the industry has effected process management by significantly reducing the overall energy consumption rate and the amount of raw materials used in production process. Furthermore, it has enabled high standard of quality products and service. Advanced control also greatly contributes to flexibility in the production process.
Initially, the technology of APC was quite complicated and expensive to use but, with improved innovations in the control systems of processes the advanced process control system is now deemed as cost-effective and easy to implement and use. Besides the obvious technological functions like valve controls and monitoring of process values, some of the innovations that have further simplified the advanced control systems include;
Ratio control
Gain scheduling
Cascade-loop control
Override control
PID tuning
PID control
Split range control
Performance monitoring control
These functional tools of advanced process control are widely applied in industries of various sizes, for instance distillation controls, product and load change processes, coupled systems control, reactor control and start up control of equipment. Therefore, the tasks that are accomplished by an advanced control system include;
Optimization of PID control
Override control for safety purposes
Smith predictor to control analysis values based on dead time
Monitoring of performance controls in large plants
Some of the benefits or advantages that accompany the use of advanced process control techniques include the following;
a) Minimization of standard deviations to within 75%
b) Significant increase in the overall throughput of the plant with an overall consistency in the quality of the product
c) Reduced operator stress due to less manual intervention of the production processes
d) There is a significant increase in the yield of the plant
e) Reduced energy consumption
f) Efficient use of raw materials
g) Improved and better understanding of the process dynamics due to process modeling
h) Increased life span of the plant due automation
i) Consistent quality in the products while minimizing the cost of production
j) There is flexibility in manufacturing process and better response from the customers
Generally, an APC system reduces production costs by controlling the variability the parameters or variables that are to be controlled. The figure below is an illustration of how APC system helps reduce costs in the production process.
Fig. 1 Control of variables by APC
The only disadvantage that significantly reduces the extent of use of the advanced control systems in the process industries is that APC requires highly specialized personnel to implement and use it effectively.
ANALYSIS
Typical examples of industries that have implemented advanced process control were examined and here are the analytical findings of the study.
A. Goodyear Tire & Rubber Company
The named company did optimize its isoprene distillation unit by use of advanced process control technology and as a result it has experienced a great balance in the operating variables, save considerable amount of process energy and significantly reduced loss of vital products. Before this technological change, the company had serious challenges in recovering the monomer in pure state. The challenges faced included variations in the composition of the feed stock which adversely affected the process operations requiring changes to be made in the set-points and temperature values.
In effect, the company implemented a new technology called SmartProcess distillation optimizer. The technology utilizes an embedded DeltaV advanced process control and online PKI calculations. SmartProcess includes a combination of modules, APC tools such as DeltaV graphics, sample configurations, documentation tools and dynamic simulator. The implementation of the new technology was done in major phases, which were;
i. Balancing of the distillation process parameters: This was initially performed on one column (first column) as a demonstration to the plant operators to nurture their acceptance of the technology and also illustrate the value of APC to management of the company.
Fig. 2 distillation column in the C5 refinery
ii. Optimizing extraction: Included the optimization of the isoprene extraction column. Four variables were considered for the demonstration; constraints, controlled variables, disturbance variables and the manipulated variables.
Fig. 3 optimized extraction process
B. APC in practice
This article mainly demonstrated the need for process engineers to adopt APC technology as a compulsory tool in today’s industries. The article lists two prominent books that are spearheading the need for companies to urgently accept the new technology for all practicing engineers. The books reviewed in the article are:
a. Basic and Advanced Regulatory Control, 2nd Edition by Harold Wade
b. Models Unleashed by Gregory K. McMillan and Robert A. Cameron
The two books have been give much credit as compulsory material for those aspiring to work in process industries as they contain core material on advanced process control.
C. Control and Optimization of Distillation Process
This article gives a general description of the process of distillation, basic optimization strategies and the manner in which advanced controls can be used to cut on costs of operation columns in the industry. The article further gives insight on the importance of controlling feed stock composition in the distillation process and the various techniques that can be used to do so.
SUMMARY AND CONCLUSIONS
The advantages of implementing advanced control systems in today’s industries are highly numerous. The technology has been simplified for ease of use and implementation to make the tools of advanced control affordable to all industry size irrespective. For instance, APC can be applied to systems in which the desired outputs are optimized in real time and on-line and to existing system models or those to be developed later.
Typical applications include;
Petrochemical industries
Semiconductor manufacturing processes
Aerospace
Robotics
Vehicle guidance processes
Radar tracking systems
In summary, the advantages and benefits of APC are;
a. Improved yield and product quality
b. Reduced process disturbances
c. Efficient utilization of manufacturing resources e.g. energy and raw materials
In contrast, there are a number of limitations that hinder wide-spread use of APC, such as:
a) Implementing an APC system is costly and time consuming
b) It requires highly specialized personnel
c) A good understanding of the process is required before implementing the system
d) Frequent training of the operators is necessary
The shortcomings notwithstanding, the technique is worth deploying for the benefits gained are far greater than the disadvantages. Below is a schematic representation of advanced control of a distillation process.
Fig. 4 APC results of a distillation column
REFERENCES
Dutton K, Thompson S & Barraclough B, The Art of Control Engineering, Addison Wesley, 1997
Marlin T, Process Control: Designing Processes and Control Systems for Dynamic Performance, McGraw Hill, 1995
Ogunnaike B. A. & Ray W. H., Process Dynamics, Modeling and Control, Oxford University Press, 1994
http://www.apc-network.com/apc/default.aspx
