When we saw an increase in production rate needed during opportunity assessments at Monsanto and Solutia, we would salivate. Impressive results could be obtained in a matter of a few days by a simple change in setpoint, tuning, and/or configuration. These were quick hits and were often done right at the end of the opportunity assessment while motivation and enthusiasm was at its peak. Once we left the plant, other priorities often sneaked into the picture. Everyone then like now had more than enough to do as a result of meetings, procedures, and paper work.
Production rate can be increased by simply increasing feed rates until affected control loops start to approach their output limits. You don’t want valves to lose sensitivity from the flattening of the installed flow characteristic near the wide open position. You don’t want valves to get into stick-slip limit cycles from high seating or sealing friction near the closed position. How close to the output limits you can operate depends upon the type of valve. Sliding stem valves with diaphragm actuator and digital positioner from a supplier, whose heritage is valve manufacturing, have a good installed characteristic and minimal stick-slip between 10% and 90% throttle positions.
Just one operating condition or disturbance that causes an operator grief at higher feed rates, results in continual gap between what is done and what is possible in production rates. The lack of online process metrics and credit for pushing rates further establishes a status quo.
A control system to push rates automatically honoring constraints and trending of properly synchronized and filtered online process metrics can get everyone on the same page for higher production rates. Valve position controllers (VPC) can be added to each process control loop whose valve is limiting production. The VPC process variable (PV) is the process control loop output (OUT). The VPC setpoint is OUT that corresponds to the maximum or minimum effective throttle position of the valve. If there are several VPC, the lowest VPC output is selected to be the feed setpoint. Slides 7, 9, and 14 in the ISA Automation Week 2011 tutorial "Biological-Chemical Reactor Control" show VPC control systems to maximize reactant feed rate. The November 2011 Control article "Don't Over Look PID in APC" summarizes the extensive possible uses of VPC for optimization.
The use of the enhanced PID developed for wireless for the APC with a threshold sensitivity setting can ignore inconsequential changes in valve position and eliminate reaction to valve limit cycles from stiction and backlash. In the setup of the VPC, the use of directional setpoint velocity limits in the analog output block and dynamic reset limit (external-reset feedback) PID option can provide a gradual approach to the optimum and fast getaway to prevent running out of valve. The VPC does not need to be retuned as the velocity limits are adjusted. The process does not get into trouble during optimization increasing operator acceptance and consequently on-stream time. See the article "The power of external-reset feedback" for the many advantages of this PID option.
The entire plant can be coordinated and load disturbances minimized by the use of plant wide feedforward control. The plant can move to new operating points smoothly taking advantages of market, energy rate, and weather fluctuations with minimized inventories. The basic setup and opportunities for continuous and fed-batch operation are discussed in "Feedforward control enables sustainable, flexible manufacturing"
Some of the many techniques to minimize batch cycle times are:
- Eliminate wait times, operator attention requests, excess process hold times, and manual actions by automation, at-line analyzers, and data analytics
- Reduce lag times by better sensor and valve design and location
- Reduce SIS interruptions by tighter control and optimization
- Minimize acquire time by improve prioritization of users and releases
- Reduce failure expression activation by better instruments, redundancy and signal selection, and more realistic expectations of instrument performance
- Improve failure expression recovery by configuration and display methods
- Eliminate steps by simultaneous actions (e.g. heat-up and pressurization)
- Increase feed and heat transfer rate by an increase in pump impeller size
- Reduce transition time by full throttle and profile rate of change control
- Minimize processing time by constraint control (override control)
- Minimize non constrained processing time by all out run, cutoff, and coast
- Minimize processing time by pairing of loop variables to maximize rate
- Minimize processing time by better end point detection
- Minimize processing time by capture of last batch's best outputs for next batch
Eliminating manual actions not only reduces batch and continuous startup time as exemplified in Nov 2009 Control Talk "Show me the Money – Part 1" but also increases process efficiency by improving product quality. See the May 2006 Control article "Full throttle batch and startup response" for a simple way to have the fastest possible setpoint response time.
Do not let accountant's reluctance to commit to the need or value of a capacity increase delay or discourage you. Plant operations know what is really needed and online process metrics will provide the evidence. Including plant operations and process support engineers in the opportunity assessment will enable the team to identify the need and value of a capacity increase. The master of opportunity assessments at Monsanto and Solutia was Glenn Mertz. Look for my June Control Talk interview conducted in Glenn's brilliant Florida home over the course of numerous beers.
An increase in process efficiency, such as a reaction yield or purification separation improvement, and a decrease in off-grade or scrap, can be taken as a decrease in raw material, reagent, utility, or recovery cost of goods (COGS) for the same production rate. Less discussed in the literature is the option to take an increase in process efficiency as an increase in production rate for the same COGS. Next week in Part 2, we will discuss quick and simple methods for increasing process efficiency.
