Fractionation Research Inc. (FRI) has been studying distillation and distillation equipment for almost 60 years on behalf of its 70 members, each of whom pay an annual subscription fee for services. FRI's two industrial-size distillation columns (Figure 1) are located on the Oklahoma State University campus in Stillwater. Three binary separation tests are typically run within those two columns:
- p- and o-xylenes from 75 mm Hga to atmospheric pressure;
- Cyclohexane and n-heptane near atmospheric pressure; and
- Isobutane and n-butane from 100 psia to 500 psia.
During test runs, the trays and packings within the columns are subjected to a very wide range of liquid/vapor and mass transfer regimes. All of the runs are logged at steady-state conditions, but steady state is maintained only long enough to collect the relevant data. The process conditions are then modified for the next test. Tests conducted include start-up, transitions within and between various modes of operations, flooding, un-flooding, setpoint changes and shutdowns.
Unsteady-state procedures at the FRI distillation units are now implemented semi-automatically using a solution called modular procedural automation (MPA). MPA was first pioneered by Yokogawa, and it initially was based on ISA88 principles. It's now being deployed using the guidelines under consideration by the ISA106 working group. MPA is a methodology that can be implemented with virtually any modern automation system, and it's effective across a wide range of processes and industries.
As implemented by FRI using Yokogawa hardware and software, MPA provides specific, on-screen instructions and information for our control room operators and technicians in areas listed in the "On-Screen Operator Guidance."
By having our operators follow these instructions and use the supplied information, MPA has produced safer operations at our test facility. Based on our experience, we believe that an MPA or similar solution might have prevented some of the largest chemical plant accidents of the past decade.
MPA Software Runs in the DCS
During 2011, the FRI control room was augmented with a data historian and MPA supervisory software supplied by Yokogawa. These tools were loaded onto the existing Yokogawa DCS, which has been in place for several years. Figure 2 depicts our control system architecture.
The historian, which collects data every second and stores it in a database, has allowed FRI to analyze unsteady data and data from operating modes other than total reflux. FRI engineers, with assistance from Yokogawa, have used the MPA software to build and execute electronic, semi-automated procedures for start-ups, transitions and shutdowns.
Semi-automated procedures based on operational experience were initially implemented using MPA, but none of the procedure algorithms were completely satisfactory when first tested in the control room. Some of the algorithms required additional process checks, some a faster or slower rate of execution, and some others needed increased flexibility.
To improve these procedures, the MPA software was run in parallel with actual operations, but in an offline mode. The procedures were then adjusted to closely mimic operator's actions. This step allowed the system to pass validation tests and, at the same time, increased operator understanding and trust.
FRI's best board operators were in the control room during these trial runs, and their experience was used to modify and improve the initial MPA instructions. Our aim was to use MPA to capture the knowledge of our best operators, so this knowledge could be used by other operators to reproduce our best practices, time after time.