Recovering a Chemical Manufacturer’s Productivity with Batch Cycle Time Reduction

The completed project made the process safer and recovered 30% of the lost cycle time, giving an ROI of well under one year.

OBJECTIVE: The chemical manufacturer needed to recover productivity losses caused by an unexpected increase in cycle time after a migration of the plant’s legacy Emerson PROVOX™ to a DeltaV™ distributed control system (DCS).

The third-party migration from legacy Emerson PROVOX™ to DeltaV™ DCS increased the cycle time on a train of eight chemical reactors by approximately 6%, costing the end user more than one million dollars per month in lost productivity. The objective was to investigate where time was being lost in the batch process due to logic changes during migration and recover as much time as possible to increase productivity.

CHALLENGE: A safety issue required a project pivot and scope change during the project. A potentially dangerous safety hazard associated with the original migration was identified and the system was operating in an unknown state.

The DeltaV™ logic was skipping over the task of confirming valve positions before moving to the next step in the batch. This issue needed to be addressed before moving forward with the cycle time reclamation aspect of the project. Addressing the lack of valve position confirmation, the additional logic forcing the DeltaV™ to wait for the valve positions to confirm caused even slower cycle times than the project started with, so the gains shown in the initial proof of concept became unattainable.

BACKGROUND:  A vinyl derived chemical manufacturer in the greater Houston area operating a fully batch process had undergone a migration from Provox to Emerson DeltaV. After the migration was completed, the cycle time of each reactor batch was up approximately 18%. Hargrove was called to assist with returning the process to its pre-migration cycle time.

A review of the new logic was undertaken and it was discovered that the logic was migrated in a manner that kept the original phase structure intact from PROVOX™ into DeltaV™. Unfortunately, key differences in PROVOX™ and DeltaV™ system architecture on how phases are loaded and executed was causing long pauses in the new DeltaV™ batch process.

The pauses occurred where devices in the unit were idle while waiting for DeltaV™ to unload the current phase and load the next phase. This issue was exacerbated by the fact that the new DeltaV™ controllers were operating beyond the ideal loading capacity, so unloading and loading phases were taking longer than they normally would. A lack of available controller processing capacity prevented the phases from being permanently loaded into memory or being allowed to execute at a faster scan rate.

SOLUTION: Move the existing logic from being executed in phases to being executed in equipment modules.

A solution was formulated to move the existing logic from being executed in phases to being executed in equipment modules. Under this scheme, each phase became one equipment module. This alleviated many of the pressure points the unit faced.

A full retest of logic was not required since the existing, fully-tested logic was only relocated and not changed.

The equipment modules were assigned to new controllers since they had no I/O associated with them. This took some strain off the existing controllers and allowed the batch logic to execute on lightly-loaded controllers.

While the execution time of phases is tied to the execution time of the unit module in DeltaV™ and requires them all to execute with the same scan rate, the individual equipment modules can have unique scan times. This allowed for some sections of code which were mostly calculations or flags to execute much faster than sections of code that communicated with the control devices in the field.

A proof-of-concept project was executed and lab testing showed that about 85% of the time associated with loading phases and executing initialization logic (setting flags) in DeltaV™ could be eliminated using this new philosophy.

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Results: Safer and More Consistent

Additional logic was implemented to wait for the confirmation of valve positions, making the operation safer and more consistent. The implemented cycle time improvements were logic driven and did not require a retest of the logic, saving cost and downtime. The logic was moved from being executed in phases to being executed in modules, allowing different scan rates for different equipment and eliminating the associated dead time. The equipment modules without I/O were assigned to new controllers, allowing the batch logic to execute more quickly on lightly-loaded controllers.

The completed project made the process safer and recovered 30% of the lost cycle time, giving an ROI of well under one year.


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