Batch to Continuous

Applying Batch Principles to Continuous Control

McEnery Automation (www.mceneryautomation.com) is a system integrator in St. Louis, Mo. Its president, Michael McEnery, PE, is the vice chairman of WBF - The Organization for Production Technology (www.wbf.org).

"We've seen a trend to convert pure batching processes to semi-continuous in-line blending, and we've implemented these systems for several customers. A major reason is accommodating an increased number of finished products,"explains McEnery.

In several of these processes, one or two base products are still created in large quantities with a typical batch operation and stored in dedicated tanks. These base products are then used to create smaller quantities of the finished products through a semi-continuous in-line blending process. The final product may be stored in a finished product tank, but more likely it will be delivered directly to a packing line or rail car (See diagram below).

This diagram shows how S88 principles can be applied to a semi-continuous, in-line blending operation.
The blending system is broken down into equipment modules, control modules and equipment phases.
(diagram courtesy of McEnery Automation)

Besides the elimination of finished-product storage vessels, there are several other operational advantages. The blending process creates the exact quantity of product required, and it can be stopped or extended with minimal reaction time. This eliminates short-shipping a production run because the batch size is too small or having extra buffer capacity in each batch. It also minimizes the amount of over run, especially if a production run is terminated early because of equipment or supply chain issues. The amount of scrap material left at the end of the run is so small it is typically sent to a common reblend tank.

From a process control perspective, these continuous or semi-continuous blending applications can use ISA-88 batch control standards. "ISA-88 ideas and principles are flexible enough to work with different processes. Once you begin to view your process equipment as a collection of equipment and control modules and your procedure as a sequence of phases, the actual process type becomes somewhat insignificant as the boundaries between batch and continuous blur," observes McEnery.

The biggest instrumentation challenge for these continuous processes is product quality testing because there is no room for error when the finished product is going directly to a packing line, truck or rail car. "Our customers have implemented sophisticated in-line instrumentation to monitor key quality parameters such as color, calorie content, alcohol content, oxygen levels, pH, flow rate and temperature," says McEnery.

"Modifying a QA department's mode of operation to real time has been a challenge as the QA process needs to react in minutes rather than hours. But with in-line QA, issues can be detected immediately, rather than after the completion of the entire batch, thus reducing the amount of scrap or rework material," he adds.

For further information on WBF's role in the application of S88 to continuous processes, see the WBF web site for archived papers and presentations. Dennis Brandl, WBF member and author of "Design Patterns for Flexible Manufacturing," will be presenting a one-day expert seminar on "Non-Stop 88― Applying Batch Methods to Continuous and Discrete Processes" at the ISA Expo 2009 on October 5, 2009. Registration information can be found at http://www.wbf.org/catalog/pages.php/page/39.

Continuous Processes Need Redundancy

An automation engineer at a pharmaceutical firm relates his experiences with converting from batch to continuous in a tablet making operation.

"The biggest improvements were to product yields because we weren't discarding material during start-ups and batch ends,"relates the engineer. "For continuous runs, I could set PID parameters to maintain tighter control responses, and the final product became more consistent because of fewer interruptions."

The plant identified mission critical devices and networks and then developed redundant controls. The plant also needed to take devices and instruments offline for calibration checks and maintenance functions without taking down production.
 
Three mission critical areas were identified: solution application, air flow and temperature. For temperature, three redundant temperature sensors were used. The control system switched from one sensor to another for maintenance.

The three temperature probes performed constant calibration checks. If a primary probe began to drift while the other two remain constant, then the controls switched from the primary to one of the backups. The controls were designed to alternate among devices at startup to ensure that the backups wouldn't fail due to lack of use.

The process also used a redundant solution skid that contained a pump along with automatic inlet and outlet diverter valves. Based on flow and solution pipe pressure, the system would automatically switch to the backup solution skid and alert maintenance of a solution skid problem.

The controls were set up so that the control variable output to the primary pump would be redirected to the backup pump. After a few seconds to allow the pump to reach speed, the diverter valves would slowly operate to accurately control solution flow.

Once the diverter valves were completely cycled, the loop control would be switched to automatic and the primary pump would be shutdown. If the solution deviated from parameters during this process, the controls would open a dump flapper to dump tablets into a reject bin until the process was brought back in control.