Batch to Continuous

Converting from Batch to Continuous Brings Great Benefits, but Also Creates Automation Challenges

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  1. Increased throughput – The continuous reactor is about one-third of the size of the remaining batch reactor, yet can produce 1.5 times as much product.
  2. Increased energy efficiency – The continuous process uses about 25% of the energy used for the batch process.
  3. More consistent product quality – Although automation has made Bayer's batch products very consistent, continuous production quality and consistency is even better.
  4. A greener process – The new catalyst technology eliminated the need for refining the reactor effluent into saleable product. In the conventional batch technology, the catalyst must be removed from the product. The new catalyst is only present in parts per million, has no deleterious effect on the product and need not be removed.

"The continuous process produces no waste, so every pound of feed is a pound of product," notes Wray.

Aside from mechanical changes, including beefing up the agitation in the reactor and adding cooling capability, instrumentation changes were needed.

Feeds previously batched into the reactor with a weigh vessel are now metered using redundant Coriolis flow meters and control valves. "We had previous experience with Coriolis meters for some reactor feeds, such as propylene oxide, but not all. The relative quantities of feeds not only control product quality, but also affect process safety by ensuring adequate catalyst feed to the reactor," explains Wray.

A second change affecting instrumentation is in level measurement, as the level instrumentation now needs to handle over-ranging caused by liquid full operation.

"Our plant was built in 1988 with a strong commitment to ISA-88 style modular batch automation, and the operations staff insisted we apply the lessons and techniques learned in batch automation to the converted continuous process. As it turns out, applying batch automation principles to continuous processes is pretty logical," feels Wray.

"There have been several papers presented at WBF - The Organization for Production Technology (www.wbf.org) conferences on this topic, and WBF continues to lead the expansion of ISA-88 principles into continuous operations. I recommend WBF as a resource for those contemplating a switch from batch to continuous. From an automation philosophy perspective, it might not be as big a change as expected," advises Wray.


 

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).

Finished Product Tank
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.

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