Chemtura Corporation, with 2006 sales of $3,5 billion, holds a leading position in the market for plastics, petroleum additives, castable urethane pre-polymers, flame retardants, and pool chemicals. Chemtura has approximately 5600 employees in research, manufacturing, sales and administrative facilities in major world markets.
The challenge
Accurate flow measurement at the beginning and end of batch runs is a common problem in industries ranging from chemical to food processing. Typically the flowmeter starts empty, suffers the hit from the onset of process fluid, meters the bulk of the fluid, and then encounters slugs and two-phase flow at the end of the batch. While no flowmeter technology performs especially well in these conditions, Coriolis meters, which are most accurate when full of fluid, are especially vulnerable to start and end conditions.
At the Chemtura chemical facility in Manchester, UK, operations people were having particular trouble with a batch line that makes a variety of water treatment chemicals for heating systems and desalination. The application involves combining three feeds into a tank, blending and pH balancing the batch, and pumping it into a storage tank using a traditional Coriolis meter to measure flow. With some batches the product needs to be filtered when it is pumped from the tank, which reduces the flow and increases metering problems.
"We had huge discrepancies between the metered inputs and the output, and it was clear that the Coriolis meters were not seeing all the product passing through," says Roger Marsden, director of Westmeade Services, who provides technical support for Chemtura. Depending on the flow rate, up to 200 kg of product was missing. And in situations where the product had to be filtered, as much as a 1000 kg went unmetered.
"'We have other clients with similar problems, and we pride ourselves in supporting the latest technology. Upon learning of the Foxboro CFT50 digital Coriolis flowmeter, we arranged a trial," says Marsden.
The Foxboro solution
Recent trials at Chemtura indicate the Invensys Foxboro CFT50 digital Coriolis transmitter can resolve the problem. The instrument has demonstrated what engineers at Westmeade say is a remarkable ability to meter accurately during the difficult start and end stages of the batch.
The Foxboro CFT50 was installed in series with the existing meter on the outlet to properly compare performance. A chart recorder was used to capture data, with a two second update time. Data collected included the start and end of a typical batch, the two mass flow readings, and the Foxboro CFT50 density reading to indicate the percentage of air in the fluid.
Prior to the onset of flow, both meters showed a zero reading, while the density reading indicated that the meters were wet and empty. Once the flow began the CFT50 started up immediately, whereas the other meter required some 16 seconds to register the flow, allowing approximately 4 kg of material to pass through unmetered.
While the batch was in full flow the two meters matched each other. At the end of the batch, some two hours later, both meters registered the drop in flow at 130 minutes, but as the flow tubes drained the other meter stalled. It failed to register the final blow-though of product.
Results
Chemtura has drawn the obvious conclusions. "Based on proven performance, we now trust the CFT50, so the other meter will be pulled out says Mark Wilkinson, Instrumentation and Evaluation manager at Chemtura, who funded the trial. "We will be recommending the CFT50 to our clients for batching applications where entrained air is an issue," adds Roger Marsden. "This is a significant extension of Coriolis capability."
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