Precision Polyethylene Batch Processing

Digital Coriolis Technology Key to Measurement and Control of Volatile Process

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Production of polyethylene typically involves flow streams containing gas and liquid, but the inability to measure this two-phase flow accurately has been linked to process interruptions, poor product quality, wasted materials and higher energy costs, any of which can sap the profitability of a production run.

 

One leading chemical producer has found a solution to the problem using Coriolis flow measurement technology that the Foxboro unit of Invensys developed in collaboration with flow specialists at the University of Oxford, England.

 

The company produces three types of quality polyethylene in both a large-scale and smaller scale reactor. The reactors receive the main ingredients, ethylene, comonomers and catalysts from bullet tanks, which are supplied by tank trunks. Between the smaller reactor and the bullet tank are seal pots, through which the ingredients flow (see Figure).

 

Polyethylene Production

Volatile catalysts flow in two phases. Accurate measurement is

key and Coriolis-based flowmeter technology ensures it.

 

The process requires volatile catalysts that are very sensitive to contact with air. The volatile nature of the catalysts requires a nitrogen gas stream to transport the catalysts through the lines, resulting in two-phase flow. Although this is a necessary part of the process, it is also the source of the problem, especially during changes in batch composition.

 

When switching is over, special precautions are taken to completely blow out the feed lines with nitrogen to avoid contaminating the next batch. This means starting with flow tubes that are completely devoid of materials, something that further complicates accurate flow measurement because conventional flowmeters do not register any materials until the flow tubes are full. But by that time, however, unaccounted materials may have already entered the reactor.

 

The inability to measure exactly how much catalyst is flowing into the reactor from the partially filled flow tube not only jeopardizes the integrity of the batch, but PE will form in areas where it should not. If this happens, the reactor has to be shut down and meticulously cleaned, a procedure that yields wasted materials, process downtime and unplanned maintenance expenditures.

 

In one approach, operators monitor process conditions manually and then compensate based on real-time pressure and temperature readings. This trial-and-error approach requires constant attention to process parameters and dictates tweaking the recipe for each batch to produce consistent product quality.

 

In another approach, operators apply radar level measurements for the seal pots that feed the smaller reactor. By measuring the catalyst level, operators are able to determine the amount going into the recipe. While this can be accurate, it requires operator intervention, which increases the likelihood of error.

 

Coriolis Measurement Provides Solution

Looking to improve production efficiencies, while reducing costs, the polyethylene processor chose to evaluate a Foxboro CFT50 Digital Coriolis Flowmeter in a beta site application that feeds the small reactor. The unit's dual digital processing system proved to be the solution. One processing system controls the meter drive sequence to provide uninterrupted flow tube operation. The second system processes the measurement data from the Coriolis flow tube, which assures precise fluid measurements.

 

Responsive to Step Changes

The flowmeter was developed in a collaborative effort with Oxford University. The flow tube drive signal is synthesized digitally, giving good control of the meter in difficult conditions. And the dynamic response is rapid. Independent trials have shown it can respond to a step change in flow within 25 ms, with sophisticated algorithms to correct for the effects of two-phase flow.

 

With the CFT50, this major U.S., polyethylene producer was able to begin measuring materials within a ½-sec. compared to a 20–30-sec. delay with other technologies. Given that for each batch there are as many as six seal pots filling at a rate of 50 gpm, that’s up to 150 gpm of expensive catalyst previously unaccounted for.

 

Digital Coriolis measurement has been online since, and plans are underway to extend it to the other seal pots on the small reactor and ultimately between the bullet tanks and the large reactor.

 

Dr. Manus Henry is a Research Fellow, Engineering Science Department University of Oxford, UK.

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