Case Study: Instruments Tame Troublesome Slurry

Two-step approach allows resin plant to more accurately dose additive.

By Kerry Haight, Lubrizol Advanced Materials, and Joe LaPoint, FCX Performance

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Lubrizol Advanced Materials’ Louisville, Ky., plant produces chlorinated polyvinyl chloride (CPVC) resin by reacting chlorine and PVC. To transport resin from vessel to vessel, the CPVC is mixed with water. A centrifuge later dewaters the slurry. Some CPVC grades then require dosing a very small amount of an additive chemical before going to a fluid-bed dryer to produce the final resin powder.

The CPVC particles are porous in structure and no two are exactly alike. This poses processing challenges. In particular, the % solids in the slurry can vary from run to run and from one type of resin to another —which made achieving the target additive dosage very difficult and resulted in manufacture of excessive non-prime product.

So, the plant set out to find a controllable and reproducible dosing method.

Originally, it relied on a simple ratio calculation based on the torque reading of the centrifuge. This method only succeeded about 60% to 80% of the time; the low success rate stems from inherent variability of the ratio calculation. First, the torque reading isn’t an exact measurement of the amount of mass going through the centrifuge. General use and wear on the centrifuge can shift the torque reading, resulting in inconsistent measurements from one run to the next. Second, the calculation uses a constant that’s, at best, a rough estimate.

The Challenge
The amount of CPVC determines the quantity of additive required (which is a wt. % of dried resin). So, it’s necessary to know the CPVC slurry’s flow rate and its % solids. It’s tough, however, to obtain accurate measurements with two-phase flow.

Faced with this challenge, Lubrizol engineers worked with their counterparts at FCX Performance, a flow control products distributor, to devise a cost-effective and accurate two-part solution. First, we chose a device to measure the slurry’s volumetric flow rate. Second, we found an instrument to measure its % solids.

We couldn’t use Coriolis technology to measure flow because such meters require the same mass in two tubes. Since the CPVC particles vary in size and shape, it’s impossible to ensure the same mass in both tubes. Flow meters that use infrared technology proved unsuccessful because the slurry’s solids level (typically greater than 25%) was too high to allow an effective reading. The plant also tried ultrasound devices but that technology couldn’t cope with the porous nature of the CPVC particles. The possible presence of gas particles trapped inside resin particles means the flow actually could have three phases — solid, liquid and gas.

We opted for the FSM4000 magnetic flow meter (magmeter) from ABB to measure volumetric flow rate. With no moving parts, magmeters are nearly maintenance free. They use small electrodes that protrude into the media stream — and so are ideal for slurries, dirty liquids, wastewaters and applications requiring small pressure drops. However, such meters generally won’t work with nonconductive fluids such as hydrocarbons. The volumetric flow rate measured by the magmeter goes to the distributed control system.

Next, the search began for an instrument that could accurately measure % solids. We tried infrared technology but it proved unsuccessful. The high percentage of solids made the slurry too noisy to get an accurate reading. We also tried a turbidity meter but it couldn’t handle the conditions either.

We discussed the problem with Lubrizol engineers; a few days later when Joe was meeting Metso factory personnel he raised the idea of using a Metso kajaaniMCA Consistency Analyzer (Figure 1). This device uses patented microwave “time of flight” technology. Flight time of the microwave sent across the pipe depends on the dielectric constant of the material in the medium. Because solids typically have a lower dielectric constant than water, the microwave signal travels faster through solids than water. This travel time across the pipe bears a linear relationship with % solids. The technology has served for years to measure stock consistency at paper mills and solids in process streams at municipal sewage plants.

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