Boosting Blending Accuracy

Optimizing operations using coriolis technology

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Coriolis flowmeters can help improve the accuracy of batch blending, dilution and continuous blending operations. These meters sense the mass flow rate at a high degree of accuracy and sensitivity, transforming the rate into an analog or frequency output.

Coriolis flowmeters offer chemical and other industrial facilities on-line measurement of mass flow, density and volumetric flow. They have an accuracy of +/-0.1 percent and are capable of multivariable measurement ," volume, mass, density and temperature. Because they have no moving parts, maintenance requirements are reduced. They are available in a wide variety of sizes and flow capacities.

This article takes a look at some of the challenges associated with batch blending, dilution and continuous blending operations, and examines how coriolis flowmeters can improve mixing accuracy and cut downtime.

Batch blending

Proper blending is important to many products because they need to have a specific taste, smell, texture, viscosity and/or appearance. For example, a slight error in ketchup blending can translate into dispensing problems at the dinner table.

Blending operations in the chemical industry produce different grades of finished products. Because many chemical facilities use the same equipment for multiple recipes, the equipment, instrumentation and control valves must be able to operate with precision across a wide range.

Traditional batch blending, however, can adversely affect a chemical plant's overall efficiencies in at least four ways.

First, the addition of one ingredient after another, called sequential addition, can amount to hours from the first addition to completion and transfer. Much of this time is required to expose the fluids to their reaction partners or to homogenize a mixture.

Second, batch blending requires a great deal of valuable floor space for the large equipment.

Third, batch blending requires a larger finished-product storage area. Space must always be available for the next batch transfer.

Finally, batch blending involves much more equipment than continuous blending. See Fig. 1.

Figure 1. Comparison of Conventional Batch and Continuous Blending

Continuous blending requires smaller equipment, less floor space and a smaller product storage area than batch blending.

 

 

Small additions cannot be accomplished accurately if a single large scale is used. If used, they can be expensive to maintain and keep in calibration. Agitators also affect weigh cells. Noise resulting from liquid turbulence must be filtered out. This equipment usage requires more maintenance than continuously operated smaller equipment.

Volumetric meters have a lower accuracy than coriolis meters, and some have longer ramp-up and decay periods, leading to inaccuracies. Additional power is consumed for pumps and agitators, and more pressure and level instruments are needed.

Dilutions

Dilutions are a major blending application. Household, agricultural and other mixtures and solutions often have one or more concentrated or powerful components that are blended into an inert carrier ," often high-purity water.

 

The reasons for dilution are many. Sodium hydroxide and other caustics can be very viscous or even freeze in mild climates. Glacial acetic acid will freeze below 62 Degrees F. Hydrogen peroxide and other peroxides can react violently when the concentrated form is contaminated. Some reagents can be distributed in more affordable piping systems when dilute. Accurate blending during dilution is important, therefore, to ensure performance and reduce cost.

Somewhere, at the use points, composition is important. The best place to dilute is as close to the unloading point as possible. Using two coriolis meters, purchased fluid, water and blended stream mass flow rates can be determined to less than 0.15 percent of rate inaccuracy, and density can be determined within 0.0005 gram per cubic centimeter (cc).

Plants potentially could handle their bills of lading and formulation needs at the same time. Plants even might be able to skip those repairs to the rail scales.

Continuous blending

Reactions have many metering needs. The traditional stirred batch reactor has not disappeared, but it has changed. In large part, competition has eliminated the separation stage after many reactors. It is much preferred to blend reactants precisely rather than to remove the overcharged component after reaction. Equipment and energy costs are eliminated, and the cycle often is improved.

The greater trend in reaction is continuous processing. With continuous blending, however, plants have only one chance to do it right. Rework is usually not an option, and co-incident precise ratios are a must. For loop, stirred and plug flow continuous reactions, even pressure/temperature-compensated volumetric metering might not yield sufficient accuracy.

Continuous blending ," measuring and mixing raw materials within the pipe as the material flows to produce exact proportions and meet specific recipe requirements ," delivers the same accuracy as ," or better accuracy than ," batch blending.

Although continuous blending can alleviate many of the shortcomings of batch blending, some facilities resist the change because of their "corporate culture." Manufacturers like to stay within their comfort zone and might be wary of an unfamiliar procedure. Moreover, many plants have invested heavily in batch blending equipment and are hesitant to replace it.

Return-on-investment (ROI) analysis could show the idled tankage could be used for surge or storage capacity. The changeover often is made after a continuous expansion or de-bottlenecking has occurred.

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