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Instrumentation & Control

Instrumentation and process control for powder and bulk solids handling operations

All powder and bulk solids processing and handling equipment or systems require some degree of monitoring and control. The more complex an industrial process becomes, the more important it becomes that it is correctly controlled and monitored. This is where instrumentation and control systems are crucial.

Silo contents measurement is a key area. Because the bulk density of silo contents may vary, level indication and level detection instrumentation is vital to prevent overfilling.  In other industrial processes, controls need to be able to react to explosions, system blockages, sifter screen breakages, metal detectors, over-heated mills, etc. and take the appropriate action to prevent system damage or compromise to product quality.

Because dry bulk solids are such an essential part of many of the products around us, measuring them accurately is critical to the efficiency of their production.

To make processes more efficient, more cost-effective, and more reliable, you have to understand what is occurring within a process to be optimized.

Ideally, that means measuring the input and output of each minor process to understand, for instance, the optimal setting for each piece of machinery, the best ratio for mixing ingredients, the best way of maximizing output while minimizing cost.

Powder and Bulk Solids Instrumentation
Picture: The Wolfson Centre for Bulk Solids Handling Technology

Process instrumentation is a range of measuring instruments to control and monitor an industrial process. Process instrumentation is used to measure and control process variables within a laboratory, production, or manufacturing area.

Process instrument

A Process instrument is a device that measures a physical quantity such as flow, temperature, level, distance, angle, or pressure and generates a signal for process controls within industrial manufacturing facilities.

Process instrumentation in dry bulk and powder handling operations

Process instrumentation is an integral part of any dry material handling/powder handling process. It allows real-time measurement and control of process variables such as levels, solids flow, pressure, temperature, dustiness, and humidity.

With the proper process instrumentation, manufacturing facilities can run effectively, efficiently, economically, and safely by integrating alarm signals.

Powder-behavior-bulk-solids-flow-properties

Powder behavior can affect manufacturing efficiency and directly affect product quality variables such as dose uniformity.

It is now possible to measure the properties of a powder reliably and relate these directly to particle morphology.

Accurate, statistically appropriate particle size and shape data have been generated using the technique of automated imaging and used to investigate the impact of particle morphology on dynamic, shear, and bulk powder properties.

Powder flow can be defined as the ease with which a powder will flow under a specified set of conditions. Some of these conditions include:

  • the pressure on the powder,
  • the humidity of the air around the powder, and
  • the equipment the powder is flowing through or from.

For some applications, ease of flow is defined by whether the powder flows or not, the so-called “go-no-go” approach. Will the powder flow through the system or not?

For other applications, the powder flow rate and consistency are essential.

Bulk solids flow

The device used to test a bulk solids flow must take the application problems and processing conditions into account to supply relevant data to the user. The powder in the test device must be in the same state as it is in the process being studied, and this ensures that the flow analysis will apply to the problem.

Flow properties of a powder or a bulk solid

Knowledge of the flow properties of a powder or a bulk solid is crucial to design silos and other bulk handling equipment, to ensure that flow problems won’t occur.

The flow properties depend on several parameters, for example:

  • particle size distribution,
  • particle shape,
  • chemical composition of the particles,
  • moisture,
  • temperature

Why test a bulk material’s flow properties?

A complete understanding of a bulk material’s flow properties and its flowability is essential for identifying the cause of poor flow, powder flooding or rate limitations, segregation, or product non-uniformity. Flow properties tests are also crucial when designing a new silo/bin/hopper, stockpile, feeder, chute, conveyor, or other bulk material handling equipment.

Powder and bulk solids properties

Understanding the powder/bulk solids properties is the first step to a good design, or efficient troubleshooting, of an industrial installation.

The design and operation of particle and powder handling and processing equipment requires specific knowledge of powders’ particle properties and bulk properties.

Particle characterization in bulk powders
Picture: The Wolfson Centre for Bulk Solids Handling Technology

Particle characterization identifies various particles by particle shape, size, surface properties, charge properties, mechanical properties, and microstructure.

A variety of commercially available particle characterization techniques can be used to measure particulate samples.

Each has its strengths and limitations, and there is no universally applicable technique for all samples and situations.

Particle size – The particle size directly affects compaction, flow, dissolution, texture. The particle size can be controlled for a specific final application or for controlling the input for further processing. Measuring the particle size is particularly important for comminution/size reduction processes.

Particle shape – Particle shape is the expression of the external morphology of the powder particle, and it includes form (overall shape), roundness (smoothness), and surface texture.

Control of the shape of the particles can be required for the needs of a specific final application. Knowing the shape of the particles can also help anticipate mixing problems.

A major problem in testing the influence of particle shape on powder properties is the separation of different particle shapes in one powder bulk. There is as yet no universal method to describe the shape of powder particles.

Particle strength – Particle strength depends on various factors such as their material and mechanical properties, their size, shape, the loading pattern, and other external conditions. Impact tests, Attrition tests, and compression tests allow good determination of the strength distribution of impact and compression strength.

Bulk density – Bulk density is a property of powder, granules, and other bulk solids. The total volume includes particle volume, inter-particle void volume, and internal pore volume.

Particle density – Particle density (or true density) is the weight per unit volume. Particle density will be relevant when the solid’s movement must no longer be considered a mass but as an individual. It is the case for some technologies of pneumatic transport.

Particle dustiness – Particles’ tendency to become airborne in response to a mechanical treatment or aerodynamic stimulus. Dustiness is affected by particle shape, size, and inherent electrostatic forces. Particularly in processes that work with powders, granular materials, or fibers, dust particles can be released into the air during conveying and transport.

Particle concentration – Particle concentration measurement is required across a diverse range of industrial processes. In many cases knowing only the particle size is not enough, as sample concentration may also affect the product’s performance.

Equipment and system designs must be based on proper material characteristics measured at representative process conditions to ensure reliable handling and processing of bulk materials. Different powders and manufacturing processes can present an array of powder handling problems, for example:

Segregation of powders

In powder technology, segregation means the separation of particles of different properties during handling, transportation, and storage, and it is a common problem for most industries processing particulate solids.

Degradation

Degradation occurs as the product moves through the system, and the particles are constantly in contact. Therefore, the material breaks down, resulting in a poor-quality product but can also cause problems with dust containment.

Powder caking

Powder caking results when particles agglomerate and form solid lumps or masses. Caking is undesirable and results in customer complaints, rejected products, and additional processing steps to delump the material.

Bridging of powders

Bridging of powders takes place when bulk materials interlock or bond together to build an arch (or bridge) above the container’s outlet (silo, hopper, IBC, mixer vessel). When this occurs, the arch holds the rest of the bin’s contents preventing discharge of the remaining powder. To overcome bridging, the wall friction must be reduced or prevented from occurring.

Ratholing

Ratholing of powders can occur in a container (silo, hopper, IBC, mixer vessel) when the powder empties through a central flow channel. Still, the material at the bin walls remains stagnant and leaves an empty hole (rathole) through the material, starting at the bin outlet.

Instrumentation bulk handling powder processing - Particle characterization in bulk powders

Samplers/sampling equipment for powders and bulk materials

Sampling can be defined as taking samples from a (dry bulk) material to obtain information about the composition of the total amount in respect of one or more attributes, such as metal content, ash content, moisture, particle size, particle shape, etc.

Particle size analytical results are most applicable when drawn samples are representative, and the appropriate dispersion techniques are used.

The majority of variation in particle sizing measurements is traceable to either incorrect sampling or sample preparation.

Sampling challenges with solids and powders

It’s frequently said that handling dry materials is more art than science. Bulk solids sampling is susceptible to conditions upstream and downstream of the area where the representative sample is drawn, chiefly due to material flow properties. Even a few degrees of moisture fluctuation, for example, can skew results.

Aeration is another hazard to flow properties. Some materials aerate well, so placing a sampler along a free-flowing stream presents no difficulties. However, those same materials may solidify as they settle. A sample taken after settling will return drastically different results than that drawn in the process stream.

Another challenge often not considered is space limitations on the process line. Depending on the application, samples vary in size and footprint, so installation can be tricky if sampling goals and the equipment needed to accomplish them are not carefully examined and aligned.

Moisture measurement is essential for manufacturing from the initial process to the end-product stage.

Measuring and controlling moisture can reduce transportation costs due to shipping excess water, and moisture control stops products from freezing during shipping and transportation.

Moisture content measurement throughout the manufacturing process also provides cost savings in energy & fuel costs and less product waste.

When one measurement is out of tolerance, a quick automatic or manual adjustment can be made to keep bulk materials moving correctly before costly time is lost and waste is produced.

Level measurement of powders and bulk solids in bins, silos, and other types of vessels comes with significant challenges.

These challenges include:

The shape of the material surface – The solids being measured can be either minute size powders or outsized having pointed edges.

Material characteristics – Bulk solids can be fine micron-sized powders or large with sharp edges.

The angle of repose of solids – Unlike all virtual liquids, the bulk solids in a vessel do not possess a horizontal plane surface. They generally have an angular surface shape known as the angle of repose, which can change according to the fill and discharge settings.

Vessel dimensions – It can be challenging to know the exact dimensions of the silo the bulk material is stored within.

Bulk density differences – It may be difficult to know an accurate value for the bulk density for materials like corn and flour that vary from season to season and are dependent on specific crop and blend.

Dust inside the vessel – Some materials generate vast quantities of dust during filling and expulsion.

These challenges deal with converting a distance or level measurement to volume or weight. Others challenge the reliability of the level measurement technology chosen.

The primary selection criterion with all bulk solids level measurement systems is the condition of the material within the vessel and its effect in terms of accuracy on level measurement and sensor contamination.

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