If the process designer for a hypothetical manufacturing plant were given the choice of working with liquids or solids as feedstocks, more often than not, he or she would likely choose liquids. Why? For most purposes, liquids are easier to work with. They are easier to move, have fewer storage problems and mix more readily. Solids have their advantages, but most are outweighed by the comparative handling difficulties.
The characteristics of solids are often most problematic in areas that depend on measuring the amount of product for use as a process feedstock, and for general inventory management. Both process engineers and accountants have their own reasons for wanting accurate measurements. So, what is the best way to measure feedstock quantities, both solids and liquids, taking stochiometric and other quality attributes into consideration?
In most cases, liquids are measured as volumes and solids by weight. These are often the default as a trip to the grocery store will verify: milk is sold in gallons or liters and flour by the pound or kilogram. Why is this the case? For liquids, volume is simply easier. In the kitchen a cook will pour the liquid into the measuring cup and get a consistent reading in fluid ounces, cups or milliliters.
Solids, on the other hand, are complex. Brown sugar is sticky and forms clumps, so cookbooks specify that it has to be packed to eliminate porosity for a volume measurement (Figure 1). Granulated sugar needs no packing and can get scooped directly into the measuring cup. Flour needs to be sifted into the measuring cup to avoid packing otherwise the result could be more flour than the recipe is calling for. All three of these would then be leveled off at the top of the cup.
Some countries, like the UK, instead use weight measurements for powders or solids in recipes. Getting a consistent measurement for solids using weight when baking a cake may definitely be advantageous, although measuring a cup of flour is certainly much simpler than using a kitchen scale, especially when the recipe is calling for a volume measurement. Converting from a volume measurement to a weight is complex as it will not be the same conversion rate for flour, brown sugar, granulated sugar, or many other powdered ingredients.
So how do we apply these kitchen concepts to a process manufacturing environment? Are there reasons why we might want to measure liquids by weight and solids by volume? The complexity in a processing plant is much greater than in a kitchen, so the answer may be yes to both depending on the circumstances. Let’s look at some possibilities.
Reversing the Measurement Method
There are situations where variability of density of a liquid can make a volume reading less valid. For example, if an oil company is purchasing crude oil at a well site, it may accept a given volume, but a temperature reading is also required to correct for density for a more accurate measurement of energy content. In these instances, measuring weight using a mass flow instrument such as a Coriolis or ultrasonic flow meter may be a simpler solution.
The comparisons with liquids are instructive, but readers of this publication are understandably more concerned about solids, so let’s think about them in greater detail. Is it valid and practical to apply the same measurement methods used with liquids for solids?
Measuring Bulk Solids
If solids are usually sold by weight, what are the normal measurement mechanisms when working at an industrial scale?
If the objective is determining how much material is stored in a given vessel by weight, one approach uses load cells under the supports, weighing the vessel and contents together. If it has six legs, six load cells are required, with some mechanism to add the readings and deduct the weight of the vessel. If material is withdrawn for use in the process or refilled with new feedstock, it is easy to see the amount as a weight. What this does not indicate is the volume in the vessel.
Say for example a silo is designed to hold five tons of a given solid product. If the load cells show that it is currently holding four tons, an operator might reasonably expect to be able to add one more ton. However, if the product is like brown sugar and doesn’t tend to pack itself, the silo may be fuller than expected, and adding the additional product would result in an overflow. Moreover, with a very large vessel, if any individual load cells are out of calibration or malfunctioning, the total weight can lose accuracy.
Measuring how much solid feedstock is being added to a process might employ a conveyor scale capable of measuring a flow rate (e.g., pounds per minute), or a totalized value just as a flow meter does for a liquid. Such systems work but can be maintenance intensive and often require frequent calibration. If the supply vessel is outfitted with load cells, it is also possible to watch the changing weight measurement as an indicator.
But, none of these methods provides the volume remaining inside the vessel beyond inference based on how much has been removed for the process. Nor do they indicate if there are areas within the vessel where poor design causes normal filling and emptying actions to leave dead product that does not move readily, potentially remaining beyond its normal shelf life.
Applying Liquid Level Technologies to Solids
One approach used commonly with liquids for determining how much has been removed from or added to a vessel is recording level readings before and after, and then calculating volume based on the dimensions. There is a wide variety of level measuring technologies suitable for liquids, many of which have a high degree of precision. Measuring the level of solids in a vessel can use many of the same technologies, including non-contact radar, guided-wave radar, nuclear, level switches, and others (Figure 2).
These work to some extent for solids. Unlike the cup measurements in the kitchen, however, that can easily be leveled off for consistent volume measurement, the tops of powders or bulk solids in a vessel are quite inconsistent. These technologies all share the same critical limitation: They provide a reading in only one spot on the surface and therefore can’t account for the peaks and valleys that form with solids. Most every solid product has an angle of repose greater than zero, so there are virtually always surface formations. Mechanisms such as thumpers mounted on the vessel walls can assist in breaking up large chunks, and additives such as silicon dioxide can assist movement, but few products spread evenly in the vessel. A spot reading may be sufficient if accuracy is not critical, but a different approach is necessary for a more precise reading, especially in larger vessels.
A 3D scanner using acoustic phased-array antennas (Figure 3) capable of measuring surface features is the only approach able to deliver a high degree of precision for solids level measurements. With the right vessel information loaded into the device, the level is converted into a highly-accurate volume reading (Figure 4). But is this sufficient to solve the measurement requirements of process applications and inventory management? Like many engineering questions, the answer is, it depends.
There is no question that such a 3D scanner can create a high-precision representation of the surface in the right vessel, regardless of the jaggedness of the surface features. Where product density is understood, a weight value can be inferred.
For process applications where the tanks are typically smaller than two meters in diameter, there is less surface variation which makes it more suitable for a spot measurement. Additionally, the 3D scanner’s sight to the surface starts getting limited by the vessel walls. Larger vessels, which are more typically focused on inventory management, are much better suited to the acoustic phased array antenna technology and vessel management software has been developed to work with them which has an assortment of options designed to help with inventory management needs.
An article published in the September 2018 issue of this magazine (https://www.powderbulksolids.com/article/Creating-a-Detailed-and-Accurate-Picture-of-Solids-Storage-09-12-2018) included a sidebar about a company that adopted the volumetric approach. It found the inventory picture created using a 3D scanner to monitor consumption of a free-flowing granular feedstock through changes of level in the storage silo was more precise than its conveyor scales. Plus, it provided an accurate indication of how much space was available in the storage tanks, making it possible to maximize storage capacity.
This works because data from the scanner can provide a continuous rolling measurement of inventory, which is a key requirement to prevent over- or under-purchasing of feedstocks. Additionally, it helps avoid the complications that can occur when ordering product based on inaccurate inventory levels.
Efficient inventory management allows companies to have the right amount of stock in the right place at the right time, ensuring that capital is not tied up unnecessarily.
Users who upgrade from single-spot level measurement devices to acoustic 3D scanners eventually gain more confidence in the inventory readings, allowing them to reduce safety stock without increasing the risk of running out of product. Most users report reductions between eight and 13 percent of on-hand inventory. Based on an annual inventory carrying cost of between 25 and 52 percent, this translates into significant savings. Acoustic phased array antenna measurement quickly pays for itself based on the reduction of annual carrying costs, typically within a year.
Being able to determine precisely bulk solids inventory levels is a vital and useful tool in ensuring accurate financial reporting. However, as discussed, measuring bulk solids volume is complex due to the uneven and shifting nature of the material surface. Therefore, it is essential to select a measurement technology appropriate for the specific application to overcome these challenges and still provide the necessary level of accuracy. 3D solids scanners remove the guesswork from volume measurement by providing a practical and cost-effective solution.
Lydia Miller is a product manager with Emerson Automation Solutions, working with Rosemount level measurement products, with a focus on radar and ultrasonic instruments and level switches. She joined the company in 2011 and has additional work experience with air-to-air energy recovery for process industries and HVAC applications. Lydia has a bachelor’s degree in mechanical engineering and English from the University of Minnesota. For more information, visit www.emerson.com.
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