The dearth of instrumentation typically built into a process design involving particulate solids can lead to major challenges in effectively working with the materials. While cost certainly plays a role in limiting the amount of instrumentation, most often the primary factor is lack of understanding how solids behave differently than pure liquids or gases. For solids, measurement of physical properties and process conditions frequently is much more difficult and instrument placement more critical.
With suspended solids, restriction of the flow (orifice plates and cone meters) and attempts at measuring pressure drop are major problems. Also, suspended solids don’t travel through piping at the same velocity as the fluid and have a tendency to settle. Even fine particles, which may move at close to the fluid velocity, can bunch up due to turbulence, making them behave like much larger particles. Fortunately, important advances have occurred in instrumentation for particulate solids, whether in slurries or suspended in gas.
Surprisingly, simplicity often is better when dealing with solids. When I was teaching graduate students, I asked a class at the start of a slurry pipeline experiment how we were going to calibrate our flow meter. After discussing several alternatives, one student sheepishly noted that in his home country the only option was to use a bucket, stopwatch and scale to establish flow rate. Advances have made suspended solids measurements more reliable, primarily due to non-intrusive or non-restrictive sensors and the durability of these devices.
Four key measurements usually are important when handling solids. So, let’s look at them and some of the best devices for getting readings.
Volumetric flow rate. My favorite inexpensive device is still the old-fashioned Pitot tube. You must place it in the top of any pipe and purge it to prevent solids from accumulating and plugging the S-type probe. You may have to resort to occasional bursts of purging fluid to dislodge any deposits. A Pitot tube will give fluid velocities but works best at low solids concentrations, where settling is unlikely. Ultrasonic instruments can handle a wide range of concentrations: Doppler devices give the particle velocity, while transit-time devices provide an average flow rate but work best at low solids concentrations; both can be clamped on slurry pipelines. Electromagnetic flow meters suit applications with turbulent flow where turndown is limited and, when properly, calibrated, are very accurate; some can be cleaned in-situ.
Mass flow rate. You can convert volumetric flow rate to mass flow rate with the appropriate physical properties; you may have to make do with estimates for these. Coriolis meters avoid this issue and often can be calibrated on a pure fluid such as water. Selection of the tube size is critical and you must consider vibration when placing the unit. Thermal flow meters are inexpensive and work best when the specific heat of the fluid and solids are the same; these devices can foul and mostly suit low solids concentrations. For bulk solids, you can use impact plates on discharge points. When feeding bulk solids, I suggest loss-in-weight designs.
Density. Vibrating tubes (which can be straight, a U-tube design or a probe) give bulk density that can be correlated to moisture content of a slurry when the particle density is well known. Particle size affects measurement very little but low flow rate gives slow response. Ultrasonic devices provide a rapid response to concentration. X-ray or -ray are best suited to concentrations above 5%. Because they are very expensive, electrochemical devices that give 3D concentrations via tomography seldom are used.
Level. Load cells are inexpensive and very accurate when calibrated for the density of the slurry system. They don’t give the best indication of level for bulk solids due to the fluidity of the solids. Advances in the last few years in ultrasonic and radio-frequency echo systems have eliminated this issue. These devices are non-invasive and unaffected by deposits. For slurries, bubblers are inexpensive but gas can cause fouling. Restrict the use of capacitive instruments to upper limit alarms and conductive materials.
These four measurements usually suffice to control your solids processing systems. Leaving them out of a design likely will cause trouble for years to come.
TOM BLACKWOOD is a Chemical Processing contributing editor. You can email him at TBlackwood@putman.net.