New Insights into Dense Phase Pneumatic Conveying

Mark Jones, Chair, Bulk Solids Handling, University of Newcastle, Australia

Pneumatic conveying is widely used in the process industry, particularly for in-plant transport over relatively short distances. This is not surprising as it has many advantages over mechanical conveying systems for these types of applications. The enclosed nature of the pipeline protects against dust generation in the working environment, as well as protecting the product to be conveyed from various types of contamination. In addition, the flexibility in pipeline routing is a particular advantage as this minimizes the space required and allows the pipeline to follow the geometry of the factory or workspace. Of course, the geometry of the pipeline must be taken into account at the design stage as pipeline routing (particularly the number and geometry of bends in the system) will have a significant impact on system performance in terms of throughput for a given pipe size, pressure drop and air flow rate.

These advantages have been recognized over many years given the prevalence of these systems in industry. The downside, of course, is the specific energy consumption of these systems, which is rather high compared with many mechanical systems. Energy consumption is particularly topical at present given the global discussion around the transformation of electricity production from fossil fuels to renewable energy sources. This gives rise to discussion of what the future might look like. In the long term there is a good chance that abundant low-cost energy may be available but, in the meantime, there are significant issues to be addressed. Storage such as battery technology--while developing at a pace--is still a long way from where it needs to be, and other forms of storage have long lead times and potentially high capital expenditure requirements. Green hydrogen certainly has potential and may be a work around storage needs. In addition, investment in smart grids to stabilize electricity distribution is another big technology challenge. These challenges are certainly not insurmountable, and much of the technology is already available. The real challenge is the scale of transformation required, the cost to implement it, and the time frame in which it needs to happen to meet net zero carbon emission commitments.

Hence, returning to our humble pneumatic conveying systems, the long-term future is bright. Clean power may be readily available. However, in the short term a focus on minimizing power consumption remains an important goal and one which can be achieved by optimizing our conveying systems. Smart system control is now so much easier than in the past and is delivering on this issue. However, the reliability of our conveying systems is still highly dependent on the properties of the materials we handle. So, no matter how intelligent our control systems are it is the inherent behavior of the materials we are handling that become the rate limiting steps for our designs and the reliability of our systems.

The ability to convey at low velocity to protect the material to be conveyed from attrition and particle breakage is often vital. In other cases, reducing the conveying velocities is important to minimize wear of the pipeline and bends when conveying erosive materials. In many cases, reducing the conveying velocity will reduce the energy requirement and often minimize the specific energy of conveying.

Understanding the modes of conveying is, therefore, increasingly important. We conveniently describe dense phase conveying as having two major modes of flow: slug (or plug) flow and fluidized or moving bed-type flow. These categorizations are useful in practical terms, but may not tell the full story. Recent research at the University of Newcastle led by Dr. Ognjen Orozovic in collaboration with Professor George Klinzing at the University of Pittsburgh suggests that these two modes of conveying may in fact be two incarnations of the same physics, which opens the prospect of a universal model for dense phase flow. This is an exciting development, so watch this space!

Professor Mark Jones holds the Chair in Bulk Solids Handling at the University of Newcastle, Australia. He is also the Director of the Centre for Bulk Solids and Particulate Technologies and Director of TUNRA Bulk Solids Handling Research Associates. He has worked in the field for more than 35 years, researching and working with industry worldwide.