Back to Basics
When it comes to maintaining product homogeneity, sometimes the simpler the mixing system is, the better
Keeping a tank in service as much as possible is at the top of the priority list. Mixing and blending requirements vary widely from product to product but, sometimes, simpler solutions can be as effective as the fanciest mechanical engineering. The less there is to go wrong, the less will go wrong.
Applications like jet mixing or bubble mixing might still be niche but, typically involving no in-tank moving parts, they have a whole range of benefits. Most notably they are relatively simple compared with the conventional side- or topentry arm and impeller arrangements used for many applications in process industries. The old adage time is money rings ever more true, as operators strive for greater competitiveness and commercial success. No one wants to drain a tank due to a loose bolt or valve, let alone spend thousands on extensive cleaning of sludge from the bottom of a tank containing product that should have been kept moving.
Jet mixing certainly has its limitations; it is usually better suited to, and more cost-effective in, larger tanks such as those designed to store crude, for example. But for tank owners looking to all but shelve those maintenance logs, reduce the need for man entry, and drastically reduce the need for spare parts, jet mixing or bubble mixing may be the answer.
According to Aidan Cumiskey, MD of Monsal Mixing, based in Mansfield, UK, and a major player in the wastewater and sludge management sector, concerns over health and safety led to a switch from conventional arm and impeller arrangements to jet or air mixing several years ago ¿ in MonsalÍs case using re-circulation. Cumiskey says the sector has seen a vast reduction in mechanical breakdowns as a result, pointing out that markets such as biomass could find jet and air mixing beneficial in comparison with conventional mixer arms.
ïThere was a lot of mechanisation in the wastewater sector until a few years ago, when concerns over access and maintenance led people to move over to non-moving parts and with all equipment outside the tank,Í Cumiskey says. ïThis led us to develop jet mixing for use in the tanks and this has now become a trend in the industry.Í
Cumiskey says the company is particularly interested in exploring applications for jet and air mixing in the growing biofuels sector, not least because of the similarities in the make-up and organic nature of biomass, biogas and wastewater. He cites ragging, whereby materials from fibrous liquids such as those commonly handled in the wastewater sector, were becoming snagged in impellers and leading to mixer malfunction and tank breakdown. Jet mixing helps to minimise this, due to the lack of a need for equipment such as impeller paddles and the use of large nozzles that deter blockage.
Jet mixing systems, including those supplied by Monsal, commonly use low energy gas compressors and boast quick blend times, between 90 minutes to two hours in MonsalÍs case. Larger tanks require more points of influence but the blend time is maintained, Cumiskey says.
Points of influence can also be added to the tank walls as well as to the tank bottom to take account of liquid height and still maintain blend rate. The larger the tank, the more likely it is that it contains shipments from several vessels, and mixing to avoid separation and sludge settlement at the bottom is crucial when it comes to integrating different density batches. Having to drain down any tank because of mechanical failure inside is highly undesirable for any tank operator. The bigger the tank the greater the disruption, so the nonintrusive nature of methods such as jet mixing is key. GermanyÍs GEA Wiegand, headquartered in Ettlingen just south of Frankfurt, provided such a system to a northwest European client looking for a non-intrusive mixing application for a very large (80,000m3) crude tank (80 metres in diameter and 20 metres high). The customer had previously used conventional shaft-driven mixer arms but found them to be lacking in the blend times achieved, and rejected air mixing due to concerns over contamination. The customer was not expecting to open the tank for 10-15 years following installation so a maintenance free system was vital.
