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Home » July Process Puzzler: Prevent the burn-out of a mixer

July Process Puzzler: Prevent the burn-out of a mixer

ChemicalProcessing.com

Keywords: mixer

Readers suggest quick solutions to a solid-liquid mixing problem in Chemical Processing's monthly Process Puzzler feature.

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Question from May's Chemical Processing

The research department is introducing a new color additive. This additive is blended with a 2-hp high-sheer mixer prior to addition to a weigh-cell feeder (Figure 1). The feeder drains to a soap plodder (mixer) and eventually product is extruded as bar soap. The specific gravity of the new liquid is 20% higher and viscosity is 10-times greater than the additive used in the past. Calculations show that the power draw by the high-shear agitator will exceed the motor limits. Because the new product will roll-out in a few weeks, we need a short-term solution — quick. What alternatives will allow production to continue?

Click to enlarge Figure 1

Add a portable mixer

Consider mixing with a portable mixer with a marine impeller. This type of impeller produces more flow than a high shear mixer — with less horsepower. An alternative might be to feed the high shear mixer with the portable mixer. At our plant, we had similar problems. Where we could not resolve the problem by modifying the process, we decreased the size of the blade by cutting some of the metal off the blades of a turbine impeller. This decreased the load on the motor. This must be done carefully to avoid a torque problem from an unbalanced impeller. We added re-circulating lines in our tanks, so we were able to eliminate dead spots on corners of the tank. Generally, soap-making has heat available (steam or electric) at hand so heating the tank might help resolve the problem. It might also be worth looking at heating the concentrated colorant, if it is in liquid form. Eventually, a different mixer should be considered.
Werner Buchmann, chief chemist
ProClean of Arizona, Inc. Phoenix, Ariz.

Reduce the mixer speed

Borrow a harmonic drive system, if available, to reduce the motor speed. High shear mixers are typically direct-drive, without a gear reduction. Reduce the speed since the power required by the agitator is related to the speed by a cubic function (P = 6.566 × 10^-14 (N_pN³D⁵r) reducing the speed by 20% will decrease the power draw by more than 50%. (Np is the manufacturer’s power number, N is the speed in rpm, D is the impeller diameter in inches and r is the solvent specific gravity.) Reducing the speed will have the less effect on the mixing time than reducing the diameter because the flow will be only slightly affected (Q = ND³N_q/231). (N_q is the impeller flow number and Q is the pumped flow in gpm.) The shearing action will be decreased, however by reducing the speed. The only problem may be the mounting plate and shafts, which might be difficult to machine in time. Another idea is to relocate the agitator off-center. This will reduce the power draw.
Lyle Bleile, design engineer
Amerock Corporation, Rockford, Ill.

Try a three-prong approach

First, have research conduct some experiments on the final color solution which includes the new additive. If the additive is non-Newtonian, the color solution may be shear- thinning. Ideally, the result of the experiments should be a shear rate versus viscosity curve at the shear that the additive will be subjected to in the high shear mixer. It will be difficult to be exact on the shear. Talk to the manufacturer about tip speed. Ask them to look at ways to reduce the viscosity and density in the solvent.

Second, assuming non-Newtonian characteristics, consider processing at a higher temperature. Viscosity is a function of temperature and is often inversely proportional to temperature. Have research provide a shear rate vs. viscosity curve as a function of different temperatures and re-calculate power draw requirements.

Third, look at ways to reduce the feed rate of the new colorant without severely affecting mixing time or labor costs.
Mohammad Babar, principal systems engineer
Unilever HPC, Jefferson City, Mo.

Use a pump

Presumably the new color additive is miscible with the soap mix, just more viscous and denser than before. We need to ensure that the color mix doesn’t settle, and that we can thoroughly disperse it. I suggest try adding by pump. Be aware that the pumping rate of a pump will never match an agitator. Install a stainless steel dip tube (It must be rigid!) adjacent to the inlet area of the high shear mixing head. The exact position will depend on the type of high shear head. The feed rate also will be important. Ensure that the rate isn’t too fast or the color material won’t be entrained into the high shear head with sufficient solvent (water). This is already done with some high shear mixers. With some manufacturers, the tube can be fixed with stainless steel fittings to one of the support rods and the outlet from the dip tube aligned so that the tube outlet is level with the inner edge of the lower support ring of the mixing head. This set-up could be more difficult for some manufacturers. The R&D laboratory could test this working scheme if they have the equipment. Obviously speeds and throughput calculations would need to be checked. They would need to check the relative feed rate of the color additive that ensures it is adequately dispersed. If done correctly, the lab phase should take about a day. Obtaining the tube, etc, and installing might take three days. The stainless steel tube would need to be angled appropriately for best results. Then, a trial run at scale would be required.