Fluid Energy manufactures three (3) complete lines of jet mills (Jet-O-Mizer, MicroJet, RotoJet) and a line of flash driers (ThermaJet). This equipment is used in the production of fine powders in industries ranging from food/pharmaceuticals to specialty chemicals, plastic fillers, additives and even propellants.
Such an expansive range of applications requires flexible testing protocols to establish feasibility through optimization. Whether dealing with a current customer looking to improve an existing material or a new customer with an unknown material, the Fluid Energy Test Center, in coordination with our technical department, work to efficiently and effectively determine the optimal milling and/or drying parameters.
Customers present their applications in several ways. Requests can be as simple as “grind as fine as possible” or “bone dry” to specifying a very select particle size distributions, rates and/or moisture levels.
Our initial step involves reviewing the request and comparing it our previous experiences. In addition to scanning our databases for previous history, Fluid Energy engineers and technicians apply their 75+ years of combined experience in the jet milling and flash drying field towards a solution.
The history allows us to use a proven method to quickly assess products that have been tested previously, while the practical experience allows us to “think outside the box” when needed.
The foundation of our new application methodology is our Test Center. Located in Telford, PA, the Fluid Energy Test Center operates lab and pilot size Jet-O-Mizer, MicroJets, and RotoJets. We also have lab and pilot size ThermaJet systems for flash drying trials. In addition to the specific Fluid Energy equipment, the Test Center is stocked with support equipment used to pre-condition feeds that are too large or too wet for our equipment, multiple collection devices, multiple heating sources and even inert gases capabilities for the safe processing of volatile materials.
Some trials last ½ day, while others run the better part of week. We often start our testing one mill or system but end up sliding over to an alternate piece of equipment. There is a lot of trial and error with jet milling and flash drying testing. Our experience and data base streamline that approach. Here are a few applications that demonstrate the problems and eventual solutions developed in our Test Center:
Suspension Drying Application
The Fluid Energy sales department was contacted by a customer that had a wet suspension with the consistency of pudding and needed a dry powder less than 3% moisture. Mechanical dewatering could only get the material to the “pudding -like” phase and it was heat sensitive. After reviewing the test data sheet supplied by the customer, we felt we could get enough heat into the product, even with the temperature limitations, but the feed was not in a form that could be metered consistently into a ThermaJet flash dryer.
Pre-conditioning the feed was our top priority. Back mixing is a method we use to mix previous dried material in with wet feed to make a “feed-able” consistency. Since we only had drums of wet material, the Test Center devised a method of manually air drying a portion of the wet feed, then de-lumping it so it could be used as seed material for the back mixer. Once enough dry back was created, the wet feed was metered into the back mixer using a positive displacement pump and mixed with the seed material generating the necessary consistency for entry into our pilot size ThermaJet.
With the back-mix loop functional, the operators performed a series of trials eventually dialing in the optimum conditions. The data and test method were incorporated into the design of a production system.
Moisture Sensitive Micronizing
With the reduction in particle size created by fluid energy jet mills, the physical characteristics of the material can change. Some materials change color, some become explosive, while many experience a dramatic shift in bulk density. We must be aware of the potential for these types of changes and design material testing to accommodate the property changes safely.
An application that required a testing redesign occurred when we were asked to micronize an extremely moisture sensitive material. The feed samples arrived in granular form, which was not sensitive to ambient moisture conditions. From previous experience we anticipated moisture pick up was going to be an issue, but we were surprised at the level and speed at which it occurred. Initial testing used compressed air passed through a desiccant air dryer. After a few adjustments, the jet mill was able to meet the particle size specification; however, the sample solidified within minutes of processing.
The decision was made to move from compressed air to compressed nitrogen. Using nitrogen, the milling worked well but the samples failed to stay in a deagglomerated form. Apparently, the stray air entering the mill inlet with the feed was providing enough ambient moisture for the superfine powder to absorb. The solution required us to mill, collect and package the samples under a nitrogen atmosphere. As system was designed in the test center that allow all facets of the process to be conducted under nitrogen and the critical material handling phases occur inside a nitrogen flooded glove box. This arrangement has been used for other application ranging oxygen sensitive to pyrophoric feed stocks.
When more the one component of a blend needs to be jet milled, we have two choices: (1) mill all components separately then blend or (2) blend first then mill. Fluid Energy was approached by a customer with a proven product but aging process equipment. In fact, the equipment being used to produce their specific blend of micronized materials was being recommissioned for other use and would no longer be available. The old blend process was not a jet milling process, so the customer’s request left the Test Center with an empty canvas to work with.
As with many applications, processing costs were being squeezed, so the goal was to match the current specification and performance with the least number of steps. The operators performed a series of trials on each component to determine individual grind-abilities. Then they combined raw materials with similar grind rates and ran combination milling trials to dial in the minimum steps needed to create the blend. Since the customer had several blend recipes and particle size requirements, multiple trials were required. Each step was documented and reviewed. One by one the Test Center was able to dial in the conditions, creating a replacement process with good economics.
The Test Center received a request to process a large batch of wet material that was a base material coated with a polymer. A review of the data sheets showed a slight level of heat sensitivity but not enough to cause concern. The test protocol called for a series of trials where the wet feed rate and temperatures would be adjusted to find the optimum throughput conditions.
After the first run was completed, the samples were analyzed for moisture, color, and consistency. During the visual inspection of the sample, flakes were found in the dried material. The final product specification called for a powder, so the flakes were unacceptable. Fearing melting was the cause, all operating temperatures were lowered but the flakes remained. The system was broken down and inspected.
The flakes were traced to the two rotary valves used to meter material in and out of the system. Some of polymer was being peeled off the base material by the blades of the rotary valve and extruded into a flake as the valves turned. Once the operators recognized the problem; the rotary valves were removed, and the system was balanced using an alternate method. By eliminating the pinch point problem caused by the rotary valves, the flake problem was eliminated allowing the optimization trial to continue. All subsequent systems drying this polymer blend follow the same “pinch less” design.
These are just a few examples of troubleshooting unconventional milling and drying processes. If your application could use this type of optimization, use this link to contact Fluid Energy Processing and Equipment.
1. David Cook, David B. Hastie, Tom Hicks and Peter W. Wypych. A Novel Approach to Rotary Valve Venting. Centre for Bulk Solids and Particulate Technologies, University of Wollongong, Northfields Avenue, Wollongong NSW 2522, Australia