Designing Mixing Systems

A mixing system consists of storage, measuring of ingredients, mixing, and delivering the product to the next operation in a useful form and meet the production required.   
    
It is better to divide the dry ingredients into three categories:  bulk in either silos or bulk bags; minors that are used in small full or partial bags, and micros –- let’s say less than 10 lb, and look at the ways they can be handled and timing.
    
Next we need to know the production required, time to be worked, and the method and time required for quality control (QC) checks, and the quality required.  
    
The quality required relates to the serving size, and how accurate the mixing has to be to meet that requirement, and how much QC is going to be checking. For example, if there has to be one to five samples from the mixer checked every batch, this time has to be allowed in the cycle time, which affects the size of equipment. When this is understood, as well as handling characteristics, it is time to do some calculating. Handling of bulk ingredients is never an exact science and there needs to be a design factor that makes sure the production per hour or day is met. We use 20%. This takes into account accuracy of calculations plus normal delays such as shortage of ingredients or bags. If you want 12 tn/hr, we would design on 15 tons. Most times we start up at design
    
The first cycle always takes more time and that is for the loading of weigh hoppers and getting other ingredients in place. Experience can put us in the ballpark and then it is a balancing act.  
    
Here is an example using a vacuum infusion mixer for coating of pet food. Experience would have us start with a cycle that looks like the one in Table 1.

At this point we have a theoretical cycle of 5.5 minutes and probably round to 6 minutes. This is 10 cycles per hour. If the volume is 12 tn/hr/0.8, the vessel has to hold 1.5 tons. If the bulk density is 25 lb/cu ft, this would be a 120-cu-ft vessel, then go to the nearest standard. With this size of vessel, we would go back and check the sizes of the holding hopper, liquid piping sizing, and valve sizes to see if it is possible to achieve our assumptions.
    
In this example the sizing is critical because the extruder will not wait. For it to produce efficiently, it has to work continuously. During the checking of each item affecting production, we may have to do some tweaking to make sure we meet the time. We know we can’t weigh too quickly because the scales just won’t gain fast enough to prevent an over shoot. But if we increase the size of the vacuum pump, we can gain time without adverse effect.  
    
The good aspect from a design point of view is that all items are automated and we have control. All we have to do is define a proper control system.
    
Let’s look at another example of dry cement mixing where there are hand-adds, and also that the job was in Asia and the sand arrived by dump truck. The sand may be screened on site and transported to the mixing system in bulk bags or front end loader. This adds another challenge. If the customer wants 100 tons per day, typically 60 tons will be sand. We will handle the sand by filling a feed hopper at ground level, to a bucket elevator and a holding hopper on load cells above the mixer.  
    
The time for hand-adds is a variable, depending on the recipe and number of workers you can use. Furthermore, it is a time that the end user has to estimate. Again with experience we start with an assumed cycle. In this case we would have weigh hoppers or holding hopper above the mixer. It is important to know about QC (see Table 2).

Cycle is 8.5 minutes; seven cycles per hour.  

Now we do the same exercise of sizing the vessel, and going back to check if we can get each segment done. Looking at available time, we have six minutes to move the sand and note we are dependent on getting the feed hopper manually filled. The Portland, calcium carbonate, and fly ash would be in silos and we can sequentially screw into a weigh hopper and we have 7.5 minutes to do that.
    
The QC allowance in this case may be to check the first batch, and the probability sample every hour, without stopping production.  
    
The design should deliberately make the bagging the bottleneck by make the rest of the system 20% faster. But if we get a tough recipe, working overtime is an option.  
    
In this system, we also have to look at dust collection and the effect it will have on our scales. We may have to use blast gates to open to atmosphere to get accurate weighments.
    
When all this fits, then we can design the control system.                                                              

A. B. (Bliss) Flower is president, A&J Mixing Company, Inc., a div. of Smico Manufacturing Inc. For more information, visit www.ajmixing.com.

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