Batch mixing versus continuous mixing

Question from a customer: What is the biggest challenge in continuous mixing of bulk materials?

When high mixing qualities are desired, continuous dosing of the powders is the biggest challenge. All mixing ingredients must continuously enter the mixer in the correct ratio. Unlike liquids, bulk solids are much more difficult to dose. Dosing errors always lead to an incorrect mixing result.

Nevertheless, continuously operating mixing systems have become established since precise powder feeders became available. Modern metering systems for bulk solids detect even small metering errors and correct them very quickly. The mass flows are accurate even at short time intervals.

In terms of mixing quality, gravimetric feeders are to be preferred. They are also called "loss-in-weight feeders". In general, the better the bulk material flows, the more accurate continuous feeders can be.

Depending on the type of continuous mixing, residence time spectra can be defined. Continuous mixers with large residence time spectra are all the better able to compensate for dosing errors that occur for a short time.

The special features of continuous mixing in the form of classic "tube flow" and classic "boiler flow" are described elsewhere in this amixon® blog.

In order not to influence the weighing signal, the inlet and outlet nozzles are connected with folding sleeves. Inside the housing, a rotating raking tool ensures that the powder is loosened evenly. The metering screw thus receives a loosened product. This favours the control behaviour. The filling level in the weighing hopper decreases continuously. In the example shown, the weighing hopper is filled pneumatically. The filling time should be as short as possible. During this time, the dosing screw delivers uncontrolled at a constant speed. In practice, such a "blind phase" can be accepted.

Question from a customer: Mixing companies have different mixing requirements depending on the product and industry. What are the criteria to be considered when deciding between continuous mixing and batch mixing?

A continuous mixing process is advantageous,

• if a defined short residence time is required
• if desagglomeration is to take place in a short time
• if the mix agglomerates spontaneously
• when the addition of liquid triggers a spontaneous chemical reaction
• when the mixing result is determined by the exact mechanical energy input
• when exothermic reactions take place and large amounts of heat have to be dissipated
• when the final product flows poorly (for example, like a highly viscous paste or cream)
• when the final product consists of only a few components
• when the recipe is standardised and the quality of the components is assured
• if large quantities of the same or similar products are produced, possibly in three shifts (bulk goods such as staple foods in grain mills, starch, animal feed, basic chemicals, metal soaps, washing powder, cellulose derivatives, semi-solid goods, plastics, building materials)
• if the mixed goods are to be packed directly without intermediate storage
• if .....

A batch mixing operation is advantageous,

• when complex preparation processes are performed, e.g. multistep mixing processes, or mixing processes with gaugepressure or vacuum application take place
• if a laboratory analysis must take place before packaging
• if quality assurance insists on batch control, and
• when cleaning must be performed after each batch
• if many components are involved. This is the case with baby food, dietary nutrients, baking agents, spice preparations, stabilizers, instant food, seasonings, ....
• when customized orders with ever changing recipes are involved (food supplements, spice preparations for meat companies, flavors for the food industry, instant meals, instant beverages,...)