Continuous Mixing

In the preceding article the concepts of batch mixing and continuous mixing were introduced. Batch mixing, its applications, advantages and limitations were highlighted.

The present article focuses on continuous mixing process. In continuous mixing, material flows steadily from an upstream process into the mixer, is retained in the mixing vessel for a specified mixing time, and discharges at the same flow rate for downstream handling. For some applications, continuous mixing has distinct advantages over batch mixing. For example, mixing in batch often leads to variation in the mixing quality amongst batches. This variation can be controlled or eliminated in continuous mixing. For higher processing capacities, continuous mixers are more compact than batch mixers. The applications, advantages, disadvantages of continuous mixing operation are detailed in the following sections.

Continuous mixing is used to mix ingredients continuously in a mixer in a single pass. In a continuous mixing process, the weighing, loading, mixing, and discharge steps occur continuously and simultaneously.

The materials to be mixed are continuously charged into the mixer as per the formulation. The process of charging the material in a continuous mixer is extremely critical and can significantly affect the quality of the final mix. Radial and axial mixing takes place as the material travels from the feeding point to the discharge point. The time taken by the material to travel from the feeding point to the discharge point is known as the retention time of the material in the mixer. Unlike batch mixers where product retention time is carefully controlled, with continuous mixers, material retention time is not uniform and can be directly affected by mixer speed, feed rate, mixer geometry, and the design of mixer internals. Material is continuously discharged at a constant rate which is generally termed as the capacity of the continuous mixer. This capacity is measured in kg/hr of mixed product. To keep track of the mix quality, a well defined sampling and material testing procedure is to be defined and observed. While discharging from the mixers, segregation can be reduced by positioning the discharge closer to the packing units or as an integral part of it.

The construction of continuous mixers requires that the charging of materials is from one end of the mixer and discharge from the opposite end. Adequate residence time within the mixer is required for homogenous mixing of the material. This is achieved through controlled movement of material from the feeding point to the discharge point. This movement is caused because of the rotation of mixer vessel as in the case of a Zig-Zag continuous mixer or by rotating mixing elements that result in motion of material in the direction of discharge, like in a continuous paddle mixer. Material handling equipment and instruments like feeders, rotary valves, belt scales, etc. are provided at the feeding point of the mixers. A schematic diagram of a continuous mixing system is shown in figure 1.

Batch mixers can also be used in a continuous production line using the following arrangements:

• Multiple batch mixers can be sequenced to handle continuous flow of material from upstream processes.
• A hopper can be placed in between the continuous line and the batch mixer. The hopper shall discharge the material and mix in batches.
• Rapid mixers can be utilized for fast mixing without causing lengthy downtime for material handling.

Continuous mixing is preferred for applications where:

• Large quantities of a single product are to be mixed.
• In a continuous process line requiring high production rate.
• Strict batch integrity is not critical.
• Smoothing out batch product variations is required.

The advantages of the continuous mixing operation, continuous mixer is as follows:

• High Capacity - Compared to batch type mixers, continuous mixers of smaller volumes and power can be used to produce large quantities of uniform mix. Hence for a given capacity they are more compact than batch mixers.
• Lower Mixing Time - The residence time in continuous mixers is lower than in batch mixers.
• Consistent Mixing Performance – With proper feeding arrangements, online instrumentation and operation controls, a consistent mixing performance and uniform product quality can be achieved.
• Suitability for Automatic Control - Operation of continuous mixers can be automated using online monitoring and measuring instruments.
• Minimum Segregation – Continuous mixers can reduce and control segregation of products as they can be located in proximity of the next processing station.
• Lower Cost of Mixers - Continuous mixers tend to be cheaper than the equivalent batch mixers because they are compact and require less space. However the cost of feeders for metering the product into the mixer, instrumentation and control may result in a higher overall cost of the system.
• Minimum Labor – Since material feeding and discharging processes are automated, minimal labor is required for continuous mixing.

• Lack of Flexibility – Continuous mixing systems are designed for a particular application and cannot be easily tailored to mix different formulations. Even if a new ingredient is to be introduced, it calls for a change in the protocol, and the system has to be recalibrated.
• Component Limitations – When a large number of ingredients are to be added, continuous mixers have limitations with respect to mixing uniformity when compared to batch mixers.
• Higher Overall Maintenance Cost – Continuous mixers heavily depend on feeders, instrumentation and online control systems. Failure, malfunction in any one component can lead to complete stoppage. Hence, overall maintenance costs for continuous mixers are higher compared to batch mixers.
• Calibration and Checking – The feeding devices in a continuous mixing require careful calibration and frequent checking for accuracy.
• Critical Applications – Continuous mixers are not suited for critical applications where product formulations need to be exact. Batch mixers are better suited to processes that require a very tight product formulation, uniform composition.

Continuous mixers are generally dedicated to a single high volume product. Continuous mixers can be designed for capacities as high as 500 Tons/hour. Even though continuous mixing is gaining popularity, selection of continuous mixers is much more complicated than batch mixers.