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Powder Dry Mixing - Double Shaft Paddle Mixers

Section summary
1. Introduction
2. Mixing principle
3. Mixing operating parameters
4. Detailed specifications
5. Mixer sizing

1. Introduction

Industrial double shaft paddle mixers have been a design of choice for process industries over the last 30 years for bulk solids dry Mixing. They offer many advantages vs other Mixers design, especially a short mixing time and a gentle handling of the product.

This webpage is focusing in the detail design of batch paddle mixers, and more especially batch double shafts paddle mixer (also called twin shaft paddle mixer or fluidized bed mixers). Most of the concepts are also valid for continuous twin shafts paddle mixers.

2. Mixing principle

Double shaft paddle mixers are convective mixers : the mixer is equipped with 2 agitators mounted with paddles that actively displaces the powder.

For the mixing to be efficient, a certain speed of rotation of the shaft must be reached. At this speed, the paddles are able to lift the solid and create a fluidization of the product in the central area of the mixer, which explains the denomination sometimes found in literature of fluidized bed mixers. This zone, called fluidization zone, is the area where mixing actually takes place.

Figure 1 : Fluidization zone in double shaft paddle mixer

The mixing speed, at which the mixing is optimal, is reached when the Froude number is greater than 1. At such a Froude number, the inertia is high enough to overcome the gravity acting on the solids particles, which allows the paddles to lift the solids and create the fluidization area. It is not necessary, and not desirable, to run a double shaft paddle mixer at Froude number higher than 1-1.1, the mixing time will not be reduced much and the energy inputs to the solids will degrade it (breakage of particles).

The optimal mixing speed for a particular mixer can be calculated, knowing the diameter of the mixing tool, and posing Fr=1.1

With :
- R is the radius of the mixing tool (center of shaft to tip of paddle) in m
- n is the mixing speed in rpm

Equation 1 : Calculated the required mixing speed for a double shaft paddle mixer

3. Mixing operating parameters

For industrial double shaft paddle mixers, the mixing time is typically 30s to 2-3 min, going longer than this, with a dry-mix, means that something in the setting of the mixer is not correct.

The mixer performance, i.e. time to reach a desired homogeneity, is a function of the following operating parameters :
- Mixing batch size : min 30-40% of mixer total volume / max 70-80% of mixer total volume
- Mixing speed : calculated and set to reach Fr=1-1.1
- Small and Minor ingredients to be introduced in the mixer after the main ingredients (or in sandwich), preferably in the central area of the mixer

The mixture desired homogeneity must be set by the manufacturer and tested during the qualification of the double shaft paddle mixer. Sampling and analysis are performed.

With :
- R is the radius of the mixing tool (center of shaft to tip of paddle) in m
- n is the mixing speed in rpm

Equation 2 : Calculation of mixing tool tip speed

4. Detailed specifications

Mixer access

The ease of access is generally considered as one of the strong points of twin shaft paddle mixers.

Depending on the manufacturers, the following options may be ordered, sorted by easiness of access for the operators :
- Extractible shafts
- Front door
- Side doors
- Opening top cover

Most of the double shaft paddle mixers available on the market have the shafts supported on both ends by bearings. However, some constructors propose also the option to have cantilevered shafts (bearings on 1 side only), which constitutes an advantage for hygienic applications (food and pharma) by allowing to extract the shaft and leave the inside of the mixer free for an easy cleaning.

Figure 2 : Accessibility options for double shaft paddle mixer (hygienic execution)

Options can be ordered together or 1 by 1.

Discharge valves

3 types of discharge valves can be found on the market, depending on the suppliers and the need of the customer. There can be 1 or 2 valves per mixer :
- Simple flap valves : discharge valves have a rectangular shape. It is the most common design.
- Hygienic round discharge valve : the valve has a round shape and, once closed, mimimizes the gaps where product can settle and avoid being mixed, contrary to the 1st type of valve that is presenting higher gaps. Those valves have a limited size that can increase the discharge time and thus reduce the mixer capacity.
- Bomb doors : the bottom of the of the mixer can entirely be opened by large flaps. The key advantage is that the mixing time is very short (less than 30 s, and that the quantity of product remaining in the mixer is very low. However, one must be careful to the tightness of the doors once closed, as well as to the access for cleaning below the doors.

Depending on the technology chosen, the mixer will discharge is a hopper that will be entirely connected to the mixer bomd doors, flap valves or that be connected through a short pipe to the mixer (round valve

Instrumentation

The following instrumentation can be found on twin shaft paddle mixers :
- Speed sensor : allows to confirm rotation and speed of the shafts
- Temperature sensors : positionned on the bearings, allow to detect abnormal heating due to broken bearings
- Flowmeter : positionned on the compressed air supply to bearing seal flush. Bearing seal flush constitute an important function to avoid ingress of product to the bearings, which would damage them or make the powder burn, which would constitute an ignition source causing dust explosion.
- Vale position sensor : allows to detect that the discharge valve is closed
- Locks : placed on each access door, ensure the safety of the machine by preventing operator to access the mixer while it runs, or start the mixer if an access point is opened.

ATEX

In order to process powders, double shaft paddle mixers, which are mixing with a mixing tool tip speed > 1 m/s, must present the following characteristics :
- Clearance in between the tip of paddles and the mixer body must be > 4 to 5 mm
- The bearing seals must be pressurized
- During loading and discharge, the mixer speed must be such that the tip speed of the paddles is < 1 m/s

The paddle tip speed can be calculated thanks to the following formula :

5. Solids mixer sizing

The mixer should be the bottleneck of the installation of mixing, which means that it should not be slowed down by the process section upstream or downstream. The capacity of the installation should be a given and a batch size should be chosen in consequence, considering as well an estimated number of batches / h

Batch size (kg) = Capacity (kg/h) / Number batches per hour (/h)

The mixing process being actually volumetric, it is necessary to know the untapped (loose) density of the mixture to size properly the mixer.

Batch size (l) = Batch size (kg) / Loose density mix (kg/l)

On top of this, it is critical to consider that the system should never be filled at 100% of its capacity, in order to allow space for particles movement.

Total mixer size (l) = Batch size (l) / 0.7

Mixers have maximum filling coefficient in between 0.65 to 0.8 usually.

Applications of double shaft paddle mixers

Double shaft paddle mixers, thanks to their working principle based on fluidization, find a lot of applications in industries where a good level of homogeneity is required with possibility to disperse minor ingredients <0.1% while having a gentle mix and short mixing time :

- Pharmaceuticals : mixing active principles
- Baby food : infant formula or infant cereals
- Food : ready to prepare mixes like coffee, cocoa...
- Food ingredients : flavors, premix
- Chemicals : mixing of reactants, premix

Some executions are however found in other industries, like in construction, environment or agriculture / animal feed / aqua feed.