SIZING OF THE MIXER FOR FEED PASTE FORMATION

As described in the preceding chapters that a slurry of EG and TPA are mixed in a stirred tank vessel equipped with a stirrer for viscous fluids (e.g. intermig) to prepare a paste with molar ratio of EG & TPA of 1:2.

When considering any mixing application it is important to realize that the optimum solution will depend on a variety of different factors. And it is crucial to specify precisely the right system configuration to best serve the total needs of a particular mixing operation.

Mixing Mechanisms

Mixing is achieved by a number of different mechanisms, as summarized in the following table. The most important mechanism will vary for any given application, and a given process may rely on any or all of these mechanisms.

In order to arrive at an optimum mixer design a detailed understanding of the various mechanisms involved and their importance in achieving the process result is required.

Convection Induced by pumping action of the impeller, Fluid moves through the different parts of vessel, preventing stratification.

Macro-mixing

Caused by turbulent flow a wide range of vortices. Smallest in the impeller region where dissipation is the highest. Separates bulk of fluid into smaller elements

Laminar shear Below the scale of macro mixing fluid elements are further dispersed by laminar shearing. Elements are stretched, distorted and folded.

Micro-mixing Final smallest scale mixing. Diffusion of reactants takes place and is driven only by concentration gradient. Takes place on scale smaller than any eddy size.

By far the most common of mixing applications are those which rely upon flow to achieve the required process result. These applications are often referred to as, 'Flow

Controlled Applications', and include such applications as Blending, Solids Suspension and Heat Transfer. For the reason that polymerization is one of the typical blending applications thus only this function is considered here.

Blending / Homogenization of Miscible Liquids

Blending involves the mixing of two or more miscible liquids to achieve a uniform mixture throughout the entire volume of the tank, usually within a specified period of time. It is important to note that the blending of liquids having widely varying density and/or viscosity, as in the case of EG & TPA, requires special attention and may require longer blend times to ensure the liquids are mixed.

Information Required for Mixer Selection o Viscosity

o Density

o Pressure & Temperature o Blend Time

o Volume (s)

o Any specific process requirement

Optimum Mixer Selection Criteria

A full and accurate specification of the mixing vessel, the process parameters, and the required mixer performance is the first crucial step to arrive at an optimum mixing operation. Some of the many other variables that can affect mixer performance and so need to be considered in arriving at the optimum mixer design are noted on these pages.

IMPELLERS

Impeller Type

The function of any mixing impeller is to convert the rotational energy of the mixer shaft into the correct combination of flow, shear and turbulence to achieve the required process result.

As no one-impeller design is capable of providing optimum performance under every process condition, optimum process performance is dependent upon selecting an impeller design that has the specific characteristics required by a given process.

Number of Impellers

The use of a single impeller is the usual preferred option on the basis of cost. However, changes in the ratio of liquid level (Z) to vessel diameter (T) can have an adverse effect on the flow patterns generated within the vessel. This can result in the need to consider the use of multiple impellers in order to achieve an economic solution.

Z/T ratio alone is not the only consideration when determining the number of impellers required. Multiple impellers may also need to be considered for other reasons including, when high viscosity fluids are involved, for mixing at low level during filling and emptying or where draw down from the liquid surface is a requirement.

Impeller Positioning

Whether utilizing a single or a multiple impeller configuration, the positioning of the impellers within the process fluid can have a significant effect on the overall process performance. Incorrect positioning can lead to staged flow patterns, poor dispersion of additives and impellers being out of the liquid at crucial stages of the process.

D/T Ratio

The ratio of mixing impeller diameter (D) to vessel diameter (T) has a very significant effect on the performance of most fluid mixers and the optimum D/T is a function of both process conditions and process requirements.

Normally the optimum D/T will be in the range 0.2 < D/T < 0.5. Some special applications however, sometimes operate outside this range.

Bottom Clearance

The impeller bottom clearance (C/T ratio) can also have a very significant effect on the overall performance of a mixer, affecting both power draw and pumping efficiency. The optimum C/T ratio is essentially dependent upon impeller type but can also be affected by process conditions.

Normally, the optimum C/T will be in the range: 0.1 < C/T < 0.3. Hydrofoils however can operate at much higher levels, up to C/T = 0.5 or more.

 VESSEL DESIGN

Vessel Geometry

When designing a vessel for mixer duty it is important to understand the role that tank geometry plays in determining the final mixer design. Poor aspect ratios and or inappropriate bottom shapes can both result in increase mixer cost and in certain circumstances make it impossible to optimize the mixer design.

Aspect Ratio

It is generally accepted that the ideal aspect ratio for most mixing tanks is one where the liquid depth (Z) is equal to the tank diameter (T) as this allows for the optimum number of impellers, optimum power input and optimum power distribution.

In practice, the optimum Z/T will be in the range 0.9 < Z/T ^ 1.2 as this does not significantly effect mixer design.

Bottom Shape

Tank bottom shape can have a significant effect on the flow patterns generated within the mixing vessel and hence the mixers ability to achieve optimum process performance.

Normally a dish-bottom tank is the preferred bottom shape. However, flat-bottoms and shallow cones (less than 15°) can be used for many processes without any particular problem. In the case of flat-bottomed tanks mixer performance can often be significantly improved by the introduction of corner fillets.

In general deep cones should be avoided especially where the requirement is solids suspension.

Baffles

The importance of proper tank baffling in obtaining optimum mixer performance should not be underestimated. In a correctly baffled tank the mixer develops the fluid regime required to achieve the optimum process result.

An incorrectly baffled tank on the other hand can lead to poor mixer performance and may even result in the mixer not being able to achieve the process result for which it was designed.

It is normally desirable to set the baffles off the wall and off the bottom of the tank to prevent solids or fluids from stagnating at those points. The optimum baffling arrangement however, will vary from process to process and is dependant upon a variety of factors including, vessel geometry, vessel internals, specific power, the required surface effects, and viscosity.

Mixing Intensity

Within the process industries in general it has become convenient to characterize the level of mixing required for a given application in terms of agitation intensity. This practice has led to the introduction of terms like mild or vigorous agitation. Whilst such general terms are convenient they can mean different things to different people and so need quantifying if they are to be of any practical use.

The following table gives a general overview of the various degrees of agitation in common use throughout the industry.