Solids Mixing

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

GBH Enterprises, Ltd.

Process Engineering Guide: GBHE-PEG-MIX-707

Solids Mixing Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Solids Mixing CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 BACKGROUND 3 5 MIXING QUALITY 4 5.1 Qualitive Mixture Quality 4 5.2 Quantitative Mixture Quality 5 5.3 Sampling of Mixtures 8 6 THE MIXING PROCESS 9

6.1 Powder Mobility 9 6.2 Mixing Free-Flowing Powders 9 6.3 Mixing Cohesive Powders 10 7 MIXER SELECTION 12 7.1 Available Equipment 12 7.2 A Selection Procedure 16 8 LATERAL THINKING 18 9 BIBLIOGRAPHY 18



FIGURES 1 DIVISION BETWEEN FREE FLOWING SOLIDS AND

COHESIVE POWDERS 4 2 REPRESENTATION OF TYPICAL BINARY POWDER

MIXTURE STATES 6 3 POSSIBLE STRUCTURES FOR BINARY COHESIVE

POWDERS 11 4 TUMBLER MIXERS 12 5 CONVECTIVE MIXERS 13 6 HIGH SHEAR MIXERS 14 7 HIGH IMPACTION MIXERS 15 8 MIXER SELECTION DECISION CHART 17 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 19



0 INTRODUCTION/PURPOSE This Guide is one of a series of Mixing Guides prepared for GBH Enterprises. 1 SCOPE This Guide covers only the mixing of dry particulate solids. 2 FIELD OF APPLICATION This Guide applies to Process Engineers in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: Scale of The quantity of mixture on which the quality of the mixture is Scrutiny judged. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 BACKGROUND The mixing of dry particulate solids differs from that of liquid and gaseous systems in three important respects: (a) There is no particulate motion equivalent to the molecular diffusion of

gases and liquids. The mobility of the mixture is dependent on an energy input and without this there will be no relative movement of the particles.



(b) Whilst the molecules of a single phase liquid system or of a gaseous system may differ and may diffuse at different rates, they will ultimately achieve a random distribution within the confines of the system. Particulate and granulate components do not have the constant properties of molecular species and these differences can cause non-random movements, or segregation with a resultant loss of mixture quality.

(c) The ultimate element of the particulate mixture is several degrees of

magnitude larger than the ultimate molecular elements of the liquid or gaseous mixture. In practical terms this means that samples withdrawn from a randomized particulate mixture shall have a coarser texture, greater content variation or poorer mixture quality than the equivalent samples taken from a gaseous or liquid mixture.

These differences accentuate the problems of mixing particulate solids. An ideal mixing system would have high mobility and fine texture of its ultimate elements. Particulate solids have poor mobility and coarse texture. Within the spectrum of industrial solids there are great variations in flow and texture. An initial division can be made between free-flowing solids and cohesive powders (see Figure 1) FIGURE 1 DIVISION BETWEEN FREE FLOWING SOLIDS AND COHESIVE POWDERS



As can be seen texture is obtained at the expense of mobility and vice versa. A ladies face powder has good texture but poor mobility whilst a synthetic detergent has high mobility and poor texture. In many cases the nature of the particulate solid can be adjusted to be more or less cohesive /free-flowing by controlling the properties, and notably the size, of the particles. For large tonnage industrial products a free-flowing solid has obvious process advantages. Flow rates can be controlled, packages can be filled consistently, dust is minimized and the customer receives an attractive product. The cohesive powder has little to commend it except that the texture or quality of mixture can be higher. This can be a dominant requirement. It is recommended that the temptation to control particulate solids properties such that the bulk flow is just free-flowing should be avoided. While potentially giving the best texture commensurate with satisfactory flow, it is a dangerous balancing point as small changes in the process could transform the flow characteristics and make the process inoperable. 5 MIXING QUALITY 5.1 Qualitive Mixture Quality When is a mixture well-mixed? This fundamental question has to be asked of all mixtures but it is especially important when coarsely textured powders are involved. Danckwerts [Ref. 1], gives some helpful qualitative ideas on mixture quality. A Scale of Segregation of a mixture measures the size of regions of unmixed material. The Intensity of Segregation measures the amount of dilution of regions of unmixed material. Evidently, the quality of a mixture is improved by reducing both the scale and intensity of segregation. As they are reduced the mixture will pass through a critical quality where it can be deemed satisfactory or well mixed. Further mixing is unnecessary. This concept of a critical mixture quality was described by Danckwerts in terms of a Scale of Scrutiny for the mixture. This is the quantity of mixture on which the customer will base a judgment of quality. Thus, in the case of dispersing a pigment in a face powder the judgment would be based on the ability of an eye to detect any patchiness when the powder is spread on the skin and the scale of scrutiny would be measured in fractions of a gram of powder.



With a detergent the scale of scrutiny is probably the contents of a cup added to a washing machine and would be up to 50 grams of powder. The smaller the scale of scrutiny the greater the mixing problem. In the limits, if the scale of scrutiny was the contents of the batch mixer then the mixture would always be perfect and if the scale of scrutiny was one particle then the mixture would be completely unmixed. It is essential to identify this scale of scrutiny for every process because: (a) it identifies the objective of the mixing process; (b) it enables the mixing problem to be defined statistically; (c) it is the scale at which the mixture shall ultimately be sampled for quality

control purposes. 5.2 Quantitative Mixture Quality Having determined the scale of scrutiny for a product it is possible to define three limiting variance values for a powder mixture: (a) the variance of a completely separated system (So

2); (b) the variance of a randomized mixture (SR

2); (c) the variance of an ideal or ordered mixture (SI

2). See Figure 2 for pictorial representation of a binary powder system



FIGURE 2 REPRESENTATION OF TYPICAL BINARY POWDER MIXTURE STATES



This can also be done for multi-component multi-sized particulate systems and provides a basis of comparison for experimental results as well as a desk-bound method of checking the feasibility of a variety of competing mixture formulations. The simplest case is that for a two-component mixture of equi-sized particles [Ref. 2 and!3]

where p and q are the proportions of the two components and A the number of particles in the sample. Lessons from this simple case can be applied to the more complex multi-component, multi-sized systems. Note that; (1) Randomization is normally the goal of an industrial mixer so that SR

2 should be as small as possible. This can be done either by having a large scale of scrutiny, (i.e. large A), or for a constant weight of sample to reduce the particle size. This is the numerical expression for the improved texture of a cohesive powder.

(2) For scale-up purposes So

2 is independent of scale of scrutiny whilst SR2 is

inversely proportional to scale of scrutiny. Between these limiting mixture values there is an unknown dependence on scale of scrutiny which makes it essential that intermediate mixtures are sampled at the required process scale of scrutiny.

(3) With free-flowing powders SR

2 is the limiting variance of the mixed product. In some circumstances with cohesive powders a positive structuring of particles can be achieved so that the zero variance of the ideal mixture SI

2, can be approached (see 6.3).

For binary systems with multi-sized particles the value of SR2 is given by:



Within the mixture and f a is the size fraction of one component of average weight Wa in a particle size range [Ref. 4]. The denominator in equation (4) is the estimate of the mean number of particles in a sample and is directly comparable with the denominator value A of equation (2). In order to estimate the limiting variance by equation (4), the size analysis of the components is required along with a knowledge of particle shape and specific gravity. This limiting equation for random mixtures has been extended to cover multi-component mixtures by Stange so that:

In this expression one component is regarded as the 'key' component. If the variance of more than one component is regarded as critical in a process it could be necessary to monitor the state of mixedness of these components independently [Ref. 5]. Equations (2), (4) and (5) are extremely important in that they provide a method of estimating the best theoretically obtainable mixture quality for any particulate mixture. Desk calculations shall show the effect of varying mixture formulations or the scale of scrutiny on attainable mixture quality. In practice this mixture quality may prove to be unobtainable due to segregation or poor mixer design but at least the boundaries of possibility in a randomizing process are established. The likeliest circumstances in which a random mixture shall not be satisfactory for its duty are: (a) When the scale of scrutiny is very small. (b) When one or more components are in a minor proportion. (c) When an ingredient is composed of large particles.



As a 'rule of thumb' if at a given scale of scrutiny a mixture contains less than 500 particles of an ingredient then the formulation is asking for a statistically difficult or impossible performance from the mixer. For more quantitative mixing statistics the reader is referred to [Ref. 6 Chapter 2]. 5.3 Sampling of Mixtures A comparison of process performance with the limiting statistical variance or quality values requires samples to be taken in order to determine the experimental variance Sex

2. Samples should be taken of a size equal to the scale of scrutiny. It is very difficult to avoid bias in the selection and retrieval of samples from free-flowing solids mixtures [Ref.7]. Rules for the sampling of free-flowing powders would be: (a) Avoid sampling from bulk and look for points in the process where the

mixture is flowing: e.g. as a batch mixer is discharging. (b) Sample the entire section of flow. (c) Sample frequently and avoid a sampling frequency which coincides with a

process frequency. The sampling of cohesive powders is less of a problem because of the reduction in segregation. A sample withdrawn from a mixture is a 'point' sample at a carefully chosen scale of scrutiny. When this sample is analyzed for quality it is unlikely that the quality required for analysis is the same as the scale of scrutiny for the process and it is vital that sub-sampling of the point sample is done in an efficient spinning riffler. That is, for analytical purposes a 'bulk' sample has to be taken from the point sample. Alternative methods of obtaining a bulk analysis of the point sample would be to dissolve the entire point sample in a liquid or to grind the sample down to a fine cohesive state.



6 THE MIXING PROCESS 6.1 Powder Mobility The mixture quality of a powder shall be improved if the scale and intensity of segregation are reduced. In order to generate inter-particulate motion and give individual particles the opportunity of relocating themselves within the mixture, the particles may be kneaded, tumbled, fluidized, sheared or scooped. The ability of particles to move freely is an important mixing characteristic and leads to a general division of particulate solid mixtures into: (a) free-flowing mixtures, and (b) cohesive or structured mixtures. The division between the two regimes is not easily defined. The bulk effect can be seen by rolling the mixture in a bottle. The free-flowing powder solids move smoothly with well defined planes of movement whilst the cohesive powder exhibits 'stick-slip' motion with irregular surface characteristics. Particle size is probably the dominant influence on the type of flow regime. The gravitational force associated with a large particle is much larger than any restraining interparticulate forces with the result that individual particles retain their freedom of movement. As the particle size decreases various interparticulate forces can potentially dominate and the particles attempt to retain a structured arrangement. For particulate solids on the boundary between free-flow and cohesivity the determination of aeratable and tapped bulk density has been found to be a sensitive guide to the structural strength or cohesivity of the powder [Ref. 8]. The identification of the flow regime of the solids is important as it determines the best type of mixer to be used. 6.2 Mixing Free-Flowing Powders The disadvantage of such mixtures is that they can be subject to segregation or 'un-mixing' on a severe scale. The very mobility which allows a particle to move smoothly and independently of its neighbors gives it the freedom to preferentially move in a particular direction. Thus a small particle shall preferentially percolate through a bed of moving particles.



A large particle shall be projected further than a fine particle given an equal initial velocity. Such preferences can result in a very large scale of segregation. The reader is recommended to view the video 'Filling a box' [Ref.9] to get a feel for the potential gravity of the problem of segregation on mixture quality. If batch mixing is a requirement for such free-flowing systems then mixers should be chosen which inhibit the freedom of movement of individual particles and promote the movement of changing groups of particles. Examples of such mixers are ribbon blenders and a variety of scooping blade machines (see Clause 7). The rolling of particles on an inclined plane is one of the chief causes of gross segregation in industrial applications and is also one of the main causes of the segregation in 'tumbler' mixers which find wide industrial application in batch processes. The rotation of the vessel provides an excellent free tumbling or percolation plane for the solids and extreme segregation can occur even with relatively small particle size differences. The potential of a mixture to segregate may be quantified by means of a segregation test in an inclined cylinder [Ref. 10]. For a suitable process continuous mixing is an excellent way of avoiding gross segregation. Within the continuous mixer the quantities of mixture at the point of mixing are minimized and the potential scale of segregation is considerably reduced. Even if a large mass of free-flowing solids is 'satisfactorily' mixed great care has to be taken in subsequently using the mixture. Many storage and handling processes can destroy the mixture quality that has been so carefully created. Only when the mixture has been 'frozen' at its final point of usage can the mixture be regarded as safe. This is the point at which the particles lose their mobility or at which segregation is no longer important and could be the formation of a tablet, the filling of a package or a point at which the powder is put into solution. The advice for the process engineer must be to locate the mixing point as closely as possible to the point of usage and if possible to avoid all bulk storage and transport between these points.



6.3 Mixing Cohesive Powders If for the free-flowing mixture the art of the process engineer is to restrict the freedom of movement of individual particles, then for the cohesive mixture the problem is reversed. The cohesive system has a natural structure which has to be repeatedly broken down in order to give individual particles within the structure an opportunity of relocating themselves. It is for this reason that the most suitable 'mixers' for cohesive powders are often to be found in the comminutor catalogues. The breaking of powder structure resembles the breakage of particles. The major reason for processing a cohesive powder rather than a free-flowing solid is to achieve high mixture quality. Sub-clause 6.2 highlighted the statistical advantage of using small cohesive particles. Another major bonus of mixing cohesive powders is that the structural nature of the powder prevents the mobility of individual particles and the gross segregation encountered with free-flowing powders is seldom met. The nature and the strength of the interparticulate forces acting within a cohesive powder structure is receiving a great deal of attention. The reason for this is that if the structure of the powder can be manipulated by the careful choice of particle shapes and sizes then an approach can be made to the ideal or ordered mixture with zero variance, SI

2. Figure 2 illustrates the basic mixture conditions for unstructured, free-flowing particles for a binary system. If the particles are structured then some interesting possibilities arise. If there is an equal structural strength between black and white particles and black and black particles then a randomly structured powder will result. If one of the ingredients is 'self loving' e.g. black particles bond by preference to black particles then the structure of Figure!3 (a) is obtained. If one of the ingredients prefers to bond to a dissimilar particle then the preferred structure 3 (b) is an approach to ordered mixing. The self loving preference of Figure 3 (a) is a common problem in the fine chemicals industry. A minor ingredient such as a pigment is well dispersed throughout the bulk of the mixture but at a small scale of segregation has a high intensity of segregation. Usually such mixtures have to be subjected to a high shear in order to break down the preferential structure. Edge runner mills or powder rollers are good examples of mixers able to achieve this (see Clause 7). The ordered structure of Figure 3 (b) offers mixtures of the highest standards. The creation of coated powders is one practical means whereby this structure can be approached.



A carrier particle provides attractive sites for very fine particles to preferentially structure and a form of positive segregation takes place in particle location. Such systems have a double advantage in that the bulk carrier particle ensures bulk free-flow characteristics and the structuring ensures a very high quality of mixture. Powder lubrication is one form of the coating process. The coated mixture is best created in an impaction mixer where the carrier particles are subjected to impaction by a high speed impeller (see Clause 7). FIGURE 3 POSSIBLE STRUCTURES FOR BINARY COHESIVE POWDERS



7 MIXER SELECTION 7.1 Available Equipment Figures 4, 5, 6 and 7 show some of the major mixer types following the mixer mechanism categorizations of 6.2 and 6.3. FIGURE 4 TUMBLER MIXERS A variety of shapes. Good process advantages. Poor mixture quality for Gentle shearing action useful for free-flowing powders. some cohesive and lubrication

processes.

A high speed impactor or Good for product integrity. intensifier can be added to give greater process flexibility. See video [Ref. 12] for more detail of mechanisms.



FIGURE 5 CONVECTIVE MIXERS Better mixture quality for Higher shear rates than tumblers free-flowing powders. can cause degradation of product. Contamination possible. Cleaning difficult. Paddles introduce air and add This speeds up mixing but can to powder mobility. blow off fines. Cohesive can be difficult to empty from this category.



FIGURE 6 HIGH SHEAR MIXERS Mechanized equivalents of the Often require pre-mixing of the mortar and pestle. bulk powder before exerting local

high nip or shear. Cause particle degradation. Useful for breaking very small

aggregates to give very high mixer quality.

Shear rate should be adjustable.



FIGURE 7 HIGH IMPACTION MIXERS Like the high shear mixers very Good for coating process. useful for breaking structures in cohesive powders. Hold up in unswept regions With all these mixer types it is can be a problem. worth examining the flexibility

of the system. How is performance affected by How would the mixer perform if degree of fill? the flow characteristics of the

powder changed? Look for mixers with a variety For example an edge runner mill of mixing mechanisms. with a fitted ribbon blender,

a tumbler mixer with a high speed impactor or a high speed impactor which also permits gentle circulation of the powder.



7.2 A Selection Procedure A mixer has to satisfy the following requirements: (a) it has to satisfy process requirements; (b) it has to produce mixtures of a satisfactory quality; (c) it has to meet these requirements at the lowest unit price. Process requirements are usually easy to identify as products and processes impose their own conditions [see Ref. 6, Chapter 3]. Producing a mixture of the correct quality is more problematical but a decision sequence would be: (1) Is the process statistically possible (see 5.1 and 5.2)? (2) What is the flow nature of the powder to be mixed (see!6.1)? (3) What mechanism of mixing should be used (see 5.3 and 6.4)? and if

possible view video [Ref. 11] 'Mechanisms of Mixing Powders'. (4) Choose a mixer. It is often desirable to choose a mixer with more then one

mixing mechanism as the process and role of the mixer might well change (see 7.1).

(5) If possible test the mixer before installation at the correct scale of scrutiny

(see!5.1) using non-biased sampling techniques (see 5.3) at a mixer capacity as close as possible to the required duty. Check performance quality and mixing time (see 5.2).

Figure 8 gives a parallel selection procedure based on a decision chart.



8 LATERAL THINKING Powder mixing can be a difficult process and for free-flowing powders it can be an equally difficult task to retain mixture quality up to the point of usage. Is there a way round the problem rather than through it? Some examples of alternative solutions would be; (a) Process orientated

(1) To avoid gross segregation of ingredients would it be possible to granulate the mixture of chemically different ingredients to provide a product which had physical but no chemical segregation? (for example granular fertilizers).



(2) Can the product be put into liquid or paste form?

(for example synthetic detergents). (b) Market orientated (1) Can the customer be encouraged to mix the raw ingredients

(for example salt package in crisp packet). (2) Would the customer accept a 'shot-filled' package at a scale of scrutiny

corresponding to its usage (for example a tea bag). Other opportunities for originality become evident on close examination of how the customer actually uses the process mixture.



9 BIBLIOGRAPHY Ref Source [1] Danckwerts, P.V., Research, London, 1963 6, 355. [2] Lacey, P.M.C., Trans. Inst. Chem. Eng., 1943 21, 53. [3] Hersey, J.A., Pow. Tech. 1975 11, 41. [4] Poole, K.R., Taylor, R.F. and Wall, G.P., Trans. Inst. Chem. Eng., 1964,

42, 166. [5] Stange, K., Chem. Ing. Tech., 1963, 35, 580. [6] Harnby, N., Edwards, M.F. and Nienow, A.W., Mixing in the Process

Industries, 1985, Pub. Butterworth. [7] Allen, T. and Khan, A.A., The Chem. Eng., 1970, 109. [8] Harnby, N., Hawkins, A.E., and Vandame, D., Chem. Eng. Sci., 1987, 42

879. [9] Harnby, N., Video 'Filling a Box with Powder'. [10] Williams, J.C., and Knan, H.I., The Chem. Eng., 1973,!1,!19. [11] Harnby, N., Video 'Mechanisms of Powder Mixing'.

Solids Mixing

Technology

Transcript of Solids Mixing