Segregation mechanisms problems

Particle Segregation Mechanisms. Segregation is the process by which an assembly of soHd particles separates as it is being handled. This often results in cosdy quaUty control problems due to the waste of raw or finished materials, lost production, increased maintenance, and capital costs required to retrofit existing faciUties. 

Segregation is a real problem in industry. Some mixers may not be able to form a random mixture of some powder mixes because of powder segregation mechanisms occurring within the mixer. Some mixers may even demix a mixture due to the dominance of segregation mechanisms acting on a particular mixer, for example, a... 

From the previous discussion about segregation mechanisms, it can be concluded that certain material properties as well as process conditions must exist for segregation to occur. Elimination of one of these will prevent segregation. It stands to reason then that if segregation is a problem in a process, one should look for opportunities to either change the material or change the process. 

An operational change to counter these awkward modes of segregation is brought about by altering the filling process to cause diffused loading. This can be achieved in many ways, but care must be taken not to introduce other segregating mechanisms to create other problems. 

The primary mechanisms responsible for most particle segregation problems are sifting, particle velocity, air entrainment, particle entrainment, and dynamic effects (14). 

Preferably, the HjS flare system should consist of a segregated header and separate line routed up the side of a conventional elevated flare stack, sharing the same structure, pilots and igniters. However, the HjS header may be tied into the regular flare seal drum if there are special mechanical design problems associated with the separate stack e.g., in the case of a flare which is to be dismantled for overhaul. Flare elevation must be sufficient to meet atmospheric pollution and ground level concentration requirements for the sulfur dioxide produced. 

The problem of taking into consideration the actual vacancy-mediated atomic exchange mechanism (rather than the direct exchange model used in most theoretical treatments) recently received some attention. In particular, possible presence of vacancy segregation at various structural inhomogeneities was discussed. However, the estimates of these effects by various authors disagree notably with each other , and there seems to be no general treatment of this problem available. 

As briefly discussed in Section 1.2, chemical-reaction engineers recognized early on the need to predict the influence of reactant segregation on the yield of complex reactions. Indeed, the competitive-consecutive and parallel reaction systems analyzed in the previous section have been studied experimentally by numerous research groups (Baldyga and Bourne 1999). However, unlike the mechanical-engineering community, who mainly focused on the fluid-dynamics approach to combustion problems, chemical-reaction... 

In this chapter, the mechanics of hopper flow and standpipe flow along with their operational characteristics are described. Problems such as segregation, inconsistent flow rate, arching, and piping that disrupt and obstruct the flow of bulk solids in hoppers are discussed. Remedies with respect to the use of flow-promoting devices such as vibrators and aerating jets to reinitiate the flow are presented. The importance of the flowability of the solids to be handled in relation to the hopper design is emphasized. 

In direct-compression formulation, there is a wide particle size distribution. Usually, the active drug is at the fine end of the range. Such a wide particle size range can easily result in significant segregation. Five primary mechanisms are responsible for most particle segregation problems [13]. Of these, only three typically occur with pharmaceutical powders sifting, entrainment of air, and entrainment of particles in an air stream. 

In this chapter, we have described the colloid chemistiy of ceramic powders in suspension. Colloid stability is manipulated by electrostatic and steric means. The ramifications on processing have been discussed with emphasis on single-phase ceramic suspensions with a distribution of particle sizes and composites and their problems of component segregation due to density and particle size and shape. The next chapter will discuss the rheology of Uie ceramic suspensions and the mechanical behavior of dry ceramic powders to prepare the ground for ceramic green body formation. The rheology of ceramic suspensions depends on their colloidal properties. 

---