Crystallizers operating problems

Crystallization processes can be used for separation, purification, or concentration of a solute, or because a particular product needs to be used in solid form, or as a component of an analytical procedure. Common requirements for accomplishing these functions are that the crystals must be produced with a particular size distribution and having a specified shape and purity. Almost all crystallizer operating problems are defined in terms of the product not meeting one of these criteria. 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

All diesel fuels eventually cause start-up and operability problems when subjected to sufficiently low temperatures. As the ambient temperature cools, high-molecular weight paraffins present in petrodiesel nucleate and form solid wax crystals which, suspended in liquid, are composed of short-chain n-alkanes and aromatics (Chandler et al., 1992 Owen and Coley, 1990 Lewtas et al., 1991 Brown et al., 1989 Zielinski and Rossi, 1984). Left unattended overnight... 

Solids size, fine Solids size, >150 /zm Compressible cakes Open cakes Dry cake required High filtrate clarity Crystal breakage problems Pressure operation High-temperature operation... 

Several crystallization objectives have been recommended to favor downstream operations or product quality. One can maximize the number-mean or weight-mean crystal size, maximize the final size of crystals grown from seed crystals, or minimize the ratio of nucleated crystal mass to seed crystal mass.f ° Other particle size-related characteristics of product crystals that have been optimized during crystallization include the coefficient of variation and the crystal shape. Although the weight-mean crystal size is the most commonly used objective in optimal control studies, the weight-mean crystal size is too insensitive to the number of small crystals that can cause filtration problems when used as an objective to optimize the crystallization operations. 

Computational fluid dynamics (CFD) is increasingly being utilized to analyze mixing systems, particularly the stirred vessels commonly used for crystallizer operation (Woo et al. 2006). The problem of modeling fluid dynamics in the presence of a solid phase is not trivial, but some workers are starting to make headway in this field. These efforts are referred in Chapter 11. 

One of the most difficult processes to scale up successfully is crystallization. Methods to achieve control of nucleation and growth are keys to development, and the degree to which they are successfully applied can be the difference between success and failure on scale-up. It is to this fundamental problem that this book is addressed, combining critically important teachings from the literature with personal experience of the authors and their colleagues in a variety of crystallization operations. 

Mixing in crystallization involves all elements of transport phenomena momentum transport, energy transport, and material transport in both the solution phase and the solid phase. In many cases, the interactions of these elements can affect every aspect of a crystallization operation including nucleation, growth, and maintenance of a crystal slurry. To further complicate the problem, mixing optimization for one aspect of an operation may require different parameters than for another aspect even though both requirements must be satisfied simultaneously. In addition, these operations are intrinsically scale dependent. 

In many cooling crystallization operations, these issues do not present a serious problem either because the metastable region is wide, the nucleation rate is low, or the compound has high growth potential. However, in this example, special measures were needed to achieve the required process outcomes. 

This book has two goals. One is to facihtate the understanding of the fundamental properties of crystallization and the impact of these properties on crystaUization process development. The second is to aid practitioners in problem-solving using actual industrial examples under real process constraints. This book begins with fundamental thermodynamic properties (Chapters 2 and 3), nucleation and crystal growth kinetics (Chapter 4), and process dynamics and scale-up considerations (Chapters 5 and 6). Subsequent chapters cover modes of crystallization operation cooling (Chapter 7), evaporation (Chapter 8), antisolvent (Chapter 9), reaction (Chapter 10), and special cases of crystallization (Chapter 11). As mentioned, real industrial examples are provided in each chapter. 

One of the major operational problems in batch (or continuous) crystallization is the fouling (or incrustation) of crystals on the heat transfer surfaces. This is because the supersaturation is normally highest near these surfaces. Once the deposition of crystals takes place on these surfaces, the accumulation of crystal deposits can increase rapidly, resulting in a severe fouling problem of the heat transfer equipment. To minimize this fouling problem, one needs to maintain sufficient agitation and a low-temperature gradient on the heat transfer surface. For batch crystallization, the... 

A robust mechanical unit that can cope with most encrustation problems is the scraped-surface heat exchanger, often referred to as a scraped-surface chiller when used in crystallization operations. The unit is essentially a double-pipe heat exchanger fitted with an internal scraping device to keep the heat transfer... 

There is also the possibility that cascade operation can influence the product CSD, although this is a complex problem to assess. For example, in a simple cascade of k vessels in which all act individually as basic MSMPR crystallizers operating at the same supersaturation, at constant crystal growth rate, and with nucleation confined to the first stage only, the mean crystal size and product CSD leaving the stage may be represented by... 

Encrustation is a serious problem in many crystallizer operations. On heat exchange surfaces, encrustation can reduce heat transfer and/or evaporation rates, and thereby reduce production rate or increase batch time. It can also lead to unstable operation. The need to remove encrustation from time to time often requires costly plant shutdown periods. Pipeline encrustation leads to greatly increased pumping power requirements and often to complete blockage. 

The mixing of saturated, or even unsaturated, flow streams in a crystallizer can produce a supersaturated mixture depending on their respective temperature, composition, flowrate and the solute solubility. Toussaint and Donders (1974) identified this particular problem, which so often leads to encrustation, and defined a mixing criterion for safe crystallizer operation which is satisfied when no supersaturation can occur whatever the ratio in which the flowstreams are mixed. 

Whilst programmed cooling (i.e. operation at constant nucleation rate within the metastable zone) increases the mean product crystal size cf. natural cooling, is it the optimum in producing the largest possible crystals The problem is to find the maximum of the integral of crystal growth over the batch time. Thus because batch operation is by definition transient, a functional has to be maximized over time rather than just a function at some point in time. Jones (1972, 1974) addressed this problem by application of a particular result in... 

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