Matthias Kind

Verification of the constant crystal growth model for attrition particles and its relevance to the modeling of crystallizers

Potash alum-water system growth experiments have been carried out in a stagnant cell as reported by Garside and Larson for attrition particles with sizes L 0 between 5 and 50 µm. Crystals with sizes Lo larger than 400 µm were investigated in a flow-through cell. In the stagnant growth cell large fluctuations of growth rate in the beginning have been observed. After about 5 to 10 min the growth rate of each crystal becomes constant. A constant linear growth rate of every fixed crystal has been measured in the flow-through cell. The growth rate dispersion according to the CCG model has been verified.

THE GROWTH OF ICE DENDRITES UNDER MIXED CONVECTION CONDITIONS

Dendritic solidification of ice from a supercooled melt is studied under conditions which include thermal convection, forced convection, and the effect of the moving solid-liquid interface, which manifests itself as apparent convection. A theory based on Oseen rectilinear flow suggests that heat transfer from the tip of a growing dendrite can be described by a linear superposition of Peclet numbers due to the individual mechanisms of convection by forced bulk motion of the fluid and by the moving boundary which creates the apparent convection effect This simple result was tested by plotting the experimental data over the full range available of rates of dendritic growth, in terms of the Nusselt number, as a function of the Peclet number, Pe*, which is based on the tip radius and the sum of the forced velocity of the fluid and the velocity of the tip of the dendrite. This procedure clearly reveals three regions in which different mechanisms control the process: 1) Dendritic growth is dominated by apparent ...

Thermodynamic Phase Equilibrium

In this chapter the thermodynamic behavior of single- and multiphase systems of pure substances and their mixtures are described in a general way.

Distillation, Rectification, and Absorption

The unit operations distillation, rectification, and absorption are by far the most important technologies for fractionating fluid mixtures. This great technical importance is founded on the fact that only fluid phases, which can be handled very easily, are involved. A further advantage is a high-density difference between the coexisting phases. High-density differences enable high velocities in the equipment and make the separation of the phases easier.

Fundamentals of Single-Phase and Multiphase Flow

Single-phase or multiphase flow in chemical engineering apparatus and reactors such as evaporators, columns, fixed and fluidized beds, and stirred vessels is decisive for the efficiency and capacity of these equipment units. The mixing of two or more liquid components without the formation of a second liquid phase leads to a one-phase flow, for instance in a stirred vessel. In the case of an evaporative crystallizer a three-phase system is moved by the stirrer and the rising bubbles. In many cases two phases are flowing in countercurrent direction, e.g., in columns for absorption, rectification, and extraction. Two phases are also moving in bubble and drop columns, froth layers on plates, fluidized beds, and stirred vessels used for suspension flow or the breakup of gases and liquids. Dealing with packed or film columns, the liquid phase runs down in films and rivulets in countercurrent to the gas. In many apparatus of chemical engineering, internal pieces of equipment are mounted and the fluid flow is passing around or through these internals.

Balances, Kinetics of Heat and Mass Transfer

Process engineering attends to yield products with a set of desired properties, given some raw materials. This aim may be achieved through several different process variants, which are found by process design. They usually differ with respect to feasibility, safety and particularly to cost effectiveness. It is thus necessary to appraise all process variants by means of process analysis.

Conceptual Process Design

Industrial separation processes typically consist of various distillative and alterna- tive separation steps that are coupled by material and energy streams. Such pro- cesses often have very complex structures caused by the properties of the systems at hand and by the constraints set by cost and energy savings. In most cases, a rather empirical approach is used for process design. Novel developments concern a conceptual process design (e.g., Douglas 1988; Smith 1995; Blass 1997; Stichl- mair and Fair 1998; Seider et al. 1999; Doherty and Malone 2001; Mersmann et al. 2005), which is based on the thermodynamic properties of the mixture at hand.

Evaporation and Condensation

Heat transfer to a liquid leads to an increase of the temperature up to the boiling temperature at which evaporation starts. The vapor pressure becomes equal to the pressure of the system. In the case of the evaporation of a liquid mixture all components or only some of them or perhaps only one component can be present in the vapor. Dealing with the evaporation of an aqueous solution of an inorganic salt with a very low vapor pressure approximately pure steam is leaving the liquid. (Note that an entrainment of small drops can take place with the result of small salt contents in the steam.) In general, all components of the liquid mixture will be present in the vapor when there are no great differences of the vapor pressure of the components.

Adsorption, Chromatography, Ion Exchange

Adsorption is the loading of solid surfaces with substances present in a surrounding fluid phase or, in other words, it is a surface effect between a solid and a fluid phase. Sometimes molecules of the fluid phase are not only fixed on the surface but can additionally enter the bulk of the nonporous solid phase according to a volume effect. This is called occlusion or absorption. When it is not known which of these two effects is dominant the term “sorption” is used. Adsorption means the loading of one or several components (adsorptives) on a solid material (adsorbent). The reverse process, e.g., the separation of adsorptives from the surface is called desorption.