Process control, liquid phase processes

DMFC modeling thus aims to provide a useful tool for the basic understanding of transport and electrochemical phenomena in DMFC and for the optimization of cell design and operating conditions. This modeling is challenging in that it entails the two-phase treatment for both anode and cathode and that both the exact role of the surface treatment in backing layers and the physical processes which control liquid-phase transport are unknown. 

This was a liquid-phase process which used what was described as siliceous zeolitic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

In addition to these mass transport steps, heat conduction can also be important in heterogeneously catalyzed processes. For exothermic reactions the heat generated at the catalytic site must be dissipated away from the catalyst and into the reaction medium while heat must be supplied to the active sites for endothermic reactions. In liquid phase processes heat transport is generally not a significant factor since the liquid tends to equalize the temperature throughout the reaction medium and, thus, facilitate temperature control. In vapor phase processes, however, heat transport can be a significant problem. 

Temperature control and heat economy may be obtained in such liquid phase processes by using the cod oil to be treated as a medium for removing the heat generated in the reaction vessel. For this purpose, the vessel is jacketed and the cool oil circulated through the annular space before passing it into the main body of hot oil.8... 

This use of diluents is especially valuable when reaction tubes of large cross section are used. With small tubes which are used in bundles of several, a somewhat different method of temperature control is available. The tubes may be immersed in a bath of a liquid which boils at or near the reaction temperature. In a manner analogous to the liquid phase process the heat of reaction is used to vaporize liquid from this bath. These vapors may be externally condensed and the liquid returned. In this way the heat of reaction may be removed externally at any desired temperature. The details of this method of operation will be presented in a later part of the book.87... 

Efficient treatment of the disparate length and time scales will be the key to the further development of predictive computational models for both plasma and electrochemical engineering. This is not limited to the plasma or liquid-phase processes. It includes the solid materials produced or modified by these technologies from the atomic level, to the microstructure to the bulk material properties. Finally, integration of modeling/simulation with design, sensors, control, optimization, safety and reliability will result in an ultimate integrated system which is based on molecular principles and which extends all the way to the factory scale ... 

The immense versatility of the sol-gel process in liquid phase enables control on the size, porosity, surface area, morphology (full or core-shell particle), and hydrophilic-lipophilic balance of the encapsulating silica cage. Not only the conditions of the process can be controlled... 

There are a large number of liquid phase processes where solid products are precipitated, e.g., salts, hydroxides, various organic products, and polymers. In many processes the solid reaction product has a low solubility, so that a recrystallization for further purification or for particle size control is often not practical. In those cases the precipitation process itself has to be controlled as well as possible. Precipitations are frequently carried out in stirred tank reactors, but other reactor types are conceivable. 

Ethylbenzene Hydroperoxide Process. Figure 4 shows the process flow sheet for production of propylene oxide and styrene via the use of ethylbenzene hydroperoxide (EBHP). Liquid-phase oxidation of ethylbenzene with air or oxygen occurs at 206—275 kPa (30—40 psia) and 140—150°C, and 2—2.5 h are required for a 10—15% conversion to the hydroperoxide. Recycle of an inert gas, such as nitrogen, is used to control reactor temperature. Impurities ia the ethylbenzene, such as water, are controlled to minimize decomposition of the hydroperoxide product and are sometimes added to enhance product formation. Selectivity to by-products include 8—10% acetophenone, 5—7% 1-phenylethanol, and <1% organic acids. EBHP is concentrated to 30—35% by distillation. The overhead ethylbenzene is recycled back to the oxidation reactor (170—172). 

Separability. One of the greatest advantages of a solid catalyst is that it can be separated easily from the products of reaction. To do this successfully requires careful control of the process conditions so that exposure of the catalyst to nonreactant liquids capable of affecting or dissolving either the catalytic material or the support is prevented or rninimi2ed. Solid catalysts typically are used in axial or radial flow beds and multitubular reactors. Many successful commercial processes maintain the reactants and products in the gas phase while in contact with the catalyst to avoid catalyst degradation problems. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Note that H is simply Henry s constant corrected for units. When the solute gas is readily soluble in the liquid solvent, Henry s law constant (H or H ) is small and Kj approximately equals k, and the absorption process is controlled by the gas film resistance. For systems where the solute is relatively insoluble in the liquid, H is large and K( approximately equals k, and the absorption rate is controlled by the liquid phase resistance. In most systems, the solute has a high solubility in the solvent selected, resulting in the system being gas film resistance controlled. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

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