Industrial high thermal effect

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Industrial Processes with a High Thermal Effect... 

In this chapter, the different types of compact heat exchangers have been reviewed and their applications described. Single-phase flow applications are now common in the process industry due to their high thermal effectiveness, compact heat exchangers are real alternatives to conventional shell-and-tube units. 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 [Lm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reaciors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCh solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

Thermal Effects in Addition Polymerizations. Table 13.2 shows the heats of reaction (per mole of monomer reacted) and nominal values of the adiabatic temperature rise for complete polymerization. The point made by Table 13.2 is clear even though the calculated values for T dia should not be taken literally for the vinyl addition polymers. All of these pol5Tners have ceiling temperatures where polymerization stops. Some, like polyvinyl chloride, will dramatically decompose, but most will approach equilibrium between monomer and low-molecular-weight polymer. A controlled polymerization yielding high-molecular-weight pol)mier requires substantial removal of heat or operation at low conversions. Both approaches are used industrially. 

Feed/effluent heat exchangers are used in many industrial processes to warm up the fluid before the reactor and to cool it down after treatment at high temperature. The conventional design of such heat exchangers is based on shell-and-tube units. But to increase the thermal effectiveness of the heat exchangers, the required heat length becomes very important, and high pressure drop will occur. 

Of the two mechanisms discussed above, thermal runaway is by far the most common cause of safety problems in chemical batch reactors, given the ability of the system to largely exceed the desired reactor temperature and, hence, the normal operative pressure with high risk of explosion. It has been estimated that an important fraction of the chemical reactions executed daily in the chemical industry has heat effects large enough to eventually cause reactor thermal runaway [16] and that ineffective temperature control has been the cause of many incidents involving batch reactors [4, 6],... 

The thermal effect of any reactions is mild and does not induce a phase change or a significant change in the temperature of the process units. Hence, the specific enthalpies of the feed and effluent streams, h and h2, of the process units are of comparable magnitude 2 = 2,s/ i,s = 0(1). (Note that this assumption is by no means restrictive rather, it reflects current industrial practice. For example, the use of adiabatic units in highly exothermic processes is avoided for safety reasons, and external cooling systems are preferred. This issue is addressed in detail in Chapter 7.)... 

Consider the reaction C02(g) + H2(g) - CO(g) + H20(flr) at 25°C. This is a reaction commonly encountered in the chemical industry (the water-gas-shift reaction), though it takes place only at temperatures well above 25°C. However, the data used are for 25°C, and the initial step in any calculation of thermal effects concerned with this reaction is to evaluate the standard heat of reaction at 25°C. Since the reaction is actually carried out entirely in the gas phase at high temperature, convenience dictates that the standard states of all products and reactants at 25°C be taken as the ideal-gas state at 1 bar or l(atm), even though water cannot actually exist as a gas at these conditions. The pertinent formation reactions are... 

Zeolites are widely used as acid catalysts, especially in the petrochemical industry. Zeolites have several attractive properties such as high surface area, adjustable pore size, hydrophilicity, acidity, and high thermal and chemical stability. In order to fully benefit from the unique sorption and shape-selectivity effects in zeolite micropores in absence of diffusion limitation, the diffusion path length inside the zeolite particle should be very short, such as, e.g., in zeolite nanocrystals. An advantageous pore architecture for catalytic conversion consists of short micropores connected by meso- or macropore network [1]. Reported mesoporous materials obtained from zeolite precursor units as building blocks present a better thermal and hydrothermal stability but also a higher acidity when compared with amorphous mesoporous analogues [2-6]. Alternative approaches to introduce microporosity in walls of mesoporous materials are zeolitization of the walls under hydrothermal conditions and zeolite synthesis in the presence of carbon nanoparticles as templates to create mesopores inside the zeolite bodies [7,8]. 

Dehydrogenation is a highly endothermic process therefore, heat has to be continuously supplied to drive the DH reaction. In industrial application, the heat is supplied by the heat exchange with other units, where the heat is generated by combustion the heat realized during the regeneration step is also used. Experimental lab-scale studies on membrane DH reactors are normally performed under isothermal conditions, with the heat supplied by electrical heaters [e.g., 33, 37]. In theoretical studies, isothermal conditions are commonly applied [e.g., 28, 30] and the thermal effects were paid a little attention with regard to membrane DH reactors. 

Thermal characteristics are also important in numerous industrial processes, and thus the development of composites with high thermal conductivity and a low coefficient of thermal expansion is important to achieve effective heat conduction (Kim et al., 2007). The use of some reinforcements such as carbon nanotubes, carbon fibers, nano silica powders, metal particles, boron nitrite and glass fibers can improve the thermal conductivity of phenolic composites (Kim et al., 2007 Simitzis et al., 2011 Srikanth et al., 2010). Kim et al. (2007) demonstrated that the homogeneous dispersion of 7 wt% carbon nanotubes in a phenolic resin acted as an effective thermal bridge between adjacent carbon fibers and enhanced the thermal conductivity (393 W m-i K-i). 

---