Endothermic reactions vaporization

The remaining unoxidized hydrocarbons react endothermically with steam and the combustion products from the primary reaction. The main endothermic reaction is the reforming of hydrocarbon by water vapor ... 

Thermal decomposition of spent acids, eg, sulfuric acid, is required as an intermediate step at temperatures sufficientiy high to completely consume the organic contaminants by combustion temperatures above 1000°C are required. Concentrated acid can be made from the sulfur oxides. Spent acid is sprayed into a vertical combustion chamber, where the energy required to heat and vaporize the feed and support these endothermic reactions is suppHed by complete combustion of fuel oil plus added sulfur, if further acid production is desired. High feed rates of up to 30 t/d of uniform spent acid droplets are attained with a single rotary atomizer and decomposition rates of ca 400 t/d are possible (98). 

Spent acid burning is actually a misnomer, for such acids are decomposed to SO2 and H2O at high temperatures in an endothermic reaction. Excess water in the acid is also vaporized. Acid decomposition and water vaporization require considerable heat. Any organic compounds present in the spent acid oxidize to produce some of the required heat. To supply the additional heat required, auxiUary fuels, eg, oil or gas, must be burned. When available, sulfur and H2S are excellent auxiUary fuels. 

FIGURE 6.9 The endothermic reaction between ammonium thiocyanate, NH4SCN, and barium hydroxide octahydrate, Ba(0H)2-8H20, absorbs a lot of heat and can cause water vapor in the air to freeze on the outside of the beaker. 

In this process use is made of an endothermic reaction between the aluminum (in the vapor form) in the crude feed material and gaseous aluminum trichloride... 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable decrease for endothermic reactors, then this is the option that would normally be chosen. If this is the case, then the feed stream to the reactor requires heating and the effluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) if the feed needs to be increased in temperature or vaporized, and a hot stream (the reactor effluent) if the product needs to be decreased in temperature or condensed. The heat of reaction appears as increased temperature of the effluent stream in the case of exothermic reaction or decreased temperature in the case of endothermic reaction. 

Cold shot. Injection of cold fresh feed for exothermic reactions or preheated feed for endothermic reactions to intermediate points in the reactor can be used to control the temperature in the reactor. Again, the heat integration characteristics are similar to adiabatic operation. The feed is a cold stream if it needs to be increased in temperature or vaporized and the product a hot stream if it needs to be decreased in temperature or condensed. If heat is provided to the cold shot or hot shot streams, these are additional cold streams. 

A change in temperature, however, does force a change in the equilibrium constant. Most chemical reactions exchange heat with the surroundings. A reaction that gives offbeat is classified as exothermic, whereas a reaction that requires the input of heat is said to be endothermic. (See Table 13-2.) A simple example of an endothermic reaction is the vaporization of water ... 

The principle applies to physical equilibria. When anhydrous sodium sulphate is dissolved in water, heat is evolved, and its solubility decreases with a rise of temp. hydrated sodium sulphate dissolves in water with an absorption of heat, and its solubility increases with rise of temp. The vaporization of water is an endothermal reaction, and hence a rise of temp, favours vaporization, for it increases the concentration of the vapour phase. 

Our specific example involves the vapor-phase cracking of acetone into ketone and methane, described by the endothermic reaction... 

Combinations of MH and ATH can give improved performance when used together, due to the increased range of endothermic reaction (180°C-400°C) and the release of water in the vapor phase.58,59 The different metal oxides produced on dehydration may also contribute to this effect. 

The second of the above requirements presupposes an assortment of heat transfer media that covers the whole temperature range of interest for gas-phase reactions in fixed-bed reactors. It is convenient to distinguish between gaseous, liquid, and vaporizing heat transfer media. Gaseous heat transfer media in the form of hot flue gases arc used in the temperature range above 500 °C exclusively to supply heat for endothermic reactions. 

Description The process consists of a reactor section, continuous catalyst regeneration (CCR) section and product-recovery section. Stacked radial-flow reactors (1) facilitate catalyst transfer to and from the CCR catalyst regeneration section (2). A charge heater and interheaters (3) achieve optimum conversion and selectivity for the endothermic reaction. Reactor effluent is separated into liquid and vapor products (4). The liquid product is sent to a stripper column (5) to remove light saturates from the C6 aromatic product. Vapor from the separator is compressed and sent to a gas recovery unit (6). The compressed vapor is then separated into a 95% pure hydrogen coproduct, a fuel-gas stream containing light byproducts and a recycled stream of unconverted LPG. 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

FTocesses for the production of tertiary amyl methyl ether (TAME) Brockwell et ah, Hyd. Proc., 70(9), 133 (1991)]. Highly endothermic reactions may require intermediate reboilers. None of these heat management issues preclude the use of reactive distillation, but must be taken into account during the design phase. Comparison of heat of reaction and average heat of vaporization data for a system, as in Fig. 13-97, gives some indication of potential heat imbalances [Sundmacher, Rihko, and Hoffmann, Chem. Eng. Comm., 127, 151 (1994)]. The heat-neutral systems [-AH (avg)]... 

Energy is provided to this endothermic reaction by the conduction, convection, and radiation from the furnace walls. This type of decomposition can also be caused on a hot substrate to produce a ceramic film. This process is called chemical vapor decomposition (CVD). 

The reactant species impinging from the vapor phase is attracted to the surface of the substrate mainly due to the instantaneous dipole moment of the substrate surface atoms. Adsorption of the condensed species is accompanied by lateral diffusion, i.e., the species moves along the surface of the substrate for a particular distance before it resides on the surface. An aggregate of condensed reactant species that reside on the surface of the substrate is termed a "nucleus". This could vary from a single atom or molecule to a cluster of species, and the process is referred to as nucleation. Nuclei are usually three dimensional island type structures formed on the surface of the substrate and nucleation is usually the initial stage of the film formation. It is worthwhile to note that, while simple condensation is usually exothermic, CVD adsorption and subsequent film formation is usually an endothermic reaction with few exceptions. 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

Hydrothermal stability. Steam or water vapor is sometimes present in the reaction mixture not necessarily as a reactant or product. It is a convenient form of heat source for endothermic reactions such as dehydrogenation of hydrocarbons. It can also oxidize carbon deposits that deactivate many catalysts. Steam is regularly used in the dehydrogenation of ethylbenzene to produce styrene for the above reasons. 

Heat consumed in system by the endothermic reaction = AH° x moles produced = 70,224 X 80 = 5,617,920kJ/h = 5617.9 MJ/h Heat to vaporize feed (gas phase reaction)... 

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. 

The chemical found in both baking powder and baking soda is sodium bicarbonate. Energy from the oven is absorbed by the sodium bicarbonate. The sodium bicarbonate breaks down into three different chemical substances, sodium carbonate, water vapor, and carbon dioxide gas, in the following endothermic reaction ... 

Negative values of standard reaction enthalpies indicate exothermic reactions, whereas positive values indicate endothermic reactions. Together with the molar volume, vaporization enthalpy is used in determining the —> Hildebrand solubility parameter. 

Cinnabar is a natural mercury(II) sulfide, HgS, found near volcanic rocks and hot springs. It is the only important source of mercury, which has many uses, including dental amalgams, thermometers, and mercury vapor lamps. Mercury is formed in the following endothermic reaction when mercury(II) sulfide is heated. Consider a system in which a sample of HgS in a closed container is heated with a Bunsen burner flame. 

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