Autothermal reactions

Steam Reforming. When relatively light feedstocks, eg, naphthas having ca 180°C end boiling point and limited aromatic content, are available, high nickel content catalysts can be used to simultaneously conduct a variety of near-autothermic reactions. This results in the essentiaHy complete conversions of the feedstocks to methane ... 

Autothermal reaction of ethanol through Pd-Ag membrane reactor prepared by sequential electroless deposition... 

Since operation in an autothermal mode implies a feedback of energy to preheat the feed, provision must be made for ignition of the reactor in order to attain steady-state operation. The ordinary gas burner and many other rapid combustion reactions are examples of autothermal reactions in which the reactants are preheated to the reaction temperature by thermal conduction and radiation. (Back diffusion of free radicals also plays an important role in many combustion processes.)... 

The situation at the upper stable point, when reaction proceeds smoothly to completion at a high temperature without an external source of heat, is described as autothermal or autothermic operation. If the volumetric flow rate of reactants, F, is increased, then the slope of the Ql line is increased and the position of the Qq is also slightly changed the principal effect, however, is that, eventually, an upper intersection of the heat generation and heat loss lines is no longer possible and the autothermic reaction is blown out . 

The main individual reactions that take place in the reformer (e.g., reactions (1), (2) and (5)) will be considered separately from the overall autothermal reaction for two reasons. First, in ATR the reactor can be considered as two plug-flow reactors in series (1) a very fast POX reaction occurs at the top of the catalyst bed and utilizes a small portion of the bed and (2) a slow SR utilizes the remainder of the reactor bed. Therefore, an optimal ATR catalyst must have excellent SR eatalytic properties. Second, there may be situations in which liquid fuels are reformed using only these individual reactions e.g., diesel fuel may be reformed using only SR (reaction (2)) or only by POX (reaction (1)). 

These two reactions occur simultaneously in commercial units in a balanced autothermal reaction because the oxidative reaction furnishes the heat to cause the dehydrogenation to take place. 

Since the incoming reaction gases in most cases must be heated to the ignition temperature of the catalytic reaction, adiabatic reaction control is often coupled with heat exchange between the incoming and exiting reaction gas resulting in so-called autothermal reaction control. This type of reaction control offers certain specific features and development perspectives, which are discussed in Section 10.1.3.4. 

Figure 24. Autothermal reaction control with direct (regenerative) heat exchange for an irreversible reaction [14], A) Basic arrangement B) Local concentration and temperature profiles prior to flow reversal in steady state C) Variation of outlet temperature with time in steady state.

The above reactions proceed also in the so-called rich-gas processes of British Gas and Lurgi/BASF, which convert naphtha with steam in autothermal reactions in a vessel filled with a special nickel-containing catalyst. It was formerly successfully used for town gas production from naphtha. This reaction may also used as pre-reformer ahead of a conventional tubular steam reforming furnace to convert higher hydrocarbons at low temperature and low S/C ratio into a methane reach gas which can than be reformed in the primary reformer with a standard methane reforming catalyst instead of an alkalized catalyst (Section 4.1.1.3.1). 

Kinetic modeling of diesel autothermal reforming is extremely complicated. Diesel fuel consists of a complex variable mixture of hundreds of hydrocarbon compounds containing paraffins, isoparaffins, naphthenes, aromatics, and olefins. To simplify the model, a steady-state power law rate expression for the diesel reforming over each type of catalyst used in this study was developed. A linearized least-squares method of data analysis was used to determine the power law parameters from a series of diesel ATR experiments. The power law rate model for diesel autothermal reaction may be written as ... 

The conversion of coal to both liquid and gaseous hydrocarbons has been commercially deployed worldwide. Gas production from coal is realized by means of a gasification medium which reacts with the coal at temperatures > 800 °C. All organic constituents of the coal will be converted with sufficiently long reaction times. If air or oxygen is injected into the gasifier, part of the coal is burnt directly leading to an autothermal reaction. 

The most widely used types of reactor for heterogeneously catalyzed reactions in the chemical and petrochemical industries are fixed-bed and fluidized-bed reactors [T26]. The most important reactors for heterogeneously catalyzed reactions are the fixed-bed reactors. They can be classified according to the manner in which the temperature is controlled into reactors with adiabatic reaction control, reactors with autothermal reaction control, and those with reaction control by removal or supply of heat in the reactor. Some of the well-known reactor designs are discussed below. 

For liquid-phase reactions, a single PFR or CSTR reactor is often used. For a single reaction at isothermal conditions, the volume of a PI is smaller than that of a CSTR for the same conversion and temperature. However, for (1) autocatalytic reactions, where the reaction rate depends on the concentration of a product, or (2) autothermal reactions, where the feed is cold, but the reaction is highly exothermic, the volume of a CSTR can be smaller than a PFR, such that axial dispersion in a tubular reactor may actually be beneficial. In general, a... 

Unique for CSTR Phases liquid, gas-liquid, hquid-liquid, hquid-catalytic solid, gas-liquid-catalytic solid, gas-liquid -biosohd. Capacity 0.0001-100 L/s and usually > 0.4 L/s volumes 1-1000 000 L. Autothermal reactions. Usually if the concentration of reactants is low, and need low concentration of reactants for selectivity. CSTR is larger and more expensive than PFTR. For multiphase, STR are characterized by high liquid holdups holdup of the reactive phase is important if the reaction is slow Ha < 1 phase ratio is easy to control. Adiabatic, CSTR usually gives higher productivity for exothermic reactions than for STR batch or PFTR. Use for large capacity, otherwise batch. Heat recovery is easier in CSTR than in a batch STR. 

The methanol reforming takes place between the membrane mbe and the second jacket. The hydrogen through the membrane is recovered as CO-free stream in the permeate side. Air is injected in the second jacket to produce heat by reaction with the unrecovered hydrogen and unconverted reactants. The produced heat is transferred by the jacket wall to the reforming zone. In this way, an autothermal reaction can be carried out with a complete conversion and integrated H2 purification. 

ATR of methane, methanol, and ethanol can be efficiently carried out in membrane-based reactors. The hotspot formation, typical of autothermal reactions, which is detrimental for the stability and selectivity of the membrane used in such reactors, can be circumvented by using FBMRs. 

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