Flow quasi-adiabatic

Quasi-adiabatic a vessel condition that allows for small amounts of heat exchange this condition is typical in testing self-heating by oxidation that is characterized by gas flows (although well-controlled in temperature) into and/or out of the test vessel this condition is typical as well in tests where heat transfer is avoided by active control, that is, the ambient temperature is kept identical to the test vessel temperature, such that an adiabatic condition is approached. 

For a quasi-adiabatic operation, the escape of heat from the temperature measuring sites to the surroundings must be reduced to a minimum. This can be achieved by the use of tube materials of low thermal conductivity on the one hand and high reaction and flow rates on the other. 

Figure 15 Porosity structure of a high-resolution single-channel calculation for an upwelling system undergoing melting by both adiabatic decompression and reactive flow (see Spiegelman and Kelemen, 2003). Colors show the porosity field at late times in the run where the porosity is quasi steady-state. The maximum porosity at the top of the column (red) is 0.8% while the minimum porosity at the bottom (dark blue) is 10 times smaller. Axis ticks are height and width relative to the overall height of the box. In the absence of channels this problem is identical to the equilibrium one-porosity transport model of Spiegelman and Elliott (1993). Introduction of channels, however, produces interesting new chemical effects similar to the two porosity models.

Fast chemical processes cannot be controlled using traditional methods, such as the consumption of the reaction heat for adiabatic heating of precooled feedstock or internal heat removal by the boiling of reactants in a reaction mixture [86]. Formation of a quasi-plug flow mode (when a reaction zone reaches the heat-conducting reactor walls) is a logical method for external heat removal. 

Thus, the zone model of a reactor implies a combination of sequentially connected adiabatic (autothermal) quasi-isothermal turbulent plug flow reactors and heat exchange elements with external heat removal. 

For average MW and MWD of the polymer obtained in the sequentially connected quasi-isothermal adiabatic turbulent plug flow reactors and heat exchangers with external heat removal, the following relations are true [2] ... 

Thus, even in an adiabatic mode of tubular turbulent chlorination reactor operation (without heat removal), the temperature growth in the reaction zone in the case of BR chlorination (12-15% solution) with molecular chlorine in a tubular reactor, operating in the optimum plug-flow mode in turbulent flows, does not exceed 2 1 °C. The process can be thought to proceed under quasi-isothermal conditions and does not require external or internal heat removal, or special stirring devices for heat and mass exchange intensification. 

Formation of the quasi-plug flow mode in the reaction zone in turbulent flows, due to the intensive convective heat exchange in proximity to the reactor walls and resulting quasi-isothermal conditions, allows efficient control of the thermal mode of the neutralisation process by external cooling. In order to decide whether it is reasonable to use external cooling for the neutralisation reaction in a tubular turbulent reactor of cylinder form, it is necessary to estimate the amount of heat which is released in the reaction zone under conditions of adiabatic heating. If the density and thermal capacity of the reaction mixture in the first approximation correspond to the values for water, then [15] ... 

T0 compute the maximum work, we need tw o other idealizations. A reversible work source can change volume or perform work of any other kind quasi-statically, and is enclosed in an impermeable adiabatic waU, so 6g = TdS = 0 and dU = S w. A reversible heat source can exchange heat quasi-statically, and is enclosed in a rigid wall that is impermeable to matter but not to heat flow, so = pdV = 0 and dU = 6q = TdS. A reversible process is different from a reversible heat or work source. A reversible heat source need not have AS = 0. A reversible process refers to changes in a whole system, in w-hich a collection of reversible heat plus work sources has AS = 0. The frictionless weights on pulleys and inclined planes of Newtonian mechanics are reversible w ork sources, for example. The maximum possible work is achieved w hen reversible processes are performed with reversible heat and work sources. 

EXAMPLE 8.8 The quasi-static adiabatic expansion of an ideal gas. Let s start with an idealization, a gas expanding slowly in a cylinder with no heat flow, Sq = 0. (Nearly adiabatic processes are common in real piston engines because the heat transfer processes are much slower than the volume changes within the cylinders.) What is the temperature change inside the cylinder as the gas expands ... 

Thus, the mathematic models for reactor design are also classified into continuous heat exchange bed and adiabatic one. Usually, the design of reactor adopts one-dimension quasi-homogeneous model which considers that when reactive gas passes the catalyst bed like a plug-flow, there exist no radial and axial return mixture, and microkinetics can be treated in intrinsic kinetics multiplied by an effective factor that involves the effects of transfer processes, and by an active coefficient that involves the effects of reduction, poisoning and declining etc. Macrokinetics can be... 

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