Flow, adiabatic external

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

For horizontal adiabatic flow with no external work, this becomes Ah + jAV2 = 0... 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

Description of a thermodynamic system requires specification of the way in which it interacts with the environment. An ideal system that exchanges no heat with its environment is said to be protected by an adiabatic wall. To change the state of such a system an amount of work equivalent to the difference in internal energy of the two states has to be performed on that system. This requirement means that work done in taking an adiabatically enclosed system between two given states is determined entirely by the states, independent of all external conditions. A wall that allows heat flow is called diathermal. 

Figure 6.2 Schematic representation of (a) an adiabatic calorimeter, (b) an isoperibol calorimeter, and (c) a heat conduction (or heat flow) calorimeter. fc and 7] are the temperatures of the calorimeter proper and the external jacket, respectively, and is the heat flow rate between the calorimeter proper and the external jacket.

When an open system in steady flow undergoes an adiabatic process without performing external work, the enthalpy of the system regains its initial value at each equilibrium state, and the entropy increases as before. Example Successive, slow expansions through porous plugs P[. Py - (Fig. 2), when we have... 

In some cases, it will be possible to consider the system as isolated (i.e., not interacting with the surroundings). In order to be isolated, the boundaries of a system must be impermeable to mass and energy. Such boundaries cannot allow any interaction with external mechanical or electrical forces. For example, if there is an external pressure, the walls of the system must be rigid so that they cannot be moved by the pressure. In addition, the system must also be adiabatic (i.e., not allowing any energy to flow through the walls in the absence of such forces). 

FIG. 19-3 Fixed-bed reactors with heat exchange, (a) Adiabatic downflow, (b) Adiabatic radial flow, low AP. (c) Built-in interbed exchanger, (d) Shell and tube, (e) Interbed cold-shot injection, (f) External interbed exchanger, (g) Autothermal shell, outside influent/effluent heat exchanger. (h) Multibed adiabatic reactors with interstage heaters, (t) Platinum catalyst, fixed-bed reformer for 5000 BPSD charge rates reactors 1 and 2 are 5.5 by 9.5 ft and reactor 3 is 6.5 by 12.0 ft temperatures 502 433, 502 => 471,502 => 496°C. To convert feet to meters, multiply by 0.3048 BPSD to m3/h, multiply by 0.00662. 

All authors, for instance, consider the jacket oil at constant temperature. This assumption, equivalent to that of infinite oil flow rate, makes it impossible to correctly compute the overall heat transfer coefficient and the thermal driving force. Since heat exchange plays an important role in the conduction of industrial reactors, where more than one third of the polymerization heat is removed through the external cooling oil (only very low conversion reactors can be assumed adiabatic, as claimed by Chen et al.), this limitation cannot be accepted. 

If we consider the change of local entropy of a system at steady state ds/dt = 0, the local entropy density must remain constant because external and internal parameters do not change with time. However, the divergence of entropy flow does not vanish div J, = . Therefore, the entropy produced at any point of a system must be removed or transferred by a flow of entropy taking place at that point. A steady state cannot be maintained in an adiabatic system, since the entropy produced by irreversible processes cannot be removed because no entropy flow is exchanged with the environment. For an adiabatic system, equilibrium state is the only time-invariant state. 

We noted earlier in this chapter that many reactions in the chemical industries are exothermic and require heat removal. A simple way of meeting this objective is to design an adiabatic reactor. The reaction heat is then automatically exported with the hot exit stream. No control system is required, making this a preferred way of designing the process. However, adiabatic operation may not always be feasible. In plug-flow systems the exit temperature may be too hot due to a minimum inlet temperature and the adiabatic temperature rise. Systems with baekmixing suffer from other problems in that they face the awkward possibilities of multiplicity and open-loop instability. The net result is that we need external cooling on many industrial reactors. This also carries with it a control system to ensure that the correct amount of heat is removed at all times. 

The advantages of the ring-shaped particles are also found for other type of reactions. To demonstrate this, consider an adiabatic plug flow reactor assuming that the external mass and heat transfer limitations are negligible. Equations for fluid-phase concentration and temperature (which are equal to the concentration and temperature at the surface of the pellet) are... 

A model for an adiabatic HDS reactor (see Fig. 4-7) with a single quench is given by Shah et al.46 Under plug-flow conditions and assuming that there are no external mass-transfer resistances, the governing material and energy-balance equations are... 

Since there is no radial flow, the reactor will be very close to adiabatic, even with small reactor diameters. There are also limited possibilities to introduce a heat exchanger between the monolith beds. The two-phase flow is sensitive to disturbances. However, in the existing plants temperature control is no problem, due to the high heat capacity of the liquid and the low conversion in each passage. An external heat exchanger in the liquid flow is sufficient to control the reactor temperature. 

New reactor types for three-phase operation are still being developed. An example is the application of structured reactors, which may have certain advantages in three-phase operation, and can be operated with co-, cross- as well as with countercurrent flow. A very recent development is the use of monolith reactors (Fig. 8.9) in three-phase operations. Their advantages are the small pressure drop, the good external mass transfer, the short diffusion distance, and the low adiabatic temperature rise. Disadvantages are the higher catalyst costs, importance of liquid distribution, and moderate catalyst load. 

The H-Oil reactor (Fig. 21) is rather unique and is called an ebullated bed catalytic reactor. A recycle pump, located either internally or externally, circulates the reactor fluids down through a central downcomer and then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually well insulated and operated adiabatically. Frequently, the reactor-mixing pattern is defined as backmixed, but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle. Thus, the reactor equations for the axial dispersion model are modified appropriately to account for recycle conditions. 

First note the phrase of itself —heat cannot of itself pass fiom a low to a high temperature We have seen that by means of an adiabatic compression of a system the temperature rises, heat being evidently thereby carried up the temperature scale But this does not contradict the above statement about the natural flow of heat, for we have, by the aid of external agency, had to do work, namely, to compress the system m order to make the temperature rise It is a known experimental fact that systems, say gases, naturally tend to expand, the molecules tend to fly apai t and not to contract This fact is evidence, of a molecular... 

The simplest heterogeneous model is that with plug flow in the fluid phase and only external mass and heat transfer resistances between the bulk fluid and the catalyst surface. More complex fluid phase behaviour can be accommodated by including axial and radial dispersion mechanisms into the mode). If tJie reactor is non-adiabatic, radial dispersion is usually more important. 

---