Plug flow reactor adiabatic operation

Continuous plug flow reactors are also unsuitable for these purposes because it is usually impossible to obtain an isothermic mode in such reactors, even for reactions with a relatively low rate of reaction. Plug flow reactors usually operate in adiabatic or intermediate modes, which are far from isothermic even with an external heat removal modification. In can be stated that almost all industrial reactors employed for fast processes are not optimally designed and are therefore ineffective. The quality of products is also far from optimal and the processes are generally not perfect from an engineering, economical, or social point of view (decrease of final product yield and quality, increase of nonrecyclable wastes, excessively high consumption of raw materials and low energy efficiency). 

Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

For a single plug flow reactor optimum conditions for adiabatic operation are, obtained by varying the feed temperature so that the Average... 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

Industrial reactors generally operate adiabatically. Cholette and Blanchet [8] compared adiabatic plug flow reactor to the CSTRmm. For exothermic reactions, they inferred that the performance of a CSTRmm is better than that of a plug flow reactor at low values of conversion, and vice-versa at high values of conversion. They further showed that the design considerations for endothermic reactions are similar to those for isothermal reactions. 

The primary reason for choosing a particular reactor type is the influence of mixing on the reaction rates. Since the rates affect conversion, yield, and selectivity we can select a reactor that optimizes the steady-state economics of the process. For example, the plug-flow reactor has a smaller volume than the CSTR for the same production rate under isothermal conditions and kinetics dominated by the reactant concentrations. The opposite may be true for adiabatic operation or autocata-lytic reactions. For those situations, the CSTR would have the smaller volume since it could operate at the exit conditions of a plug-flow reactor and thus achieve a higher overall rate of reaction. 

For adiabatic operations, HTN = 0 (no heat-transfer area), and for plug-flow reactors Eq. 5.2.55 reduces to... 

For comparison, for adiabatic plug-flow reactor (R = 0) of the same volume, the outlet extent is Zout — 0.742, and 9out =1.135. The production rate of product B is 1,131 mol/min. For adiabatic CSTR R = oo) of the same volume, the outlet extent is Zout = 0.841, and 9out= 1.132. The production rate of product B is 1260 mol/min. Note that for both isothermal and adiabatic operation, a recycle reactor provides a higher production rate of product B than a corresponding plug-flow reactor and a CSTR. 

R9.5. Adiabatic Operation of a Batch Reactor R9.6. Unsteady Operation of Plug-Flow Reactors... 

Dehydrogenation of ethylbenzene to styrene is normally accomplished in a fixed-bed reactor. A catalyst is packed in tubes to form the fixed bed. Steam is often fed with the styrene to moderate the temperature excursions that are characteristic of adiabatic operation. The steam also serves to prolong the life of the catalyst. Consider the situation in which we model the behavior of this reactor as an isothermal plug flow reactor in which the dehydrogenation reaction occurs homogeneously across each cross section of the reactor. The stoichiometry of the primary reaction is... 

J. M. Castro, S. D. Lipshitz, and C. W. Macosko [AIChE J., 28, 973 (1982)] modeled a thermosetting polymerization reaction in a laminar flow reactor under several different operating conditions. Demonstrate your ability to simulate the performance of a plug flow reactor for this reaction under both isothermal and adiabatic reaction conditions. In particular, determine the reactor space times necessary to achieve 73% conversion for both modes of operation and the following parameter values for a (3/2)-order reaction (r = kc - ). 

The aforementioned researchers have utilized experimental and computer simulation studies of this reaction to assess the performance of adiabatic reactors subjected to various modes of operation. An analysis of the performance of a plug flow reactor operated adiabaticaUy with a feed entering at 20°C that is 0.4 M in thiosulfate and 0.6 M in hydrogen peroxide indicates that the space time necessary to achieve 70% conversion of the limiting reagent is 38.9 s. 

The performance of a reactor is very sensitive to the tempraature of the inlet Stream. This is especially true for plug-flow reactors, and for reactors that operate adiabatically. 

The tray temperatures of the trap-out trays are the inlet temperatures to the external reactors. The ordinary differential equations of the plug flow reactor for steady-state adiabatic operation are integrated from 0 to the total reactor volume Vr, keeping track of how the component compositions and temperature change along the reactor. These equations are the following ... 

The size of reactor needed for a given duty is found as follows. For plug flow tabulate the rate for various Xp along this adiabatic operating line, prepare the y -rp) versus Xp plot and integrate. For mixed flow simply use the rate at the conditions within the reactor. Figure 9.8 illustrates this procedure. 

Figure 9.8 Finding reactor size for adiabatic operations of plug flow and mixed flow reactors.

No real reactor is either perfectly mixed or in pure plug flow. An interpolating procedure to bridge the factor of 3 is needed for the rational design and safe operation of adiabatic reactors. 

---