For gas-phase PFRs

Computational Scheme for Gas-Phase PFRs. A general procedure for solving the reactor design equations for a piston flow reactor using the marching-ahead technique (Euler s method) has seven steps ... 

TTie two reactors with recycle shown in (i) and (j) can be used for highly exothermic reactions. Here the recycle stream is cooled and returned to the reactor to dilute and cool the inlet stream thereby avoiding hot spots and runaway reactions. The PFR with recycle is used for gas-phase reactions, and the CSTR is used for liquid-phase reactions. The last (wo reactors, (k) and (I), are used for thermodynamically limited reactions where the equilibrium lies far to the left (reactant side)... 

Solution of the design equations for liquid phase PFRs is usually easier than for gas phase reactors because pressure typically has little effect on the fluid density or the reaction kinetics. Extreme pressures are an exception that theoretically can be handled by the same methods used for gas phase systems. The difficulty will be finding an equation of state. For ordinary pressures, the mass density can usually be estimated as a simple function of composition. This leads to easy and direct use of Equation 3.2. 

Plug-flow reactors are used for gas-phase and. liquid-phase reactions. If the PFR is filled with a porous catalyst and the fluid flowing in the void space is turbulent, the reactor is referred to as a fixed-bed... 

Equation 4.74 is used with the mole balance, Equation 4.72, to solve gas-phase PFR problems under the ideal-gas assumption. To evaluate the concentrations for use with the reaction rate expressions, one then... 

In addition, we can consider the differential stream reactor (DSR) as a generalisation of PFR model, in which the inlet and outlet streams enter or leave at different side points. In practice DSR can be met as cold-shot reactors for methanol and ammonia synthesis, or as multi-feed tubular reactor for gas-phase ethylene polymerisation. However, the approach is not restricted to homogeneous systems. In fact, the most incentive is the field of heterogeneous reactor when several phases (gas, liquid, solid) are involved. 

Stirred gas phase PFR characteristic allows for fast grade transfer More complex reactor design, higher capital cost... 

A plug-flow reactor (PFR) may be used for both liquid-phase and gas-phase reactions, and for both laboratory-scale investigations of kinetics and large-scale production. The reactor itself may consist of an empty tube or vessel, or it may contain packing or a fixed bed of particles (e.g., catalyst particles). The former is illustrated in Figure 2.4, in which concentration profiles are also shown with respect to position in the vessel. 

Calculate (a) the residence time, t, and (b) the space time, r, and (c) explain any difference between the two, for the gas-phase production of C2H4 from C2H6 in a cylindrical PFR of constant diameter, based on the following data and assumptions ... 

In this chapter, we develop the basis for design and performance analysis for a plug flow reactor (PFR). Like a CSTR. a PFR is usually operated continuously at steady-state, apart from startup and shutdown periods. Unlike a CSTR, which is used primarily for liquid-phase reactions, a PFR may be used for either gas-phase or liquid-phase reactions. 

Consider the gas-phase decomposition of ethane (A) to ethylene at 750°C and 101 kPa (assume both constant) in a PFR. If the reaction is first-order with kA = 0.534s-1(Fro-ment and Bischoff, 1990, p. 351), and r is 1 s, calculate /a- For comparison, repeat the calculation on the assumption that density is constant. (In both cases, assume the reaction is irreversible.)... 

A gas-phase reaction between butadiene (A) and ethene (B) is conducted in a PFR, producing cyclohexene (C). The feed contains equimolar amounts of each reactant at 525°C (T,) and a total pressure of 101 kPa. The enthalpy of reaction is — 115 kj (mol A)- , and the reaction is first-order with respect to each reactant, with kA = 32,000 13,850/7 m3 moi-l S 1. Assuming the process is adiabatic and isobaric, determine the space time required for 25% conversion of butadiene. 

Consider the following gas-phase reaction that occurs at 350 K and a constant pressure of 200 kPa (Lynch, 1986) A- B + C, for which the rate law is (-/a) = 0.253cA/(l + 0.429cA)2, where (- rA) has units of mol m-3 s-1 cA has units of mol m-3. Pure A is fed to a reactor at a rate of 8 mol s-1. The desired fractional conversion, fA, is 0.99. A recycle PFR is proposed for the reaction. When the recycle ratio, R, is zero, the recycle reactor is equivalent to a PFR. As R approaches infinity, the system is equivalent to a CSTR. However, it is generally stated that the recycle reactor behavior is close to that of a CSTR once R reaches approximately 10 to 20. Furthermore, it is often stated that, for an equivalent fractional conversion, the volume... 

A first-order gas-phase reaction, A - C, is conducted in a nonisothermal PFR, operating isobarically. Equimolar amounts of A and an inert species are fed to the reactor at a total rate of 8 L min-1, with cAo = 0.25 mol L-1. Determine the residence time required for 50%... 

For a certain homogeneous, gas-phase reaction taking place in a PFR, suppose it has been... 

For a gas-phase reaction represented by A - B + C carried out (separately) isotheimally in a constant-volume BR, and isothermally and isobarically in a PFR, show that t r and tpF, for a feed of pure A,... 

For a gas-phase reaction in a PFR, the pressure should be relatively high, if... 

The PFR tubular reactor is used for both liquid and gas phases. The reactor is a long vessel with feed entering at one end and product leaving at the other end. In some applications the vessel is packed with a solid catalyst. Some tubular reactors run adiabatically (i.e., with no heat transferred externally down the length of the vessel). The heat generated or consumed by the reaction increases or decreases the temperature of the process... 

The reactions are elementary and take place in the gas phase. The reaction is to be carried out isothermally and as a first approximating pressure drop will be neglected. The feed consists of hydrogen gas, carbon monoxide, j carbon dioxide, and steam. The total molar flow rate is 300 mo /s. The entering pressure may be varied between 1 atm and 160 atm and the entering temperature between 300 K and 400 K. Tubular (PFR) reactor volumes between 0.1 m and 2 m are available for use. 

Table 8-3. (continued) PFR/PBR Algortthm for Heat Effects 3. Stoichiometry (gas phase, no APy. 

PFR/PBR Solution Procedure for a Reversible Gas-Phase Reaction Living Example Problems... 

For the case of isothermal operafion with no pressure drop, we were able to obtain an analytical solution, given by equation B, which gives the reactor volume necessary to achieve a conversion X for a gas-phase reaction carried out isothermaliy in a PFR, However, in the majority of situations, analytical solutions to the ordinary differential equations appearing in (he combine step are not possible. Consequently, we include POLYMATH, or some other ODE solver such as MATLAB, in our menu in that it makes obtaining solutions to the differential equations much more palatable, ... 

---