Pressure reactors

Now consider the effect of pressure. For reversible reactions, pressure can have a significant effect on the equilibrium conversion. Even though the equilibrium constant is only a function of temperature and not a function of pressure, equilibrium conversion can still be influenced through changing the activities (fugacities) of the reactants and products. 

Consider again the ammonia synthesis example from Examples 6.1 and 6.2 

Example 6.4 Following Example 6.2, the reactor temperature will be set to 700 K. Examine the effect of increasing the reactor pressure by calculating the equilibrium conversion of hydrogen at 1 bar, 10 bar, 100 bar and 300 bar. Assume initially ideal gas behavior. 

Solution The ideal gas equilibrium constants can be corrected for real gas behavior by multiplying the ideal gas equilibrium constant by K,f as defined by Equation 6.23, which for this problem is  

The fugacity coefficients fa can be calculated from the Peng-Robinson Equation of State. The values of fa are functions of temperature, pressure and composition, and the calculations are complex (see Pohling, Prausnitz and O Connell6 and Chapter 4). Interaction parameters between components are here assumed to be zero. The results showing the effect of nonideality are given in Table 6.9  

Increasing the pressure of irreversible vapor-phase reactions increases the rate of reaction and hence decreases reactor volume both by decreasing the residence time required for a given reactor conversion and increasing the vapor density. In general, pressure has little effect on the rate of liquid-phase reactions. 

The selection of reactor pressure for vapor-phase reversible reactions depends on whether there is a decrease or increase in the number of moles and whether there is a system of single or multiple reactions. 

This is an endothermic reaction accompanied by an increase in the number of moles. High conversion is favored by high temperature and low pressure. The reduction in pressure is achieved in practice by the use of superheated steam as a diluent and by operating the reactor below atmospheric pressure. The steam in this case fulfills a dual purpose by also providing heat for the reaction. 

Multiple reactions producing byproducts. The arguments presented for the effect of pressure on single vapor-phase reactions can be used for the primary reaction when dealing with multiple reactions. Again, selectivity is likely to be more important than reactor volume for a given conversion. 

If there is a significant difference between the effect of pressure on 

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This was a Hquid-phase process which used what was described as siUceous zeoUtic catalysts. Hydrogen was not required in the process. Reactor pressure was 4.5 MPa and WHSV of 0.68 kg oil/h kg catalyst. The initial reactor temperature was 127°C and was raised as the catalyst deactivated to maintain toluene conversion. The catalyst was regenerated after the temperature reached about 315°C. Regeneration consisted of conventional controlled burning of the coke deposit. The catalyst life was reported to be at least 1.5 yr. 

Hydrocarbon, typically natural gas, is fed into the reactor to intersect with an electric arc stmck between a graphite cathode and a metal (copper) anode. The arc temperatures are in the vicinity of 20,000 K inducing a net reaction temperature of about 1500°C. Residence time is a few milliseconds before the reaction temperature is drastically reduced by quenching with water. Just under 11 kWh of energy is required per kg of acetylene produced. Low reactor pressure favors acetylene yield and the geometry of the anode tube affects the stabiUty of the arc. The maximum theoretical concentration of acetylene in the cracked gas is 25% (75% hydrogen). The optimum obtained under laboratory conditions was 18.5 vol % with an energy expenditure of 13.5 kWh/kg (4). 

The Amoco reactor operates at 70—80°C and 2 MPa (300 psi) reactor pressure. The existence of several partially isolated compartments allows a semi-iadependent control of temperature as well as comonomer and hydrogen concentrations within each section, which ia turn offers a substantial control of the molecular weight and MWD of resias. Amoco technology also accommodates a large variety of polymerization catalysts, including Phillips and Ziegler catalysts. 

Fig. 4. Higher yields from lower pressures reactor pressure A, 791 kPa (100 psig) B, 1480 kPa (200 psig) C, 2515 kPa (350 psig). Data for Arabian...

In the modern unit design, the main vessel elevations and catalyst transfer lines are typically set to achieve optimum pressure differentials because the process favors high regenerator pressure, to enhance power recovery from the flue gas and coke-burning kinetics, and low reactor pressure to enhance product yields and selectivities. 

The reactor effluent is rapidly quenched with aqueous mother Hquor in specially designed equipment operating at pressures essentially equal to the reactor pressure. This operation yields an off-gas consisting of ammonia and carbon dioxide vapor and a crystalline melamine slurry saturated with ammonia and carbon dioxide. The slurry is concentrated in a cyclone mill. The mother Hquor overflow is returned to the quenching system. The concentrated slurry is redissolved in the mother Hquor of the crystallization system, and the dissolved ammonia is stripped simultaneously. 

Other types of selective systems employ multiple final control elements or multiple controllers. In some applications, several manipulated variables are used to control a single process variable (also called split-range control). Typical examples include the adjustment of both inflow and outflow from a chemic reactor in order to control reactor pressure or the use of both acid and base to control pH in waste-water treatment. In this approach, the selector chooses from several controller outputs which final control element should be adjusted (Marlin, Process Control, McGraw-Hill, New York, 1995). 

A forward pressure regulator (FPR) reduces the pressure and keeps the reactor at 7.5 psig. Reactor pressure is Indicated and recorded by (PIR) and temperature by (TIR). Discharge flow from the reactor is measured by a flow transmitter (FT) and after correction for temperature and pressure the Flow is Recorded on (FR). 

Let Pg(atm) be the initial reactor pressure. Prove that ly2, the time required to achieve 50% conversion of A in the reactor, equals RT/kpg. Assume an ideal gas behavior. 

The decomposition of nitrous oxide (NjO) to nitrogen and oxygen is preformed in a 5.0 1 batch reactor at a constant temperature of 1,015 K, beginning with pure NjO at several initial pressures. The reactor pressure P(t) is monitored, and the times (tj/2) required to achieve 50% conversion of N2O are noted in Table 3-19. Use these results to verify that the N2O decomposition reaction is second order and determine the value of k at T = 1,015 K. 

Procedural The same reactor described in Example 3 above, but without the 5 psig high pressure interlock. Instead, the operator is instructed to monitor the reactor pressure and stop the reactant feeds if the pressure exceeds 5 psig. There is a potential for human error, the operator failing to monitor the reactor pressure, or failing to stop the reactant feeds in time to prevent a runaway reaction. 

Warning If pipeline valves are not closed oil or solvent may flow back due to possible mischarges or high reactor pressures. 

Warning High reactor pressures may cause backflow... 

Valve after reactor Pressure buildup/e.xplosion/ Feed flow ... 

The sources of steam in the reactor vapor are lift steam to the standpipe, atomization steam to the feed nozzles, dome steam, and stripping steam. Some units may have other streams and the feed may contain water. Depending on the reactor pressure, approximately 25% to 50% of the stripping steam is entrained with the spent catalyst flowing to the regenerator, which should be deducted. 

The reactor pressure is not directly controlled instead, it floats on the main column overhead receiver, A pressure controller on the overhead receiver controls the wet gas compressor and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor. 

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