Tubular reactor systems

In a tubular reactor system, the temperature rises along the reactor length for an exothermic reaction unless effective cooling is maintained. For multiple steady states to appear, it is necessary that a... 

Verwijs, J. W., H. van den Berg, and K. R. Westerterp (1996). "Startup Strategy Design and Safeguarding of Industrial Adiabatic Tubular Reactor Systems. AIChE Journal 42, 2 (February), 503-15. 

Selective oxidation of p-xylene was carried out over the temperature range of 450-590°C at an atmospheric pressure using an 8-channel parallel tubular reactor system made in-... 

Adiabatic with Intermediate Heat Transfer. Many tubular reactor systems use a series of adiabatic reactors with heating or cooling between the reactor vessels. For example, naphtha reforming has endothermic reactions of removing hydrogen from saturated cyclical naphthene hydrocarbons to form aromatics. The process has multiple adiabatic reactors with fired furnaces between the reactors to heat the material back up to the required reactor inlet temperature. 

STEADY-STATE DESIGN OF TUBULAR REACTOR SYSTEMS... 

The discussion above points out one of the most important tradeoffs in chemical reactor design. The smaller the reactor size, the larger the recycle flowrate. This reactor/recycle tradeoff dominates the steady-state economics of the design of tubular reactor systems, as we will illustrate in several examples in this chapter. It also has a major impact on dynamic control, as we will see in the next chapter. 

This chapter presents a comparison of the steady-state economics of four alternative tubular reactor systems. The entire process will be considered, not just the reactor in isolation, because the optimum economic steady-state design can be determined only for the entire plant. The type of recycle, the phase of the reaction, and the heat transfer configuration all affect the optimum design. 

SINGLE ADIABATIC TUBULAR REACTOR SYSTEMS WITH GAS RECYCLE 265... 

The final tubular reactor system considered is one in which a single cooled reactor is used. Figure 5.19 shows the flowsheet. The cooled reactor, which is assumed to be simply a shell-and-tube heat exchanger, has catalyst packed inside the parallel tubes. Steam is generated on the shell side, serving as a coolant. The liquid level in the shell is controlled by bringing in BFW to keep the tubes covered. The steam-side temperature is constant at all axial locations in the reactor because the BFW is vaporized at a constant temperature. The temperature of the steam is assumed to be equal to the reactor inlet temperature, and the temperature of the BFW is assumed to be equal to the steam temperature. 

The design of tubular reactor systems is dominated by the classical tradeoff between reactor size and recycle flowrate. Gas phase systems are particularly affected because of the high cost of compression. 

The four types of tubular reactor systems designed in Chapter 5 are investigated for dynamic controllability in this chapter. The four flowsheets are given in Figures 6.1 -6.4 with stream conditions and equipment sizes shown. These are the optimum economic flowsheets for the expensive catalyst cases. A three-bed cold-shot system is shown, but a seven bed system is the optimum steady-state design. As we will show, the seven bed system is uncontrollable. 

Figures 6.5-6.8 show the control structures used for the four alternative tubular reactor systems. In Figures 6.6 and 6.7 (the multistage adiabatic systems with interstage...

Openloop Response The openloop responses of a single adiabatic tubular reactor system to +20% step changes in recycle flowrate FR are shown in Figure 6.9. The solid lines represent increases in recycle flow and the dashed lines, decreases. The results show that the system produces limit cycle behavior, alternating between high temperatures and low temperatures. This type of dynamic response is called openloop-unstable behavior in this chapter. 

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