Open chemical reactor

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

In an open system such as a CSTR chemical reactions can undergo self-sustained oscillations even though all external conditions such as feed rate and concentrations are held constant. The Belousov-Zhabotinskii reaction can undergo such oscillations under isothermal conditions. As has been demonstrated both by experiments [1] and by calculations 12,3] this reaction can produce a variety of oscillation types from simple relaxation oscillations to complicated multipeaked periodic oscillations. Evidence has also been given that chaotic behavior, as opposed to periodic or quasi-periodic behavior, can take place with this reaction [4-12]. In addition, it has been shown in recent theoretical studies that chaos can occur in open chemical reactors [11,13-17]. 

Fail-safe is a concept used to specify the position of process instrumentation in the event of power, air pressure, or other utility failures. For instance, the valve supplying cooling water to a chemical reactor would fail in the open position ( fail open ) in the... 

The efficiency of exergy utilization in an open system (chemical reactor) is then defined by Eq. 11.4 ... 

Example 4.8 Chemical reactions and reacting flows The extension of the theory of linear nonequilibrium thermodynamics to nonlinear systems can describe systems far from equilibrium, such as open chemical reactions. Some chemical reactions may include multiple stationary states, periodic and nonperiodic oscillations, chemical waves, and spatial patterns. The determination of entropy of stationary states in a continuously stirred tank reactor may provide insight into the thermodynamics of open nonlinear systems and the optimum operating conditions of multiphase combustion. These conditions may be achieved by minimizing entropy production and the lost available work, which may lead to the maximum net energy output per unit mass of the flow at the reactor exit. 

Chemical reactors are inherently nonlinear in character. This is primarily due to the exponential relationship between reaction rate and temperature but can also stem from nonlinear rate expressions such as Eqs. (4.10) and (4.11). One implication of this nonlinearity for control is the change in process gain with operating conditions. A control loop tuned for one set of conditions can easily go unstable at another operating point. Related to this phenomenon is the possibility of open-loop instability and multiple steady states that can exist when there is material and/or thermal recycle in the reactor. It is essential for the control engineer to understand the implications of nonlinearities and what can be done about them from a control standpoint as well as from a process design standpoint. 

Here we have dealt with the control of chemical reactors. We covered some of the fundamentals about kinetics, reactor types, reactor models, and open-loop behavior. In particular we have shown that reactors with recycle or backmixing can exhibit multiple steady states, some of which are unstable. Nonlinearities in reactor systems also frequently give rise to open-loop parametric sensitivity. 

On the other hand, well engineered manufacturing operations depend on the availability of manipulated variables for real-time feedback control. These variables usually operate at macroscopic length scales (e.g. the power to heat lamps above a wafer, the fractional opening of valves on flows into and out of a chemical reactor, the applied potential across electrodes in an electrochemical process). The combination of a need for product quality at the molecular scale with the economic necessity that feedback control systems utilize macroscopic manipulated variables motivates the creation of methods for the simulation, design and control of multiscale systems. 

Below, we apply the energy balances for macroscopic systems. First, we derive the energy balance equation for closed systems (batch reactors) and then for open systems (flow reactors). Microscopic energy balances, used to describe point-to-point temperature variations inside a chemical reactor, are outside the scope of this book. 

If an open system in a stationary state is considered, e.g. a chemical reactor which is exchanging heat and matter with the surroundings at Tq, then, at constant p, according to the First and Second Laws of thermodynamics the following balances pertain ... 

Most processes are open loop stable. However, the exothermic irreversible chemical reactor is a notable example of a process that can be openloop unstable. 

Open systems consist of vessels from which the volatile pyro-products can leave the heated reactor thus avoiding secondary reactions. Closed systems are sealed reactors that confine all reactants and products to a restricted volume. The removal of gaseous products from the open system reactor can be enhanced by purging with inert or chemically active... 

In the Mars Polar Lander loss, the software requirements did not include information about the potential for the landing leg sensors to generate noise or, alternatively, to ignore any inputs from the sensors while the spacecraft was more than forty meters above the planet surface. In the batch chemical reactor accident, the software engineers were never told to open the water valve before the catalyst valve and apparently thought the ordering was therefore irrelevant. 

Since they are open systems that can exchange chemical species with their surrounding solvent, gels can also play the role of chemical reactors. In this framework, the design of open spatial gel reactors has allowed well controlled experimental studies of chemical patterns such as chemical waves or Turing structures (2). They are made of a thin film of gel in contact with one or two continuous stirred tank reactors that sustain controlled nonequilibrium conditions. 

PI has inspired the development of new equipment as well as new innovative processes. Until now, most efforts were directed toward improved transport properties in chemical reactors to achieve processes, Hmited only by the inherent chemical kinetics. Further activities concern the use of alternative energy resources and nonconventional fluids. The development of structured catalysts and reactors can be considered as a mean for a significant increase of product yield and productivity. It fadhtates the transformation of batch to continuous processes and opens new process windows for the use of high temperature and high reactant concentrations and allows increased process flexibility. 

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