Reactants accumulation

Is reactant accumulation possible Steady state concentrations Reaction calorimetry combined with analysis... 

The released energy might result from the wanted reaction or from the reaction mass if the materials involved are thermodynamically unstable. The accumulation of the starting materials or intermediate products is an initial stage of a runaway reaction. Figure 12-6 illustrates the common causes of reactant accumulation. The energy release with the reactant accumulation can cause the batch temperature to rise to a critical level thereby triggering the secondary (unwanted) reactions. Thermal runaway starts slowly and then accelerates until finally it may lead to an explosion. 

Elow intensification is made with the use of apparatuses in which flow follows a perfect plug flow the internal parts of the reactor have to be designed accordingly. Indeed, dead zones, that is, reactant accumulation, must be avoided not only in order to have better selectivity and yield but also to avoid formation of hot spots, which would generate safety problems. 

According to the literature [77], a process is considered to be low hazard from the thermal standpoint if the normal operating temperature or temperature due to upset is 50°C or more lower than the ARC onset temperature, and the maximum process temperature is held for only a short period of time. However, other factors must be considered in evaluating the thermal hazard of a process such as total enthalpy of reaction or decomposition, potential for reactant accumulation, the boiling point of the reaction mass, and the rate of reaction. The testing must involve all appropriate materials including reactants, intermediates, and products. In some cases, though, the 50°C differential... 

It may be possible to minimise the time spent on relief system assessment and design if the basis of safety for a multi-purpose reactor can be changed to prevention rather than emergency pressure relief. For example, if it can be arranged for all the reactions to operate in semi-batch mode with no significant reactant accumulation, then the use of a trip system of sufficient integrity may provide a suitable basis of safety. (This may not always be possible.)... 

Heat flow calorimetry indicated that failure of the heater control to switch off and of cooling water to switch on (case (e)) was not a problem, since the steam heating is unable to exceed 100°C and, for this semi-batch reaction, the total temperature increase only reduces further any reactant accumulation. This means that cases (b), (f) and (d) are all worse than case (e). 

If relief sizing is for a continuous or semi-batch reactor, then it may be appropriate to use isothermal calorimetry to determine the amount of reactant accumulation under worst case conditions. The mass of the accumulation, rather than the "all-in 1 batch mass, can then be used for relief system sizing and this can reduce the required relief system size. It should sbe noted that it will still be necessary to carry out suitable adiabatic tests, as described below. Further information is given by Singh1101. 

This non-converted reactant B is called the reactant accumulation. It results from the mass balance, that is, the feed rate as input and the reaction rate as consumption. In other words, a low accumulation is obtained when the feed rate of B is slower than the reaction rate. Since, as defined in Equation 7.4, the reaction rate depends on both concentrations CA and CB, this means that both reactants must be present in the reaction mixture in a sufficiently high concentration. For fast reactions, such as those with a high rate constant, even for low concentrations of the reactant B, the reaction will be fast enough to avoid the accumulation of unconverted B in the reactor. For slow reactions, a significant concentration of B is required to achieve an economic reaction rate. Thus, two cases have to be considered fast reactions and slow reactions. 

If the deviation was an uncontrolled temperature increase, the temperature increase will continue and accelerate the reaction until the accumulated reactant has been converted. Therefore, it is important to know quantitatively the degree of reactant accumulation during the reaction course, as it predicts the degree of conversion, which may occur after interruption of the feed. This can be done by chemical analysis or by using a heat balance, for example from an experiment in a reaction calorimeter [4]. Since the accumulation is the result of a balance between the amount of reactant B introduced by the feed and the amount converted by the reaction, a simple difference between these two terms calculates the accumulation [5, 6]. 

For a single reaction, the determination of reactant accumulation can be done directly by using calorimetric methods the conversion is replaced by the thermal conversion defined by... 

Besides the heat release rate, the feed rate also affects the maximum reactant accumulation, a further important safety related parameter. The accumulation governs the temperature (T ) which may be reached in the case of a cooling failure. If the feed is immediately halted at the instant the failure occurs, the attainable temperature is expressed by... 

The thermal characteristics of a reaction, including its heat production rate, the necessary cooling power, and the reactant accumulation, are fundamental for safe reactor operation and process design. A successful scale-up is achieved, only when the different characteristic time constants of the process, such as reaction kinetics, thermal dynamics of the reactor, and its mixing characteristics are in good agreement [9]. If we focus on the reaction kinetics and thermal dynamics, that is, we consider that the reaction rate is slow compared to the mixing rate, in principle, there are two ways to predict the behavior of the industrial reactors ... 

For the designer, understanding the mass balance of the plant is a key requirement that can be fulfilled only when the reactor/separation/recycle structure is analyzed. The main idea is that all chemical species that are introduced in the process (reactants, impurities) or are formed in the reactions (products and byproducts) must find a way to exit the plant or to be transformed into other species [4]. Usually, the separation units take care that the products are removed from the process. This is also valid for byproducts and impurities, although is some cases inclusion of an additional chemical conversion step is necessary [5, 6]. The mass balance of the reactants is more difficult to maintain, because the reactants are not allowed to leave the plant but are recycled to the reaction section. If a certain amount of reactant is fed to the plant but the reactor does not have the capacity of transforming it into products, reactant accumulation occurs and no steady state can be reached. The reaction stoichiometry sets an additional constraint on the mass balance. For example, a reaction of the type A + B —> products requires that the reactants A and B are fed in exactly one-to-one ratio. Any imbalance will result in the accumulation of the reactant in excess, while the other reactant will be depleted. In practice, feeding the reactants in the correct stoichiometric ratio is not trivial, because there are always measurement and control implementation errors. 

We say that the inventory is self-regulating. Similarly, the plantwide control can fix the flow rate of reactant at the plant inlet. When the reactant accumulates, the consumption rate increases until it balances the feed rate. This strategy is based on a self-regulation property. The second strategy is based on feedback control of the inventory. This consists of measuring the component inventory and implementing a feedback control loop, as in Fig. 4.2(b). Thus, the increase or decrease of the reactant inventory is compensated by less or more reactant being added into the process. 

The first inequality characterizes recycle systems with reactant inventory control based on self-regulation. It occurs because the separation section does not allow the reactant to leave the process. Consequently, for given reactant feed flow rate F0, large reactor volume V or fast kinetics k are necessary to consume the whole amount of reactant fed into the process, thus avoiding reactant accumulation. The above variables are grouped in the Damkohler number, which must exceed a critical value. Note that the factor z3 accounts for the degradation of the reactor s performance due to impure reactant recycle, while the factor (zo — z4) accounts for the reactant leaving the plant with the product stream. 

Let us consider a small, positive deviation of the reactor-inlet flow rate, from the steady state B. At the right of point B, the amount of reactant fed in the process is larger than the amount of reactant consumed. Reactant accumulation occurs, leading to a further increase of the recycle and reactor-inlet flow rates hence the steady state B is unstable. This is independent of the dynamics, because the proof is based only on steady-state considerations. 

Whilst this technique does not directly provide data on the course of a reaction, the shape of the trace does give an indication. For example, the time for the temperature rise to occur will seem to increase on subsequent additions if the reaction rate is falling off. The technique can also be used to investigate typical maloperations. Examples are the effect of a double charge — does reactant accumulation occur — and the effect of agitator failure — is there any build-up of reactants or does the reaction continue ... 

Where reactant accumulates because tbe chosen reaction temperature is too low, the reaction continues after the end of the addition (work-off). In these cases a hazardous situation could occur if cooling were lost. The following example illustrates this phenomenon and shows how it can be eliminated if the reaction is carried out at a higher temperature to reduce the amount of accumulation. 

A process designed near the turning point of the Da - za,i map can suffer from operability problems. If the reaction kinetics is over-estimated, or the feed flow rate deviates from the nominal design value, the operating point falls at the left of the turning point, in the region where no steady state exists. As a result, infinite reactant accumulation occurs, and the plant has to be shut down. 

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