Thermally safe operation of a semibatch reactor for liquid-liquid reactions. Slow reactions

Abstract

Accumulation of the reactant supplied to a cooled semibatch reactor (SBR) will occur if the mass transfer rate across the interface is insufficient to keep pace with the supply rate. Then, due to a low starting temperature or supercooling, the reaction temperature does not rise fast enough to the desired value. This accumulation may eventually lead to a temperature runaway. We investigated the possibility of such an event for reactions of the type "chemically enhanced mass transfer" or "fast" and found that only low distribution coefficients, i.e. or lower, can lead to accumulation. At higher distribution coefficients, the mass transfer rate across the interface of a well-mixed dispersion is generally sufficient to prevent accumulation. A thermal runaway in the fast regime exerts a moderate effect, because the effective activation energy is halved. Calculations for the "instantaneous" reaction regime, regarded as a special case of fast reactions, show that there is no runaway possible.* Dr. ir. M. Steensma,

Thermally safe operation of a semibatch reactor for liquid‐liquid reactions–fast reactions

“…The calculations for the fast reaction category show the existence of the same thermal phenomena of QFS-reaction, marginal ignition and temperature overshoot as for slow reactions [2]. The boundary diagrams for fast reaction regimes are given in Figs 4 and 5 for reactions in the dispersed and the continuous phase, respectively.…”

“…The conversion rates are known from literature [3] and for fast reactions in the continuous and dispersed phases are given by It is important to mention the variability of some factors in Eqs (1) and (2). The interfacial area is given by the standard expression: a = 6 Ed/db = 6 ~8 / ( 1 +~8 ) .…”

Temperature diagrams for preventing decomposition or side reactions in liquid–liquid semibatch reactors

“…If the initial temperature is too high, a situation can arise in which the coreactant accumulation is confined below critical values but the initial temperature itself is too close to the MAT value for avoiding the overshooting of the threshold temperature. For the identification of low accumulation operating conditions in liquid-liquid SBRs in which exothermic reactions occur, Steensma and Westerterp (1988, 1990, 1991 firstly developed the so-called boundary diagrams method. Such diagrams provide, in a suitable dimensionless space defined through an "exothermicity number" (E x ) and a "reactivity number" (R y , see Fig.…”

“…In fact, even if operating conditions belonging to the excessive accumulation region of the boundary diagram are normally characterized by a higher reaction temperature rise than the so-called quick onsetfair conversion-smooth temperature profile (QFS) conditions (Steensma and Westerterp, 1990), it must be stressed that for a higher initial temperature even a lower temperature rise can cause the maximum reaction temperature to exceed the MAT value.…”

Safe operation of stirred‐tank semibatch reactors subject to risk of thermal hazard

“…According to Steensma and Westerterp,21,25,26 SBR is safe if a sudden temperature excursion is prevented and if reactant accumulation is avoided. Because of the difficulties of formulating an appropriate accumulation criterion, these authors based safety on the evolution of temperature of the reacting mixture.…”