Reactor volume continuous-flow reactors

From this general mole balance equation, we can develop the design equations for the various types of industrial reactors batch, semibatch, and continuous-how. Upon evaluation of these equations, we can determine the time (batch) or reactor volume (continuous-flow) necessary to convert a specified amount of the reactants into products. 

Fig. 6. Breakthrough curves for aqueous acetone (10 mg 1" in feed) flowing through exnutshell granular active carbon, GAC, and PAN-based active carbon fibers, ACF, in a continuous flow reactor (see Fig. 5) at 10 ml min" and 293 K [64]. C/Cq is the outlet concentration relative to the feed concentration. Reprinted from Ind. Eng. Chem. Res., Volume 34, Lin, S. H. and Hsu, F. M., Liquid phase adsorption of organic compounds by granular activated carbon and activated carbon fibers, pp. 2110-2116, Copyright 1995, with permission from the American Chemical Society.

Continuous-flow reactors are usually preferred for long production runs of high-volume chemicals. They tend to be easier to scaleup, they are easier to control, the product is more uniform, materials handling problems are lessened, and the capital cost for the same annual capacity is lower. 

Micro reactors are continuous-flow devices consuming small reaction volumes and allowing defined setting of reaction parameters and fast changes. Hence they are ideal tools for process screening and optimization studies to develop solution-based chemistries. 

Although the concept of mean residence time is easily visualized in terms of the average time necessary to cover the distance between reactor inlet and outlet, it is not the most fundamental characteristic time parameter for purposes of reactor design. A more useful concept is that of the reactor space time. For continuous flow reactors the space time (t) is defined as the ratio of the reactor volume (VR) to a characteristic volumetric flow rate of fluid (Y). 

The ideal continuous stirred tank reactor is the easiest type of continuous flow reactor to analyze in design calculations because the temperature and composition of the reactor contents are homogeneous throughout the reactor volume. Consequently, material and energy balances can be written over the entire reactor and the outlet composition and temperature can be taken as representative of the reactor contents. In general the temperatures of the feed and effluent streams will not be equal, and it will be necessary to use both material and energy balances and the temperature-dependent form of the reaction rate expression to determine the conditions at which the reactor operates. 

The reactor has facilitated a diverse range of synthetic reactions at temperatures up to 200 °C and 1.4 Pa. The temperature measurements taken at the microwave zone exit indicate that the maximum temperature is attained, but they give insufficient information about thermal gradients within the coil. Accurate kinetic data for studied reactions are thus difficult to obtain. This problem has recently been avoided by using fiber optic thermometer. The advantage of continuous-flow reactor is the possibility to process large amounts of starting material in a small volume reactor (50 mL, flow rate 1 L hr1). A similar reactor, but of smaller volume (10 mL), has been described by Chen et al. [117]. 

For a continuous-flow reactor, such as a CSTR, the energy balance is an enthalpy (H) balance, if we neglect any differences in kinetic and potential energy of the flowing stream, and any shaft work between inlet and outlet. However, in comparison with a BR, the balance must include the input and output of H by the flowing stream, in addition to any heat transfer to or from the control volume, and generation or loss of enthalpy by reaction within the control volume. Then the energy (enthalpy) equation in words is... 

Reactors have volume V. Continuous-flow reactors have volumetric flow rate V, and constant-density reactors have residence time X = V/v. Until Chapter 8 all continuous reactors are either completely mixed (the CSTR) or completely unmixed (the PFTR). 

A variable-volume batch reactor is a constant-pressure (piston-like) closed tank. On the other hand, a variable-pressure tank is a constant-volume batch reactor (Fogler, 1999). Thus, in batch reactors, the expansion factor is used only in the case of a constant-pressure tank whereas and not in a constant-volume tank, even if the reaction is realized with a change in the total moles. However, in continuous-flow reactors, the expansion factor should be always considered. In the following section and for the continuous-flow reactors, the volume V can be replaced by the volumetric flow rate Q, and the moles N by the molar flow rate F in all equations. 

So far in dealing with tubular reactors we have considered a spatial coordinate as the variable, i.e. an element of volume SV, situated at a distance z from the reactor inlet (Fig. 1.14), although z has not appeared explicitly in the equations. For a continuous flow reactor operating in a steady state, the spatial coordinate is indeed the most satisfactory variable to describe the situation, because the compositions do not vary with time, but only with position in the reactor. 

We consider a reaction of type A——>P the CSTR (Figure 8.1) is continuously fed with a stream at an initial conversion X0. Thus, the concentration of the reactant A in the feed stream is CA0 and at the outlet of the reactor is at its final value Ctt = CA= CA0 (1 — XA), which is also equal to the concentration inside the reactor volume. If the reactor is operated at steady state, the molar flow rate of A, FA the mass balance can be written for the reactant A ... 

Fig. 11.9 Types of linear continuous-flow reactors (LCFRs). (a) Continuous plug flow reactor (CPFR) resembling a batch reactor (BR) with the axial distance z being equivalent to time spent in a BR. (b) A tabular flow reactor (TFR) with (tq) miscible thin disk of reactive component deformed and distributed (somewhat) by the shear field over the volume, and (b2) immiscible thin disk is deformed and stretched and broken up into droplets in a region of sufficiently high shear stresses, (c) SSE reactor with (cj) showing laminar distributive mixing of a miscible reactive component initially placed at z = 0 as a thin slab, stretched into a flat coiled strip at z L, and (c2) showing dispersive mixing of an immiscible reactive component initially placed at z — 0 as a thin slab, stretched and broken up into droplets at z — L.

The selective oxidation of primary alcohols is notoriously difficult to achieve selectively as further oxidation of the product to the respective carboxylic acid occurs rapidly, as depicted in Scheme 50. Wiles et al. (2006) therefore proposed that it should be possible to isolate either the aldehyde or the carboxylic acid, depending on the reaction times employed. To demonstrate this, the authors constructed a packed-bed reactor [3 mm (i.d.)x 5.0 cm (long)] containing silica-supported Jones reagent (0.15 g, 0.15 mmol), a Cr(VI)-based oxidant, and by exploiting the high surface to volume ratio obtained within continuous flow reactors, the... 

Heterogeneous reactions lend themselves to continuous flow reactors, which are desirable as they minimise the reacting volume. This reduces operation risks, and allows smaller, more efficient plants to be built. Flow reactors designed for fluorous reactions with both liquid and gaseous substrates have been demonstrated to be effective, at least on a bench scale [49]. Fluorous solvents have also recently found applications as liquid membranes to control the rate of addition of reagents and so control exothermic reactions such as alkene bromination (Fig. 6), and demethylation of anisoles by reaction with boron tribromide [50], This has potential as a clean route as the kinetic control gives improved selectivity. 

Residence Time of Continuous Flow Reactors. The definition of the residence time of a continuous flow reactor is simply the ratio of its volume to its volumetric flow rate t = (VR/ V). It is the time that every reaction component element, i.e. every molecule, stays in the reactor. 

It is obvious that, for continuous flow reactors, designing the reactor means estimating its residence time rather than volume. Continuous flow reactors of the same type with different volumes but the same residence time will give the same conversion. 

Normally, conversion increases with the time the reactants spend in the reactor. For continuous-flow systems, this time usually increases with increasing reactor volume consequently, the conversion X is a function, of reactor volume V. If is the molar flow rate of species A fed to a system operated at steady state, the molar cate at which species A is reacting within the entire system will be... 

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