CSTR volume

It is well understood that mixing and mass transfer are affected by agitation and aeration rates. Typically, in an agitated vessel, the working volume is about 75% of nominal CSTR volume. 

ILLUSTRATION 8.6 DETERMINATION OF REQUIRED CSTR VOLUME UNDER ISOTHERMAL OPERATING CONDITIONS—LIQUID PHASE REACTION... 

In Illustrations 8.3 and 8.6 we considered the reactor size requirements for the Diels-Alder reaction between 1,4-butadiene and methyl acrylate. For the conditions cited the reaction may be considered as a pseudo first-order reaction with 8a = 0. At a fraction conversion of 0.40 the required PFR volume was 33.5 m1 2 3, while the required CSTR volume was 43.7 m3. The ratio of these volumes is 1.30. From Figure 8.8 the ratio is seen to be identical with this value. Thus this figure or equation 8.3.14 can be used in solving a number of problems involving the... 

Number of CSTRs Volume of one reactor (m3 ) Residence time (h) Conversion... 

The CSTR volume necessary to achieve 80% conversion at the specified tempera-ture and pressure is 555 dm . 

Next use this polynomial and an ODE solver to plot fte conversion down Uie length (i.e., volume) of a PFR and find the CSTR volume for 80% converison for an entering molar flow rate of 5 mol/s. 

How might you increase the conversion and decrease the time of operation For example, what conversion would be achieved if a second 1000-ga CSTR were placed either in series or in parallel with the CSTR For the same temperature and feed conditions as part (a), what CSTR volume would be necessary to achieve a conversion of 85% for a molar feed rate of A of 1 lb mol/min ... 

L s calculate the CSTR volume necessary to achieve 40% conversion. The mole balance is... 

We see that the CSTR volume (1 m ) to achieve 40% conversion in this adiabatic I reaction is less than the PFR, volume (1.15 m ). 

We can rearrange Equation (2-12) to determine the CSTR volume necessary to achieve a specified conversion X. 

CDP2-Ca Use Levenspiel plots to calculate PFR and CSTR volumes. 

The adiabatic CSTR volume is less than the PFR volume... 

Note that the resulting single CSTR volume would have been too large — try to increase the temperature and/or use a series of CSTRs. 

Thus, the ratio of backmixed reactor (CSTR) volume to plug-flow reactor (PFR) volume is 594/27.63 = 21.5. The CSTR not only has 21.5 times the PFR volume, it also needs snfficient impeller power to achieve backmixing. Obvionsly, from flxed as well as operating costs points of view, PFR is a better option. However, let us now consider the length-to-diameter (L/H) ratio for PFR behavior. 

The CSTR volume necessary to achieve 80% conversion is 6.4 m when operated at 500 K, 830 kPa (8.2 atm), and with an entering molar flow rate of A of 0.4 mol/s. This volume correspond.s to a reactor about 1.5 m in diameter and 3.6 m high. It s a large CSTR. but this is a gas-phase reaction, and CSTRs are normally not used for gas-phase reactions. CSTRs ire used primarily for liquid-phase reactions. 

The CSTR volume was 6.4 m- and the PFR volume wa.s 2.165 When we combine Figures E2-2.1 and E2-3. i on the same graph, we see that the crosshatched area above the curve is the difference in the CSTR and PFR reactor volumes. 

For isothermal reactions greater than zero order (see Chapter 3 for zero order), the CSTR volume will usually be greater than the PFR volume for the same conversion and reaction conditions (temperature, flow rate. etc.),... 

We see that the reason the isothermal CSTR volume is usually greater than the PFR volume is that the CSTR is always operating at the lowest reaction rate (e.g., -Ta = 0.05 in Figure E2-4.l(b)), The PFR on the other hand starts at a high rate at the entrance and gradually decreases to the exit rate, thereby requiring less volume because the volume is inversely proportional to the rate. However, for auto-catalyiic reactions, product-inhibited reactions, and nonisothermal exothermic reactions. these trends will not always be the case, as we will see in Chapters 7 and 8. 

From Example 2-2, the CSTR volume was 6.4 m and the correspondin space time and space velocity are... 

CDP2-Db Use Levenspiel plots to calculate PFR and CSTR volumes. (PDF) (3rd Ed. P2-91... 

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