Tubes in Parallel

Scaling in parallel gives an exact duplication of reaction conditions. The number of tubes increases in direct proportion to the throughput  

Equation (3.31) is satisfied with Sr = Sl=. Equation (3.32) is satisfied the same way, but with the added provision that the inlet and outlet pressures are the same in the large and small units. Scaling in parallel automatically keeps the same value for t. The scaleup should be an exact duplication of the pilot plant results but at S times the flow rate. 

There are three, somewhat similar, concerns about scaling in parallel. The first concern applies mainly to viscous fluids in unpacked tubes. The second applies mainly to packed tubes. 

Will the feed distribute itself evenly between the tubes This is a concern when there is a large change in viscosity due to reaction. The resulting stability problem is discussed in Chapter 13. Feed distribution can also be a concern with very large tube bundles when the pressure drop down the tube is small. 

Will the distribution of flow on the shell side be uniform enough to give the same heat transfer coefficient for aU the tubes  

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Parallel—put 128 identical tubes in parallel using a shell-and-tube design. The total length of tubes will be 1536 ft, but they are compactly packaged. All operating conditions are identical on a per-tube basis to those used in the pilot plant. 

The above computation is quite fast. Results for the three ideal reactor t5T)es are shown in Table 6.3. The CSTR is clearly out of the running, but the difference between the isothermal and adiabatic PFR is quite small. Any reasonable shell-and-tube design would work. A few large-diameter tubes in parallel would be fine, and the limiting case of one tube would be the best. The results show that a close approach to adiabatic operation would reduce cost. The cost reduction is probably real since the comparison is nearly apples-to-apples. ... 

Tube-to-tube interactions. The problems of velocity profile elongation and thermal runaway can be eliminated by using a multitubular reactor with many small-diameter tubes in parallel. Unfortunately, this introduces another form of instability. Tubes may plug with pol5nner that cannot be displaced using the low-viscosity inlet fluid. Imagine a 1000-tube reactor with 999 plugged tubes ... 

Koshelov et al. (1970) also reported tests results on a bank of three heated tubes in parallel. The phase shifts of flow oscillations were quite different for various tubes. Sometimes the flow oscillations in two tubes were in phase, while the flow oscillation in the third tube was in a phase shift of 120° or 180°. The amplitudes of the in-phase oscillations were different, that is, high in one tube and negligible in the other. Sometimes phase shift between individual tubes took place without apparent reason, but there were always tube in which the flow oscillations were 120° or 180° out of phase. 

The total mass flow rate is then 29.58(89.86) = 2658 g/sec or 2.658 kg/sec. For 1000 tubes in parallel the available cross section for flow is given by... 

Heat transfer and friction are reduced by placing tubes in parallel. If neither factor is critical, either parallel or series placement is feasible. Series arrangement will be trombone type because of the great length. 

The actual configuration of the reactor may take various forms depending on the precise requirements of the process. For example, for a high-temperature homogeneous gas-phase reaction such as naphtha cracking, the reactor may be simply a long tube in a furnace [Fig. 6(a)]. In other cases, the single tube is replaced by a number of tubes in parallel as shown in Fig. 6(b). 

Steam-cracking reactors typically consist of several steel tubes, perhaps 100 m long and 4 in. in diameter in a tube furnace with reactants and steam fed through the several tubes in parallel. The ceramic fined furnace is heated by burning natural gas at the walls to heat the tubes to 900°C by radiation. The reactor is fed by ethane and steam in a ratio of 1 1 to 1 3 at just above atmospheric pressure. The residence time in a typical reactor is approximately 1 sec, and each tube produces approximately 100 tons/day of ethylene. We will return to olefins and steam cracking in Chapter 4. 

In order to design an efficient reactor using a faUing film reactor, we would need to have many small tubes in parallel so that the interfacial area can be large. This is difficult to accomphsh with flow down tubes, but it is easy to accomplish with rising bubbles or faUing drops. The interfacial area is not now the area of the cyYilid T between gas... 

As with the falling film reactor, the rate of mass transfer to the catalyst goes as R, while the size of the reactor goes as R, so this reactor becomes very inefficient except for very small-diameter tubes. However, we can overcome this problem, not by using many small tubes in parallel, but by allowing the gas and liquid to flow (trickle) over porous catalyst pellets in a trickle bed reactor rather than down a vertical wall, as in the catalytic wall reactor. 

Fuel oil of 18,500 Btu/lb is fired with 13% excess air at 80°F. Flue gas leaves at 410°F. A simplified cross section of the boiler is shown. Heat and material balances are summarized. Tube selections and arrangements for the five heat transfer zones also are summarized. The term Ag is the total internal cross section of the tubes in parallel. (Steam Its Generation and Use, 14.2, Babcock and Wilcox, Barberton, OH, 1972). (a) Cross section of the generator (b) Heat balance ... 

Such exchangers are made up of a number of tubes in parallel and series through which one fluid travels and enclosed in a shell through which the other fluid is conducted. 

The pyrolysis reactor can be simulated in Aspen Plus as PFR with power-law kinetics and temperature profile or heat duty. To validate the kinetic data, we consider an initial flow rate of 73000kg/h EDC at a reaction temperature of 530°C and 18 bar. The reactor consists of 16 tubes in parallel with an internal diameter of... 

In two test rigs materials like Inconel 625, Haynes 214, Hastelloy C-276, Nicrofer 5923 and 6025 were tested in aqueous solutions containing 0.5 mol/kg oxygen and 0.05 to 0.5 mol/kg HC1. One of the rigs offers the possibility to test five tubes in parallel at pressures between 240 to 400 bars and at temperatures up to 600 °C. Practically all types of material showed heavy local corrosion after some tens of hours. More encouraging results were obtained for oxide ceramics, small samples of which are presently under test. 

L = heated length of straight tube or length of heat-transfer surface, ft if tubes in parallel are involved, L is the length of one tube n = constant, dimensionless... 

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