Pressure drop tubular reactor

PRESSURE DROP (AP) IN TUBULAR (PLUG FLOW) REACTORS... 

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes. 

A summary of the nine batch reactor emulsion polymerizations and fifteen tubular reactor emulsion polymerizations are presented in Tables III IV. Also, many tubular reactor pressure drop measurements were performed at different Reynolds numbers using distilled water to determined the laminar-turbulent transitional flow regime. 

Chapter 2 developed a methodology for treating multiple and complex reactions in batch reactors. The methodology is now applied to piston flow reactors. Chapter 3 also generalizes the design equations for piston flow beyond the simple case of constant density and constant velocity. The key assumption of piston flow remains intact there must be complete mixing in the direction perpendicular to flow and no mixing in the direction of flow. The fluid density and reactor cross section are allowed to vary. The pressure drop in the reactor is calculated. Transpiration is briefly considered. Scaleup and scaledown techniques for tubular reactors are developed in some detail. 

The emphasis in this chapter is on the generalization of piston flow to situations other than constant velocity down the tube. Real reactors can closely approximate piston flow reactors, yet they show many complications compared with the constant-density and constant-cross-section case considered in Chapter 1. Gas-phase tubular reactors may have appreciable density differences between the inlet and outlet. The mass density and thus the velocity down the tube can vary at constant pressure if there is a change in the number of moles upon reaction, but the pressure drop due to skin friction usually causes a larger change in the density and velocity of the gas. Reactors are sometimes designed to have variable cross sections, and this too will change the density and velocity. Despite these complications, piston flow reactors remain closely akin to batch reactors. There is a one-to-one correspondence between time in a batch and position in a tube, but the relationship is no longer as simple as z = ut. 

Consider the gas-phase decomposition A B -b C in an isothermal tubular reactor. The tube i.d. is 1 in. There is no packing. The pressure drop is 1 psi with the outlet at atmospheric pressure. The gas flow rate is O.OSSCF/s. The molecular weights of B and C are 48 and 52, respectively. The entering gas contains 50% A and 50% inerts by volume. The operating temperature is 700°C. The cracking reaction is first order with a rate constant of 0.93 s . How long is the tube and what... 

This section has based scaleups on pressure drops and temperature driving forces. Any consideration of mixing, and particularly the closeness of approach to piston flow, has been ignored. Scaleup factors for the extent of mixing in a tubular reactor are discussed in Chapters 8 and 9. If the flow is turbulent and if the Reynolds number increases upon scaleup (as is normal), and if the length-to-diameter ratio does not decrease upon scaleup, then the reactor will approach piston flow more closely upon scaleup. Substantiation for this statement can be found by applying the axial dispersion model discussed in Section 9.3. All the scaleups discussed in Examples 5.10-5.13 should be reasonable from a mixing viewpoint since the scaled-up reactors will approach piston flow more closely. 

Nerve gas is to be thermally decomposed by oxidation using a large excess of air in a 5-cm i.d. tubular reactor that is approximately isothermal at 620°C. The entering concentration of nerve gas is 1% by volume. The outlet concentration must be less than 1 part in lO by volume. The observed half-life for the reaction is 0.2 s. How long should the tube be for an inlet velocity of 2m/s What will be the pressure drop given an atmospheric discharge ... 

Illustrations 8.3 and 8.4 indicate the application of the above analysis to isothermal tubular reactors with negligible pressure drop. 

If one desires to design a pilot scale tubular reactor to operate isothermally at 500 °C, what length of 6-in. pipe will be required to convert 90% of the raw feedstock to methyl acrylate The feedstock enters at 5 atm at a flow rate of 500 lb/hr. Ideal gas behavior may be assumed. A 6-in. pipe has an area of 0.0388 ft2 available for flow. Pressure drop across the reactor may be neglected. 

The pressure drop through a tubular reactor of length AL may be expressed in terms of the friction factor / as... 

The gas phase reaction, A => 2B, is conducted at 600 R in a tubular flow reactor of diameter 0.2 ft. The feed contains 50 mol% A of molecular weight 40 and the balance inert of molecdular weight 20. Charge rate is 9000 lb/hr, inlet pressure is 5 atm, specific rate is 6000/hr and viscosity is 0.05 lb/ft-hr. Obtain the relation between conversion, pressure drop and volume of the reactor. 

Normally, the pressure drop in a tubular reactor is small compared with the operating pressure and in this case, the fluid can be treated as incompressible. Then the pressure drop in a tube of length L and diameter d is given by... 

In any real situation, reactants only flow through the reactor because there is a difference in pressure between the inlet and the outlet. Methods for calculating the pressure drop in pipes and packed beds have been outlined in Chap. 1. Often, the pressure drop is negligible compared with the total pressure and it is usual to assume that a tubular reactor with plug flow operates at constant pressure. 

In Sect. 3.2, the development of the design equation for the tubular reactor with plug flow was based on the assumption that velocity and concentration gradients do not exist in the direction perpendiculeir to fluid flow. In industrial tubular reactors, turbulent flow is usually desirable since it is accompanied by effective heat and mass transfer and when turbulent flow takes place, the deviation from true plug flow is not great. However, especially in dealing with liquids of high viscosity, it may not be possible to achieve turbulent flow with a reasonable pressure drop and laminar flow must then be tolerated. 

Tubular reactors are operated under pressures of 250 - 330 MPa measured at the reactor inlet. When a pressure drop of 0.01 - 0.03 MPa/m is taken into account the pressure at the reactor outlet is lower by 30 - 60 MPa. 

Significant amounts of CH4 and C2H2 are also formed but will be ignored for the purposes of this example. The ethane is diluted with steam and passed through a tubular furnace. Steam is used for reasons very similar to those in the case of ethylbenzene pyrolysis (Section 1.3.2., Example 1.1) in particular it reduces the amounts of undesired byproducts. The economic optimum proportion of steam is, however, rather less than in the case of ethylbenzene. We will suppose that the reaction is to be carried out in an isothermal tubular reactor which will be maintained at 900°C. Ethane will be supplied to the reactor at a rate of 20 tonne/h it will be diluted with steam in the ratio 0.3 mole steam 1 mole ethane. The required fractional conversion of ethane is 0.6 (the conversion per pass is relatively low to reduce byproduct formation unconverted ethane is separated and recycled). The operating pressure is 1.4 bar total, and will be assumed constant, i.e. the pressure drop through the reactor will be neglected. 

For a homogeneous tubular reactor, the pressure drop corresponding to the desired flowrate is often relatively small and does not usually impose any serious... 

Figure 4.24. Nonmonotonic dependence of a pressure drop on flow rate, showing possible instability in flow through a tubular reactor.

The fifth difference between a CSTR and a tubular reactor is pressure drop. A CSTR has essentially no pressure drop. A tubular reactor can have very substantial pressure drop,... 

However, a small-diameter tube gives more pressure drop for a given flowrate through each tube and a given tube length. Of course, a larger number of parallel tubes that are shorter can be used to keep pressure drop at a reasonable level, but this increases the shell diameter of the reactor, which increases the cost. Mechanical problems also limit the minimum tube diameter. Typical tube diameter in cooled tubular reactors is 0.03 m. Typical tube diameter in a furnace-fired heated tubular reactor is 0.15 m. 

Tubular flow reactors (TFR) deviate from the idealized PFR, since the applied pressure drop creates with viscous fluids a laminar shear flow field. As discussed in Section 7.1, shear flow leads to mixing. This is shown schematically in Fig. 11.9(a) and 11.9(b). In the former, we show laminar distributive mixing whereby a thin disk of a miscible reactive component is deformed and distributed (somewhat) over the volume whereas, in the latter we show laminar dispersive mixing whereby a thin disk of immiscible fluid, subsequent to being deformed and stretched, breaks up into droplets. In either case, diffusion mixing is superimposed on convective distributive mixing. Figure 11.9(c) shows schematically the... 

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