Plug Flow Tubular Turbulent Reactors

divergent-convergent tubular turbulent reactors which can be used for the formation of both a quasi-plug flow mode in turbulent flow (the main condition of any chemical process  

Tubular turbulent units, used for creating rapid chemical processes during industrial production, provide continuous energy- and resource-saving technologies, meeting ecological requirements. 

---

Taking into consideration the fact that fast polymerisation processes are characterised by inequality of chemical reaction time and transfer time ( chem < it is clear that an increase of facilitates the decrease of and both these processes are comparable in duration. The increase in linear flow rate V, i.e., the intensification of heat and mass exchange in the system, is equivalent to a slow dovm of the polymerisation reaction itself, compared with the transfer process. Therefore, the conventional approaches to external heat removal, which normally have such a restrictive effect on conventionally designed fast polymerisation processes implemented in stirred tank reactors, play an essential role at both high V and values when quasi-plug flow tubular turbulent reactors are used. In this case, control of the external temperature can be significantly enchanced due to zone-type catalyst loading. 

The flow patterns, composition profiles, and temperature profiles in a real tubular reactor can often be quite complex. Temperature and composition gradients can exist in both the axial and radial dimensions. Flow can be laminar or turbulent. Axial diffusion and conduction can occur. All of these potential complexities are eliminated when the plug flow assumption is made. A plug flow tubular reactor (PFR) assumes that the process fluid moves with a uniform velocity profile over the entire cross-sectional area of the reactor and no radial gradients exist. This assumption is fairly reasonable for adiabatic reactors. But for nonadiabatic reactors, radial temperature gradients are inherent features. If tube diameters are kept small, the plug flow assumption in more correct. Nevertheless the PFR can be used for many systems, and this idealized tubular reactor will be assumed in the examples considered in this book. We also assume that there is no axial conduction or diffusion. 

The use of cylinder tubular turbulent reactors with different diameter ratios d /dj, showed [3] that a decrease of d /d leads to quasi-plug flow mode formation at a lower consumption of reactant, introduced through the dj diameter axis branch pipe. In particular, upon the decrease of djd from 0.44 to 0.13, the axial volume flow rate, at which the quasi-plug flow mode starts to form, decreases by 60%. This allows us to use more concentrated reactant solutions in the chemical process. 

Thus, Equations 4.1-4.3, in the case of the homogeneous mixing of liquid flows, characterised by density and viscosity, provide an efficient commercial application of tubular turbulent reactors at almost any stage limited by mass exchange. The optimal operation conditions of the tubular turbulent reactors in the quasi-plug flow mode can be calculated due to the changes of the physical characteristics of the liquid flow, the kinetic parameters of the fast chemical reactions and the reaction construction parameters. 

The results of the experimental tests gave the following dependence of the conditions of quasi-plug flow mode formation in a tubular turbulent reactor (Vj, V2 = 0.1-0.8 m/s, and d /dj = 0.44) ... 

Formation of the quasi-plug flow mode in the reaction zone in turbulent flows, due to the intensive convective heat exchange in proximity to the reactor walls and resulting quasi-isothermal conditions, allows efficient control of the thermal mode of the neutralisation process by external cooling. In order to decide whether it is reasonable to use external cooling for the neutralisation reaction in a tubular turbulent reactor of cylinder form, it is necessary to estimate the amount of heat which is released in the reaction zone under conditions of adiabatic heating. If the density and thermal capacity of the reaction mixture in the first approximation correspond to the values for water, then [15] ... 

Table 5.2 Technical and economic running of a tubular turbulent reactor under quasi-plug flow conditions in turbulent flows in comparison with a perfect mixing reactor ...

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases, and consequently closer to the ideal (Fig. 2). Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer... 

There will be velocity gradients in the radial direction so all fluid elements will not have the same residence time in the reactor. Under turbulent flow conditions in reactors with large length to diameter ratios, any disparities between observed values and model predictions arising from this factor should be small. For short reactors and/or laminar flow conditions the disparities can be appreciable. Some of the techniques used in the analysis of isothermal tubular reactors that deviate from plug flow are treated in Chapter 11. 

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. 

We will now find the RDT for several models of tubular reactors. We noted previously that the perfect PFTR cannot in fact exist because, if flow in a tube is sufficiently fast for turbulence (Rco > 2100), then turbulent eddies cause considerable axial dispersion, while if flow is slow enough for laminar flow, then the parabolic flow profile causes considerable deviation from plug flow. We stated previously that we would ignore this contradiction, but now we will see how these effects alter the conversion from the plug-flow approximation. 

Plug Flow Reactor. A PFR is a continuous flow reactor. It is an ideal tubular type reactor. The assumption we make is that the reaction mixture stream has the same velocity across the reactor cross-sectional area. In other words, the velocity profile across the reactor is a flat one. In a PFR there is no axial mixing along the reactor. The condition of plug flow is met in highly turbulent flows, as is usually the case in chemical reactors. 

Backmix flow reactor or continuously stirred tank reactor. The conversion rate is lower than for plug-flow reactors because the reagent is immediately diluted on being introduced into the reactor. Many flow reactors, e.g. tubular reactors, and especially in the turbulent regime are in this class. 

In addition to the CSTR and batch reactors, another type of reactor commonly used in industry is the tubular reactor. It consists of a cylindrical pipe and is normally operated at steady state, as is the CSTR. For the purposes of the material presented here, we consider systems in which the flow is highly turbulent and the flow field may be modeled by that of plug flow. That is, there is no radial variation in concentration and the reactor is referred to as a plug-flow reactor (PFR). (The laminar flow reactor is discussed in Chapter 13.)... 

Gas-phase reacdotis are carried out primarily in tubular reactors where the flow is generally turbulent. By assuming that there is no dispersion and ttiere are no radial gradients in either temperature, velocity, or concentration, we can model the flow in the reactor as plug-flow. Laminar reactors are discussed in Chapter 13 and dispersion effects in Chapter 14. The differential form of the design equation... 

The gas flow through tubular reactors is of particular importance because the composition at any point is influenced by the linear velocity of the gas, the size of the reactor and the size of the catalyst particles. When gas flows through a pipe at low linear velocity (low Reynolds number), the radial velocity is not uniform. As the linear velocity increases, turbulence increases and the velocity profile approaches what is called plug flow. However, in a packed bed, plug flow can never be completely attained because of the high voidage near the reactor wall. 

---