External heat removal

If one doesn t take into account the longitudinal heat transfer by turbulent thermal conductivity inside the reactor, then temperature change (cooling) at the expense of external heat removal is determined as following  

It is consequent from (1.30) that heat removal efficiency is in inverse proportion to rector radius at constant linear speed V or in direct proportion at constant volumetric speed w of reaction mixture flow (productivity). That is one should increase R maintaining apparatus productivity approximately constant and trying to improve external heat removal. However at that reagents concentrations and temperature gradients appear (torch regime is formed) otherwise flow regime transforms from turbulent into laminar. Mentioned factors limit the radius of tubular turbulent reactor from the top [1, 60]. 

Modification of catalyst introduction into reaction zone in particular realization of multi-step catalyst introduction into tubular turbulent apparatus ( zone model) is the convenient way of polymer MM and MMD control [62]. For zone model realization in turbulent regime the following conditions should be fulfilled [1, 27, 66]  

reaction zones should not be crossed, i.e. the distance between adjacent points of catalyst introduction should be Li V/kj  

intensity of heat evolution is not high in each zone, and heat-transfer coefficients should be such to fulfill the requirement of temperature constancy in each reaction zone (quasi-isothermal regime). 

The decrease of temperature gradients in the fast polymerisation processes exerts a substantial influence on the molecular characteristics of the polymers. Fleat removal by a heat-conducting wall determines, in many cases, the possibility of the practical implementation of the process. Changing the input method of the reactants into a reaction zone, under conditions of external heat removal, affects the MW characteristics of the products of fast polymerisation processes [30, 31, 95, 96]. 

An increase of the reactor radius during polymer production creates a more perceptible effect on changes of the polymer MW characteristics, the P valne for method I does not demonstrate significant changes with an increase of the intensity of heat removal through a metallic wall. The polydispersity coefficient stays constant in this case. Catalyst input method II (model II) shows a substantial decrease of PJP , which approaches exponential behaviour and simultaneous growth of P . 

A quasi-plug flow mode description, in cylindrical tubular turbulent devices, requires the development of an equation for the calculation of a temperature field in a reaction volume of a fast chemical reaction (assuming 100% at the reactant input points). In this case, a thermal balance equation for the surface element dF will be the following [38]  

Where G,., G o i, Cp Cp .o(,i are the mass flow rate and heat capacity of a reaction mixture, and a cooling agent respectively. 

Where Kj is the coefficient of the heat transfer through a wall, therefore Equation 2.81 can be transformed in the following way  

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The reactor design must account for heat removal as well as chemical reaction. For monoliths, the unique situation exists that because of the low-pressure drop, external heat removal in a liquid loop is feasible 38,39). Alternatively, structured packings can be applied that improve radial transport 124,126). 

Catalyst coolers (internal and external heat removal). 

According to computerized numerical calculations, polymerization under high-temperature conditions is a non-steady-state process substantially dependent on external heat-removal and heat conduction of the substance. 

CHAPTER 4. EXTERNAL HEAT REMOVAL UNDER FAST CHEMICAL... 

Selection of thermal conditions control mode under fast liquid-phase reactions in turbulent regime on the account of external heat removal 96... 

Formation of quasi-plug-flow mode in turbulent flows under fast chemical processes when reaction zone reaches heat exchanging reactor walls determines possibility of application of effective external heat removal that allows regulating of molecular characteristics of resulting polymer [62-65]. 

At external heat removal (Tch = const) reaction mixture temperature change AT=Tad-Tr along cooling zone length Lcooi from maximum Tad = To+ATad (To - initial temperature of reaction mixture) up to required temperature Tr is determined as following [1,40,41] ... 

Thus, the application of tubular turbulent apparatus with local hydrodynamic resistances including divergent-convergent design for realization of fast polymerization processes allows additionally to increase of external heat removal at the expense of intensification of convective hest-exchange. 

As in industry including butyl rubber chlorination and piperylene cationic oligomerization we often have to use liquid flows with different physical characteristics (density, viscosity) it is necessary to study the influence of these parameters on conditions of reaction plan front formation - quasi-plug-flow mode, and consequently on the effectiveness of external heat removal. 

Formation of quasi-plug-flow mode in tubular turbulent apparatus and consequently of optimal conditions for application of external heat removal requires creation in apparatus volume of high turbulent mixing level determining by value of Dt, and Dt significantly depends on reaction zone geometry (see 3.1.2). 

Guaranteed formation in reaction zone of quasi-plug-flow mode determines effectiveness of thermostating of fast polymerization processes at the expense of external heat removal. This determined the expediency of working out of the ways of temperature field control in reaction zone at turning on of heat removal through heat exchanging reaction zone surface. 

As a consequence it is necessary to study heat regime in reaction zone and to reveal the ways of effective regulating of temperature profile in tubular turbulent apparatus under realization of fast chemical processes at the expense of external heat removal in technically admissible conditions. 

We may estimate the length of cooling zone at external heat removal required for maintenance in reaction volume of necessary temperature Tr by solution of (4.9) in relation to Lcooi ... 

Equation (4.13) determines possible ways of temperature regime control in tubular apparatus of jet type under fast chemical processes at the expense of external heat removal, in particular by change of To, All, Tc, R and V at condition of turbulence of reacted flow. 

Figure 4.10. Temperature change (Tr) along apparatus length (L) at conditions of external heat removal by the example of interaction of sulphuric acid with water in turbulent regime with the use of one- (1) and two-zone (2) models. (To = 283 K, Tc = 283 K, V = 1 m/sec, flow parameters are in Table 4.1).

With reference to piperylene polymerization transforming (4.9) we may obtain correlation for calculation of temperature along reaction zone length L at external heat removal ... 

Figure 5.5. Temperature change in reaction zone along tubular reactor under piperylene oligomerization at adiabatic conditions (1-5) and at external heat removal (6-10). Catalysts TiCU (1, 10), TiCl4-Al(i-C4H9)3 (2, 9), AlC2H5Cl2 0(C6H5)2 (3, 8), AIC2H5CI2 (4, 7), AlCl3-0(C6H5)2...

Relative disadvantages of compact tubular turbulent apparatus of the first generation are fast decay of turbulent diffusion coefficient Dt in reaction zone and along its length (over 1-2 calibres) (see Fig. 3.6a) and low effectiveness of external heat removal (see Fig. 4.20). In a number of cases they succeeded in intensification of external heat removal. Shell-and-tube turbulent reactors were... 

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