Tubular Reactor

ABSTRACT/SUMMARY

For tubular reactor model BP 101-B, we carried out two experiments. In the first experiment, we have examined the effect of pulse input in a tubular flow reactor and we have constructed a residence time distribution (RTD) function for the tubular flow reactor. In this experiment the deionised water are let to continue flowing through the reactor until the inlet and outlet conductivity values are stable at low levels. We need to maintain flow rate of deionized water at approximately 700mL/min. Then, the salt solution is let to flow for 1 minute. Both the inlet and outlet conductivity values at the regular intervals of 30 seconds are recorded. In the second experiment, our aims are to examine the effect of step change in tubular flow reactor and we have constructed a residence time distribution (RTD) function for the tubular flow reactor. Only the deionized water is allowed to continue flowing until the inlet and outlet conductivity values are stable at low levels. We continued record the conductivity values until all readings are almost constant. Graphs of conductivity versus time and RTD function versus time were plotted and analyse for both pulse input and step change input. From the RTD function calculated, the mean residence time (t m), variance (σ2) and skewness (s3) values were calculated using appropriate numerical method for both experiment which gives the value of 1.9713min, 0.3662min2 and 0.2123min3 respectively for pulse input and 1.7788min, 1.236785min2 and 1.7371min3 respectively for step change input.

INTRODUCTION

Tubular reactors of BP101-B are often used when continuous operation is required but without back mixing of products and reactants. It is one of three reactor types which are interchangeable on the Reactor Service Unit. Reactions are monitored by conductivity probe as the conductivity of the solution changes with conversion of the reactants to products. The unit is small in scale for ease of operation but capable of demonstrating the principles of industrial reactor behaviour. The unit includes a 10 litres reactor vessel as a water jacket, and is equipped with a variable speed stirrer, inlet and outlet ports for the feed and product streams, sampling, conductivity measurements and temperature measurements and control. A cooling coil and immersion heater are provided inside the vessel to provide constant reaction temperature. The desired reaction temperature is achieved by controlling the heating using a digital temperature controller located on the front panel. Two non-corroding feed storage vessels are supplied, together with chemically resistant pumps and flow meters. A product collection vessel is also provided and if necessary, the products are neutralised before discharging to the laboratory drains. The tubular reactor is a coil of long tubing wound around a cylinder inside the vessel to give a total reactor volume of approximately 0.4 litres. The spiral design is practically the best approximation to plug flow conditions, as the secondary flow ensures good radial mixing while minimising longitudinal dispersion. Two reactants are pre-heated prior to mixing and entering the reactor In the tubular reactor, the reactants are continually consumed as they flow down the length of the reactor. Flow in tubular reactor can be laminar, as with viscous fluids in smalldiameter tubes, and greatly deviate from ideal plug-flow behaviour, or turbulent, as with gases. Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer are improved. For slow reactions and especially in small laboratory and pilot-plant reactors, establishing turbulent flow can result in conveniently long reactors or may require unacceptable high feed rates. However, many tubular reactors that are used to carry out a reaction do not fully conform to this idealized flow concept. In an ideal plug flow reactor, a pulse of tracer injected at the inlet would not undergo any dispersion as it passed through the reactor and would appear as a pulse at the outlet. The degree of dispersion that occurs in a real reactor can be

assessed by following the concentration of tracer versus time at the exit. This procedure is called the stimulus-response technique. The nature of the tracer peak gives an indication of the non-ideal that would be characteristic of the reactor. For most chemical reactions, it is impossible for the reaction to proceed to 100% completion. The rate of reaction decreases as the percent completion increases until the point where the system reaches dynamic equilibrium (no net reaction, or change in chemical species occurs). The equilibrium point for most systems is less than 100% complete. For this reason a separation process, such as distillation, often follows a chemical reactor in order to separate any remaining reagents or by products from the desired product. These reagents may sometimes be reused at the beginning of the process, such as in the Haber process. Tubular flow reactors are usually used for this application which are: 1 2 3 4 5

Large scale reactions Fast reactions Homogeneous or heterogeneous reactions Continuous production High temperature reactions Residence Time Distribution (RTD) analysis is a very efficient diagnosis tool that can

be used to inspect the malfunction of chemical reactors. It can also be very useful in modelling reactor behaviour and in the estimation of effluent properties. This technique is, thus, also extremely important in teaching reaction engineering, in particular when the nonideal reactors become the issue. The work involves determining RTDs, both by impulse and step tracer injection techniques, and applying them to the modelling of the reactor flow and to the estimation of the behaviour of a nonlinear chemical transformation. The RTD technique has also been used for the experimental characterization of flow pattern of a packed bed and a tubular reactor that exhibit, respectively, axially dispersed plug flow and laminar flow patterns (FEUP). The concept of using a ‘tracer’ species to measure the mixing characteristics is not limited to chemical reactors. In the area of pharmacokinetics, the time course of renal excretion of species originating from intravenous injections in many ways resembles the input of a pulse of tracer into a chemical reactor. Normally, a radioactive labelled ( 2H, 14C, 32P, etc.) version of a drug is used to follow the pharmacokinetics of the drug in animals and human. Another important field of RTD applications lies in the prediction of the real reactor performance, since the known project equations for ideal reactor are no longer valid. Now the

concepts of macro and micro mixing are fundamental. For each macro mixing level, expressed in the form of a specific RTD, there is a given micro mixing level, which lies between two limiting cases, complete segregation and perfect micro mixing.

Residence time distribution factor The Residence time distribution factor (RTD) of a reactor is a characteristic of the mixing that occurs in the chemical reactor. In a plug flow reactor, there is no axial mixing. Thus the omission is reflected in the RTD exhibited by the reactors.

OBJECTIVES

EXPERIMENT 1

:

To examine the effects of a pulse input in a tubular flow reactor To construct a residence time distribution (RTD) function for the tubular flow reactor.

EXPERIMENT 2

:

To examine the effects of a step change input in a tubular flow reactor To construct a residence time distribution (RTD) function for the tubular flow reactor.

THEORY

In a pulse input, for a short time as possible, an amount of tracer, N o is suddenly injected in one shot into the feed stream entering the reactor. The effluent concentration-time curve is referred to as the C curve in the RTD analysis. If we select an increment of time ∆t sufficiently small that the concentration of tracer, C(t), exiting between time t and t + ∆t is essentially constant, then the amount of tracer material, ∆N, leaving the reactor between time t and t + ∆t is where, v is the effluent volumetric flow rate. ∆N

= C (t) υ ∆t

And now divide it by the actual amount of material that was injected into the reactor, NN, we obtain new equation for a pulse injection, ∆ N vC (t) = ∆t No No vC ( t ) No

E(t) =

Rewriting the above equation in the differential form, dN= C(t)vdt After integrating, we obtain: ∞

No =

∫ vC ( t ) dt 0

υ = constant, so C(t ) E(t) =

∞

∫ C ( t ) dt 0

The integral in the denominator is the area under the C(t) curve. However, for a step input, it consider a constant rate at tracer addition to a fed that is initiated at time, t = 0. Thus, CN (t) = 0

t