Online diagnosis of flow maldistribution in packed bed reactors.

Abstract

Massive adiabatic fixed bed reactors are common in the process industries for conversion of raw materials into valuable products. The unit usually consists of a vessel filled with catalyst particles through which the process gas flows. In general, the operation of this unit is akin to the ideal plug flow reactor, i.e. there are no radial gradients of concentration or temperature. During the normal operation of the unit, the catalyst particles can undergo attrition and dusting, resulting in regions of the bed that have different void fractions. These differences in bed voidage can result in non-uniform flow through the packed bed. When the process gas finds a path of least resistance it can flow through this path with little contact with the catalyst. This is referred to as channelling. Channelling leads to low conversion of reactants to product as there will be little contact with the catalyst. The maldistribution of flow through packed bed reactors can be determined by taking the unit offline and carrying out a residence time distribution test, for example a pulse of tracer is injected into the vessel inlet and the concentration of the tracer is measured at the exit. Although valuable information can be obtained from these measurements, it cannot be performed in most cases whilst the unit is online. Experimental work was done to determine if there is an appreciable difference in the pressure fluctuations of pressure vessels during correct and incorrect flow through packed beds and to assess if the effect of pressure fluctuations can be used to develop a method for diagnosing flow maldistribution in packed bed reactors online. Correct and incorrect flow through packed beds were achieved by using uniform and non-uniform packing arrangements, to induce non-uniform flow during the operation of the packed bed reactor. To develop a model for analysing flow maldistribution in packed bed reactors online, experiments were done using a small and large vessel. Rachsig rings were used as packing material in the laboratory packed bed reactor. The pressure fluctuations were converted to an analysable form using the Fast Fourier Transform, resulting in dominant frequency for the oscillations. The dominant frequency data could not give a distinguishable difference in the pressure fluctuations data. Experimental work done using various packings showed a significant difference in the measured amplitude as packing arrangement changes. Large(uniform) packing arrangement was seen to minimize pressure loss through the packed bed. The large packing also had the largest magnitude of pressure fluctuations. There was an inflection point in the amplitude vs flowrate data. The amplitude of pressure fluctuation first increased with an increase in gas flowrate up to a flowrate of 2 dm3/minute. From 2 dm3/minute to 2.33 dm3/minute there was an inflection point and the amplitude dropped. There was a further decrease in amplitude with a further increase in gas flowrate. The non-uniform packing had the highest amplitude below 2.33 dm3/minute but, after 2.33 dm3/minute the uniform packing had the largest amplitude followed by the large and small packing at the side while the large and small packing at the centre configuration had the least amplitude at high flowrates. This can be used online to check for particle breakage and the resulting maldistribution in flow in packed bed reactors. A model was developed to analyse pressure fluctuations in packed bed reactors. The model was developed using equations, found from literature, for non-ideal flow in packed beds. MATLAB software was used to solve and analyse the model. The pressure fluctuations obtained from experimental work agreed with the simulated data. The uniform packing had the highest amplitude while the nonuniform (large and small at the centre) had the least amplitude. The model was in agreement with experimental data, as it was seen that the amplitude of pressure fluctuations can be used for diagnosing flow maldistribution in packed bed reactors.