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Apr-2017

Catalyst filling in a fixed bed tubular reactor

A reactor filling methodology ensures even reactant distribution and pressure drop

Subrata Das and Naresh Kumar Singh
Fluor Daniel
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Article Summary
This article discusses a catalyst filling methodology for a multi-tubular fixed bed reactor system for a gas phase reaction process to ensure uniform distribution of the reacting fluid in all of the reactor tubes. It provides a suitable procedure for filling the catalyst in the reactor as per the catalyst dilution pattern required for a reactor system. A reaction used to model the filling methodology is the production of 3-cyanopyridine (3-CP) from 3-methylpyridine (3-MP).

It is good practice to maintain overall pressure drop across each reactor tube within +/-5% deviation of the reference pressure drop. The reference pressure drop can be derived by passing an equivalent gas flow rate (corresponding to the actual reactant flow rate in the tube) through the reference tube. The typical value of the reference pressure drop measured for this reactor system is 85 mmHg. All the tubes need to be identified and marked following an agreed and suitable marking pattern. The tubes with thermocouple elements are also to be marked a priority and these are filled with catalyst separately to ensure proper catalyst distribution and a steady reaction temperature profile.

Each of the reactor tubes is filled with diluted catalyst (with different ranges of dilution across the reactor height). The reactor is provided with suitable nozzles and baffles in the shell side for circulating thermal fluid for the removal of reaction heat. The reactor feed in this case consists of 3-MP, ammonia, air and demineralised water in a vapour state at a temperature of 325-345°C.

This is an exothermic reaction carried out in the presence of solid catalyst in the form of cylindrical pellets with a silica-alumina base. To ensure homogeneous distribution of reactant gas and a similar temperature profile in all of the tubes it is necessary to follow a suitable catalyst filling strategy.

Process outline
The reactor feed enters the top dome of the reactor from a side nozzle. The material is allowed to pass through a bed of inert material placed on a supporting screen above the reactor tube sheet to ensure uniform flow of the feed to all of the tubes. Each tube is filled in a uniform manner, with catalyst material having different dilution patterns at different reactor heights. Heat of reaction is removed by thermal fluid circulating in the shell side. Temperature is measured at different points in the tubes’ radii and at different axial heights of the tubes. Some of the temperature sensing elements are linked to a temperature controller which adjusts the circulation of thermal fluid on the shell side. The reactor is provided with a safety relief system, feed cut-off and purging facility.

Catalyst filling activity
In view of the six stages of catalyst dilution for each tube, different aspects of the catalyst filling exercise are covered in the following sections. The inert/catalyst mixture zones in a reactor tube are the top and bottom inert zones and four different catalyst inert mixture zones.

Safety measures and precautions
The top as well as the bottom dome of the reactor need to be opened and placed at the side of the reactor to provide an empty work space for a smooth and safe catalyst filling operation. Appropriate lifting arrangements are required for raising and relocating the catalyst/inert container from the ground floor to a platform at the level of the reactor top tube sheet. The reactor top tube sheet working height needs to be checked for any additional platform requirement, if needed.

During sieving and blending of the catalyst/inert mixture, some emission of fine particles is unavoidable. This release of fine carbonaceous material is due to attrition of the coated catalyst during handling.

As catalyst dust is harmful to the respiratory tract, screening, blending or sieving operations need to be performed in a ventilated dry atmosphere on the platform adjacent to the reactor top.

Preparation of the reactor tubes
In this reactor, there are 1678 reactor tubes each 5m in length with an internal diameter of 1.5in and 12 gauge thickness. The reactor has a 1850 mm internal diameter and a total length of 7500 mm at the tangent line. Each of the reactor tubes is provided with a 120 mm spring to support the catalyst load.

On studying the tube sheet orientation it was noted that only two identical rows with eight tubes each at the edge are placed side by side in a straight line. So it was deemed logical to number the tubes, keeping one of these as the base row and to mark all the other tubes by row and number of tubes per row accordingly. The tube sheet layout is shown in 
Figure 1.

Marking of tubes containing thermocouples

There are a total of 25 reactor tubes with thermocouple elements for the measurement and control of reactor temperature. Each of these tubes is identified and marked clearly on the reactor top tube sheet.

The thermocouples used in the reactor system are of two categories. The first category of thermocouples (20 in number) has four temperature sensors located 1.25m from each other along the thermocouple pocket (reactor tube length). The other variety has only one fixed sensing point (totalling five in number). Hence a total of 85 sensing points along the reactor length is available to measure temperature variation. The orientation of the thermocouple tubes is made in such a way that each sensing point of a particular thermocouple is at the same height (either from the top or bottom of the tube sheet) with respect to the sensing point of another thermocouple. Such an arrangement is helpful in providing radial variation of temperature profile across the reactor cross section and also to identify possible hot spot locations in the event of a malfunctioning reactor.

Pressure drop measurement
In order to ensure uniform flow of reactant feed through all the tubes it is essential to achieve very close values of pressure drop across each of the reactor tubes. For this purpose, a pressure drop measurement system across the reactor tube is suggested and shown in Figure 2. A specific quantity of gas is allowed to flow through the reactor tube and pressure drop is measured in the manometer. The quantity of gas flow is based on equivalent volumetric flow of gas as in actual reactor operation. A blank experiment was carried out to find the pressure drop when there is no catalyst packing inside the tube. A few reference tubes are chosen and the pressure drop across these tubes was measured after filling these with different quantities of diluted catalyst and inert material.

Pressure drop across all of the other tubes is measured for the same gas flow rate and some quantity of inert material and catalyst is added or removed to ensure acceptable pressure drop. As good practice, the measured pressure drop across all of the other tubes should be within +/-5% of the reference value.

Preparation of catalyst
It is generally observed that, for an exothermic gas-solid reaction with a uniform concentration of catalyst, the heat release rate varies with reactor length and gives rise to the creation of a hot spot. The hot spot is initially located at the reactor top and with time it gradually moves in a downward direction, mainly due to loss of catalyst activity with time. In order to achieve a moderately uniform bed temperature, it is common practice to use a dilution pattern across the reactor length. This is due to controlled reactor heat liberation in the upper zones because of higher dilution with inert material in the upper zones. Although the reactant concentration in the reactor inlet zones is higher compared to those in the lower zones, catalyst activity is moderated by using larger quantities of inert material.
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