Heat multitube reactor

In the Godrej-Lurgi process, olefins are produced by dehydration of fatty alcohols on alumina in a continuous vapor-phase process. The reaction is carried out in a specially designed isothermal multitube reactor at a temperature of approximately 300°C and a pressure of 5—10 kPa (0.05—0.10 atm). As the reaction is endothermic, temperature is maintained by circulating externally heated molten salt solution around the reactor tubes. The reaction is sensitive to temperature fluctuations and gradients, hence the need to maintain an isothermal reaction regime. 

The reactions are highly exothermic and very rapid. Consequently conventional practice in the design of fixed bed reactors for phthalic anhydride production has been based on the use of multitube reactors to ensure good heat transfer and good temperature control. These are required to ensure good selectivity. Often a thousand or more small diameter tubes may be... 

Reactions that are strongly exothermic, such as selective oxidations, or those that are strongly endothermic, such as oxychlorinations, are usually carried out in multitube reactors. The catalyst is dumped into tubes of limited diameter—typically to 1 in. (25 mm)— such as to permit adequate radial heat transfer to/from the liquid bath surrounding them as a result of their limited cross section, the overall number of such tubes is very large (typically on the order of 25,000). Minimization of pressure drop and maximization of... 

With honeycombs (monoliths), pressure drop could be reduced by a factor of 100 or more, with corresponding reductions in equipment and operating costs. The total absence of radial flow in such structures, however, precludes their use in multitube reactors the bulk heat generated by the reaction would not be transported to the tube walls, the selectivity of the process would decrease dramatically, and prevention of reaction runaway would prove difficult. Furthermore, the lack of radial vectors means that inhomogeneities in radial velocity profiles would be maintained these inequalities in residence time would reduce selectivity and result in poor utilization of the catalyst in many of the channels. 

A gas-phase exothermic reaction is carried out in a multitube reactor with the catalyst in 1-in. tubes and boiling water in the jacket. The feed temperature and the jacket temperature are 240°C. The average reactor temperature rises to 250°C a short distance from the inlet and then gradually decreases to 241°C at the reactor exit. The resistance to heat transfer is about equally divided between the bed and the film at the wall. If the tube diameter were increased to 1.5 in. with the same catalyst, what should the jacket temperature be to keep the peak reactor temperature at 250°C Sketch the temperature profiles for the two cases. What pressure steam would be generated for the two cases ... 

A multitube reactor is used to carry out an exotbermic gas reaction. Catalyst is packed in 0.025-m-diameter tubes and boiling water is used in the reactor jacket. Feed and jacket temperatures are 116°C. During operation the average reactor temperature rises to 121°C a short distance into the reactor and slowly goes down to 116.2°C at the end of the reaetor. Heat transfer resistance is about the same for the bed and the wall. If the tube diameter is increased to 0.038 m (same catalyst), what jacket temperature should be used to keep the peak of 121°C Sketch both cases temperature profiles. Estimate steam pressures. 

The dehydrogenation of 2-butanol is conducted in a multitube vapor-phase reactor over a zinc oxide (20—23), copper (24—27), or brass (28) catalyst, at temperatures of 250—400°C, and pressures slightly above atmospheric. The reaction is endothermic and heat is suppHed from a heat-transfer fluid on the shell side of the reactor. A typical process flow sheet is shown in Figure 1 (29). Catalyst life is three to five years operating in three to six month cycles between oxidative reactivations (30). Catalyst life is impaired by exposure to water, butene oligomers, and di-j -butyl ether (27). 

Some reactors are designed specifically to withstand an explosion (14). The multitube fixed-bed reactors typically have ca 2.5-cm inside-diameter tubes, and heat from the highly exothermic oxidation reaction is removed by a circulating molten salt. This salt is a eutectic mixture of sodium and potassium nitrate and nitrite. Care must be taken in reactor design and operation because fires can result if the salt comes in contact with organic materials at the reactor operating temperature (15). Reactors containing over 20,000 tubes with a 45,000-ton annual production capacity have been constmcted. 

Fixed-bed reactors resemble multitube heat exchangers, with the catalyst packed in vertical tubes held in a tubesheet at top and bottom. Reaction heat can be removed by generating steam on the shell side of the reactor or by some other heat-transfer fluid. However, temperature control is more difficult in a fixed-bed than in a fluulized-bed reactor because localized hot spots tend to develop in the tubes. 

If the catalyst deactivates rapidly or whenever good heat transfer properties are essential and where local hot spots cannot be accepted, the slurry reactor should also be considered. Finally, the slurry reactor can be an attractive alternative to multitube gas-solid packed-bed processes, particularly where large heat effects ask for thousands of thin tubes to control the reactor temperature in the packed beds. Also here, the much better heat transfer characteristics of slurries relative to gas-solid packed beds are deciding. 

It is desired to determine how large the reactor tubes can be and still not exceed the maximum temperature of 673 K. The reactor will be designed to operate at 95 percent conversion of CioHg and is expected to have a production rate of 10,000 Ib/day of phthalic anhydride. It will be a multitube type reactor with heat transfer salt circulated through its jacket at a temperature equivalent to the feed temperature. 

The latest technologies (especially Toyo Soda) do not include the separation and intermediate purification of acrolein. They employ two trains of reactors in series, operating in different conditions, with catalysts of distinct compositions based on molybdenum oxide, and through which the reaction medium flows. These are multitube systems with molten salt circulation (sodium and potassium nitrites and nitrates) on the shell side, to remove heat generated by the transformation, ensure effective temperature control, and the production of low-pressure steam. The catalyst is placed in a fixed bed in the tubes. 

This process is technologically more difficult to implement, because it requires the use of multitube reaction systems with heat transfer fluid flow outside the tubes. However, it is justified by the energy gains and the better performance achieved by operation at a lower reactor feed inlet temperature, and consequently with a lower steam ratio than with adiabatic operation. 

When a latest-generation multitube falling-film reactor [23] is adopted for AO suifonation, the reaction heat is almost completely evolved over a few seconds after the contact between the reactants, and to remove the reaction heat it is necessary to ensure, particularly in the first part of the reactor, an efficient cooling capability, without negatively affecting the viscosity profile of the mass undergoing suifonation. 

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