Reverse flow from reactors

In one incident paraffin wax and chlorine were reacted at atmospheric pressure. Some paraffin traveled from the reactor back up the chlorine line and reacted with liquid chlorine in a catchpot, which exploded with great violence. Bits were found 30 m away [3]. 

A more serious incident occurred at a plant in which ethylene oxide and aqueous ammonia were reacted to produce ethanolamine. Some ammonia got back into the ethylene oxide storage tank, past several check valves in series and a positive pump. It got past the pump through the relief valve, which discharged into the pump suction line. The ammonia reacted with 30 m of ethylene oxide in the storage tank. There w as a violent rupture of the tank, followed by an explosion of the vapor cloud, which caused damage and destruction over a wide area [4], 

Another somewhat similar incident occurred when butadiene from a reactor flowed in the wrong direction up a line used for adding emulsifier. The check valve, which should have prevented the flow, was obstructed. The emulsifier tank was in a building and had an open vent. Butadiene came out and exploded. The explosion was heard 15 km away, but damage was minimized by the light construction of the building, w hich ruptured at the junction of the roof and walls [13]. 

When such violent reactions can occur, it is not sufficient to rely on check valves. In addition, either  

The reactant(s) should be added via a small break tank so that if reverse flow occurs only a small quantity will react and not the main stock, or 

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Sulfur from H2S (Claus process) 96 Reverse-flow adsorptive reactor... 

The reverse-flow tank reactor is based on the use of a high-pressure tank with hot and cold zones (Fig. 4). The high-pressure feed is injected downward into the center region where oxidation of the organics and precipitation of salts occur. The produced gases move up and leave through the exit at the top. The lower section is cooled by injecting cold water, and hence remains in the liquid phase in which salts dissolve. The salts are easily removed off the reactor as brine from the... 

The most serious incidents resulting from reverse flow have occurred when reactant A (Figure 18-5) has passed from the reactor up the reactant B feed line and reacted violently with B. 

Figure 18-5. Reverse flow of A occurred from the reactor into the B stock tank.

Fig. 8. Observed temperature profiles in a single, flow-reversal S02 oxidation reactor processing (I) 40,000 m3/h, (II) 20,000 m3/h of a smelter effluent containing 2.3 vol% S02. (Figure adapted from Matros, 1989, with permission, 1989 Elsevier Science Publishers.)...

Several pilot plants have been built to test periodic flow direction reversal. Pilot-scale reactors with bed diameters from 1.6 to 2.8 m were operated with flow reversal for several years. The units, described by Bunimovich et al. (1984,1990) and Matros and Bunimovich (1996), handled 600 to 3000 m3/h and operated with cycle periods of 15 to 20 min. Table VIII shows the performance of these plants for different feeds and potassium oxide promoted vanadia catalysts. The SVD catalyst was granular the IK-1-4 was in the form of 5 (i.d.) x 10-mm cylinders, while the SYS catalyst was... 

Airco A modification of the Deacon process for oxidizing hydrogen chloride to chlorine. The copper catalyst is modified with lanthanides and used in a reversing flow reactor without the need for external heat. Developed by the Air Reduction Company from the late 1930s. U.S. Patents 2,204,172 2,312,952 2,271,056 2,447,834. 

Figure 2.21 Temperature and species concentration profiles in a reverse-flow reactor (a) flow direction is left to right (b) flow is reversed to be from right to left (c) periodic quasi-steady state [47] (by courtesy of ACS).

A. M. De Groote, G. F. Froment, Synthesis Gas Production from Natural Gas in a Fixed-Bed Reactor with Reversed Flow. Can. J. Chem. Eng., 1996, 74, 735-742. 

VOC emissions from printing and chemical plants are oxidized in reverse flow reactors that couple reaction with regenerative heat transfer. The concept here is to maintain a catalyst zone in the center of a packed bed with inert heat-transfer packing on either side. 

Reverse-flow operation for Sulfur Production over Bauxite Catalysts by the Claus Reaction has been considered in Refs 9 and 31. The rate of H2S oxidation by SO2 on bauxite catalysts is very high even at ambient gas inlet temperature, but sulfur condensing at low temperatures blocks the active catalyst surface, and the reaction stops because of catalyst deactivation. In a reverse-flow reactor the periodic evaporation of condensed sulfur from the outlet parts of the catalyst bed occurs. Although it is difficult to remove all the sulfur condensed within the catalyst pellets at the bed edges, after a certain time a balance between the amount of sulfur condensed and evaporated is attained. Using a reverse-flow reactor instead of the two-bed stationary Claus process provides an equal or better degree of... 

Step 6. Three liquid levels need to be controlled reactor, separator, and stripper base. We must use the Buckley strategy of level control in. the reverse direction to flow since the stripper base product B is fixed by production rate. Therefore liquid flow from the separator (L) must be used to control stripper base level. To control level in the separator, we select the cooling water flow to the condenser (CWcj. 

The cleaning of flue gases from stationary sources is another field in which the application of monolithic catalysts will certainly rise. There will be no versatile catalyst for cleaning all off-gases. Therefore tailor-made catalysts with zeoliths of various types for specific applications will be developed. Incorporated-type monolithic catalysts are likely to prevail in this field. Since cleaning usually requires a set of equipment items in series (e.g., converter, heat exchangers), multifunctional reactors (reverse-flow reactors, rotating monoliths) will become more common. 

Figure 15 Flowsheet diagrams for a reverse-flow SCR 1 = reactor, 2 = switching valves, 3 catalyst, 4 = inert layer (From Ref 50)...

De Groote, A.M. and Froment, G.F. Synthesis gas production from natural gas in a fixed-bed reactor with reversed flow. Canadian Journal of Chemical Engineering, 1996, 74, 735. 

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