Fixed bed plants

Fixed bed plants. In this type of plant, the process flow for all three feeds looks like the plant in Figure 20—3. The feed and compressed air are mixed, vaporized in a heater, and then charged to the fixed-bed reactor, a bundle of rubes packed with the catalyst. The ratio of air to hydrocarbon is generally about 75 1 to keep the mixture outside the explosive range, always a good idea. The feed temperature is 800-900°F, depending on the feed. The reaction time is extremely quick, so the feed is in contact with the catalyst for only 0.1 to 1.0 second. 

Utilization of catalyst would be more efficient. Channeling has sometimes been experienced in fixed-bed plants, resulting in ineffectiveness of a portion of the catalyst bed. Trouble due to channeling has sometimes been experienced during regeneration. 

Commercial Fixed-Bed Plant Design. The commercial fixed-bed MTG plant is very similar in design concept to the 4 B/D demonstration plant. A typical design of the reaction section of a commercial plant is shown in Figure 10. The feedstock may be distilled or crude methanol. 

In the early days of development of continuous ion exchange processes the benefits to water treatment of high efficiency and low leakage were paralleled by the rapidly advancing designs of counterflow fixed bed plant. Thus the more complex hydrodynamic requirements of CIX, resin losses due to mechanical attrition, and perhaps a conservative attitude towards availability of plant during periods of unscheduled maintenance meant that, in the UK at least, the CIX... 

At least two fixed beds ate required to give continuous service of purified water but three are normally used for recovery processes. This ensures ihet barren effluent is alweys below the limit and that each bed is fully loaded with product before regeneration lakes pi nee, Automatic control of a fixed-bed plant regulates the times of the velions cycles and controls feed rates at protkrtermined values. For fully automatic control of cydea, up to six automatic shut-off valves per vessel are required. 

Operating experience with the Couitaulds plant has shown the recovery efficiency to be at least as high as for a fixed-bed plant and steam consumption a little lower. Carbon attrition has proven to be somewhat of a problem as the accumulation of flnes in the bed increases the residence time, which in turn leads to a greater adsorption of water at the expense of CS2. This problem can be partially resolved by drawing off fines collected from the exit gas instead of returning them to the bottom adsorber tray as shown in the flow diagram. However, this. solution leads to an increased requirement for makeup carbon. 

An economic comparison was made of the Polyad FB process, an activated-carbon fixed-bed plant, and a thermal incinerator system for purifying 6,300 scfm of air from a gravure... 

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

At the end of Wodd War II the butynediol plant and process at Ludwigshafen were studied extensively (67,68). Vadations of the original high pressure, fixed-bed process, which is described below, are stiU in use. However, ak of the recent plants use low pressures and suspended catalysts (69—75). 

Pre-Keformer A pre-reformer is based on the concept of shifting reforming duty away from the direct-fired reformer, thereby reducing the duty of the latter. The pre-reformer usually occurs at about 500°C inlet over an adiabatic fixed bed of special reforming catalyst, such as sulfated nickel, and uses heat recovered from the convection section of the reformer. The process may be attractive in case of plant retrofits to increase reforming capacity or in cases where the feedsock contains heavier components. 

Regenerative heat exchangers of both the fixed-bed and moving-bed types (67) have been considered for MHD use. The more recent efforts have focused on the fixed-bed type (68), which operates intermittently through recycling. A complete preheater subsystem for a plant requites several regenerators with switch-over valves to deflver a continuous supply of preheated air. The outlet temperature of the air then varies between a maximum and a minimum value during the preheat cycle. 

Data for the production and sales of maleic anhydride and fumaric acid ia the United States between 1979 and 1992 are shown ia Table 5. Production of maleic anhydride during this time grew - 2% on average per year. Production of fumaric acid has declined during the same period as customers have switched to the less cosdy maleic anhydride when possible. All production of maleic anhydride in the United States in 1992 was from butane-based plants which used fixed-bed reactor technology as shown in Table 6. The number of fumaric acid producers has been reduced considerably since the early 1980s with only two producers left in the United States in 1992 as shown in Table 6. Pfizer shut down its fumaric acid plant at the end of 1993. However, Bartek of Canada will start up an expanded fumaric acid faciUty to supply the North American market for both their own and Huntsman s requirements. 

Ammonium Ion Removal. A fixed-bed molecular-sieve ion-exchange process has been commercialized for the removal of ammonium ions from secondary wastewater treatment effluents. This application takes advantage of the superior selectivity of molecular-sieve ion exchangers for ammonium ions. The first plants employed clinoptilolite as a potentially low cost material because of its availability in natural deposits. The bed is regenerated with a lime-salt solution that can be reused after the ammonia is removed by pH adjustment and air stripping. The ammonia is subsequentiy removed from the air stream by acid scmbbing. 

Oxidation. Naphthalene may be oxidized direcdy to 1-naphthalenol (1-naphthol [90-15-3]) and 1,4-naphthoquinone, but yields are not good. Further oxidation beyond 1,4-naphthoquinone [130-15-4] results in the formation of ortho- h. h5 ic acid [88-99-3], which can be dehydrated to form phthaUc anhydride [85-44-9]. The vapor-phase reaction of naphthalene over a catalyst based on vanadium pentoxide is the commercial route used throughout the world. In the United States, the one phthaUc anhydride plant currently operating on naphthalene feedstock utilizes a fixed catalyst bed. The fiuid-bed process plants have all been shut down, and the preferred route used in the world is the fixed-bed process. 

The quahty of naphthalene required for phthaUc anhydride manufacture is generally 95% minimum purity. The fixed plants do not require the high (>98%) purity naphthalene product and low (<50 ppm) sulfur. The typical commercial coal-tar naphthalene having a purity ca 95% (freezing point, 77.5°C), a sulfur content of ca 0.5%, and other miscellaneous impurities, is acceptable feedstock for the fixed-bed catalyst process based on naphthalene. 

Future Developments. The most recent advance in detergent alkylation is the development of a soHd catalyst system. UOP and Compania Espanola de Petroleos SA (CEPSA) have disclosed the joint development of a fixed-bed heterogeneous aromatic alkylation catalyst system for the production of LAB. Petresa, a subsidiary of CEPSA, has announced plans for the constmction of a 75,000 t/yr LAB plant in Quebec, Canada, that will use the UOP / -paraffin dehydrogenation process and the new fixed-bed alkylation process (85). 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

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