Vertical pipe-loop reactor

The right picture in Figure 228 shows a similar stage of swelling in a commercial reactor. A "log" of about 2-ft diameter was sawed into sections for disposal. In this case, the log solidified into a hard cylinder that shrank slightly upon cooling and therefore could be easily pulled out of the vertical pipe-loop reactor section. 

U.S. Patent 3,152,872 issued to J.S. Scoggin and Harvey S. Kimble on October 13, 1964, and assigned to Phillips Petroleum Company, provides a detailed description of the separation system used to isolate the solid polyethylene granular particles from the liquid diluent used in the vertical pipe-loop reactor. The contributions of Scoggin and Kimble were important in the design and start-up of the first commercial loop reactor in Pasadena, Texas, in 1961. It should be noted that the first vertical loop reactor used n-pentane as the slurry solvent, which was later changed to isopentane and then to isobutane in about 1970. 

The 95-gallon, pilot-plant, vertical pipe-loop reactor was designed to eliminate the problem of polymer build-up on reactor surfaces found in the stirred tank auoclave designs by circulation of the reacting slurry around a closed loop at a velocity sufficient to keep the polymer particles suspended. This design was the critical technical development that led to the introduction in 1961 of the Phillips vertical-loop, particle-form reactor on a commercial scale, which is illustrated in Figure 5.12. 

In June of 1958, a 95-gallon vertical pipe-loop, pilot plant reactor was installed and placed into operation with the objective of solving the reactor wall-fouling problems encoxmtered with the stirred-tank autoclave reactors. 

The first United States patent that described the vertical pipe-loop slurry polymerization reactor illustrated in Figure 5.11 was issued to Donald D. Norwood on April 26,1966, as U.S. Patent 3,248,179 and assigned to Phillips Petroleum Company. Earlier applications filed in 1959 were abandoned, thus explaining the relatively late issue date on the Norwood patent. Although Donald Norwood was the only name on the first United States patent issued to Phillips Petroleum, Philips Petroleum credits three process engineers, D. D. Norwood, S. J. Marwil and R G. Rohling, for the design of the vertical loop reactor [24]. 

The agitators necessary in the autoclave reactor were no longer required in the vertical pipe-loop system as a new design was disclosed in U.S. 

The loop reactors, which are recycled tubular reactors, are used by the Phillips Petroleum Co. and Solvay et Cie. The Phillips process is characterized by the use of a light hydrocarbon diluent such as isopentane or isobutane in loop reactors which consist of four jacketed vertical pipes. Figure 1 shows the schematic flow diagram for the loop reactor polyethylene process. The use of high-activity supported chromium oxide catalyst eliminates the need to deash the product. This reactor is operated at about 35 atm and 85-110° C with an average polymer residence time of 1.5 hr. Solid concentrations in the reactor and effluent are reported as 18 and 50 wt %, respectively. The reactor diameter is 30 in. (O.D.) and the length of the reactor loop is about 450 ft. 

Many different loop reactor configurations are used in industrial processes. The loop can be either in a vertical (Phillips and Spheripol processes [72, 73]) or in a horizontal position (USl process). The pipe can also be bent into multiple legs to increase the reaction volume. Several loops can be arranged in series to produce bimodal polymer, as in the case of Spheripol and Borstar processes [74]. Alternatively, the series of loop reactors can be operated as one single unit to increase average residence time and throughput. 

In airlift bioreactors the fluid volume of the vessel is divided into two interconnected zones by means of a baffle or draft-tube (Fig. 5). Only one of these zones is sparged with air or other gas. The sparged zone is known as the riser the zone that receives no gas is the downcomer (Fig. 5a-c). The bulk density of the gas-liquid dispersion in the gas-sparged riser tends to be less than the bulk density in the downcomer consequently, the dispersion flows up in the riser zone and downflow occurs in the downcomer. Sometimes the riser and the downcomer are two separate vertical pipes that are interconnected at the top and the bottom to form an external circulation loop (Fig. 5c). External-loop airlift reactors are less common in commercial processes compared to the internal-loop designs (Fig. 5a, b). The internal-loop configuration may be either a concentric draft-tube device or an split-cylinder (Fig. 5a, b). Airlift reactors have been successfully employed in nearly every kind of bioprocess—bacterial and yeast culture, fermentations of mycelial fungi, animal and plant cell culture, immobilized enzyme and cell biocatalysis, culture of microalgae, and wastewater treatment. 

A detailed description of the Phillips pilot plant pipe-loop slurry reactor (Figure 5.11) is found in U.S. Patent 3,248,179. The design consisted of three 4.5 foot, 10 inch, inner-diameter sections and one 4.5 foot, 12 inch inner-diameter pipe which contained the reactor pumping unit as one of the two vertical sections of the design, with either n-pentane or n-hexane as the slurry solvent. 

One of the most efficient implementations of the slurry process was developed by Phillips Petroleum Company in 1961 (Eig. 5). Nearly one-third of all HDPE produced in the 1990s is by this process. The reactor consists of a folded loop with four long (- 50 m) vertical mns of a pipe (0.5—1.0 m dia) coimected by short horizontal lengths (around 5 m) (58—60). The entire length of the loop is jacketed for cooling. A slurry of HDPE and catalyst particles in a light solvent (isobutane or isopentane) circulates by a pump at a velocity of 5—12 m/s. This rapid circulation ensures a turbulent flow, removes the heat of polymeriza tion, and prevents polymer deposition on the reactor walls. 

Figure 7.11b shows an EL airlift reactor, in schematic form. Here, the downcomer is a separate vertical tube that is usually smaller in diameter than the riser, and is connected to the riser by pipes at the top and bottom, thus forming a circuit for liquid circulation. The liquid entering the downcomer tube is almost completely degassed at the top. The liquid circulation rate can be controlled by a valve on the connecting pipe at the bottom. One advantage of the EL airlift reactor is that an efficient heat exchanger can easily be installed on the hquid loop hne. 

The polymerization reactor is made of large diameter pipes assembled in a long vertical loop configuration. Early designs consisted of four- or six-leg loops. The reactor walls are made of carbon steel and are equipped with water jackets to control temperature in the isothermal reactor. Ethylene is polymerized under a total pressure of about 25 bars to 40 bars and at a temperature between 75°C and 110°C (167°F and 230°F). The recycle diluent, fresh monomer, comonomer, hydrogen, catalyst and co-catalyst (when required), are fed to the reactor. Polyethylene is formed as discrete particles in a rapidly circulating diluent-polymer slurry. On average, 98% of the ethylene is polymerized. 

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