Gas circulation loop

From a practical point of view, one of the most challenging applications to date has been in the experimental fluidised bed shown in Figure 3, which is a scaled-down version of a polyethylene production unit, of overall height approximately 2 m, with a conical expansion section 1 m above the distributor the diameter of the lower section is 0.154 m. The bed is constructed of 316 stainless steel and is placed within a gas circulation loop allowing operation at pressures up to 20 bars. 

Catalyst samples ( 0.3 g) were pretreated in a quartz cell connected to an evacuable gas-circulation loop (F= 880 cm ). The pretreatment was carried out at 750-800" C in a flow oxygen and under vacuum by the procedure described in detail in [6]. 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

In the fluid circulation loop shown on the right the ammonia gas from the evaporator is adsorbed into Active bed 2 at low pressure. Heat must be removed... 

Derive the cloud thickness and volume ratio of cloud to bubble for a single bubble in a fluidized bed in terms of the Davidson-Harrison (1963) model. Since the cloud is the region established by the gas circulating in a closed loop between the bubble and its surrounding, the following assumptions can be employed (1) zero radial gas velocity outside the surface of the cloud sphere (r = Rc) (2) uniform gas velocity at far distance from the bubble, i.e., Uboo... 

Figure 2. Circulating loop venturi reactor (a) reactor setup (b) scheme of ejector (1 = ejector, 2 =main holding vessel, 3 =gas recycle (adapted from Ref. 8).

In order to prevent any sulfur or other poisons, such as hydrogen cyanide, from reaching the methanol synthesis catalyst, the make-up synthesis gas is passed over an absorbent catalyst and finally cooled for the suction of the synthesis gas circulator in the methanol synthesis loop. 

Gas recycle loops are normally set at maximum circulation rate, as limited by compressor capacity, to achieve maximum yields (Douglas doctrine). 

The principle of experimental semp is presented in Figure 7.5. In addition to a loading stage, it mainly consists of a gas-pressurized cross-flow hltration circulation loop where the membrane allows the separation of the feed in two streams the permeate and the retentate. Each stream is further separated into gas and liquid parts in the separation stage. Membranes are inorganic to ensure hard-operating conditions without failure. 

The apparatus and methods are described in detail in the previous paper (4) and elsewhere (5-6). It is of the dual recirculation-type and is comprised, in part, of a central pressure vessel through which both liquid and vapor phases are continuously recirculated. Samples of the liquid and vapor phases are removed from the circulation loops and analyzed by gas chromatography. 

A pilot test plant with hydrogen production capacity of about 30 Nm /h is being designed. The pilot test plant will be made of industrial materials and operated using an electrically heated high-pressure helium gas as the heat source. Figure 10 shows a tentative scheme of the pilot test facility, which consists of the IS process pilot test plant and a helium gas circulation facility (He loop)... 

Scale-up of CFBs is generally less of a problem than with bubbling beds [111], Moreover, the higher velocity in CFB means higher gas throughput, which can minimize the reactor costs. Several CFB loop designs have been proposed for getting smooth steady state circulation of solids. Basically, there are two basic types of solids circulation loops distinct in that some include a reservoir of solids while others do not. The solids circulation loops which do not include a reservoir of solids (hopper) are less flexible in operation compared to the circulation systems with reserviors. 

Soot recycle Steam boiler compressor Loop gas circulator... 

Type (b) models are at present the most useful, though still requiring more hydrodynamic data. A successful example considers the liquid and gas circulations in a stirred tank as loops of connected mixed zones (see Figure 15.23) with feed and offtake where appropriate. The zones can have different and gas hold-ups if these have been determined experimentally. The mean circulation flow and relevant number of zones have to be supplied from experiments, so the model is again not in itself predictive. However, the parameters have a physical meaning and so they are more amenable to scale-up. 

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