Recirculating gas

A recirculation design (Fig. 10) returns the gas to the classifier through the fan after the fine particles are removed from the gas stream. Such an arrangement requires an excellent soHd/gas separator otherwise the classification becomes less efficient. A perfect soHd/gas separator would be a device having a = 1. If the recirculated gas is entered through a secondary coarse stream classification section, then the classification is not less efficient unless the secondary classification is very iaefficient. 

To maintain a low level of inert gas (nitrogen) a part of the recirculating gas stream is withdrawn. The sulfur dioxide is washed out by water. The resulting aqueous solution is warmed, and the sulfur dioxide stripped by a stream of oxygen and fed back to the reactor. 

Suppose the following data on the iodination of ethane have been obtained at 603 K using a recirculating gas-phase reactor that closely approximates a CSTR. The indicated concentrations are partial pressures in atmospheres and the mean residence time is in seconds. 

Condensed mode operation. To avoid accumulation of liquid in the reactor the dew point of the gas must be kept above reactor temperature. When operating in condensed mode, higher-boiling components in the recirculating gas are allowed to condense in the heat exchanger, and this liquid is fed back to the reactor where it evaporates. 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

Figure 5.2. Calculated gas flow fields in the near-nozzle region of free-fall atomizers. Primary gas pressure 0.140 MPa secondary gas pressure 0.189 MPa angle of secondary gas nozzle relative to the spray centerline 10° angle of primary gas nozzle relative to the spray centerline (a) 0°, (b) 22.5°, and (c) 30° designed for minimizing recirculation gas flow. (Reprinted from Ref. 612.)...

The size of the bubbles produced in the reactor and the gas volume fraction will depend on the agitation conditions, and the rate at which fresh hydrogen is fed to the impeller, as shown in Fig. 4.20. (Some hydrogenation reactors use gas-inducing impellers to recirculate gas from the head-space above the liquid while others use an external compressor.) For the purposes of the present example, the typical values, db = 0.8 mm and eg m 0.20, will be taken. Also dependent to some extent on the agitation conditions are the values of the transfer coefficients kL and k, although these will depend mainly on the physical properties of the system such as the viscosity of the liquid and the diffusivity of the dissolved gas. The values taken here will be kL = 1.23 x 10 5 m/s and k, = 0.54x 10"3 m/s. 

Fig. 2.13. Commercial metal glove box. An aluminum glove box with a recirculating gas-purification system. (Reproduced by permission of the copyright ow ner Vacuum Atmospheres Corp., North Hollywood, Calif.)...

If the synthesis gas contains traces of carbon oxides, ammonium carbamate will form upon mixing with the ammonia in the recirculating gas from the synthesis loop. The carbamate will clog and/or corrode downstream equipment. To avoid this condition, consider controlling the carbon oxides level in fresh makeup gas at less than 5 ppm88. 

The micromixing state in an agitated vessel is connected with coalescence of bubbles. Coalescence occurs mainly in the stirrer zone, where the recirculated gas bubbles partially mix with fresh gas in the cavities. It also occurs to a lesser extent in the highly turbulent stream leaving the stirrer, but it is virtually absent in other parts of the vessel because the low kinetic energy of the bubbles cannot stretch out the liquid film between a pair of bubbles to reach the coalescence thickness. 

The liquid flow is driven by gravity, and a small pressure increase is obtained at the bottom of the reactor. The pressure difference can be used to recirculate the gas. This recirculated gas is mixed with the fresh gas at the inlet. Some of the recirculated gas is also entrained by the liquid in the lower monoliths (Fig. 11). 

The liquid is recirculated with an external pump that gives a controlled volumetric flow rate qi. The recirculated gas flow is determined by the amount that can be entrained by the liquid into the monolith. The total linear velocity in the channels is controlled by gravity. The gas entrained by the liquid is given by the difference between the total linear velocity and the linear velocity of the liquid in the channels and can be calculated from the relationship... 

A schematic of the 2kD system is shown in Figure 2. The title 2kT> is derived from the design capacity of 2k ton per day charcoal output. The basic trailer-mounted system is comprised of three subsystems 1) a reactor, 2) a recirculating gas loop to bring heat into the reactor by indirect contact of the recirculating gas with hot products of combustion in a heat exchanger and 3) the combustion system. Ancillary subsystems such as a condenser to obtain liquid distillates (shown in sketch) and secondary heat exchangers (not shown) are not critical to the operation of the 24D system. 

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