Recirculation gas flow

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 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... 

Fig. 19.8 Recirculation gas flow pattern for different tower geometries and entrainment volumetric gas flow dependent on the distance from the atomizer (L atomizer = 200m /h, only gas phase)...

A general flow map of different hydrodynamic conditions (Fig. 23) consists of regions of flooding, dispersion, and recirculation on a plot of N vs for a Rushton turbine. For a low viscosity aqueous/air system, the gas flow numbers for the three conditions are given hy FI = 30Fr[D/TY for flooding, = 0.2Fr° (F/r)° for complete dispersion, and =13FF D/TY for recirculation. 

Chemical additives for gas-based drilling fluids are limited to surfactants (qv), certain polymers, and occasionally salts such as sodium or potassium chloride. An aqueous solution of the additives is iajected iato the air or gas flow to generate a mist or foam. No additives are used ia dry air or gas drilling operations. Gas-based fluids are not recirculated and materials are added continuously. As the fluid exits the well, air or water vapor escapes to the atmosphere, gas and oil are burned, and water and formation soflds are collected into a pit for later disposal. Stable foams must be destabili2ed to separate the air from the Hquid phase for disposal. 

In high speed dusted, premixed flows, where flames are stabili2ed in the recirculation 2ones, the turbulent flame speed grows without apparent limit, in approximate proportion to the speed of the unbumed gas flow. In the recirculation 2ones the intensity of the turbulence does not affect the turbulent flame speed (1). 

Figure 2.2 Schematic of a modem four-pass, packaged horizontal FT boiler, with low NOx emission, showing path of hot combustion gas flow, and air/combustion gases recirculation system.

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). 

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

Possible solutions to overcome this problem are (1) decrease the residence time the decrease of conversion is more than compensated by an increase of selectivity (due to the lower extent of methacrylic acid combustion), and in overall the productivity increases (2) increase the total pressure, while simultaneously increasing both the oxygen and the isobutane partial pressure, as well as the total gas flow (so as to keep a constant contact time in the reactor). A higher pressure also implies smaller reactor volume, and hence lower investment costs. Under these circumstances, productivity as high as 6.4 mmol/h/gcat was reached, which is acceptable for industrial production. The additional heat required for the recirculation of unconverted isobutane and for increased pressure would be equalized by the higher heat generated by the reaction. 

Mixing. Mixing can be accomplished by mechanical recirculation, agitation or by controlled gas flow methods. The practice is desired to facilitate on intimate contact between methane forming bacteria and their substrate and to prevent the formation of surface scum in the digester. 

In Figure 11.2 a schematic view of a stirred vessel is given. The vessel is cyhndrical with a height (m) and a diameter T (m). Usually is equal to or greater than 2 T. It is equipped with a stirrer in the lower compartment. TTiis stirrer is mounted near the bottom, usually at a distance equal to the stirrer diameter. At a lower position the stirrer and bottom interact, leading to a decrease in power consumption. At a higher position hquid circulation problems can occur because, at increased gas flow rate in case of aeration, the bubbles will not be recirculated in the lower compartment. Sometimes the upper compartment (s) are also equipped with a stirrer. The vessel is equipped with baffles to prevent rotation of the contents as a whole. For aeration an air sparger is mounted below the stirrer. For mass transfer... 

The feed gas is introduced near the rotor axis. Enriched and depleted gases are extracted by stationary pitot-like scoops. The location and shape of these tubes, and the baffles within the rotor, greatly effect the gas flow which recirculates within the rotor, reaching enrichment equilibrium at a given feed rate. A vacuum is maintained around most of the rotor. The UF leakage around the stationary axial post is confined to the top of the case by the use of a molecular pump. 

Bubble slurry column reactors (BSCR) and mechanically stirred slurry reactors (MSSR) are particular types of slurry catalytic reactors (Fig. 5.3-1), where the fine particles of solid catalyst are suspended in the liquid phase by a gas dispersed in the form of bubbles or by the agitator. The mixing of the slurry phase (solid and liquid) is also due to the gas flow. BSCR may be operated in batch or continuous modes. In contrast, MSSR are operated batchwise with gas recirculation. 

The key to smooth operation of a CFB system is the effective control of the solids recirculation rate to the riser. The solids flow control device serves two major functions, namely, sealing riser gas flow to the downcomer and controlling solids circulation rate. Both mechanical valves or feeders (see Figs. 10.1(a) and (d)) and nonmechanical valves (see Figs. 10.1(b) and (c)) are used to perform these functions. Typical mechanical valves are rotary, screw, butterfly, and sliding valves. Nonmechanical valves include L-valves, J-valves (see Chapter 8), V-valves, seal pots, and their variations. Blowers and compressors are commonly used as the gas suppliers. Operating characteristics of these gas suppliers which are directly associated with the dynamics and instability of the riser operation must be considered (see 10.3.3.2). 

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