Liquid recirculation

The horizontal natural circulation systems do not use a kettle design exchanger, but rather a 1-2 (1 shell side, 2 tube-side passes) unit, with the vaporized liquid plus liquid not vaporized circulating back to a distillation column bottoms vapor space or, for example, to a separate drum where the vapor separates and flows back to the process system and where liquid recirculates back along with make-up feed to the inlet of the horizontal shell and tube reboiler. See Figures 10-96A-C. 

The recirculation ratio for a unit is the lb rate of liquid leaving the outlet compared to the lb rate of vapor leaving. The liquid recirculation flow rate entering the unit is set by the differential pressure driving the system. 

The variation of removal efficiencies of benzene according to the inlet loading rate and the liquid recirculation rate is shown in Fig. 3. As shown in the figure, the removal efficiency of benzene decreases with increasing the inlet loading rate and decreasing the liquid recirculation rate. At an inlet loading rate less than 40 g/m -hr, benzene removal efficiency shows a little variation as the liquid recirculation rate increases. However, the removal efficiency... 

Fig. 11. Liquid recirculation in a flooded system where A = refrigerant circulated through coils as brine, B = direct expansion coils, C = flooded evaporators, andD = special heat-exchanger designs.

The results presented in Fig. 27 represent an exciting advance because they demonstrate that the GERVAIS pulse sequence allows direct imaging of regions of liquid recirculation in both the z- and. vv-vclociiy fields. However, a word of caution is needed. Because GERVAIS is an EPI-based technique, it requires that variations in magnetic susceptibility within the sample be minimized. In this system, plastic spheres were used, which were naturally susceptibility-matched with water. If this... 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

Liquid plug flow produces a horizontal (i.e., flat) flow profile (Fig. 7.7a), Channeling produces a U-shaped flow profile (Fig, 7.8a). The liquid moves fast at the tray center and slow near the walls. Wide stagnant zones, a steep U shape, and liquid recirculation in the stagnant zones (Fig. 7.35) signify a highly channeled flow profile. The... 

Higher gas flow rates flatten the U shape, narrow the stagnant regions, and reduce liquid recirculation in the stagnant regions... 

Step 2. There are 26 control degrees of freedom in this process. They include three feed valves for oxygen, ethylene, and acetic acid vaporizer and heater steam valves reactor steam drum liquid makeup and exit vapor valves vaporizer overhead valve two coolers and absorber cooling water valves separator base and overhead valves absorber overhead, base, wash acid, and liquid recirculation valves gas valve to CO removal system gas purge valve distillation column steam and cooling water valves column base, reflux, and vent valves and decanter organic and aqueous product valves. 

Step 8. Several control valves now remain unassigned. Steam flow to the trim heater controls reactor inlet temperature. Cooling water flow to the trim cooler is used to control the exit process temperature and provide the required condensation in the reactor effluent stream. Liquid recirculation in the absorber is flow-controlled to achieve product recovery, while the cooling water flow to the absorber cooler controls the recirculating liquid temperature. Acetic acid flow to the top of the absorber is flow-controlled to meet recovery specifications on the overhead gas stream. Cooling water flow to the cooler on this acetic acid feed to the absorber is regulated to control the stream temperature. Cooling water flow in the column condenser controls decanter temperature. 

The reaction is initiated with nickel carbonyl. The feeds are adjusted to give the bulk of the carbonyl from carbon monoxide. The reaction takes place continuously in an agitated reactor with a liquid recirculation loop. The reaction is run at about atmospheric pressure and at about 40°C with an acetylene carbon monoxide mole ratio of 1.1 1 in the presence of 20% excess alcohol. The reactor effluent is washed with nickel chloride brine to remove excess alcohol and nickel salts and the brine—alcohol mixture is stripped to recover alcohol for recycle. The stripped brine is again used as extractant, but with a bleed stream returned to the nickel carbonyl conversion unit. The neutralized crude monomer is purified by a series of continuous, low pressure distillations. 

The liquid is separated from the gas and recirculated by an external pump. This liquid recirculation flow is mixed with fresh reactants and added at the top of the monolith. 

Figure 1. Schematic Diagram of the Dual-Sampling Vapor-Liquid Equilibrium Apparatus. Components are labeled as follows 6PG, Gas pressure generator GRP, Gas recirculation punqp HZ, Heated zone LPG, Liquid pressure generator LRP, Liquid recirculation pump P, Pressure gauge RV, High pressure sanqpling valve SPV, Sapphire pressure vessel VG, Vacuum gauge. Hatchmarks on the lines in the saxtqpling section of the apparatus indicate where heaters have been wrapped on them. The path of vapor saxtqples is indicated by light arrows and the dark arrow shows the path of liquid sanples.

Several measures can be taken to control the reactor temperature. For instance, cooling jackets or internal cooling coils can be used. External liquid recirculation through a heat exchanger is another possibility. An elegant solution is to operate at the boiling point of the liquid mixture. In bubble columns and stirred tanks thus near isothermal operation can be achieved by evaporation of one or more of the liquid components present. 

In order to reduce attrition, recirculation of the particles can be achieved by the gas bubbles (gas lift) instead of by mechanical agitation (bubble column, see Fig. 8.2). External liquid recirculation offers the possibility of improving the gas-liquid mass transfer by special liquid ejector types. In the case of gas and liquid upflow a three-phase fluidized bed reactor is produced. Here particles of a few millimeters are needed to retain them in the reactor and allow separation from the gas/liquid phase. 

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