Overhead Pressure

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

Because trichlor is 99.9% overhead, use it only to select boiling point from vapor pressure curves at 10 psig overhead pressure = 223°F (1,280 mm Hg abs). 

Evaluate the effects of choosing trays, random packing, or systematic packing. It is recommended that the top column pressure be set for the lowest reasonable overhead pressure consistent with the use of a water- or air-cooled condenser. In this case, pressure would be 50 mmHg, corresponding to a temperature of 53°C (127°F). 

A complicating feature in any a-olefin process where C12 and C14 olefins must be separated from triethylaluminum (TEA) is the difficulty encountered in eflFecting this separation. Because of the small volatility differences among dodecene, tetradecene, and TEA, good separations will require eflScient, multiplate columns. With such equipment, high still pot temperatures will be encountered, even at very low overhead pressures. Under such conditions, backreaction of TEA with the dodecene and tetradecene occurs rapidly and excessive olefin and TEA loss will result. 

Pressnre control loops are nsed to maintain system pressure for distillation columns, reactors, and other process nnits. A pressnre control loop for maintaining overhead pressure in a column is shown in Fignre 15.31. The hnal control element is a control valve on the vent line, and the sensor is a pressnre sensor monnted on the top of the column. The output from the pressure controller goes directly to the control valve on the vent line. The objective of this loop is to maintain the column overhead pressnre at or near setpoint for changes in condenser duty and changes in vapor flow rate up the column. 

After equilibrium in the extractive distillation column was reached, the overhead pressure stabilized and the column temperature gradient increased smoothly from tray to tray down the column. Samples were taken, and column conditions were recorded at two-hour intervals. When the desired data had been obtained at one solvent-to-C4 feed ratio, solvent and feed flows were adjusted to a new ratio, and additional data were obtained. Table IV and Figures 3 and 4 show typical data obtained for each solvent. 

Rather than setting the pressure of a distillation column at a level sufficiently high to permit the use of cooling water in the overhead condenser, one may specify a lower pressure and use a refrigerant in the condenser. For example, the separation of propylene-propane, as specified in Fig. 17.2, can be conducted by low-temperature distillation at a 100-psia column overhead pressure, as shown in Fig. 17.8, if a feed system such as shown in Fig. 17.9 is provided. There, the feed mixture is compressed in two stages with an intercooler. A refrigerant-cooled condenser, which follows a water-cooled aftercooler, prepares a saturated liquid feed for the distillation operation. In Fig. 17.8, refrigerant must be supplied to the partial condenser to condense the overhead vapor to obtain reflux at 43 F. At... 

Bicarbonate is considered in equilibrium with dissolved CO2, with the pH decreasing from 5 to 4.2 during the fermentation. Desorbed CO2 leaves the fermentor when the overhead pressure reaches the maximal tank pressure. Under these conditions... 

Finally Figure 8 shows the measured change with time of dissolved CO2 concentration, fermentor overhead pressure, and CO2 outlet fow rate at 14° C in the pilot plant. As demonstrated by the good agreement between the experimental points and the calculated curves, equations 14 to 16 provide an adequate modeling of CO2 production, dissolution and desorption. 

Figure 8. Experimental and theoretical time variation of dissolved COg, overhead pressure, and COg exit flow rate at I4°C.

Increase the bottoms temperature gradually until the overhead pressure is about normal. 

When the overhead pressure reaches 3.4 barg as measured by PIC-221, stop the temperature... 

The permissible overhead pressure range is 3.0—4.0barg. If the pressure deviates outside this range, refer to Instruction XYZ. 

Steam ejectors Operability of steam ejectors is very sensitive to the stability in the motive fluid (steam) pressure. Prefer vacuum pumps to steam ejectors (H). Keep diameter of pipes = diameter of inlet and discharge flanges of ejectors. As the column overhead mass flowrate increases above design, so will the column overhead pressure and vice versa. Compression ratios per ejector 6 1 to 15 1. If inlet gas temperature < 0 °C or below the triple point of water (0.61 Pa) then add steam jacketing to cope with ice formation. Seal for the hot well submerge > 30 cm. The volume in the hotwell between the pipe and the overflow weir should be 1.5 times the volume in the down spout sealed. Replace any nozzles or diffusers where the area is > 7 % larger than design. 

The vented tube separator, in turn, can be improved with the introduction of a valve, which allows the overhead pressure to be varied. The valve would introduce an additional circulation control mechanism. As the valve closes, the overhead pressure would increase, and the gas would not disengage as readily. The assumption is that gas disengages too easily in the vented state, while it almost fails to disengage in the tube separator (Jones, 2007 van Benthum et al., 1999a), which a closed valve would imitate. Hence, the optimum would be somewhere in the middle, and a valve adjustment would allow this operational point to be reached. If the initial assumption does not hold true for the design, the valve addition to the vented tube separator would not improve operation. 

The stabilizer contains 20 trays and a total condenser. Feed enters at tray 10. The normal column overhead pressure is 700 kPa and there is a 20 kPa pressure difference that is evenly distributed between the condenser and the reboiler. Each tray is 2.0 m in diameter with a 0.10 m weir, which is 1.6 m long. 

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