Condenser outlet line

The flow of steam to a reboiler can be controlled by using a control valve on either (1) the steam inlet line or (2) the condensate outlet line. 

Controlling heat input by a control valve in the reboiler condensate outlet line may adversely affect reboiler stability, especially in vacuum reboilers. This valve varies condensate level in the heating side of the reboiler. It therefore varies the point at which heating is first applied to the process side. This is equivalent to operating the process side at a fluctuating reboiler sump level. Further details of this and alternative control schemes are in Sec. 17.1.2. 

Adequate sizing of condensate outlet lines is important (134, 290 see also Sec. 15.12). 

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

Figure 17.5/ shows a flooded condenser scheme similar to that of Fig. 17.5a, but with the control valve located at the condenser inlet. This method is inferior compared to Fig. 17.5a (77). It requires a larger control valve, is more difficult to understand, and it affects condensation at a lower temperature. The condenser outlet line must enter the reflux drum well below the liquid level. A pressure-equalizing line as in the method shown in Fig. 17.5a is also required.

Figure 21.5. Process of recovery of plasticizer from PVC full-scale installation. 1 reactor, 2 burner, 3 hot-gas line, 4 feed line, 5 discharge line, 6 discharge line, 7 condenser, 8 condenser outlet line, 9 condensate pump, 10 line, 11 line, 12 heat exchanger, 13 line, 14 offgas line, 15 branch line, 16 branch line, 17 circulating-gas line, 18 circulating-gas compressor, 19 branch line. [Adapted from Lyding Q Pille M, von Plessen H, Semel J, US Patent 5,464,876.]...

Check the pressure drop from the condenser outlet to the reflux drum inlet. Subtract the calculated static pressure loss (i.e., the height of liquid) from the measurement. If the result is more than one-half psi, consider enlarging the condenser outlet line. 

The most common problem with reftigeration condensers is condensate backup, rather than fouling (see Chap. 16). This may happen if the condenser outlet line is imdersized. Let s say that the pressure drop due to piping friction losses in this line is 10 psig. The refrigerant is isobutane. The isobutane cannot flash as it enters the refrigerant receiver. 

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Water hammer can also occur in steam mains, condensate return lines, and heat exchange equipment where steam entrapment can take place (Fig. I). A coil constructed and installed as shown here, except with just a steam trap at the outlet, permits steam from the control valve to be directed through the center tube(s) first. Steam then gets into the return header before the top and bottom tubes are filled with steam. Consequently, these top and bottom tubes are fed with steam from both ends. Waves of condensate are moved toward each other from both ends, and steam can be trapped between the waves. 

Elevation increase promotes subcooling. Once upon a time many years ago, a tragic event occurred in Louisiana. A rat entered the condenser outlet pipe shown in Fig. 13.4. The condenser had been off line for cleaning. The rat, having crawled up the riser pipe to the reflux drum, got its head stuck in the drum s inlet nozzle. Your author, unaware of the rodent s predicament, put the exchanger back into service. The condensed butane now flowed across the rat. The rat died. Well, we all must come to that end eventually, although perhaps not quite that exact end. Such is the way of all flesh. 

Does this mean, then, that the condenser outlet temperature is lower than the reflux drum outlet temperature If I were to place my hand on the reflux pump suction line, would it be warmer than the condenser outlet Answer—yes ... 

When the first ring seal has been made, the glass within the inner tube end is blown out to a neat hemisphere of normal wall-thickness. The water outlet tube is then joined to the bulb and bent over and to one side of the jacket so that the point handle is at 45° to the condenser centre line. This bend must be neat and strong, and as close to the bulb as possible, since water tubes are subjected to quite large forces when rubber tube is attached and detached from them. 

The reactor effluent proceeds to a cooler where dissolved ethylene is separated from the butenes stream. A reflux drum is provided for the condensation of solvent vapor and the liquid is recycled back to the reactor. The mixture of ethylene and butene-1 proceeds from the reflux drum into the outlet line. After 5 hrs of reaction time, ethylene conversion reached 25.7 percent and the product distribution was mainly butene-1 at 95.5 percent selectivity and small amounts of hexenes and other oligomers but without any polymer formation. The unit ratio for the grams of butene-1 produced per gram of triethylaluminum was about 159. 

To a three-neck, 250-mL round-bottomed flask equipped with a water-jacketed condenser and nitrogen inlet and outlet lines were charged 75 g of ethanol, hexane, or 2-ethylhexanol solution. Magnetic stirrers were used to maintain constant agitation. In cases where reductions were accomplished at an elevated temperature (45°C), flasks were immersed in constant-temperature oil baths. Solutions were allowed to equilibrate with the surroundings before appropriate reducing agents were introduced. 

Small quantities of liquid should always be expected in column vapor outlets. The origin of this liquid can either be entrainment from the column (fine droplets can pass even through mist eliminators and coalesce in the overhead line), or atmospheric condensation. Low points in vapor outlet lines should be avoided as these tend to trap and accumulate liquid. The accumulated liquid back-pressures the column, causing instability, erratic operation, and slug flow into the downstream unit one case history where this occurred has been reported (203). Vapor outlet lines should be sloped (i.e., self-draining) ei-... 

It has been recommended (354, 355) to size top vapor outlet lines for a maximum velocity of 60 ft/s for atmospheric and pressure columns, increasing to 200 ft/s as pressure is lowered below 10 psia, and to confirm that pressure losses between column and condenser are acceptable. In medium- and high-pressure columns (>100 psig), pressure drop considerations usually dominate and velocities well below 60 ft/s are normally used. 

Blown condensate seal. When this occurs, uncondensed vapor blows and channels right through the reboiler and out the condensate drain line. Heat transfer slumps and water hammer may follow. Experience shows that as much as half the reboiler duty is lost by a small amount of vapor blowing (239). Throttling the reboiler outlet reestablishes the seal. Installation of a condensate seal drum can cure this problem. Additional discussion is in Sec. 17.1.2. 

If no liquid level is maintained in the condenser, and the liquid is not substantially subcooled, vapor bubbles are likely to be entrained in the liquid condensate. The outlet line should therefore be sized for self-venting flow (Fig. 4.5). Moore (290) has suggested a slightly more conservative rule of thumb for gravity drainage of condensers (assuming no liquid accumulation)... 

If no liquid level is maintained in the condenser, the liquid outlet line should enter the vapor space of the reflux drum and should not be submerged. 

The condensate outlet scheme is often troublesome. If the condensate valve cannot handle the amount of condensate that the reboiler can generate, a m lximum vapor flow rate may be reached with condensate still covering a portion of the tubes. The remedy depends on the cause of the problem. If there is ample pressure difference between the reboiler and the condensate system downstream of the valve, it may be sufficient to resize the valve or condensate line. If the pressure difference is small, a condensate pot with a pump (Fig. 17.1c) may be needed to overcome the problem. 

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