Distillation Tower with Reflux

At the lop of the tower any intermediate components going out with the gas are condensed, separated, pumped back to the tower, and sprayed down on the top tray. This liquid is called reflux. and the two-phase separator that separates it from the gas is called a reflux tank or reflux drum. The reflux performs the same function as the cold feed in a cold-feed stabilizer. Cold liquids strip out the intermediate components from the gas as the gas rises. 

The heat required at the reboiler depends upon the amount of cooling done in the condenser. The colder the condenser, the purer the product and the larger the percentage of the intermediate components that will be recovered in the separator and kept from going out with the gas. The hotter the bottoms, the greater the percentage of light components will be boiled out of the bottoms liquid and the lower the vapor pressure of the bottoms liquid. 

A condensate stabilizer with reflux will recover more intermediate components from the gas than a cold-feed stabilizer. However, it requires more equipment to purchase, install, and operate. This additional cost must be justified by the net benefit of the incremental liquid recovery, less the cost of natural gas shrinkage and loss of heating value, over that obtained from a cold-feed stabilizer. 

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A plant has two streams containing benzene and toluene, one with 37 percent benzene and one with 68 percent benzene. About equal amounts of the two streams are available, and a distillation tower with two feed points is proposed to produce 98 percent benzene and 99 percent toluene in the most efficient manner. However, combining the two streams and feeding at one point would be a simpler operation. For the same reflux rate, calculate the number of ideal stages required for the two cases. 

Certain refinery wastewater streams are treated separately, prior to the wastewater treatment plant, to remove contaminants that would not easily be treated after mixing with other wastewater. One such waste stream is the sour water drained from distillation reflux drums. Sour water contains dissolved hydrogen sulfide and other organic sulfur compounds and ammonia which are stripped in a tower with gas or steam before being discharged to the wastewater treatment plant. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

Figure 6-5 shows a stabilizer with reflux. The well fluid is heated with the bottoms product and injected into the tower, below the top, where the temperature in the tower is equal to the temperature of the feed. This minimizes the amount of flashing. In the tower, the action is the same as in a cold-feed stabilizer or any other distillation tower. As the liquid falls... 

Figure 3.13. Crude oil vacuum tower. Pumparound reflux is provided at three lower positions as well as at the top, with the object of optimizing the diameter of the tower. Cooling of the side streams is part of the heat recovery system of the entire crude oil distillation plant. The cooling water and the steam for stripping and to the vacuum ejector are on hand control.

The raw methylphenyldimethoxysilane from collector 9 and the intermediate fraction from receptacle 15 self-flow into tank 10. After the loading is finished, the heater of the tank is filled with vapour. After the reflux appears in the box, the tower operates in the self-serving mode for 1 or 2 hours then the rest of methyl alcohol and toluene are distilled. The vapours pass through tower 11, refluxer 12 and cooler 13 the condensate enters receptacle 14. Methyl alcohol and toluene are distilled when the tern-... 

A typical flow diagram of a two-stage crude oil distillation system is shown in Fig. 18.14. The crude oil is preheated with hot products from the system and desalted before entering the fired heater. The typical feed to the crude-fired heater has an inlet temperature of 550°F, whereas the outlet temperature may reach 657-725°F. Heater effluent enters the crude distillation (CD) column, where light naphtha is drawn off the overhead tower. Heavy naphtha, kerosene, diesel, and cracking streams are sidestream drawoffs from the distillation column. External reflux for the tower is provided by several pumparound streams.12... 

The next important aspect of tuning a multivariable controller is to determine what happens when different constraint sets become active. Eor example, assume that you have designed a distillation tower controller with constraints on overhead composition, bottom composition, and tower AP. The MVs are reboiler duty and reflux rate. The foUowing are examples of the types of constraint tradeoffs that you need to consider ... 

Packed distillation towers can often be operated over a moderate range of flow rates at nearly constant separation efficiency. Data for isooctane-toluene separation at total reflux are shown in Fig. 22.25. The three Intalox metal (IMTP) packings numbered 25, 40, and 50 correspond to nominal sizes of 1,1.5, and 2 in., respectively. As the capacity parameter increases, both the liquid rate and the vapor rate increase, which explains why HETP is nearly constant. The gas film has the controlling resistance to mass transfer, and Hoy increases with the 0.3 to... 

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