Refinery crude distillation tower

Here is our problem. A refinery crude distillation tower is producing gasoline, truck diesel, and a gas oil. The diesel is contaminated with the gas oil. Also, the gas oil is contaminated with the lighter diesel. As shown in Fig. 12.5, the vapor flow into the tower is constant. Our job is to improve the degree of fractionation between diesel and gas oil. Our objective is to remove the relatively heavy gas oil from the diesel and to remove the lighter diesel from the gas oil. 

Figure 49.4 shows the overhead system of the Coastal Refinery crude distillation tower in Aruba. The island of Aruba is a beautiful country in the Caribbean Sea off the coast of Venezuela. 1 often take my wife, Liz, on a romantic vacation to this tropical paradise. During one such exotic trip I was assigned by Coastal to find a plan to improve fractionation between naphtha and jet fuel. The problem was that the naphtha contained 20 percent jet fuel. A computer simulation of the crude tower indicated that the apparent tray efficiency of the upper five trays was... 

This contradicts common sense that flooding is promoted by higher vapor rates. But I have stopped flooding in a crude distillation tower recently in a Kansas refinery by increasing the stripping steam rate. Also,... 

Contract maintenance workers often will not replace the tray manways unless the tray manway is adjacent to a tower external manway. They reason that once the tray manways that are visible from the tower manway are closed, there is no way for someone to inspect the other trays. This problem is not just common— it is imiversal. The maintenance force at the Good Hope Refinery pulled this nasty trick on me at the coker fractionator. Equipped with my crescent wrench, I opened the tray internal manway below the side tower manway. 1 discovered that the 12 trays below this point had their manways stacked in their downcomers. In 1990,1 worked on a project to improve fractionation at the Chevron Refinery crude distillation unit in El Segundo, California. When the tower was opened to implement my design, the tray manways were found lying on the tray decks below the diesel draw tray. The lesson is, inspect each tray and then witness the closure of each tray manway, separately. 

Figure 1.1. Crude fractionation tower at the Phillips Oil Refinery in Woods Cross, Utah. In this crude fractionator, 25,000 barrels per day of incoming crude oil is divided into six fractions by distillation. (Photo by Alexis Kelner.)...

Crude oil enters the refinery and is stored and excess salt removed prior to distillation. Often several different crude oils are co-mingled before distillation in the atmospheric distillation tower. This has the duty to separate the crude oil into fractions according to distillation range. 

In older refineries, high-pressure de-ethanizing distillation towers were often used, especially for straight-run crude oil distillation or thermal cracking plants. 

Crude oil is known to contain volatile mercury because mercury is found concentrated in the liquefied petroleum gas (LPG) and naphtha fractions of the atmospheric distillation when oil is refined. It is known conclusively that elemental mercury is at least one of the volatile mercury species in crude oil because it sometimes is found condensed in trays in refinery distillation towers and condensed in cryogenic heat exchangers that liquefy petroleum gases. What is not known is whether elemental mercury is the only volatile species. 

To test alternative control strategies, Prett and Morari (1986) provide a linearized model, referred to as the Shell process, of a distillation tower to separate crude oil into fractions in a refinery. Part of the model describes the dynamics of the two top compositions as a function of the manipulated variables (the two top draw rates) and two key disturbances (the heat removal loads in pump-around streams used to remove heat and create intermediate reflux). For this example, it is sufficient to examine die matrices specific to the nominal model ... 

An existing refinery at which the primary crude oil atmospheric distillation tower was replaced afl r October 31, 1973. 

A combination unit is a special type of unit that was developed to reduce the investment for a small refinery. In effect, one main distillation unit serves as a crude fi-actionator as well as the cat unit primary fractionator. This same tower also serves the naphtha reformer and visbreaker. A schematic diagram of a combination unit is shown in Figure 2. Crude oil is topped (material boiling below 650°F is removed) in the atmospheric tower, and the topped crude is sent to the combination tower along with cat products and naphtha reformer products. These latter streams provide heat to distill the topped crude and also, being more volatile than topped crude, provide a lifting effect which assists in vaporizing more of the crude. 

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