Plant atmospheric distillation unit

In terms of throughput, the biggest unit in most plants is the crude distillation unit Figure 9). Many downstream conversion units also use distillation for production separation. For example, in a coker, hydrocracker, or FCC unit, an atmospheric tower, a vacuiun tower, and a multi-column gas plant may be required. 

As shown in Figure 1, hydrocracking often is an in-between process. The required hydrogen comes from catalytic reformers, steam/methane reformers or both. Liquid feeds can come from atmospheric and/or vacuum distillation units delayed cokers fluid cokers visbreakers or FCC units. Middle distillates from a hydrocracker usually meet or exceed finished product specifications, but the heavy naphtha from a hydrocracker usually is sent to a catalytic reformer for octane improvement. The fractionator bottoms can be recycled or sent to an FCC unit, an olefins plant, or a lube plant. 

This chapter serves as a guide to model atmospheric distillation section of the crude distillation unit. We provide relevant process, operational and modeling details to model the atmospheric column. We also discuss methods to estimate missing data for model development We provide step-by-step instructions to model a particular column in Aspen HYSYS. We discuss how to validate the model predictions with plant data and how to use the model to perform industrially useful case studies. 

The vertical thermosiphon reboiler is a popular unit for heating distillation column bottoms. However, it is indeed surprising how so many units have been installed with so little data available. This indicates that a lot of guessing, usually on the very conservative side, has created many uneconomical units. No well-defined understanding of the performance of these units exists. Kern s recommended procedure has been found to be quite conservative on plant scale units yet it has undoubtedly been the basis for more designs than any other single approach. For some systems at and below atmospheric pressure operation, Kern s procedure gives inconsistent results. The problem is in the evaluation of the two-phase gas-liquid pressure drop under these conditions. 

Chemical Conversion. In both on-site and merchant air separation plants, special provisions must be made to remove certain impurities. The main impurity of this type is carbon monoxide, CO, which is difficult to separate from nitrogen using distillation alone. The most common approach for CO removal is chemical conversion to C02 using an oxidation catalyst in the feed air to the air separation unit. The additional C02 which results, along with the C02 from the atmosphere, is then removed by a prepurification unit in the air separation unit. 

The processes used in industrial air-separation plants have changed very little in basic principle during the past 25 years. After cooling the compressed air to its dew point in a main heat exchanger by flowing counter current to the products of separation, the air feed, at an absolute pressure of about 6 MPa, is separated in a double distillation column. This unit is kept cold by refrigeration developed in a turbine, which expands a flow equivalent to between 8 and 15% of the air-feed stream down to approximately atmospheric pressure. 

Wood creosotes are derived from beechwood (referred to herein as beechwood creosote) and the resin from leaves of the creosote bush (Larrea, referred to herein as creosote bush resin). Beechwood creosote consists mainly of phenol, cresols, guaiacols, and xylenols. It is a colorless or pale yellowish liquid, and it has a characteristic smoky odor and burnt taste (Miyazato et al. 1981). It had therapeutic applications in the past as a disinfectant, laxative, and a stimulating expectorant, but it is not a major pharmaceutical ingredient today in the United States. Beechwood creosote is obtained from fractional distillation (200-220 °C at atmospheric pressure) of beechwood or related plants. The mixture has only recently been characterized to any significant extent (Ogata and Baba 1989). Phenol,/ -cresol, and guaiacols... 

A second method for the industrial production of heavy water, used by the Manhattan Project in the United States [M5], was the distillation of water. Three plants having a total capacity of 13 MT DjO/year were built at Army Ordinance plants. Because the relative volatility for separating Hj 0 from HDO is only 1D3 at atmospheric pressure, the size of equipment and the heat consumption of these plants per unit of D O produced was very high, and the cost of heavy water was greater than in other processes. Nevertheless, the distillation of water was attractive as a wartime production method because the process needed little development work and used standard equipment. These plants were shut down after the war. Mote recently, distillation of water has come to be one of the most satisfactory methods for ftnal concentration of heavy water. 

Sharing of past major incidents with other oil and gas industries provides useful input data for similar process industries in order to identify the most critical barriers and improve their safety processes. One poignant example highlights this matter. In 1998 there was an accident in the gas compression stage of a Middle East oil and gas plant which caused 7 dead as a result of fuel accumulation and vapor cloud explosion which was very similar to the Texas City Refinery disaster on March 23, 2005 in which a distillation tower was overfilled and an uncontrolled release of hydrocarbons led to a major explosion and fires. Fifteen people were killed and 180 were injured in the worst disaster in the United States in a decade. In both incidents, excess hydrocarbons were diverted into a pressure relief system that included a blowdown stack. In the Iranian case, it was equipped with a flare, but one which the operator didn t ignite in Texas City the blowdown stack was not equipped with a flare to burn off hydrocarbons as they were released. As a result, the flammable overflow from the tower entered the atmosphere. Ignition of the escaped hydrocarbons was enabled by startup of a nearby vehicle resulted in the explosion and subsequent fires (Hopkins, 2008). This example shows the repetitive patterns of accidents, and root causes of events all over the world in this sector. The lesson of this paper is that accidents in one country, where the scenarios are very similar, can and should serve as lessons to prevent the same scenario being actualized in other countries. 

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