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Apr-2014

Maximising the use of process energy

Integrating a flash drum in an atmospheric and crude distillation unit offers the potential for savings and capacity enhancement

SUNIL KUMAR, SHRIKANT NANOTI and M O GARG
Csir-Indian Institute of Petroleum

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Article Summary

The crude distillation unit has the highest processing capacity in a refinery. It is highly energy intensive and consumes around 2% of the total crude processed to meet its energy requirement.1 Various methods have been reported for better heat recovery, such as the application of pinch analysis and exergy analysis, and for achieving energy savings, for instance the installation of a flash drum or pre-fractionation column.2,3-8 The energy requirement of a pre-flash integrated crude distillation unit is slightly lower than for the design without a pre-flash device.5,6 However, yields of valuable distillate products from the atmospheric distillation column are decreased3,5 and the yield of residue is increased. Processing of more residues in a vacuum column requires a significantly larger 
diameter column as one pound of vaporised oil at 10 psi pressure occupies 70 times more space than at atmospheric pressure. Energy consumption is also increased due to greater residue flow and the requirement to generate more vacuum.9

Benali et al7 reported a decrease in exergy destruction up to 14% as a whole and more than 21% in the furnace alone due to integration of a pre-flash drum with the atmospheric distillation unit. It is also reported that the overall change in the energy requirement for a pre-flash integrated design is marginal; however, the level of this energy changes significantly.8 Substantial amounts of energy can be saved utilising low level heat. However, the finding that low level heat utilisation can increase the crude temperature to the furnace is to be read with utmost caution. This will be true only if the pinch temperature is below the temperature of this low level energy. It is essential to note that application of low level energy below the pinch point will rather increase the cold utility requirement without saving high level energy (furnace duty).2,10-12 In view of the energy intensiveness and carbon footprint of a crude distillation unit, it is important to emphasise that a small improvement in its energy efficiency will be of great importance.

This study focused on the quantitative evaluation of savings in furnace duty, carbon dioxide emissions and fuel expenditure, and the capacity for potential enhancement of an atmospheric and vacuum integrated crude distillation unit by installation of flash drum, without compromising on quality and yields of distillate products. Pinch technology, a well-proven tool to minimise utility consumption in a crude distillation unit, was used to estimate the utility targets and excess process heat available in the process. Insight from the principles of pinch technology was applied to create the scope for utilising excess process heat to save furnace duty. Area targeting for each case considered was also carried out to understand the qualitative effect of energy savings on a heat exchanger network.

Conventional crude distillation unit
It is essential to establish a base case to understand the quantitative benefits of any improvement or modification in the process. In this study, an atmospheric and vacuum column integrated crude distillation unit with a capacity of 5.0 million t/y was selected to represent the base case. This base case was used to establish the basis for yields of distillate product and their quality, and for separation criteria for products obtained in other cases to be studied.

Description
Figure 1 is a simplified schematic of a conventional crude distillation unit. Crude is first heated in a heat exchanger network using the hot products and pumparound streams before entering the desalter. Water is fed to the desalter to remove water soluble salts from the crude. The desalted crude enters another heat exchanger network and receives heat from hot streams. The preheated crude enters the furnace. Partially vaporised crude from the furnace is fed to the flash zone of the atmospheric column. The vapour from the flash zone moves upwards in the column whereas liquid falls to the bottom of the column. The vapour is then fractionated into distillate products such as naphtha, kerosene and diesel in the upper section of the column. To recover heat at a different temperature, heavy naphtha, kerosene and diesel pumparound circuits are used along with the overhead condenser. The distillate products withdrawn from different trays of the column are then stripped by steam in their respective side strippers for removing lighter components to meet ASTM D-86 distillation specifications. The liquid falling downwards from the flash zone is stripped out using steam at the bottom of the column to remove lighter, diesel-range material from the residue.

The residue from the atmospheric distillation column is fed to the furnace along with the coil steam. Partially vaporised crude from the furnace is fed to the flash zone of the vacuum column. The vapour from the flash zone is then fractionated into distillate products such as vacuum diesel, light vacuum gas oil and heavy vacuum gas oil. The liquid from the flash zone is stripped using the steam at the bottom. Heat at a different temperature from the vacuum column is recovered using the vacuum diesel, light vacuum gas oil and heavy vacuum gas oil pumparound circuits. The combined flow of pumparound and product is taken from the specific column tray and cooled to the desired pumparound return temperature. The combined stream is then divided into product and pumparound streams. The pumparound stream is returned to the upper tray in the column. The product stream further exchanges heat with the cold streams and is finally cooled in a water cooler to attain the desired run-down temperature.

Simulation
An atmospheric and vacuum integrated crude distillation unit, selected in the study to represent the base case, is shown in Figure 1. Simulation was carried out using Aspen Hysys. The Grayson-Streed thermodynamic model was used in a simulation for the prediction of stream properties.3,13-14 The properties of crude used in the study are given in Table 1. The details of important information/parameters used in the simulation of atmospheric and vacuum distillation columns are given in Table 2 and Table 3 respectively. Crude was heated to a typical furnace coil outlet temperature of 364.5°C. Partially vaporised crude was fed to the flash zone of the atmospheric distillation column containing 40 trays. Stripping steam (8500 kg/hr) was used at the column bottom to obtain liquid distillate yields of 66.11 vol% predicted from the true boiling point curve of the crude, with a final boiling point temperature of 370°C.

The temperatures corresponding to 95 vol% of top product, heavy naphtha, kerosene and diesel were fixed at the values 110°C, 160°C, 245°C and 370°C respectively, to predict the distillate yield of each product. The values of stripping steam used in the strippers to remove the lighter material are given in Table 2.

Residue obtained from the bottom of the atmospheric distillation column was mixed with steam and an ethane-propane mixture (4:1). This mixed stream was heated in the furnace to 365°C and fed to the flash zone of the vacuum distillation column. Vacuum diesel, light vacuum diesel and heavy vacuum diesel streams were drawn from the column.

Energy targeting
Pinch technology is a proven tool for the estimation of minimum hot and cold utility targets in a process.2,10 The technology is based on the first and second laws of thermodynamics and is straightforward to implement in the design and revamping of plant operations to minimise energy consumption. Composite and grand composite curves are generated in pinch technology to estimate hot and cold utility targets and their temperature level. A pinch point in the composite curve divides the whole process into two sections. The section above the pinch point is in heat deficit and requires hot utilities to heat the cold streams to their target temperature, whereas the section below the pinch point is in heat surplus and requires cold utilities to cool the hot streams to their target temperature. The pinch point temperature is governed by the enthalpy of the hot and cold streams. The grand composite curve provides an estimate of the amount of excess heat available below the pinch and its temperature level.


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