Revamping a crude distillation unit

A revamp project aimed to increase process safety and reduce high energy consumption in a CDU. It also delivered better product separation and higher diesel output.


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Article Summary

Tüpraş İzmir refinery is one of the biggest refineries operated by Turkish Petroleum Refinery Corporation. It started operation in 1972 with 3 million t/y capacity. After the second crude distillation unit started up in 1987, the capacity of İzmir refinery increased to 11 million t/y. The refinery’s two crude distillation  units are U-100 and U-7000, both designed to process 29-36 API crudes. The units have similar process flows and they produce light and heavy naphtha, kerosene, light and heavy diesel, and atmospheric residue that is processed in the vacuum distillation units.

U-7000 has the highest crude oil processing capacity among the Tüpraş refineries. Hence, initiating a revamp project in the unit would affect balances throughout the Tüpraş fleet. So the benefits and drawbacks of the project would need to be examined in detail. After long engineering studies, a revamp project was implemented to increase process safety and reduce high energy consumption. In the revamp project, some critical points would need to be improved:
• The thermal efficiency of the furnaces was about 75%
• The stack gas temperature was about 450°C
• There was no air preheater system to warm the furnace air
• Fuel consumption was nearly 112 Gcal/h and needed to be reduced
• The burners had maintenance problems due to severe leakages

By means of the revamp project, the aim was to make further attempts to increase process safety and reduce energy consumption. The project consisted of renewing the fired heaters, and renewing and modifying some heat exchangers and air coolers according to pinch analysis.

Moreover, of the seven columns of the unit, the trays in six were renewed with higher efficiency designs. By adding a new preflash drum, flashed hydrocarbons could be sent directly to the atmospheric distillation tower so that the duty of the furnace would decrease greatly.

As a result of the revamp project, the inlet furnace temperature increased from 240-282°C, which significantly decreases energy consumption. The overall energy saving is about 298.9 Gcal/y and the decrease in CO2 emissions is about 96.8 t/y, which means a 0.02% reduction in Turkey’s CO2 emissions according to 2016 values.
Process description
U-7000 was constructed in 1987. Following the revamp project, the charge of the unit is 1000 m3/h. The main purposes of the unit are removing salt and water in the crude and separating the crude into fuel gas, LPG, naphtha, kerosene, light diesel, heavy diesel, and atmospheric residue fractions according to their boiling point differences. The separated fractions are sent to their related units as charge.
U-7000 consists of four main sections:
• 7100: crude oil desalting section
• 7200: atmospheric distillation section
• 7300: naphtha stabilisation section
• 7400: chemical substances section

Crude oil reaches the unit as charge then enters the preheat heat exchanger section where it is heated to 110-120°C for desalting. The unit’s desalters are two-stage designs and every stage consists of two desalters. After the removal of salt and water in the desalters, the crude oil heats up to 230°C in the second preheat heat exchanger and then goes to the preflash column as feed. In the preflash column, light hydrocarbons (fuel gas, LPG, naphtha) move up the column as vapour and after cooling are sent to the light hydrocarbons collector drum as liquid.

Preflash column bottom product, crude oil without the light hydrocarbons, splits into two streams. The first stream goes to the preflash column bottom reboiler to heat up to 363°C then goes back to the bottom of the column in order to control the bottom temperature. The second stream goes to the preflash drum. When the feed enters the preflash drum, which operates at 1.8 kg/cm2g, it flashes and most of the kerosene is taken from the top of the drum before the furnace. Flashed hydrocarbons can be sent directly to the kerosene section or flash zone of the main distillation column. Preflash bottom feed goes to the furnace and heats up to 371°C in order to become the charge of the main distillation column.

In the main atmospheric distillation column, crude oil can be separated into heavy naphtha, kerosene, light diesel, heavy diesel, and atmospheric residue fractions. Heavy naphtha is obtained at the top of the column; after it is cooled, it reaches the reflux drum. Heavy naphtha splits into two streams and the first stream is used as reflux to the main distillation column to control the top temperature. The second stream is sent to the absorption column to absorb the LPG that remains in the fuel gas. Heavy naphtha with absorbed LPG goes back to the light hydrocarbons collecting drum.

At the main distillation column, kerosene, light diesel, and heavy diesel are sent to side strippers where their flash point is adjusted with stripping steam. After that, kerosene is sent to the kerosene Merox units, and light and heavy diesel are sent to the hydroprocessing units. Atmospheric residue, the bottom product of the column, is sent to the vacuum distillation units. All products can be sent to the tank farm.

Light hydrocarbons from the light hydrocarbons collector drum are sent to the debutaniser column. Within the debutaniser, LPG and fuel gas are obtained from the top of the tower and naphtha is obtained from the bottom of the column. LPG is sent to the LPG Merox unit, fuel gas is sent to the absorber column, and naphtha is sent to the CCR and isomerisation units (see Figure 1).

U-7000 revamp project
The U-7000 revamp project aimed to increase process safety standards and change the existing process furnaces which did not have air preheaters, resulting in low efficiency. Also, by means of the project, the unit charge rate increased from 950 m3/h to 1000 m3/h.

The U-7000 revamp project was based on new furnaces, new heat exchangers and new air coolers, changing some heat exchanger services and new trays for columns which are more efficient than the previous design. Three new furnaces take part in three different services: one furnace for heating the main distillation column feed, one for the reboiler of the preflash column, and one for the reboiler of the debutaniser column. With the help of the new preflash drum, the duty of the furnace has decreased significantly by sending flashed hydrocarbons directly to the main column. In addition, new Compabloc heat exchangers and air coolers were added according to pinch analysis which helped to improve heat efficiency in the preheat train.

Performance evaluation
A test run was performed between 30 July and 3 August 2018 with the aim of analysing the overall performance of the unit after revamp. The test run was performed with Iran Heavy (IHV) crude oil, the feed basis of the revamp project.

During the test run, steam optimisation, heavy diesel maximisation, naphtha maximisation-minimisation gap overlap in all products, percentage of C4 remaining in naphtha, and propane remaining in fuel gas were observed and examined.

The preflash column top temperature was gradually increased by keeping all other process parameters constant. Table 1 shows the effect of change in the top temperature on products.

The T95 value of WSRN naphtha increased as expected due to the increase in the preflash column top temperature. When the top temperature of the preflash column was increased from 150°C to 154°C, the kerosene T95 value and flash point increased. However, there was no significant change in kerosene flash point and T95 value when the temperature was adjusted from 154°C to 160°C. The reason for this is that the kerosene draw was kept within specification limits.

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