Jan-2022

Turnaround at a crude unit

Improvements in performance and energy efficiency were achieved during the turnaround of a crude distillation unit.

GÜLS¸EN S¸AHIN ANDAS¸, SINAN MUTLU and ZEYNEP YES¸ILKAYALI
Tupras Izmir refinery

Article Summary

Increasing reliability and energy efficiency are key strategies for Tupras. Because the crude distillation unit supplies the charge for further processing in other units of the refinery and is one of the most energy consuming processes in the refinery, its reliability and energy efficiency directly affects refining margin. In order to increase competitiveness at Tupras Izmir refinery, improvement projects and maintenance activities were planned, organised and completed during a turnaround period. Projects carried out included modernisation of the desalter for increased reliability and performance, modification of the main distillation column overhead line, improvements to the overhead heat exchangers and chemical injection system, modifications to furnace burners, cleaning of crude oil-atmospheric residue preheat exchangers, and upgrading the insulation material on the furnace outlet transfer lines and atmospheric residue lines. The outcomes are increased energy efficiency through decreased fuel consumption, for a lower energy intensity index, with enhanced unit sustainability.

The CDU
The objective of the crude distillation unit is rejection of major contaminants and initial fractionation of crude oil into streams of fuel gas, LPG, naphtha, kerosene, diesel, and residue for further processing. The unit consists of four major parts:
- Preheat and desalting
- Atmospheric distillation
- Naphtha stabilisation
- Naphtha splitter

Fractions up to naphtha are distilled in the main column and the overhead product is cooled and sent to the stabiliser column. In the stabiliser column, the overhead products are fuel gas and LPG and the bottom product is a naphtha mix which is further sent to the splitter column and separated as light and heavy naphtha. A basic process sketch is shown in Figure 1.

Turnaround projects
Desalter modernisation
Two electrostatic coalescers operating in parallel were renewed for more reliable and efficient operation. Each desalter vessel was provided with a power unit and transformer with bushing failure indication (pressure switch and level gauge installed on the bushing chamber), a temperature/level switch on the transformer, and PSV installed on the transformer. There is a housing for each transformer. In case of deterioration of the desalter bushing, hydrocarbons cannot escape to atmosphere or penetrate the transformer to cause any damage. A local control panel, special emulsifying valves, and a mud wash system are proposed. A crude oil/water mixture enters through a bottom mounted nozzle and is distributed horizontally at the centre line level. Residence time and equalised flow distribution of the oil/water emulsion in the high voltage field are maximised. The inlet header is fitted with special distributors located at the centre line of the vessel to provide effective separation. The electrical field set-up is strong enough to coalesce and separate water droplets (see Figure 2).

Main distillation column overhead
Crude oil arriving from the tanks is first preheated with the main distillation column overhead product at the overhead heat exchangers. These heat exchangers used to have a leakage problem requiring bundle changes at six- to nine-month intervals.

The tube side is crude oil and the shell side is overhead product. Since the tube side has a higher pressure, when leakage occurs crude oil mixes with the overhead product and sometimes with the kerosene cut. In order to re-tube, the unit charge must be decreased and the heat exchangers must be bypassed. A charge decrease leads to margin loss and more energy is required for the crude oil furnaces since there is less preheat. A team was appointed to investigate the main causes of this problem. A comprehensive review of all the relevant parameters was conducted and the required actions were listed. All the required projects were planned and completed during the unit turnaround. The projects are detailed below.

Overhead line modification
Two parallel overhead lines leaving the main distillation column enter the heat exchangers to preheat the crude oil charge. When isometric drawings were analysed in detail, it was found that the overhead lines were not symmetrical, leading to different corrosion rates.

When the overhead line was cut and analysed at turnaround, corrosion and accumulation was found at the non-symmetrical elbows (see Figure 3).

The overhead lines were renewed to overcome this problem. In Figure 4 the previous, non-symmetrical lines are shown as light yellow and the new symmetrical lines are shown as light blue.

In addition, the overhead lines were previously uninsulated; this led to condensation of overhead products in the overhead line before the heat exchangers. The column overhead temperature was exposed to atmospheric temperature changes and therefore was unstable. The overhead lines were therefore insulated during the turnaround.

Overhead line water wash
Overhead wash was previously performed using demineralised water containing oxygen. Since this might be a cause of corrosion, the supply of wash water was changed to boiler feed water.

A simulation of the column overhead line was performed and the percentage of liquid phase material remaining in the overhead injected water was calculated. Because this was below a suggested 25% rate, the water injection lines and control valves were enlarged to supply enough water.

Furthermore, the aim is to improve chemical and water injection for more homogeneous mixing by adding new injection nozzles for corrosion inhibitor and boiler feed water.

Renewing the overhead heat exchangers bundle
The design and material of the heat exchangers bundle were investigated and improvements were applied. The bundle’s design had been causing vibration, a mechanism that could increase leakage. The carbon steel bundle was changed to duplex stainless steel (see Figure 5).

Burner modification
Four main furnaces heat the crude feed before it enters the main distillation column. Within the scope of this project, all burners in the four heaters were renewed (see Figure 6). NFPA 85/86 standards were taken as reference for high operational safety and reliability. In addition, the main furnaces’ emergency shutdown system (ESD) was modernised, an automatic burner ignition system was introduced, and plugging problems in the pilot burners were solved.