Jun-2017

Revamping crude towers to meet bitumen specifications

A revamp of crude column internals improved bitumen quality, increased output of light products, saved on energy costs and improved ease of operation

Seçil Uzun and Murat Ercan
TupraÅŸ

Article Summary

TupraÅŸ Batman refinery carried out a major project to enhance bitumen specifications in two crude distillation units. Mass loss, which is one of the bitumen specifications, determines the quality of the bitumen and a certain quality is necessary for sales. Mass loss is connected with 5% of distillation cuts of bitumen and the existence of light hydrocarbons inside it. This quality parameter is crucial to ensure sufficient bitumen sales and to provide customer satisfaction.

In accordance with laboratory results, the mass loss specification of bitumen in crude towers needed to be improved. In this respect, a test run study was performed in order to observe a column’s performance by changing the operational parameters and so demonstrate that the bitumen specifications could not be improved with the current atmospheric distillation columns. For this reason, the decision was made to initiate a revamp project by studying a simulation of the modifications that had been decided upon. Within the scope of the revamp, changes were proposed for the stripping trays and heavy gas oil section, feed inlet, stripping steam and other tray side design aspects of the columns.

These changes resulted in an improvement of 70% in the mass loss change on heating of bitumen, meaning that the 5% boiling point of bitumen has been increased along with improvement in the separation and yield of light products (kerosene, diesel and naphtha).

Furthermore, this revamp resulted in a 7 m3/h capacity increase in the units, shutting down the vacuum unit mentioned in the coming section of article, reducing operational cost by 2.04 Gcal/h and greater ease of operation. The most important output of this project is to meet all bitumen specifications.

Crude units before revamp
Batman refinery is one of four refineries operated by TupraÅŸ. It opened in 1954 as the first in Turkey and was designed to meet the petroleum product demands of Turkey’s western regions with a crude oil processing capacity of 330000 t/y. With the aim of overcoming production bottlenecks, new crude units were commissioned in 1972 when the processing capacity of Batman refinery reached 1.1 million t/y.

The refinery was designed to process domestic crude oil extracted by Turkish Petroleum Anonym Partnership (TPAO) in Batman. Hence, the API of the crude oil is between 18 and 20. The refinery has the advantage of being located near domestic sources of crude oil.

In Batman refinery, there are two crude units which are very similar to each other, each having an atmospheric tower that produces light and heavy naphtha, kerosene, light and heavy diesel and atmospheric residue (bitumen). In the larger capacity unit, atmospheric residue can be sent directly to storage as bitumen although in the smaller unit some of the atmospheric residue had to be sent to the vacuum distillation unit to meet the required bitumen quality in the combined atmospheric residue.

Basic flow diagrams of the units are shown in Figures 1 and 2.

By processing low API crude oil, column bottom long residue can be used in the bitumen pool to prepare different grades of bitumen. All bitumen specifications (50/70, 70/100 and 160/220 grades) are produced from the distillation units, directly from the larger capacity unit and with the help of the vacuum unit for the smaller capacity unit. The sales specification of the different grades of bitumen is shown in Table 1.

The approach
The refinery’s produced bitumen meets all of the specifications shown in Table 1. However, it was realised that mass loss could be further improved to increase customer satisfaction. Mass loss is directly related to the lighter components in bitumen. It means that the produced bitumen’s 5% boiling point has lighter components and weight loss occurs after heating the bitumen. Because kerosene and even naphtha exist inside the bitumen from the existing column, a 5% distillation cut of bitumen is quite low and the mass loss property of the bitumen needed to be improved. 

For the purposes of improving bitumen quality, a test run study was carried out by changing some operating parameters, chiefly the furnace outlet temperature, stripping steam rate and amount of diesel produced. The results of the study showed that penetration value is directly influenced by the >5% distillation cuts and mass loss on heating value, and is also related to the 5% boiling cuts of the bitumen. In the existing column, it was very difficult to obtain the mass loss specification without reducing the penetration below target even if maximising furnace outlet temperature, stripping steam rate and diesel output since there was no liquid flow from the flash zone tray above. As Table 2 shows, there were found to be no results that sufficiently provided both mass loss and penetration on specification at the same time.

After evaluation of the field operating parameters and the test run results, TupraÅŸ decided to revamp the stripping and flash zone sections of the existing columns.

Scope of modifications in the atmospheric towers
In order to meet the requirement for upgrading the bitumen, it was proposed to make modifications to the crude towers. For this purpose, a simulation program was studied using the test run values from the crude units with the aim of recovering light hydrocarbons from the bitumen and so improve its quality through modifications to the columns. Additionally, the modifications aimed to enhance the separation and increase the yield of light products. 

Within the simulation study, the performances of both columns were observed, and the liquid and vapour loads of the trays were compared with real values. To determine the scope of the revamp, all condenser and pumparound duties were calculated, and the capacity of the pumparound pumps was examined. A simple description of the planned revamp is shown in Figure 3.

As can be seen in Figure 3, the scope of the revamp covered a feed inlet device to reduce entrainment, replacing stripping with higher efficiency trays, moving the diesel draw tray to the side reflux line and using a steam sparger to distribute steam effectively. Additionally, it was decided to alter the heavy gas oil draw with a chimney tray and the addition of a downflow pipe.

All of these modifications were applied to the smaller crude columns, while changes and additions applied to the stripping trays, chimney tray, steam sparger and inlet pipe applied to the other crude columns. A more detailed description of modifications completed in the crude towers follows.