Jul-2015

Revamping a hydrocracker’s overhead condenser

A stepwise approach solved seasonal issues affecting plans to increase a hydrocracker’s capacity

Mustafa Gören and Ahmet Bebek
Tüpraş Kirikkale Refinery

Article Summary

The hydrocracking unit is one of the most important units in a refinery because of its high potential for profitability compared with other units. TüpraÅŸ Kirikkale Refinery’s hydrocracking unit was commissioned in 1993 with a capacity of 2305 m3/day.

A capacity test run was conducted in 2010 to determine the unit’s potential for increased capacity. The results of the test run demonstrated that unit capacity could be increased from 2305 m3/day to 2500 m3/day. However, the capacity test run was carried out in winter when weather conditions meant that there were no cooling problems in the fractionator section. In the summer, the hydrocracking unit’s feed rate has to be adjusted, depending on the capacity limit of the fractionator section’s overhead condenser. In addition, kerosene and diesel yields can be changed, depending on demand for hydrocracking operations. When demand for diesel increases, its yield increases, but kerosene yield is reduced as a consequence by leaving kerosene in the diesel draw tray. In this case, the kerosene bottom reflux flow rate needs to be higher while the kerosene top reflux rate becomes lower and the amount of heat released from the column by the kerosene top reflux is diminished. Previously, this could not be achieved effectively because of limited column overhead condenser capacity. This article explains how those problems were solved, the steps needed to achieve this, and how the cooling capacity of the fractionator column overhead was increased stepwise.

Phase 1: Using sour water stripper clean water for the unconverted oil cooler
The fractionator column bottom product is called ‘unconverted oil’ (UCO); this is the unconverted component of the feed to the hydrocracking unit. UCO has to be routed to its tankage area at temperatures above 75°C in view of its high freezing point. Originally, it was pumped to the kerosene stripper reboiler, then to the fractionator closed loop water circulation system, or ‘tempered water cooler’. This step was designed to keep the UCO rundown temperature stable at 75-80°C. The tempered water cooler is shown in Figure 1; demineralised water is circulated through a heat exchanger cooling the UCO product, then routed to an air cooler to lower its temperature to 50°C.

The sour water stripper (SWS) unit’s clean stripped water temperature is around 45-50°C. This clean water is used as desalter wash water in the crude unit. After a check of the design parameters and limits of heat exchanger E1 in the tempered water cooler (see Figure 1), it was calculated that the cooling capacity of SWS clean water is suitable for the UCO product heat exchanger. This new design has two advantages: first, it preheats SWS clean water to increase the crude heater inlet temperature before it is injected as desalter wash water. Since the tempered water cooler is not used for cooling the UCO product, the air cooler and pump shown in Figure 1 are not needed to cool tempered water circulating in the system. After commissioning of the new lines, 0.165 Mkcal/h of energy was saved by increasing the temperature of the desalter wash water and, thereby, the crude heater inlet. The new system, which cools UCO product with SWS clean water, is shown in Figure 2.

Phase 2: Using the tempered water system’s air cooler to cool the kerosene top reflux stream
During the summer, the fractionator column’s overhead condenser lacked the capacity to maintain the overhead reflux drum at its design operating temperature of 71°C. The temperature, pressure and opening of the pressure control valve PV1 in the years 2011-2013 are shown in Figures 3 and 4 for the summer season, to show the capacity limitation at the overhead cooling capacity of the fractionator column. Figure 3 shows that the pressure of the reflux drum was increased to 1 kg/cm2g whereas the design operating pressure of the reflux drum is 0.73 kg/cm2g. Therefore the separation index of the column was influenced by operating the column at a pressure higher than 0.73 kg/cm2g. It can be seen from Figure 4 that opening of the pressure control valve on the reflux drum was nearly 100% because of a temperature of around 90°C (compared with the design operating temperature of 71°C).

After the commissioning of Phase 1, the air cooler of the tempered water closed loop system could be utilised to increase the fractionator overhead cooling capacity. The kerosene pumparound network of the fractionator column is shown in Figure 5. Here, further cooling of the kerosene top reflux can be maintained to decrease the column reflux flow rate so that the column overhead flow is decreased. A management of change study was carried out in Kirikkale Refinery regarding Phase 2; this involved a team comprising an inspection engineer, a mechanical engineer, a process engineer and operations engineer. The hazardous area classification of the air cooler was checked and declared suitable for kerosene service. The operating parameters for kerosene service and design values for the tempered water air cooler are shown in Table 1.

The study showed that the air cooler could be used for this type of service, and that the metallurgy of the air cooler is also suitable for kerosene service. The cooling capacity of the tempered water air cooler is 0.65 gcal/h and, after using the air cooler for cooling the kerosene top reflux, the fractionator overhead condenser duty is decreased in line with the capacity of the tempered water air cooler. There was one concern: the hydraulic network of the kerosene pumparound would be changed because one air cooler was added to the kerosene pumparound’s hydraulic network (see Figure 5). Therefore, a final decision was taken on condition that the flow rate of the kerosene pumparound network was checked after the tempered water air cooler was commissioned and, if necessary, the impeller size of the kerosene pumparound pump would be increased accordingly. A simple scheme of Phase 2 is shown in Figure 5. The pressure difference in the new lines was calculated at 0.9 kg/cm2, based on 3in lines. Therefore the pressure difference in the lines is recalculated with 4in lines and found to be 0.3 kg/cm2g. Based on these hydraulic calculations, the line sizes were increased from 3in to 4in in order not to lose further flow rate from the kerosene pumparound pump.
The kerosene pumparound flow is split into two components: the kerosene top and bottom refluxes. Further cooling by the tempered water air cooler was applied to the kerosene top reflux because the kerosene bottom reflux directly affects the tray temperature of the kerosene draw and the kerosene product specifications, whereas the kerosene top reflux directly influences the fractionator column overhead flow and reflux rate. Therefore, the decision was taken to further cool the kerosene top reflux stream. Hysys was used to calculate the outcome of using the tempered water air cooler to further cool the kerosene pumparound top reflux. The results obtained from the model are shown in Table 2. The column reflux rate and internal flows are decreased by further cooling the kerosene top reflux, and the heat released from air cooler EC1 is directly reflected in the capacity decrease from column overhead condenser EC2. The final temperature of the kerosene top reflux before entering the column, 62°C, can also be seen in Table 2.

In order to establish the necessary line connections to the kerosene top reflux, the shutdown of the hydrocracking unit was required. Therefore, the connection points were changed to make the necessary connections without a shutdown (see Figure 6). The connection points, which are upstream and downstream of the tempered water air cooler (EC1), were changed from the configuration in Figure 6 back to the original orientation (see Figure 5) at the first shutdown of the unit because of the reliable operation of FV1.

First of all, the block valves of the control valve were checked for leakage, then T connections were manufactured for the necessary line connections with their block valves. Afterwards, the kerosene top reflux control valve was closed and all the necessary mechanical isolations were carried out for one hour while the two T connections were installed. After the construction work was completed, tests and inspection of the new lines were carried out and the system was commissioned.