Apr-2012

Optimising pumparound reflux for zero-cost energy savings

By increasing pumparound ratios in principal units, refinery engineers were able to recover energy for process heating and reduce stack emissions

Ahmet Bebek and Sule Seda Ay
Turkish Petroleum Refineries Corporation

Article Summary

Energy efficiency projects have gained importance in recent years, not only for their role in decreasing operating cost, but also for addressing global warming issues. Therefore, most industrial facilities are searching for energy-saving projects.

In this case, engineers at TüpraÅŸ Kirikkale refinery came up with the idea of increasing pumparound ratios in the crude distillation column to transfer more available heat to crude in the heat exchanger network — a system of exchangers that heats crude fed to the crude distillation unit using process heat — and thus save energy at no cost. The heat removal sections in the column use subcooled liquid; a portion of the side liquid draw is cooled against incoming crude and refluxed back to the higher section of the crude column.

Moreover, the engineers are now working on the addition of one more pumparound reflux to the crude column, a project that is now in the construction phase. Energy from the new pumparound will enable the removal of the naphtha splitter reboiler furnace and its replacement with a shell-and-tube heat exchanger reboiler. Similar work has also been done in the hydrocracker unit of the refinery subsequent to the operation carried out in the crude distillation column, with increased pumparound ratios in the main fractionator column.

The refinery
Kirikkale refinery is one of the four refineries operated by TüpraÅŸ, at Izmit, Izmir, Kirikkale and Batman in Turkey. The refinery was established in 1986, with 5 million t/y crude oil processing capacity, in order to meet the petroleum demands of Turkey’s Central Anatolia, Eastern Mediterranean and Eastern Black Sea regions. After expansion of the hydrocracker, isomerisation, deep diesel desulphurisation and CCR reformer units, it became a refinery with mid-level complexity according to Mediterranean standards.

Although the refinery was designed to process Kirkuk crude oil (36 API), it has the capability to process crude oils with API numbers ranging from 23 to 36. The crude unit has a one-stage single desalter and two furnaces in parallel. The crude distillation column has two pumparounds (naphtha and kerosene), and whole naphtha is taken from the top of the column. The atmospheric distillation column is followed by the naphtha splitter and debutaniser.

Pumparounds
Pumparounds allow the utilisation of available column heat via the column feed heat exchanger network, maintaining the stability of the vapour loading of the column. Maintaining the amount of liquid traffic in the column for effective fractionation is another important concept that needs to be considered. Vapour-liquid load can be quantified by reviewing the average vapour-liquid molar traffic in any zone of the column.

Pumparounds also control and distribute fractionation between products, since they adjust the flow of vapour and liquid within the column. If the rate of pumparound (that is, heat removal by pumparound) is decreased by the operator, the vapour load to the top will be increased. The top reflux rate will also increase in order to keep the top temperature constant. If there were no pumparounds, the highest liquid load would be at the top of the column due to an increase in top reflux. In addition, the amount of heat lost to the atmosphere from the column’s top condenser would be higher.

The capital cost is affected by the addition of a pumparound in view of the associated additional requirement for pumps and exchangers. The additional use of reflux trays for heat removal instead of fractionation service also increases the capital cost. Crude distillation columns usually have three pumparounds, since this configuration is found to be the most cost-effective geometry, but columns can have two to four pumparounds in general.

Kırıkkale refinery’s CDU
The design capacity of the refinery is 18 000 cu m/day. There are two pumparounds below the product draw in the crude distillation unit of the refinery; namely, the naphtha pumparound and the kerosene pumparound. A schematic of the arrangement is shown in Figure 1.

Both of the pumparounds remove heat from the column via heat exchangers in the crude preheat train. Naphtha pumparound/crude heat exchangers are located upstream of the desalter, and kerosene pumparound/crude heat exchangers are located downstream of the desalter (upstream of the furnaces). Thus, pumparound exchanger duties via these heat exchangers affects the desalter and furnace inlet temperatures. In other words, pumparound rates affect desalting efficiency in the desalter and fuel consumption in the furnaces.

The optimum working temperature range of the desalter is 130-140°C, depending on the crude type being processed. The average operational temperature during 2010 was 125°C. The average furnace inlet temperature in 2010 was 229°C, ranging between 222°C and 233°C, depending on capacity and operational changes. Thus, the refinery was searching for opportunities to increase the desalter and furnace inlet temperatures.
The vapour flow from the top of the column is condensed by the top condenser, and available heat in the column is given to air by this air cooler. The top reflux rate controls the top temperature. Thus, in the event of an increase in the top temperature, the top reflux rate is increased by the temperature controller.

The approach
The ratios of the pumparound volumetric flow rates to crude charge were optimised at 0.885 and 1.09 for the naphtha and kerosene pumparounds respectively at the design stage of the crude distillation unit. The capacities of the naphtha pumparound pump and kerosene pumparound pump were enough for the flow increase, which means there was an opportunity to increase the desalter inlet temperature and furnace inlet temperature by raising pumparound rates within the hydraulic limits of the unit, considering the vapour-liquid loads of the column and the degree of fractionation. The increase in pumparound ratios would also decrease the top vapour load (the top condenser duty, which is transferred to air) and would recover this heat to the crude via the heat exchanger network.