Enhancing energy efficiency in the CDU
Revisiting your crude distillation unit may expose hidden potential for major energy savings
OSMAN KUBILAY KARAN and CANSU INER
TÃ¼praÅŸ Kirikkale Refinery
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Crude distillation is probably the oldest process in any refinery configuration. In the early days of refining, ‘black oil’ in simple batch stills was used to produce lighting oil (kerosene) as the main product, beside asphalts to meet the needs of the times. Following the development of the internal combustion engine, the need for improved fractionation led to the use of simple fractionation columns. Demand for increased throughputs and higher quality products resulted in the development of continuous fractionation units.
A simple crude distillation unit constructed in earlier times included a basic heat exchanger train in which generally hot atmospheric residue was used for preheating, as well as a furnace and a column which would not have any pumparound flows. The only aim was to meet product demands and produce higher quality products. Lowering the energy consumption of the unit would not be considered an important issue in a time of abundant and low-cost crude.
Over the course of time, energy saving has become one of the most essential topics in the oil industry because of environmental considerations and high production costs due to fluctuations in the price of products. Some studies such as the pinch method, developed at the end of the 1970s and extended to heat and power systems, were accepted among standard applications for â€¨refinery projects related â€¨to energy savings after the 1980s.
In refineries, crude distillation units (CDUs) are the largest energy consumers, utilising around 20% of a refinery’s total energy consumption, depending on configuration and type of crude processed.
TüpraÅŸ Kirikkale Refinery is a medium-sized refinery with a capacity of 5 million t/y which began operating in 1986 with a Romanian designed CDU. It is a conversion refinery whose other units were developed to meet local product demands for Turkey. It may be considered a typical example of â€¨its type and was a perfect candidate for an energy efficiency programme begun â€¨in 2006 after the privatisation of Turkish Petroleum Refineries.
As the cost of energy became a major item, Kirikkale Refinery researched a wide variety of opportunities for the CDU as well as other parts of the refinery to maintain the productivity of the plant. â€¨Many projects were evaluated according to their economic benefits and many of them were applied to the site between 2008-2014. Following completion of the new â€¨projects, a dramatic decrease in energy consumption in the CDU was achieved (see Figure 1). This article describes â€¨some of these energy saving projects.
Kirikkale refinery’s CDU
Kirikkale Refinery was designed to process 5 million t/y of 36 API Kirkuk crude. The unit design is a daily 18 000 m3 of crude oil. A simple flow scheme is shown in Figure 2.
Energy efficiency projects for â€¨the CDU
Change in diameter of air preheater tubes
As the regulations for emissions from industrial plants have tightened in the last 20 years, so the quality of the fuel oil burned in CDU furnaces has improved with reduced levels of sulphur and other impurities such as heavy metals. It was found that the two crude charge furnaces’ air preheater tubes were designed with a large diameter (3in) to avoid plugging by the ‘dirty’ fuel oil utilised in the 1980s. A project was developed to reduce the diameter to 2in and to increase the number of air preheater tubes in order to increase the heat transfer area in the air preheaters. In this way, furnace efficiency was increased by transferring more heat to the air fed to the burners. This modification resulted in an increase in heat surface area of 283 m2 in both furnace A and furnace B. By changing the heat surface area, the stack gas temperature decreased by almost 40°C and fuel consumption in the furnaces decreased by 150-200 kg/h.
This decrease in fuel oil consumption corresponds to 1.7 Gcal/h of energy savings per furnace and a reduction â€¨in emissions of about â€¨22 000 t/y.
Optimisation of naphtha and kerosene pumparounds
The CDU’s main column has two pumparounds – the kerosene pumparound and the naphtha pumparound – that are used to preheat crude oil before and after the desalter by removing heat from the column. The amounts and temperatures of the pumparound flows are crucial and should be optimised according to crude charge flow and type since they affect the inlet temperature of the desalter and furnaces and hence are a key parameter for fuel consumption in the heater. Design values of the kerosene and naphtha pumparound ratios were 1.09 and 0.885, respectively. An optimisation study for the pumparound flows against crude flow were carried out to increase heat transfer through the first and second preheat trains, to achieve energy savings in the furnaces. In particular, vapour-â€¨liquid equilibrium in the column and product specifications were taken into account. After a simulation study, the naphtha and kerosene pumparound ratios were increased to 1.32 and 1.42; the crude flow and inlet temperature of the desalter and furnaces also increased by 6.2°C and 7.8°C, respectively. Energy savings were about 1.9 Gcal/h in the furnaces.
Additionally, an increase in pumparound levels resulted in a decrease in top reflux since extra heat removal reduced heat losses from the column top air coolers. As a result of reducing the amount of top reflux, a pump was shut down since only one pump was needed to transfer liquid as top reflux and splitter column feed.
Installation of variable speed drive
Crude oil from the desalter is sent to the furnaces via G-1102 A and B pumps (see Figure 2), one of them (A) with an electric drive while the other has a steam turbine. Now that the rate of crude charge changes frequently in response to economic conditions, a 6 kV variable speed drive was installed in the electric motor. Variable speed drives for that level of voltage are known to be expensive but a feasibility study showed that the return on investment was satisfactory. After installation of the variable speed drive, electricity consumption decreased by more than 50% (from 392 kWh to 180 kWh) for the same operating conditions.
Use of antifoulant
Crudes with a high asphaltene content cause more fouling in the second preheat train (after the desalter) because of its higher temperature compared to the first preheat train. Fouling in a preheat train directly affects the charge furnace inlet temperature and hence leads to greater fuel consumption. The loss of inlet temperature is measured as a decay rate of C/day and, by using an appropriate antifoulant in the second preheat train, it was found that the decay rate reduced to 0.0175 C/day from 0.08 C/day. The economic return on use of antifoulants is satisfactory and calculated at around $910 000/y. Using antifoulant also reduces cleaning time for the exchangers as a side benefit.
Tempered water system for â€¨cooling residues
In the original design of the CDU and VDU at Kirikkale refinery, cooling of atmospheric residue to storage was carried out by air coolers while vacuum residue run-down was cooled by a tempered water system. Cooling with air is not an efficient approach, especially for heavy products, since it depends on ambient temperatures which sometimes cause bottlenecks on hot summer days. A project was developed to combine the residue cooling systems of the CDU and VDU with tempered water cooling. Installing a combined tempered water circulation system removed the bottleneck in residue cooling and the recovered waste heat, about 3 Gcal/h, was transferred to crude oil before the first preheat train.
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