Energy savings in a coker’s water system
Revamping a delayed coking unit’s water system enabled significant energy savings. Energy consumption is an important indicator of the economic performance of plant, and there is much room for improvement in the energy performance of a unit’s water system.
WANG ZHIGANG, JIA YIN, LIXIAO CHANG and SONG KANG
Sinopec Luoyang Company
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This article describes energy consumption in the water system in Sinopec Luoyang Company’s delayed coking unit A, where the energy consumption taken by circulating water, deoxygenated water and desalting water accounts for 78%, 19% and 3% respectively of the total energy consumption of water. A comparison of energy consumption in the water systems of unit A and an advanced coking unit B shows that the energy consumption of circulating water, deoxygenated water and desalting water of unit A are 0.12 kgoe/t, 0.06 kgoe/t and 0.1kgoe/t respectively, higher than their equivalents in unit B. Taking into account that the condensate water from unit A is drained and not included in the calculation of energy consumption, and the utilisation ratio of purified water in Luoyang Company is less than 50%, and by analysing the properties of different kinds of water and production conditions, unit A’s water system can by optimised through the introduction of condensate water and purified water.
The following adjustments need to be carried out: introducing condensate water instead of deoxygenated water into the steam generators; replacing deoxygenated water with purified water to wash ‘fat’ gas, injecting purified water rather than circulating water into water-sealed tanks; and rearranging recuperative coolers in series rather than in parallel. Implementation of these revamps produces a good result: the energy consumption of the water system in unit A has been reduced by 20.15% in total. These rearrangements can be referenced for the optimisation of other plant.
The delayed coking unit is characterised by relatively low capital outlay and quick returns, and is one of the main technologies for secondary processing of raw materials as the quality of crude oil declines.1 A unit’s energy consumption is an important indicator to measure its economic performance.2 As the consumption of energy as fuel gas and steam represents a large proportion of a device’s total energy consumption, many experts’ attention has been focused on optimisation of this aspect of performance. Until recently, few measures have been available to further such optimisation. Although the energy consumption of the water system accounts for only 5-6% of the total3 there is much room for optimisation. This article analyses steps taken with the water system of a coking unit in order to reduce coking energy consumption and to provide an example for optimisation of other plant.
Composition and problems of a coking unit’s water system
A coking unit’s water system is composed of fresh water, circulating water, deoxygenated water, reclaimed water and desalting water. The energy consumption of circulating water, deoxygenated water and desalting water account for 78%, 19% and 3% respectively of the total energy consumption of the water system. Fresh water and reclaimed water are not taken into account in view of their low level of consumption in the unit. Circulating water is mainly used in coolers; deoxygenated water is used to produce steam in steam generators; and desalting water is used to wash fat gas.
A comparison of energy consumption in the water systems of units A and B in Table 1 reveals a big gap between the two units. The gap in energy consumption between the circulating water systems of the two units is 0.12 kgoe/t, the highest. However, in this study deoxygenated water is given the highest priority for optimisation measures because of its high coefficient of energy consumption. Desalting water should not be neglected considering that it also has a high coefficient.
Overall analysis of the water system indicates that condensate water generated by the coking unit itself is not utilised rationally but discharged directly after undergoing treatment in downstream units, hence its energy consumption is excluded. Purified water produced by the sewage stripper is also drained and its energy consumption is not taken into consideration. The highest energy consumption results from the circulating water system, which can be addressed by arranging the recuperative coolers in series. Therefore, making full use of condensate water and purified water and replacing deoxygenated water and desalting water that have a high coefficient of energy consumption is an effective way to deal with the energy consumption of the water system.
Rearrangements in the coking unit’s water system
Condensate water replacing deoxygenated water
Deoxygenated water is mainly used to generate steam at 0.4 Mpa with diesel, light wax oil and heavy wax oil as heat sources in the E1108 diesel steam generator, in the E1109 steam generator for light wax oil, and in the E1110 steam generator for heavy wax oil. The coking unit’s operating design stipulates that the phosphate content of water used in the steam generators should be 5-15 mg/L and the water’s pH value should be in the range 9-11, to protect the steam generators against damage caused by impurities in the water and against corrosion arising from acidity. An analysis of the condensate water shows that it is suitable to replace deoxygenated water for use in the steam generators (see Table 2).
As the flow diagram in Figure 1 shows, condensate water generated by the coker itself flows through steam traps and is introduced into the condensate drain outlet of steam generator V1105 by an extension pipeline and finally to V1105 itself. Such a modification can be easily implemented at low cost.
Using purified water in the coking unit
Purified water is produced by the sewage stripper with a yield of 200 t/h. It is mainly used for electric desalting in the atmospheric and vacuum distillation units, or it is injected into aerial coolers in the wax oil hydrogenation unit, or it is delivered into the spray purifier of the gas holder. However, the utilisation ratio of purified water is less than 50%, and most of the water is wasted and directly discharged. Introducing purified water into the coking unit can effectively reduce energy consumption.
Purified water can be adopted in the coking unit to replace the following kinds of water: desalting water, used to wash fat gas in the coker in order to absorb and wash off impurities contained in the gas such as sulphides, salts and coke powder; and circulating water, used for sealing in three water-sealed tanks: V1407, the coke cooling water surge tank, V2101, the acid water tank, and V1303, the lean solution tank. Coke cooling water and filtered decoking water in the coke pot are stored in the coke cooling water tank to cool coke in the coke drums (see Figure 2).
The washing process for fat gas has a strict requirement for the quality of wash water. Calculations show that the amount of desalting water consumed in the process is 3 t/h; if purified water is used, the amount used is increased to 5 t/h to meet the same requirement. By increasing its consumption, the purified water can also reach the standards required for water in water-sealed tanks and for cooling coke.
As Figure 2 shows, purified water is supplied from other units and is introduced separately for fat gas to be water washed, as well as into water-sealed tank V1407 ( the surge tank for coke cooling water), water-sealed tank V2101 (the acid water tank), water-sealed tank V1303 (for lean solution) and coke cooling water tank V1404. Through this arrangement, purified water can be taken into the production process to lower operating costs.
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