Process Simulation for Improved Energy Efficiency


Table 4: Summary of Results of Investigated Cases for Medium-type Crude Oil


Property Feed


Capacity Case 5


Duty of HEN (MW) 51.75 HEN outlet


temperature (°C) 256


Firing rate of furnace (MW)


Tonnes CO2 /tonne crude


51.3 0.038 Case 6 Base 271 45.9 0.034 HEN = heat exchanger network; MW = megawatt. 56.65 Case 7


Crude ‘B’ Crude ‘B’ Crude ‘B’ Base


258 57.4 0.037 Case 8 Crude ‘B’


Base + 20% Base + 20% 59.37


64.38 273 52.5 0.034


However, the required firing rate in the atmospheric furnace was higher for Crude ‘B’ in every case investigated. The reason for this is that the rate of evaporation is higher for the lighter crude, requiring a higher heat load in the heater.


Table 4 shows that Crude ‘B’ can be processed at base capacity without any modification of the HEN because the required firing rate is lower than the maximum allowable of 53.34MW.


of the processing of medium-type crude oil (Crude ‘B’) at base and increased capacity was also carried out.


The first step of this part of the study was to investigate the distillation column for processing of Crude ‘B’. Table 1 shows the yield of atmospheric products was higher for Crude ‘B’ than for Crude ‘A’. This means a higher internal mass flow rate in the atmospheric column. Based on the calculation results, it can be stated that Crude ‘B’ can be processed at base capacity without violating any design parameters, considering the current tower internal arrangement.


On the contrary, the results of the calculation prepared with PRO/II software for introducing Crude ‘B’ at higher capacity (Base + 20%) showed that the flooding factor exceeded the 80% design limit in the upper, narrowed section of the column. Tray hydraulics were also investigated with KG–TOWER software. The programme gave warnings for the jet flood in the top tray and the downcomer flood in the top pumparound trays, which exceeded the design limits. This strengthens the previous results, indicating that trays in the top section should be changed.


One possible solution to overcome this bottleneck is to replace the currently applied conventional trays with high-capacity and high-performance trays. We recalculated the critical column section applying Superfrac® trays and all of the obtained parameters were below the design limits.


After the investigation of the distillation column, the crude oil preheating line and the atmospheric furnace were studied. During the calculation of processing Crude ‘B’ at base and increased capacity, fouling factors recommended by TEMA were applied and bypass of heat exchangers was not considered. The results of the study of HEN are summarised in Table 4.


Comparing the results summarised in Table 2 and Table 4 indicated that, at the same HEN configuration and feed rate (e.g. Case 1, Case


1. 2. 3.


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42 4. 5. 6.


At increased capacity (Case 7) the calculated temperature of crude oil leaving the preheat line was 258°C and the total rate of heat transfer to the crude was 59.37MW, respectively. However, the results of the investigation of the atmospheric furnace showed that a coil inlet temperature of 265°C was needed to obtain the required coil outlet temperature (395°C) at a normal firing rate of 53.34MW. This clearly showed us that a fired heater is inadequate to heat up a higher-rate feed to the necessary temperature.


The aforementioned modifications of the HEN (based on pinch analysis) were added to the simulation model and their viability for processing Crude ‘B’ was also checked. Results showed (Case 8) that the total duty being exchanged between products and crude was increased by 5.01 to 64.38MW.


These data displayed that more than 50% of the maximum possible energy recovery was achieved. After the modification, the coil inlet temperature of the atmospheric furnace was 273°C, well above the required value. Based on the data summarised previously, it can be seen that the bottleneck of the furnace can be resolved by improving the efficiency of the HEN for processing Crude ‘B’ as well.


Additionally, implementation of modifications of the HEN improves the energy efficiency of the ADU considerably (see Case 6 in Table 4),


at base capacity, too. Comparing the specific CO2 emissions (tonnes CO2/tonne crude) of Case 6 with Case 5, it can be seen that CO2 emission can be decreased by 10.5%.


Conclusions


This article presents the results of a feasibility study for increasing crude processing capacity and improving energy efficiency and feed flexibility of a crude oil atmospheric distillation unit. Results show that boosting the distillation capacity was solved while improving the flexibility of the unit for processing different types of crude oil. Analysis of the HEN contributed to remove the bottleneck in atmospheric furnace capacity constraints as well as to a significant reduction in fuel consumption


and in CO2 emissions (11.3% for heavy-type crude and 10.5% for medium–type crude). n


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5, etc.), the rate of heat exchange between the crude and products (duty of HEN) was similar for both crude oils.


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HYDROCARBON WORLD – VOLUME 6 ISSUE 1


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