October 2021

Special Focus: Plant Safety and Environment

CO₂ emissions reduction via a pinch study in a vacuum distillation unit

The project detailed here was carried out on a vacuum distillation unit (VDU) in the TÜPRAŞ İzmir refinery.

The project detailed here was carried out on a vacuum distillation unit (VDU) in the TÜPRAŞ İzmir refinery. Decreasing the carbon footprint of the facilities and being eco-friendly are some of the main strategies for TÜPRAŞ. Because the process is one of the largest units in the refinery, any energy improvement in the unit significantly affects the refinery’s efficiency and carbon footprint. To determine the optimum energy efficient application, a pinch analysis of the unit was executed.

The refinery staff performed pinch studies with five different cases and also changed a number of shells for new heat exchangers in end of run (EOR) and start of run (SOR). Within a VDU, SOR and EOR mean the clean stage and dirty stage of heat exchangers in a turnaround cycle, respectively. The reasons for comparing two different cases in the project are that the VDU processes heavy products; therefore, fouling occurs in the heat exchangers between turnaround periods and the heat efficiency of the pre-heat train changes dramatically. Finally, 38 different scenarios were achieved for heat integration of the unit.

During the pinch study for each scenario, new operating conditions were determined and compliance with the temperature/pressure design criteria of the existing equipment were evaluated. In addition to the above evaluations, the most suitable scenario was selected by considering equipment costs and fuel consumption in all alternatives.

With the selected case, the project cost was reduced marginally because no new/additional heat exchangers were required. The fired heater duty was decreased approximately 4.6% for EOR and 2.4% for SOR conditions. This improvement is equivalent to ~1,700 tpy reduction of carbon dioxide (CO₂) emissions, which requires approximately 4,150 trees to offset.

The re-arrangement of the preheat train (or pinch study, in general) of process units via simulation tools promises huge potential. Such holistic solutions have been proven to decrease fuel consumption, EII index and CO₂ emissions for energy intensive industries.

Importance of the VDU

VDUs are among the largest units in refineries. The VDU separates atmospheric residue (AR) and contains valuable products that cannot be obtained in crude distillation units (CDUs) without high temperatures. Atmospheric columns may cause uncontrolled cracking after a certain temperature, which limits the maximum temperature. Uncontrolled cracking of hydrocarbons in distillation columns is undesirable, as this operation must occur only in well-designed reactors. Vacuum conditions lower the boiling point of hydrocarbons so valuable products in the atmospheric residue can be obtained by distillation without cracking. VDU products can be produced by diesel hydroprocessing (DHP) units, hydrocrackers, fluid catalytic cracking units (FCCUs) and a lube oil plant.

The VDU products mainly used in the lube oil plant (LOP) are spindle distillate, light distillate and heavy distillate. Additionally, the unit produces light vacuum gasoil (LVGO) and vacuum residue (VR). LVGO is sent to the diesel hydroprocessing unit and VR is sent to the propane deasphalting unit. Spindle distillate, light distillate and heavy distillate products are processed in the lube oil plant to produce spindle oil, light oil and heavy oil products.

The need for this study

Energy efficiency is always a hot topic in petrochemical plants and refineries—high-capacity production necessitates the search for energy reduction, so new technologies are implemented and potential improvements are always evaluated.

VDU energy consumption is immense, as is the unit’s impact on a refinery’s total energy requirements. Energy reduction projects are high priority, and it was considered that potential might exist to improve the authors’ refinery’s pre-heat section. A preliminary version of the study with a commercial rigorous simulation model showed valuable improvement possibilities, so a pinch analysis in the unit was executed to determine the optimum energy efficient application for the pre-heat section.

VDU simulation and validation

A commercial rigorous simulation program was used to model the VDU with the goal of optimizing the heat exchanger network and the fired heater. The appropriate thermodynamic fluid package for this simulation was Peng Robinson. For characterization of the streams (AR, spindle, light and heavy distillate and VR), an oil manager was used. The unit model was configured by using field data (distillation, flowrates, operational data, etc.) and equipment process data sheets. The main focus was optimizing the heat network, which is why the rigorous models were considered for exchangers. For the analysis, the exchangers were modeled in both cases for SOR as clean and also for EOR as dirty. The main hydraulic and thermal checks were crucial steps in the study and were the main assumptions before validation.

VR from the VDU and LOP feed were integrated via a heat exchanger and the lube oil plant feed entered the fired heater after this heat exchanger, as shown in FIG. 1. Both fuel consumption effects for the VDU and the LOP fired heater should be considered before performing a heat integration study. The fired heaters efficiencies were assumed as 90% and 80%, based on the design for the VDU fired heater and lube oil unit fired heater, respectively.

For the first step of validation, the base model of the system was created. Heat and material balance were checked on the selected data set for this model. This model was then analyzed based on simulation outputs according to predetermined verification limits. In each step of validation, the exchangers design data were considered for detailed comparison with the datasheet values for duties, hydraulics and fouling factors. The acceptable criteria were ± 5% based on possible measurement errors and general refinery applications.

The base model

The base model of the system is shown in FIG. 1. The scope of this pinch analysis only covered the atmospheric straight-run fuel oil (ASRFO) heat network; the remaining sections of the unit were not modeled in this study. All alternative cases were based on the model discussed below.

CASE STUDY

Pinch studies were conducted for five different cases and the numbers of shells for new heat exchangers in EOR and SOR were changed. As previously stated, 38 different scenarios were achieved for heat integration of the unit.

Analysis of energy efficiency alternatives in the simulation model

After model validation, five different cases were generated to analyze fuel savings and CO₂ emissions reduction alternatives. Details of these alternatives are summarized here.

Case 1. In this case, the target was the addition of a new heat exchanger to the preheat train between the Feed-PA and the Feed-LT distillate for the heating charge of the unit via the VR stream, shown in FIG. 2. The number of shells in the new heat exchanger were changed to determine the effects on energy savings and CO₂ emissions reduction rates. The re-arrangement of the existing Feed-VR heat exchangers from four shells to 2 × 2 shells as an alternative scenario for the new system was also studied.

Case 2. In this case, the target was the rearrangement of the heat exchanger hot side services for the Feed-HV distillate-2 from the HV distillate stream to the VR stream, as shown in FIG. 3. All HV distillate streams were diverted to the Feed-HV distillate-1 heat exchanger—in this way, the Feed-HV distillate-2 heat exchanger is revised as the Feed-VR heat exchanger. The energy savings potential of the rearranged pre-heat train was studied to determine the effects on fuel consumption of the fired heater and CO₂ emissions reduction rates.

Case 3. In this case, the target was the addition of a new heat exchanger to the pre-heat train between the Feed-LT distillate and the Feed-HV distillate-1 for the heating charge of the unit via the VR stream, as shown in FIG. 4. The number of shells in the new heat exchanger were changed to determine the effects on energy savings and CO₂ emissions reduction rates. The rearrangement of the existing Feed-VR heat exchangers from four shells to 2 × 2 shells as an alternative scenario for the new system was also studied.

Case 4. In this case, the target was the addition of new heat exchangers to two separate locations for heating charge of the unit via the VR stream. The first addition is between the Feed-PA and the Feed-LT distillate, and the second addition is between the Feed-LT distillate and Feed-HV distillate-1, as shown in FIG. 5. The number of shells in the new heat exchanger were changed to determine the effects on energy savings and CO₂ emissions reduction rates. The rearrangement of the existing Feed-VR heat exchangers from four shells to three shells was studied, and that one shell was used between the Feed-PA and the Feed-LT distillate or between the Feed-LT distillate and Feed-HV distillate-1 as alternative scenarios for the new system.

Case 5. In this case, the target was the rearrangement of the heat exchanger hot side services for the Feed-HV distillate-2 from the HV distillate stream to the VR stream, as well as the addition of the modified heat exchanger between the Feed-PA and Feed-LT distillate heat exchangers, as shown in FIG. 6. All HV distillate streams were diverted to the Feed-HV distillate-1 heat exchanger; in this way, the Feed-HV distillate-2 heat exchanger was revised as the Feed-VR heat exchanger. The energy savings potential of the rearranged pre-heat train was studied to determine the effects on the fuel consumption of the fired heater and CO₂ emissions reduction rates.

Checking equipment adequacy

Energy savings alternatives were analyzed through validated unit simulation models. In these analyses, the goal was to identify possible equipment that could create bottlenecks. The list of equipment adequacy controls included:

• Heat exchangers—Design temperature and design pressure controls were performed according to revamp operation conditions; and ρV² and vibration controls were also considered for all alternative scenarios.
• Pumps—Adequacy controls were performed on the rated capacity values in the datasheets of the pumps, and it was determined whether the pumps could meet the corresponding differential pressure.
• Fired heaters—Fuel savings and CO₂ reduction rates were analyzed for each case, and the effects of the cooled down VR stream on the lube oil plant unit fired heater were analyzed to determine if any bottleneck might occur.

Results

The VDU energy efficiency performance, fired heater fuel savings data and CO₂ emissions rates regarding the pinch analysis were checked for each alternative case. In the alternatives, SOR and EOR operation parameters and results were obtained from simulation models and the economic feasibility and/or CO₂ emissions reduction rates were calculated individually.

In Case 2 and Case 5, using the revised Feed-HV distillate heat exchanger as the Feed-VR heat exchanger proved an inadequate solution due to the hot side design pressure and hot side design temperature limitations, respectively. Accordingly, these two cases were eliminated from the pinch study results.

The rates of increase on both VDU, lube oil unit fired heater duties and fuel savings were tabulated in TABLE 1 and TABLE 2 for Case 1, Case 3 and Case 4 at SOR and EOR operating conditions, respectively.

Takeaway

The impetus for this project feasibility evaluation was achieving minimal CAPEX, a simple payback period with higher potential fuel savings, and a minimal impact on additional fuel consumption of the LOP fired heater. Accordingly, Case 3 (the modification of the Feed-VR HEX) was selected, as shown in TABLE 1 and TABLE 2. The lowest project cost and optimum solution for site applications (construction works, equipment erection, etc.) were also attained.

Decreasing the carbon footprint of its facilities is one of the main TÜPRAŞ strategies. The pinch study showed that fuel savings of 4.6% for EOR and 2.4% for SOR conditions and a significant CO₂ emissions reduction of ~1,700 tpy were achieved. HP

The Authors

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