A. Srivastava, R. BAGROLA and J. BIST, Fluor, Haryana, India
Propane-propylene splitters are present in significant numbers across petrochemical complex units, such as ethylene crackers and propane dehydrogenation units. Separating propane from propylene requires significant energy and several trays due to the close boiling point. Top-vapor compression heat pumps are used in most units based on heat source availability. Depending on the compression power cost, a top-vapor compression distillation column can be operated at a lower pressure.1
Top vapor is the economical choice in case a heat source is unavailable, leading to high heating and cooling utility costs. The author’s company analyzed the energy savings using the bottom flashing approach in a propylene splitter vs. the proven top-vapor compression approach, which is present in various refinery and petrochemical complex units. The study showed an approximate 23% savings compared to the top-vapor recompression approach. The bottom streams were flashed to lower pressure to achieve low temperatures. This can be used as a utility in the condenser, ensuring the 5°C approach is maintained. After the heat is transferred in the condenser, vapor and liquid are separated in the knockout (KO) drum. The vapors are compressed to the column bottom pressure, and the liquid pressure is also increased to the column bottom pressure using a pump. They are then mixed and cooled to maintain the vapor load to the last stage.
Design considerations. A simulation toola was used to set up the top-vapor compression and bottom-flashing schemes. The simulation was set up with the Peng-Robinson property package. This state-model equation adequately predicts the light hydrocarbon mixture’s equilibrium based on past project experiences. Both process flow schemes were simulated for the same feed composition, product specification, column profile and operating pressure to compare the energy savings in the bottom flashing approach with the top-vapor compression. The design parameters were kept constant in both flow schemes for the comparative study (TABLE 1).
A standalone distillation column was simulated to fix the column profile and reflux ratios and meet the targeted composition for products. The flow scheme was then developed to integrate the heat pump and bottom flashing approach.
FLOW SCHEME AND HEAT AND MASS BALANCE (HMB)
Top vapor compression. In this flow scheme, the column overhead is compressed to a high pressure (FIG. 1). The compressed overhead is sent as a hot stream to the reboiler, which is condensed to provide the re-boiling duty to the column. After condensing, the overhead is flashed to the accumulator pressure and sent to the reflux drum as a mixed phase. The balanced stream from the compressor discharge is cooled in a trim condenser and flashed to the reflux drum pressure. The overhead from the reflux drum is sent to the compressor suction, the liquid is sent as reflux back to the column and the product is sent to the downstream unit (TABLE 2).
Bottom flashing. In this flow scheme, the column bottom is flashed to a lower pressure to achieve a lower temperature, which is used as a cooling utility in the overhead condenser to condense the overhead stream (FIG. 2). After condensing the overhead, the mixed-phase stream is flashed in a downstream separator. Overhead from this separator is compressed to the column bottom operating pressure and the liquid is pumped to the column operating pressure, and both streams are then recycled back to the column bottom (TABLE 3).
Results and discussion. Based on the design considerations described, a simulation was performed, fixing the operating parameters specified in TABLE 1 and the energy requirements for both schemes are tabulated in TABLE 4.
The energy consumption of both flow schemes shows that replacing the top-vapor compression approach with the bottom flashing approach will lead to operational expenditure (OPEX) reductions. Reduced OPEX in the bottom flashing scheme can be attributed to lower compressor power because the flashed vapor is only compressed compared to the total overhead from the column in the top vapor compression scheme. This offers significant energy savings and is advantageous to the bottom flashing approach.
Equipment’s capital expenditure (CAPEX). The CAPEX was computed only for equipment, which is different in each scheme, to analyze the difference in cost between the two flow schemes. The exchanger sizes were estimated using an exchanger design toolb, and the vessels were sized based on a 5-min hold-up time for the reflux drum and 10 min for the compressor KO drum, using standard vapor-liquid separation criteria based on Stokes law. The costs were based on the year of analysis and not current material data (TABLE 5).
Based on the cost estimation, TABLE 6 summarizes the cost for each flow scheme to highlight the cost difference (FIG. 3). The following considerations were made to estimate the utilities’ OPEX:
Takeaway. Based on the simulation results, the bottom flashing approach for C3 systems consumes less energy than heat pump configurations. Additionally, the investment required for the bottom flashing approach is less. Therefore, if a heat pump scheme is proposed for economic consideration, bottom flashing should also be considered as an alternative option because it offers an economic advantage. HP
NOTES
a Aspen HYSYS
b HTRI Exchanger Suite
LITERATURE CITED
ANAND SRIVASTAVA is a Process Engineer for Fluor India. He has 15 yr of experience in executing front-end engineering design (FEED) and detailed engineering activities across refining, petrochemical and specialty chemical projects. Srivastava earned a BS degree from the Indian Institute of Technology Kanpur.
RAKESH BAGROLA is a Process Engineer for Fluor India. He has 20 yr of experience in operation, basic engineering, FEED and engineering, procurement and construction for refining and petrochemical projects. Prior to joining Fluor India, Bagrola worked with UOP India and Haldia Petrochemicals. He earned a BE degree in chemical engineering from Ravishankar Shukla University in Raipur, Chhattisgarh.
JYOTI BIST is a Process Engineer with Fluor India. She has 12 yr of experience in FEED and detailed engineering for refining projects. Bist earned a BS degree from the University of Petroleum & Energy Studies in Dehradun, India.