An Excel workbook was used to determine and compare the economics of the design options considered. The team proposed a design that showed a higher NPV for the plant than that of the original base case design. With a good idea of the equipment in the process, the first part to analyze is the reactor part.
Due to the nature of the process and equipment limitations, there are some conditions that must be maintained in the plant. Determining NPV required calculating the costs of utilities, equipment, labor, raw materials, buildings, and land. As part of our initial analysis, we determined which factors had the greatest impact on the facility's value.
Additionally, we were able to reduce the number of columns required for the separator section by changing the design of the second one. The liquid behavior of the catalyst allows the process stream to better interact with it.
Pressure vs Styrene Production
With that change the increase in styrene production becomes 33 kmol/hour, which requires 725 oC and 2.27 bar at the inlet, with 6 tubes.
Temperature vs Styrene Production
Introduction
We were assigned to Unit 500, which is the part of the polystyrene plant that reacts ethylbenzene to produce styrene. The packed-bed adiabatic reactor cannot exceed an inlet temperature of 726.9 ºC due to the temperature limitation of the reactor catalyst. Since the polystyrene plant requires 100,000 tons of styrene to produce the main product polystyrene, it is reasonable for Unit 500's NPV to be negative.
Thus, we want to increase the net present value (NPV) from the base case NPV of -$616 million to make unit 500 more profitable within the framework of. Based on the increase in NPV, our group recommends moving forward to establish a more complex design of Unit 500. First, we optimized an adiabatic packed reactor and a shell-and-tube reactor to determine which reactor produced the highest yield of styrene at the lowest price.
Our team focused on yield as a unit of reactor evaluation because it evaluates how much of the ethylbenzene feed reacts to form styrene, our desired product. We sought to make temperature and pressure changes in the flash drum and distillation column to separate styrene from ethylbenzene in one step. When we finished the optimization of the separation part, we integrated the heat exchanger system in Unit 500 to reduce the need for heat supplies.
Assumption and ConstraintReasonAssumption and ConstraintReason The inlet temperature and pressure of the ethylbenzene feed, Stream 1, are 136 °C and 210 kPa The temperature and pressure of 136 °C and 210 kPa, respectively, are the given condition that our company buys the Adiabatic rector ethylbenzene feed. is zero and the inlet temperature to the Stream 9 reactor is 725 °C. The adiabatic reactor had a lower annual equivalent operating cost than the shell-and-tube reactor, the inlet temperature limit is 726.9°C, so we checked for a temperature of 725, giving our system a temperature and pressure of buffer inlet 1°C of low pressure steam, Stream 4, are 159°C and 600 kPa. The thermodynamic package of Flash Drums V-501 and V-511 is SRK02SRK02 accounts for the separation of the two liquid phases The inlet ethylbenzene feed, Stream 1, is 98% ethylbenzene, 1% toluene, and 1% benzene The composition of Stream 1 is given of food that our company buys the thermodynamic distillation column package is IDEA01.
The pressure drops are the same as those from the base case and the drop within the heuristics in Table 11.11 under number 5 in Analysis, Synthesis and Design of Chemical Processes written by Richard TurtonPumps P-501 and P-505 have an outlet pressure of 200 kPa200 kPa is the required press to treat and sell waste water and styrene respectively. The steam to ethylbenzene ratio entering the reactor, Stream 9, must remain at 15.6. The ratio 15.6 is set to maintain a minimum velocity in the reactor of 2 m/s at less than 725°C, 4 m diameter and 6 m length of the reactor Compressor, C-511, has an outlet pressure of 90 kPa. The reason for the discharge compressor pressure is to recover ethylbenzene. A maximum velocity in the reactor of 2 m/s Maximum velocity of 2 m/s gives a reasonable pressure drop of 30 kPa over the reactor. The outlet pressure of the P-506 is 210 kPa. 210 kPa in stream 39 is the same as stream 1, the ethylbenzene feed. The process concept diagram illustrates the two general sections of Unit 500: the reactor and the separation system. Apart from the yield and conversion, we assumed that there would be perfect reaction and separation based on the stoichiometric ratio of the reactants and products to produce 120.02 kmol/hr of styrene.
Results and Discussion
The styrene distillation column separates ethylbenzene in the distillate from styrene in the bottoms to obtain a styrene product with a purity of 99.5% by weight for Unit 500. The bottoms of the distillation column is heated in the reboiler, E-513, using of the effluent vapor of R-511, stream 13, to maintain a constant flow rate throughout the column. Our team chose SRK-SIMSCI as the default thermodynamic package in the Pro/II simulation software.
Water is the only substance that can hydrogen bond and is inert in the system. The SRK-SIMSCI is capable of handling two immiscible liquid phases, which is useful for the separation of a water and organic phase in the flash drum. Styrene is present because it is difficult to separate in the distillation tower, and superheated inert steam is present to bring the temperature of the feed to R-511 and act as a diluent for the reaction.
Although the NPV remains negative, Unit 500 can be used to produce polystyrene, and ethylbenzene can be another product in the polystyrene plant, making the optimized design of Unit 500 a possible solution to increase the profitability of the polystyrene plant. Additionally, we viewed the fuel gas as a positive resource that our team could sell or use in the fired heater, H-501, or any other part of the polystyrene plant. Since the distillation columns accounted for a significant portion of the equipment costs in the base case, our team wanted to reduce the number of columns.
The distillation column included in the base case of Unit 500 contained titanium towers and trays. Since styrene production increases as the time it spends in the catalyst bed decreases, also called contact time, the reactor has a higher yield of styrene when the catalyst bed is shorter. The figure shows an increase in the yield of styrene as the reactor pressure decreases.
The figure also shows a greater increase in the conversion of ethylbenzene than the decrease in the yield of styrene. The product of conversion and yield is defined as the amount of styrene produced in the reactor per mole of ethylbenzene fed to the reactor. Our change also reduced the number of heat exchangers required in Unit 500 from 9 to 6, while leaving only cooling water as a required resource in heat exchangers E-514, E-509 and E-516.
The specifications of the stern and shortcut column remained the same as the other turret in the base case, the T-502. To reduce the non-condensable gases in the distillate of the distillation tower, we created a vapor stream, Stream 33, that leaves the condenser to mix with the vapor stream, Stream 16, and exits the flash drum V-501.
Conclusions and Recommendations
Other Sections
Appendices
