Liquid Ammonia (NH3) Plant Design
Rev 1: Basic Mass Balance
This simple PFD allowed me to scope the basics of the design, determine the main process flows, and construct a simple mass and energy balance to determine basic process conditions. My ammonia plant had to be able to produce 432 mol/s of liquid ammonia while taking take feedstocks of H2 at 92% purity and N2 at 97% purity. The process was modeled and optimized in excel.
Stream N2-0 and H2-0 represent the feed streams of N2 and H2 respectively. Stream S4 contains pure liquid ammonia and stream S6 is the purge stream. This system uses a recycle stream feed S7 in order to achieve and overall N2 gas conversion of 98.53%
It is also important to note that Mix-1 must fulfill an adiabatic mixing condition. This mix temperature affects the rest of the downstream process conditions
Rev 2: Expanding PFD & Parameter Refinement
This new PFD incorporate the location of some key control valves and other important process equipment. The separation vessel (SV-01) used to separate the ammonia from feedstocks is a reboiler. The Knockout Drums (KOD) and Heat exchanger's used a the feed streams are to remove impurities such as water from the gas feed. I have opted for air-cooled heat exchangers as this is the direction the industry is moving towards as governments tighten regulations on heat exchanger cooling water temperatures.
Rev 2: Storage Tank Unit Operation P&ID
The detailed Piping & Instrumentation Diagram (P&ID) of the ammonia storage tank is shown on the right. Some important design considerations were made in designing the control systems and equipment choices for this unit operation. The design objective was that the tank had to be able to store and deliver liquid ammonia on demand whenever needed, maintaining the liquid state of ammonia to the output.
The outlet flow of ammonia leaves the tank at the bottom and flows through a heat exchanger followed by a pump and then a 3 way valve. This heat exchanger & pump combo serve 2 main purposes. Firstly, when the feed of the 3 way valve feeds back to the tank, the system constantly chills the liquid ammonia to maintain its liquid state. Secondly, the system delivers liquid ammonia to the next process on demand. Since the ammonia is stored at the saturation pressure, even the smallest pressure drop when passing through the pump will cause the ammonia to vaporize resulting in cavitation. Hence, the heat exchanger cools the ammonia by at least 5 degrees lower than the boiling point. This ensures that the ammonia never vaporizes and prevents cavitation in the pump.
CHE324-Assignment4&final project report - Portfolio.docxFinal Report
The original client requested design of 432 mol/s ammonia plant was found to not be profitable even after 25 years. After conducting an NPV analysis, PlantsRUs recommends a minimum 853 mol/s target production of ammonia in order to break-even at 25 years, and scale up of up to 1300 mol/s for an even more profitable plant with an even earlier breakeven point of 13 years. The recommended minimum plant will cost $36M in initial expenses but will have a NPV of $11M at the end of 25 years. Meanwhile, the recommended scaled up design will cost $78M in initial expenses but will have a NPV of $400M at the end of 25 years. It is important to note that there is about a ±30% uncertainty in this CAPEX estimation. This large disparity is in returns is why PlantsRUs recommends the client go with a target production somewhere in between 853 and 1300 mol/s rather than the original 432 mol/s. The plant design is easily capable of being scaled up in order to take advantage of the scale of production and generate much bigger revenues simply from higher production rates.
The plant parameters have been optimized in order to maximize the overall N2 conversion without sacrificing safety and minimizing waste gasses. This provides an overall 95% N2 conversion when using a 5% excess H2 feed. Since H2 is much cheaper to manufacture than N2, it has been chosen to be the excess reagent. Additionally, it is targeted that the reactor have a single pass N2 conversion of about 50% but the reactor was designed to have a target 62% N2 single pass conversion in order to provide a conservative estimates. The reactor was also targeted to operate at about 300 ̊C, 167 bar, 1.5% ammonia in the purge and a 5% purge.
If you would like to take a look at the spreadsheets and simulations or learn more about this project, please feel free to contact me at my email address down below!