Cryogenic Air Separation For the Production of Nitrogen
April 29, 2013 Authored by: Jaclyn Demyan, Todd Siwik
Abstract ................................... ................................................... .................................. ................................... .................................. ................................. .......................... .......... 2 Introduction Introduction .................................. ................................................... .................................. .................................. ................................... ................................... .................... ... 3 Project Description Description ................................ ................................................. ................................... ................................... .................................. .......................... ......... 3 Safety, Regulations Regulations and Codes ................................... ................................................... .................................. ................................... ....................... ...... 4 Technical Discussion ................................ ................................................. ................................... ................................... .................................. .......................... .........6 Basis of Design ................................... .................................................... .................................. .................................. .................................. ............................. ............6 Alternatives Alternatives ................................ ................................................. .................................. .................................. ................................... ................................... .................... ... 6 Process Description Description.................................. ................................................... .................................. .................................. ................................... ........................ ...... 7 Compressor .................................. ................................................... .................................. .................................. .................................. ................................ ...............9 Removal of Impurities Impurities ................................. .................................................. .................................. .................................. ................................. ................10 Heat Exchanger.............................................. ............................................................... .................................. .................................. .............................. .............11 Distillation Column .................................. ................................................... .................................. .................................. ................................... ....................11 Turbine ................................ ................................................. ................................... .................................. .................................. ................................... ..................... .... 12 Scale-up .................................. .................................................. .................................. ................................... .................................. ................................. ........................ ........ 13 Process Description Description.................................. ................................................... .................................. .................................. ................................... ...................... .... 13 Compressors ................................... .................................................... .................................. .................................. .................................. ........................... .......... 13 Removal of Impurities Impurities ................................. .................................................. .................................. .................................. ................................. ................14 Turbine ................................ ................................................. ................................... .................................. .................................. ................................... ..................... .... 14 Cold Box ................................. .................................................. .................................. .................................. ................................... ................................... ................... 14 Control System .................................. ................................................... ................................... ................................... .................................. ........................ .......15 Piping .................................. ................................................... ................................... .................................. ................................. .................................. ...................... ..... 17 Storage and Transportation Transportation of Liquid Nitrogen Nitrogen ................................. .................................................. .............................. .............19 Carbon Footprint .................................. ................................................... .................................. .................................. ................................... ...................... ....20 Land and Utility Requirements....... Requirements........................ .................................. .................................. .................................. .............................. .............20 Economic Analysis............................................. .............................................................. .................................. ................................... ................................. ...............21 Conclusion and Recommendations Recommendations .............................................. ............................................................... .................................. ........................ ....... 23 1
Nitrogen gas is a very common requirement in several industries today. The purpose of this report is to provide the design of a cryogenic air separation plant for the production of nitrogen gas. First, a pilot plant must be constructed to provide all calculations and proof of concept. Then, the pilot plant will be scaled up to a full production plant. The pilot plant must produce 20,000 SCFH of nitrogen gas with a minimum purity of 99 volume percent. In addition to the nitrogen gas, liquid nitrogen must be produced at a rate of 10% of the gas production. After completion of the pilot plant design, it must be scaled up to a full size production plant producing at least 600,000 SCFH of nitrogen gas with the same minimum purity. After design of the pilot plant, equipment was selected for the full production plant. The system first compresses air from the atmosphere to about 11 bar with an Ingersoll Rand model number C3000 centrifugal air compressor. After compression, the feed stream enters a temperature swing adsorber (TSA) to remove remaining water, carbon dioxide, and contaminants in order to prevent damage to cryogenic equipment. 13X and F-1 alumina zeolites will be used in the TSA. The feed stream proceeds to the multi-cell heat exchanger to reduce the temperature to 110 K by utilizing the waste and product streams from the distillation column. After the feed stream is cooled, it enters the distillation column, with a temperature of 110 K and a pressure of 10.1 bars, where nitrogen is separated from the remaining components. The mostly pure nitrogen stream leaves the top of the column as the oxygen rich waste stream is drained from the bottom. The waste stream is flashed to further reduce its temperature and fed to the column condenser to liquefy some of the product gas. After the condenser, the waste stream enters the heat exchanger, cooling the feed. The nearly pure distillate leaves the column and is partially condensed in the column condenser. The liquid nitrogen is sent to on-site storage tanks. This liquid nitrogen is stored to be vaporized into nitrogen gas to be used during plant start-up as well as sold during plant shut-downs. The remaining nitrogen gas, with a purity of 99vol%, is sent to the heat exchanger to further cool the incoming feed air. A small portion of this gas is removed to provide the nitrogen purge necessary for the coldboxes. The remaining nitrogen gas is pumped through a pipeline to be sold to local customers. After the design of the full production plant was complete, an economic analysis was performed to ensure profitability. The process was found to be profitable and construction of the plant is recommended. 2
More than three fourths of the Earth’s atmosphere is made up of nitrogen gas. Just as it is common within the atmosphere, nitrogen gas is also common in many industrial processes. Nitrogen is an inert gas and limits oxidation when used in place of standard air. Because of its properties, one of the highest industrial uses of nitrogen is within the drilling industry. “Used both in onshore and offshore situations, applications for nitrogen include well stimulation, injection and pressure testing, Enhanced Oil Recovery (EOR), reservoir pressure maintenance, nitrogen floods and inert gas lift. Additionally, nitrogen can be used to help prevent flammable gases from igniting and protect tubulars from downhole (sic) corrosion.” (Generon IGS, 2010) In addition to its use within the drilling industry, nitrogen is vital to the production of electronics, polymers, and within fuel systems on military aircrafts. There are several options for companies who require nitrogen gas. When large quantities of nitrogen are required at a plant, the nitrogen can be delivered as a liquid or gas and stored on-site in storage tanks. Nitrogen gas can also be delivered by local pipeline, as with this case study. The final option is to separate air on-site to obtain the required nitrogen. For this case study, a plant will be designed for the purpose of providing nitrogen gas to local companies via a pipeline. Additional profits can also be made by transporting liquid nitrogen to customers. A detailed description of the project specifications can be found below.
Company A has requested the design of a nitrogen plant within the Akron/Canton, Ohio. The production requirements of the plant are 600,000 SCFH of nitrogen with a minimum purity of 99 volume percent. The design must also allow for the production and storage of liquid nitrogen at about 10% of the gas production. This liquid nitrogen shall be available such that it can be vaporized into gaseous nitrogen to off-set high peaks in demand as well as plant shutdowns. Company A requires the construction of a pilot plant with the ability to produce 20,000 SCFH of nitrogen at the above stated specifications. This will be the basis of the plant design. This pilot plant will then be scaled to the required 600,000 SCFH. Below you will find the results of the pilot plant study as well as the recommendations for scaling to the required full plant specifications. One thing must be noted in regards to measurements within this report. Standard cubic feet per hour (SCFH) are a volumetric flow rate of gas corrected to “standardized” 3
temperature and pressure. However, there is no universal definition of what this standard temperature and pressure should be. (SCFM versus ACFM and ICFM) For the purpose of this paper, the standard temperature and pressure will be 60 degrees Fahrenheit ( 289 K) and 14.696 pounds per square inch (1.01 bar), respectively.
Cryogenic materials are very cold substances. Hazards associated with cryogenic fluids include personnel exposure, high pressure gases, material and construction compatibility, and asphyxiation. Cryogenics can present physiological hazards such as severe cold burns or frostbite which may be inflicted if the cryogenic material or cooled materials comes into contact with the human body. Consideration must be taken in the selection of materials due to the effects of low temperatures on the properties of those material. Thermal stresses, over pressurization of equipment, and brittle fracture are all hazards which can occur due to the cryogenic process. ASME codes B31.1 through B31.7 contain relevant information regarding materials compatible with the products and temperatures they encounter. Potential hazards exist in highly compressed gases because of the stored energy. Cryogenics liquids vaporize with a change ration of volume up to 900, this can cause violent changes in pressure especially if in confined areas. All cryogenic systems are equipped with pressure relief devices to prevent excessive pressure buildup. The most significant risk of cryogenic liquids is death by asphyxiation where a spill or leakage depletes the atmospheric oxygen. If the oxygen concentration falls below 18% adverse effects will occur resulting in loss of mental alertness and performance combined with distortion of judgment. In atmospheres containing less than 10% oxygen death by asphyxiation is rapid: just two breaths of oxygen-free air kills. Transportation of cryogenic substances is covered in the US Department of Transportation (DOT) 49 CFR 173, which covers mass/volume of goods that may be transported, correct packaging and labeling as well as vehicle usage, driver training and duties of responsibility.
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Training should be given in all aspects of the handling of cryogenic materials. It is important that personnel have an understanding of the risks involved and where to obtain information. Proper personal protective equipment (PPE) should be worn during the normal handling of cryogenic materials. Some major environmental laws which were taken into consideration were the Clean Air Act regulates sources of air pollution such as manufacturing plants and the Clean Water ACT (CWA) is an amendment to the Federal Water Pollution Act in 1977 which regulates the discharges of pollutants to waters.
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The design of this nitrogen production plant is based on the production of 20,000 SCFH of nitrogen with a minimum purity of 99 volume percent. For simplification purposes, the feed air is assumed to be at 283 K and 1.01325 bars. Air contains nitrogen, oxygen, carbon dioxide, argon, water, and trace amounts of various other components. The pie graph below shows the makeup of these components by volume percent. The water content of air is dependent on the temperature and will vary from day to day operation of the plant.
0.03%
0.93% 20.95%
Nitrogen Oxygen 78.09%
Carbon dioxide Argon
Although cryogenically separating air was suggested for this plant, time must be taken to fully eliminate other options. There are two widely used processes to separate air: adsorption and membrane separation. Within adsorption processes, there are three methods to divide air into pure gases. These include pressure swing adsorption (PSA), vacuum swing adsorption (VSA) and a combination of the two, known as vacuum pressure swing adsorption (VPSA). All of these adsorption methods utilize aluminosilicate minerals, also known as zeolites, to separate the product gas from the remaining air. Generally, multiple beds of zeolites are used so that one bed filters the air while the other bed is regenerating. Pressure swing systems operate at near-ambient temperatures and rely on the properties of the zeolites and specific pressures to adsorb the unwanted gaseous components. While under high pressure, the unwanted gases are adsorbed by the packed bed and the product gas 6
continues out of the system. The pressure between the beds is then switched and the first bed releases the waste gas from the zeolite under lower pressure. This swing between pressures allows for continuous production of the target gas. Although vacuum swing adsorption systems are similar to pressure swing, they allow for greater efficiency, reduction in maintenance, as well as cost savings. A vacuum swing system operates at near ambient temperature and pressure. The feed gas is drawn into the system over the zeolite bed to separate the gases. When the system switches to the second bed, the initial zeolite is regenerated by subjecting it to a vacuum. The lower pressure allows for the waste gas to be released and removed from the system. Maintenance costs are reduced on this system because the difference in pressure between the two stages is lower than the difference in a pressure swing system, limiting the deterioration of the zeolite beds and reducing the amount of dust particles that leave the system. By combining these two types into a vacuum pressure swing adsorption (VPSA) offers the best efficiency. In VPSA systems, the feed gas is drawn into the system at a higher pressure and regenerates the waste bed under a vacuum. Although any three of these systems are capable of providing the purities required for the case study, they are unable to produce gaseous nitrogen in the required amounts. The last possible alternative is membrane separation of air. Membrane separation is based on the difference of permeation components of the feed gas. Depending on the required product gas, a membrane is selected. The driving force for the separation is provided by the difference of partial pressures on each side of the membrane. The air is pressurized and then fed over the hollow-fiber membrane where the product gas diffuses through the membrane while the waste gases exit the system. Membrane separation produces lower purity gases compared to the other alternatives discussed. Each of these alternatives is not suitable for this case study because they are unable to produce nitrogen at the required purities at the necessary production needs. An extremely simplified illustration depicting the cryogenic separation of air can be found in the appendix.
In cryogenic distillation, air is drawn into the system from the atmosphere and liquefied by compressing and cooling the air. This liquefied air can then be distilled to separate nitrogen from the remaining components of air. First, the air is compressed to about 10 bar. Then the high pressure air is fed to a temperature swing adsorber where carbon dioxide, contaminants, and remaining water are removed. The “clean” air is then 7
cooled to cryogenic temperatures by utilizing the waste and product streams within a four chamber heat exchanger. The feed stream is then sent to a distillation column that produces a distillate of nearly pure nitrogen gas and a bottom product of oxygen rich liquid. The liquid is flashed to reduce the temperature to provide refrigeration for the condenser. After the condenser, the bottoms product goes to the heat exchanger to cool the incoming feed. To recoup some energy to power the compressor, the waste stream is expanded through a turbine. It is then fed through the heat exchanger a second time. The waste stream is then released to the atmosphere. The nearly pure nitrogen product stream is fed to the heat exchanger to cool the feed stream. The product stream is then split. Nitrogen gas is required for the coldbox surrounding the heat exchanger and distillation column and additional nitrogen gas is liquefied for storage. The remaining product stream is pumped through pipelines to local customers. Figure 1 below shows the process flow diagram. This is followed by a detailed analysis of each piece of equipment for the design of the pilot plant.
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To compress the air, a few variables must be taken into consideration including required flow and required pressure. The necessary outlet pressure is about 10 atm with a flow rate of about 55,000 SCFH. The correct type of compressor must be chosen to achieve these specifications. There are three different types of compressors; reciprocating, rotary screw, and centrifugal. Reciprocating and rotary screw compressors are both positive displacement compressors. Positive displacement compressors increase the pressure by decreasing the volume of the gas. Reciprocating compressors contain a piston within a cylinder to reduce the volume of the gas. These compressors are simple in design and have lower installation costs but are not designed to run at full capacity 24 hours a day, 7 days a week. Rotary screw compressors decrease the volume of air with the use of two or more rotors, or screws. Rotary screw compressors are designed to run 100% of the time and are the most popular type of compressor within plants. The last type of compressor, a centrifugal compressor, is considered dynamic because it relies on a transfer of energy to increase the pressure of a gas. Angular momentum from a rotating impeller spinning at high speeds is transferred to the gas, producing a higher pressure. These compressors can be designed to reach flows of over 20,000 cfm at 125 psig. (Types of Air Compressors) For the design of the pilot plant, the best selection would be a rotary screw compressor. This compressor can be either oil free or oil flooded to reduce the heat created by compressing air as well as lubricate the internal mechanisms. A comparison of these methods is listed below in Table 1.
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Rotors turn at high speeds
Rotors turn at lower speeds
High compression temperatures
Low compression temperatures
Higher maintenance requirements and
Lower maintenance requirements and
cost
costs
Requires larger HP motor
Smaller HP motor and more energy efficient
Water cooling required
Air cooling
No additional filtration needed
Additional filtration required to remove oil from product
Although using an oil flooded compressor will introduce contaminants to the feed gas, full package filter systems built into the compressor are available that are guaranteed to remove all contamination. The higher cost of a filtration system will be offset by the savings in maintenance costs and energy efficiency. With a smaller motor, less energy is required. Theoretically, 170 HP is required to produce air flow of 55,000 cfh at a pressure of 10 bar. By adding an additional 20% to compensate for friction and real life conditions, the required HP is a little over 200. (Horsepower required to Compress Air, 2011) This cost of energy is one of the biggest concerns when selecting a compressor. To reduce this energy need, the process will also contain a turbine to recoup some energy from the waste stream to power the compressor.
Air quality is critical to cryogenic processes. Because of the extremely low temperatures of the main components within the system, it is critical to remove all water and contaminants from the incoming air stream. If water is allowed to enter the cryogenic equipment, it will freeze, damage equipment, and reduce the flow of the feed stream. Contaminants, especially hydrocarbons, can damage cryogenic equipment as well as reduce the ability of the system to effectively separate nitrogen. To remove contaminants from the air before it is cooled, a temperature swing adsorber (TSA) will be used. This TSA functions much like the pressure swing adsorbers described in the alternatives section of this report. Two separate zeolite beds are used to remove water, carbon dioxide, and other contaminants. The feed air flows through the first bed where 10
contaminants are removed. This stream continues on to the heat exchanger. When breakthrough, the point when the zeolites are near full capacity, is about to occur, the first bed is taken off-line, and the feed stream is directed to the second, fresh bed. While the second bed removes contaminants, the first bed is regenerated with hot purge gas at about 520 K. This purge gas is released to the atmosphere. When all contaminants are removed from the first bed, a cool down period begins to return the bed to operating temperatures. At this time, the feed stream is switched back to the first bed and the cycle begins again. F-200 Alumina and 13X will be used to remove water, carbon dioxides, and other contaminants. From an incoming feed rate of 18,670 lb/hr and assumed cycle length of 12 hours, 890 cubic feet of alumina is necessary to remove water within the feed air and 14 cubic foot of 13X is necessary to remove carbon dioxide and any remaining water. This will require a tower with working dimensions of about 11.4 feet high with a diameter of 5 feet for alumina and about 0.2 feet high with the same diameter for the 13X. It is assumed that all contaminants are removed by this TSA system and the feed stream that leaves the packed bed is composed of nitrogen, oxygen, and argon. Using the Ergun Equation, the pressure drop across the tower can be calculated. The pressure drop must be determined to ensure that the feed stream enters the distillation column at the correct pressure to produce the specified product. Using this equation, it was determined that the pressure drop across the packed tower is negligible.
The compressed dry air is cooled to cryogenic temperatures in a series of four consecutive heat exchangers which are combined as one main heat exchanger unit in the cold box. This main heat exchanger utilizes the product and waste stream thermal properties to cool the air to near its liquefaction temperature (around 110 K) before it enters the distillation column. The distillate product stream minus the 10% which is liquefied, and the bottoms product from the distillation column will be used to cool the air process stream.
The main piece of equipment in the process is the distillation tower which is the largest unit of equipment and allows for the separation of nitrogen to take place. The distillation column will have a feed of compressed cooled air at 10 atm and – 163 oC. The column contains 25 stages and the feed is introduced into the column above tray 17. The column will be several stories high. The column produces distillate product of nitrogen at 11
99.5% purity which comes out as a gas at -169 C. The bottoms waste product, which will be °
recycled for cooling contains less than 3% nitrogen and exits the column at a temperature of -153 C. This bottoms stream which comes out as a liquid will be used to condense part of °
the distillate stream to produce liquid nitrogen.
Energy can be recovered from the waste stream to offset the cost of running the compressors. Gas expansion turbines are often used for this purpose. A turbine is used to convert internal energy of a high pressure stream into shaft work. An expansion turbine is considered a reaction turbine that produces torque by “reacting” to the pressure of the incoming gas stream. The outlet pressure from the turbine will be 1 atm in order to recover the most energy. The incoming pressure to the turbine is about 5.7 atm at a temperature of 130 K. Knowing the incoming pressure, desired outlet pressure, and incoming temperature, the outlet temperature and recovered power can be calculated. Assuming a turbine efficiency of 95%, the outlet temperature is 81.6 K with about 18 kW of energy recovered. Although this does not seem like a substantial amount of energy recovered compared to the 140 kW requirement of the compressor, using a turbine will reduce energy needs as well as reduce the overall carbon footprint.
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When a pilot plant is scaled up, the designing engineers face many challenges. Scaling up from a pilot plant to a full production plant is not a linear transformation. Pilot plants are used to establish a basis for designing a full production plant and offer a “proof of concept.” Within pilot-scale equipment, certain aspects of a process can be examined and optimized to provide a better foundation for the full production plant. By using the previously discussed analysis of the pilot plant, a solid design for the full production plant can be made. Each piece of equipment is reanalyzed to ensure the product specifications are reached.
In the pilot plant, a rotary screw oil filled compressor was used to compress the air to about 10 bars. When scaling up, a larger flow of air is needed. There are two different ways to achieve a larger flow of air. Rotary screw compressors are limited to about 150,000 cfh. Seven compressors would need to be run in parallel to reach the required 900,000 cfh flow required for the process. The other option is to purchase a centrifugal air compressor which is capable of producing higher flow rates at the required pressure. A brand new 250 hp Chicago Pneumatic Rotary Screw Air Compressor, model number CPF-250, will cost about $98,500. Seven of these units will cost $689,500. One multistage centrifugal air compressor, manufactured by Ingersoll Rand, will cost $124,300. In addition to the cost difference between the two options, energy requirements, noise production, and footprint are substantially less for the centrifugal air compressor. For these reasons, the Ingersoll Rand model number C3000 air compressor will be purchased for the plant. Table 2 below lists some standard features and benefits to purchasing this air compressor.
Ingersoll Rand Centac C3000
Compact package on rigid baseplate
No special foundation required
Mounted intercoolers and aftercooler
Compact efficient design
Baseplate mounted control panel
Pre-wired and factory tested
Fewest electrical hook-ups
Minimal installation time and cost
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To scale up the TSA for use on a 600,000 SCFH plant, the increased air flow must be considered. The incoming air flow for the full size plant is 68,682 pounds per hour. This will require a tower with working dimensions of about 29 feet high with a diameter of 6 feet for the alumina and 0.5 feet high with the same diameter for the 13X. Again, the pressure drop is calculated to ensure the desired specifications are met. The pressure drop in the larger column is larger, as expected, but can still be considered neglible. The pressure drop is about 0.3 bar. The TSA can be constructed or purchased. Parker’s Finite Airtek Filtration Division sells a heat reactivated desiccant dryer, model number TWB9000, which can be loaded with the 13x and F-200 alumina. When purchasing a pre-fabricated unit for TSA, the working volume for each zeolite must remain the same regardless of design. At the time of this report, financial data and necessary design required was not available. From known specifications of the TWB9000, required flow and output requirements can be met. (Parker Hannifin Corporation, 2012) For a constructed TSA, the Guthrie method was used to determine the cost of construction. Both the cost of the vertical vessels and the cost of the adsorbents must be accounted for. Because the TSA is before the process streams are at cryogenic temperatures, carbon steel can be used for construction. The vessels will cost about $56,500. The adsorbents will cost about $200,000 and can be purchased from BASF. (BASF Catalysts LCC, 2007) The adsorbents will have to be replaced on a regular basis.
Although the incoming temperatures and pressures will remain about the same for the scaled up plant, the amount of air flow will increase, increasing the energy produced by the turbine. With preliminary calculations, the turbine will produce about 100 kW of energy. The selected compressor requires over 2,000 kW of energy. The cost of purchasing and installing a turbine to expand the waste stream is more expensive than the amount of energy produced.
Insulation must surround several pieces of process equipment in order to ensure they are able to perform at cryogenic conditions. The heat exchanger, distillation column, and piping must be enclosed in what is commonly known as a “cold box” to reduce heat gain. Cold boxes generally consist of a carbon steel casing and insulation material in an inert 14
atmosphere. The most common type of insulation is perlite. Perlite is a siliceous volcanic rock that expands when heated. Although the design of a cold box is extremely complex, the full design is not within the scope of this project. Cold boxes can be purchased with all of the necessary connectors, valves, monitoring equipment, as well as ladders and platforms. Two separate cold boxes will be purchased. One will surround the heat exchanger while the other will contain the distillation column. Two boxes will be used to reduce damage to the perlite insulation and equipment in the case of an emergency. The cold boxes can be purchased from Chart Industries. Chart can provide a BAHX (brazed aluminum plate-fin heat exchanger) with the cold box as one unit ideal for air separation. A specific volume of perlite will be necessary for the BAHX. Chart Industries can also manufacture a cold box for the distillation column. The cold box must also contain a nitrogen purge system that will provide a continuous nitrogen supply during operation to eliminate moisture and air from the interior. If air comes into contact with the process equipment, it will liquefy and create oxygen-rich pools that if warmed, can vaporize and destroy the perlite insulation. The nitrogen purge also prevents moisture from entering the area. Moisture within the cold box can also destroy the insulation. (Asia Industrial Gases Association, 2005) Industry standards call for the continuous purge to be set at a rate of one volume change of the cold box every 24 hours. This nitrogen purge can come from the product stream of the system. (Chart Energy and Chemicals, Inc. , 2009) The foundation for the coldboxes must be monolithic. This means that the foundation must be one continuous slab of concrete with no seams or joints. The area directly underneath the walls of the coldbox must also be reinforced.
Controls for the system will be purchased from ABB Inc. Their Symphony Plus™ system is a distributed control system that targets key focus areas of the plant including plant productivity, energy efficiency, operation security, plant safety, and cost of ownership. The system will include all require hardware, software, training, and installation. Sensors are needed to measure conditions and provide output information to the control system. The control system then uses this information to provide feedback to the input actuators that will make necessary changes to ensure that the process remains within control and producing the product at the required specifications. Figure 2 below depicts the basic piping and instrumentation drawing. 15
In this process, there are very few actuators that can be adjusted to correct process control. Although this is true, almost every stream within the process must be measured to ensure proper equipment operation. There are several critical areas within the process that must be under constant surveillance. These include the entrance to the heat exchanger, the cold boxes, and the product stream. The stream entering the heat exchanger must be free of water and contaminants in order to eliminate damage to sensitive cryogenic equipment. If water is allowed to enter the extremely low temperature areas, it could freeze and ruin the most expensive pieces of equipment. The same is true for the cold boxes. A constant nitrogen purge is fed to the cold boxes to eliminate moisture that could damage the insulation. Damage to the insulation would require a full plant shut-down that would be detrimental to plant profits. The product stream must be watched carefully as well to guarantee the correct product specifications are available for customers. Negative public relations caused by the delivery of inferior products to customers could easily shut down plant operations. 16
By installing a continuous control system, such as the Symphony Plus controls, the process can be observed and necessary changes be made in real time to safeguard the equipment and products. The controllers provided are a high performance and high capacity process controller. The controllers contain the control algorithms and user configurations to provide input to the actuators throughout the system. The controllers and most communication lines within the system are “hot stand -by” redundant. The purpose of this redundancy is to provide a constant back-up in the case of failure within the primary module. Another benefit of using the Symphony Plus system is the capability to interface with thirdparty devices. ABB systems have the ability to communicate with a variety of communication protocols and communication media. Several pieces of equipment, such as the air compressor, can be purchased with standalone controls. ABB’s system can connect to these built in systems to reduce the cost of hardware and provide central control of the process. This system also includes “S+ Operations,” a human machine interface that provides information integration and user navigation within a Windows environment. S+ Operations provides the users with detailed process overview displays, flexible reports, performance monitoring, and streamlined maintenance. Data from the system is stored in a “historian” to allow for archival of system collected data. This data can be accessed instantaneously to and can be used to analyze disturbances, compare trend data, and compile reports. S+ Operations also provides an emergency shut-off protocol that when instructed, will halt production. This emergency shut-off is in addition to programed shut-off procedures that would be activated under specified unsafe process conditions. (ABB, 2013) The complete turnkey control system, including control room furnishings, necessary training, and installation, can be purchased from ABB for about $476,400.
Piping, as simple as it sounds, requires additional attention when designing a cryogenic process. The materials used in this process are exposed to a wide range of temperatures and pressures. Common construction materials such as standard carbon steel and aluminum are not suitable for all areas of the plant. Common piping solutions for cryogenic processes include insulating with foam or vacuum. Figure 3 below shows a comparison of foam insulated piping, dynamic vacuum insulation, and static vacuum piping. (Weiler, 2004) Although there are several piping systems in existence, the most efficient systems utilize vacuums. There are two main types of vacuum systems. The first utilizes a 17
pipe within a pipe design and is known as the dynamic vacuum system. The inner pipe is constructed of copper and is insulated by a vacuum produced by a continuously running pump. Although the vacuum decreases heat loss, the cost of such system due to installation and maintenance requirements makes this option ineffective. This type of system would be more effective for small scale plants. The second option is a static vacuum jacketed system. Similar to the dynamic system, a pipe within a pipe is used to insulate the process streams with a vacuum. The difference is in how this vacuum is applied. In a static system, the piping is constructed in sections, with each section independently sealed. The piping is installed with bayonet style fittings that provide very low energy loss. Although the initial construction and installation costs are more expensive for a static vacuum system, the cost savings from drastically reduced energy usage and maintenance make this the best selection for cryogenic piping. Chart Industries can provide a full “Modular Vacuum Insulated Pipe System constructed of T304 Stainless steel inner pipe and outer jacket. The system utilizes vacuum insulated bayonets for quick and flexible installation without welding and cutting.
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Storage tanks are required on-site to house the liquefied nitrogen. As discussed earlier, this liquid nitrogen can be sold as production allows as well as vaporized into nitrogen gas during plant maintenance, unforeseen shut-downs, and increases in market demand. The storage tanks for the proposed facility will be purchased from Universal Industrial Gases, Inc. based out of Pennsylvania. The cryogenic tanks contain perlite insulation between an ASME U rated inner vessel and an SA-36 carbon steel outer vessel. The capacity of the selected tanks, model 80000, is 78,406 gallons. If 14 of these tanks are stored on-site, they will house enough liquid nitrogen to provide about 1 week of nitrogen gas at standard production rates to the plant. It will take approximately 70 days for these tanks to be filled at regular production rates. The fill rate of the storage tanks is also affected by the rate of transporting liquid nitrogen off-site. Transportation of liquid nitrogen requires specialized equipment. Universal Industrial Gases, Inc. can also provide ASME U rated transportation trucks. These trucks contain all necessary piping and pumping drives necessary to deliver liquid nitrogen to customers. The trucks contain 3 transverse baffles within the inner vessel, surrounded by composite insulation and a high strength carbon steel outer jacket. The evaporation rate of the vessels is less than 0.6%. UIG can provide trucks ranging from 6,000 gallons to 8,600 gallons, depending on transportation needs. One truck with a capacity of 8,600 gallons is capable of transporting half of the daily produced liquid nitrogen. If this liquid nitrogen is transported on a daily basis from the beginning of production, it will take a little over five months to completely fill the 14 storage tanks on-site under ideal conditions. If it is assumed that one week of planned maintenance will be completed every six months, this scheme of 14 storage tanks and one transportation truck will be sufficient. Additional research should be done to verify the needs of the current liquid nitrogen market within the Akron/Canton area. The design of the tank farm is very elaborate and must take many things including safety and hazard potential into consideration. This design is not within the scope of this project. Estimates can be made however to determine the area needed for the tank farm. According to industry standards, a three foot buffer zone is needed between the tanks. The tank model selected has dimensions of 125 feet long by 12 feet wide with a height of 12.4 feet. With the need of 14 tanks, the tank farm will be about 0.6 acres. To account for the 19
necessary area of the concrete pad as well as a containment system, this value will be estimated as 0.8 acre.
Climate change is a major challenge facing the global population and the natural environment. A carbon footprint measures the total greenhouse gas emissions caused by an organization. Causes of these emissions include electricity production in power plants, heating with fossil fuels, transportation operations and other industrial processes. A carbon footprint can be quantified using indicators like the Global Warming Potential (GWP). A GWP as defined by the Intergovernmental Panel on Climate Change (IPCC) , is an indicator that reflects the relative effect of a greenhouse gas in terms of climate change. The GWP’s for different emissions can be added together for an overall contribution to climate change from the emissions. Although the cryogenic air separation plant will release some carbon dioxide back into the atmosphere the main carbon footprint will be due to the energy consumption of the plant. All of the processes for air separation are very power intensive. Accounting for the energy which the ASU consumes and the carbon dioxide that will be released the ___ . This is below the nessasary amount to even require a permit which under ___ is needed for __ This noted a company should know their carbon footprint.
The footprint of the proposed 600,000 SCFH plant is about 5 acres. When determining a suitable location for the construction of the plant available utilities, correct zoning requirements, as well as vicinity to major transportation should be considered. A possible location was located on Kropf Ave in Canton, Ohio. This location is zoned I-1 and I-2 (industrial) and is 14.5 acres. The property has access to water and sewer and is within 2 miles of Interstate Highway 77. This particular location contains a 4,500 square foot metal building that can be converted into office space. The advertised price is currently at $699,000 but because this lot has been on the market for over 6 months, and considering the current economy, the reduction of this price is expected. The other benefit to purchasing this large plot of land is the ability to sell or lease unused acreage and also have the capability to expand.
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Estimating the cost of equipment and capital investment can play a crucial role in selecting the design of a process. Attempt was made to procure an estimate for each of the units in the process, land, materials, and utility requirements. Where a quote was not obtained the pricing was calculated through other means. The cost of the heat exchanger was estimated from the heat transfer area required, the purchase cost of the distillation column was estimated from the height and diameter of the tower using the method of Mulet, Corripio, and Evans which is a method based on the weight of the shell. The method includes an allowance for platforms, ladders, and a nominal number of nozzles. (Seider et al) The purchase cost of each of the units can be viewed in the chart below. Initial investment
Cost per year
Heat Exchanger
$594,641.22
Distillation column
$2,363,517.69
TSA
$2,389,739.89
Storage Tanks
Labor
$1,518,458.40
Power/Utility
$4,922,126.10
$359,471.36
Cold box
$2,769,137.59
turbine
$33,636.01
pumps
$700,000.00
filter
$73,803.94
Land
$699,000.00
Nitrogen Sales per year
$11,592,000.00
Taking the initial investment, the cost per year on labor and power and utilities, and the sale of nitrogen per year the following chart shows the net profit per year and return on investment. Year 0 1 2 3 4 5 6 7 8 9 10
Net profit per year -$9,982,947.69 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50 $5,151,415.50
Return on Investment -$9,982,947.69 -$4,831,532.20 $319,883.30 $5,471,298.79 $10,622,714.29 $15,774,129.79 $20,925,545.28 $26,076,960.78 $31,228,376.28 $36,379,791.77 $41,531,207.27
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The following two graphs show the net profit per year and the return on investment. As you can see from the second graph and the table above the ASU will take only two years to return on the initial investment. From this analysis the ASU proves to be a good investment with a short term return. It should be noted that this analysis is done for a ten years however replacing material such as the packing in the TSA and perlite was not taken into account which will lower the net profit in the 5-10 year period.
$10,000,000.00 $5,000,000.00 $0.00 0
1
2
3
4
5
6
7
8
9
10
-$5,000,000.00 -$10,000,000.00 -$15,000,000.00
$50,000,000.00 $40,000,000.00 $30,000,000.00 $20,000,000.00 $10,000,000.00 $0.00 -$10,000,000.00 0
2
4
6
8
10
12
-$20,000,000.00
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o
In conclusion, the proposed design of construction for the 600,000 SCFH nitrogen production plant will be profitable.
o
Additional profits can be realized with the sale of liquid nitrogen. The local market within the Akron/Canton area should be analyzed to correctly regulate the production and sale of the liquid nitrogen.
o
Other alternatives to the temperature swing adsorber should be researched further.
o
The production of additional pure components is possible with the addition of multiple columns. The available market for pure oxygen and argon, as well as the cost of additional equipment should be analyzed.
o
It is also possible to expand the waste stream as discussed in the paper to recover energy. Expansion of the product stream is also an option. An analysis of expanding multiple streams should be done to determine if energy recovery is profitable.
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