Design and Simulation of Cryogenic Columns in Air Separation Unit

Abstract

An Air Separation Unit separates atmospheric air into its primary components, typically Oxygen, Nitrogen and sometimes also Argon and other rare inert gases. The most common method for Air Separation is cryogenic distillation. Other methods such as membrane, pressure swing adsorption (PSA) and vacuum pressure swing adsorption (VPSA), are commercially used to separate a single component from ordinary air. Cryogenic air separation processes are used in medium to large scale plants to produce Oxygen, Nitrogen and Argon as gases and liquid products. They are preferred for technology in producing very high purity Oxygen and Nitrogen. It is the most cost effective technology for high production rate plants. The complexity of the cryogenic air separation process, the physical sizes of equipment, and the energy required to operate the process all vary with the number of gaseous and liquid products, required product purities, and required delivery pressures. The cryogenic separation process requires a very tight integration of heat exchangers and separation columns to obtain a good efficiency and all the energy for refrigeration is provided by the compression and expansion by expander of the air at the inlet of the unit. To achieve the low distillation temperatures an air separation unit requires a refrigeration cycle that operates by means of the Joule–Thomson effect, and the cold equipment has to be kept within an insulated enclosure (commonly called a "cold box"). The cooling of the gases requires a large amount of energy to make this refrigeration cycle work and is delivered by expansion turbines. The output of the expander helps drive the air compressor, for improved efficiency. The air is pre filtered, compressed and passed to the absorbers for removal of water vapor content and carbon dioxide. Process air is passed to the integrated heat exchanger which is brazed aluminum plate fin-heat exchangers. These have been installed between the HP-LP column to separate feed gas into its constituents. The products and waste streams are then re-warmed against the feed streams. From the midway argon with 2.8% purity is used for further argon processing. Thus, this one plant unit air separation giving rise to formation of product in gases and liquid form that is LIN,LOX,HP-GOX,MP-GOX,HP-GAN, LAR. This thesis has been carried out, keeping in mind to satisfy the consumer or industrial needs with the process technology. For this, the survey of on-running process train-3 concept is utilized. Based on the on-running process stoichiometry data was calculated. The train-3 process production is about 1*1700 TPD and it is operated under two cases.(1*1700 TPD means one train of air separation unit into the production of gases obtained from the process unit).Case-1 describes about the pumped LIN case. In this case the production of HP-GAN,LIN,HP-GOX,LOX were obtained.Case-2 describes about Liquid case. Over here production of only GOX and LOX is achievable. Purity of Nitrogen obtained by on-running process train is about 99.9999%,Oxygen-99.5% and Argon-99.9999%. Objective is to make production upto 1*3400 TPD along with the same purity of on-running process train.To satisfy the requirement that is purity in components along with increasing production capacity, data to accomplish this aim was obtained from on-running plant. It was observed that the on-running plant was first verified and simulated with the design parameters which were obtained from train-1 and train-2.From this, double production of products will be simulated, that is LOX,MPGOX, HP-GOX, LAR will be obtained by increasing in number of plates within column. This has lead to increase in height of column and also changes in feed stages for feed flow rates to the process system. As well as designing of plate fin heat exchanger was also carried out keeping in mind for the liquefication process to carry out vapor-liquid distillation. Stoichiometry data was incorporated for the air separation process for production of 1*3400 TPD with the present conditions that was obtained from on-running plant process. Based on that, designing of train-3 process was carried out using Aspen.HYSYS. The simulated results were obtained. In the results it was observed that the composition in purity for nitrogen in stream 26 as waste stream is 99.58% from low pressure column. Whereas the on-running process plant showed up 97% purity of nitrogen in low pressure column for the waste stream. Analyzing and comparison of results are made between the two design process of train-3.The flow rate of product LIN is about 4718Kg/hr,HP-GAN:51740 Kg/hr, LOX- 133700Kg/hr and HP-GOX: 25960Kg/hr,MP-GOX:103880Kg/hr have been found in the results as production that was simulated in HYSYS. Now the content of stream 26 which acts as waste nitrogen gas showed up purity >97%. Hence, this pure nitrogen gas can be used in coke oven process. This thesis has aimed towards productivity that will help as a carrier and purge gas in steel production where nitrogen is used to prevent oxidation and can be used in the heat treating process. Oxygen is a component of metal processing, and is used to replace or enrich air, ultimately increasing combustion efficiency in both ferrous and non-ferrous metal production. A key component of stainless steel refining, argon helps to prevent oxidation of molten steel as is used as a stirring agent.