Basınç salınımlı adsorpsiyon ile hidrojenin saflaştırılması
The purification of hydrogen by pressure swing adsorption
Yazar: BENGİSU SEVİNÇ BERBER
Danışman: PROF. DR. MEHMET GÖKTUĞ AHUNBAY
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Günümüzde yaşanılan sera gazı problemleri dünyanın karşı karşıya kaldığı çevresel değişikliklerin başlıca nedenidir. Özellikle petrol, doğalgaz ve kömür gibi fosil yakıtların kullanılması küresel problemlerin ve sera gazı emisyonlarının artmasına neden olmaktadır. Ortaya çıkan emisyonlardan dolayı atmosfer sıcaklığı da giderek artmaktadır. Paris Anlaşması ve çevresel kaygılar ile beraber fosil yakıtlara alternatifler araştırılmaya başlanmıştır. Bulunan alternatiflerin teknik olarak uygulanabilir, ekonomik açıdan rekabetçi, çevresel olarak da tatmin edici olması gerekmektedir. Bu noktada hidrojen, yüksek özgül enerji içeriğine sahip olması ve evrende bol bulunması ile öne çıkmaktadır. Hidrojenin yanması ile birlikte elde edilen enerji miktarı fosil yakıtların kullanıldığı duruma göre daha fazladır ve azot oksitler, uçucu organik bileşikler, kükürt oksitler ve partikül emisyonları da görülmez. Gelişmiş ülkeler, çevre dostu olan hidrojenin sürdürebilir kalkınmaya önemli bir katkısı olacağı düşüncesiyle bu alanda çalışmalarına devam etmektedir. Artan hidrojen talebi de geleneksel yollarla hidrojen elde etme yöntemlerinde gelişmeler sağlamıştır. Hidrojenin kimya endüstrilerinde, rafinerilerde kullanım oranı oldukça fazladır. Hidrojen üretmek için petrol, doğalgaz , kömür gibi hidrokarbonların kullanımı yaygındır. Ancak bu prosesler ile üretilen hidrojenin yanı sıra karbondioksit, karbonmonoksit, metan, azot gibi gazlar da ortaya çıktığı için yüksek saflıkta hidrojen eldesi için gaz karışımların ayrılması ve saflaştırılması gerekmektedir. Basınç salınımlı adsorpsiyon prosesi gaz ayırma ve saflaştırma için kullanılan bir tekniktir. Basınç salınımlı adsorpsiyon ile %60-90 mol oranında hidrojen içeren bir gaz karışımından yüksek saflıkta hidrojen eldesi petrokimya ve yakıt pili uygulamalarında gittikçe önem kazanmaktadır. Basınç salınımlı adsorpsiyon prosesi ile çeşitli gaz karışımlarından %70-90 oranında geri kazanım oranı ile yüksek saflıkta hidrojen elde edilebilmektedir. Bu çalışmada bileşimi H2/CH4/CO (60/29/11 mol %) olan gaz karışımdan basınç salınımlı adsorpsiyon prosesi ile hidrojenin saflaştırılması amaçlanmıştır. Aspen Adsorption programı kullanılarak çalışma simüle edilmiştir. İlk olarak tek kolon sistemi modellenerek kullanılan parametrelerin doğruluğu incelenmiştir. Ardından simülasyonda kullanılan parametreler değiştirilerek sisteme etkileri incelenmiştir. Adsorban olarak aktif karbon kullanılan iki kolonlu basınç salınımlı adsorpsiyon sisteminin simülasyon çalışmasından elde edilen sonuçlar ( adsorpsiyon atılım eğrileri, hidrojen geri kazanımı oranı (%70), hidrojen saflık yüzdesi (%95), sistem basınç eğrileri) literatür verileri ile kıyaslanmış ve uyumlu olduğu görülmüştür.

Abstract:
Global warning which is increased due to greenhouse gas emissions caused by human activities is the main problem of the world. The use of fossil fuels which are oil, natural gas and coal increase greenhouse gases emissions and causes global problems. Atmospheric temperature is increasing gradually owing to emissions. Firstly, the importance of reducing CO2 emissions caused by the combustion of fossil fuels was emphasized with Paris Climate Conference United Nations agreement in 2015. Due to environmental concerns and Paris Agreement, the search for alternatives to fossil fuels has accelerated day by day. The alternative sources should be technically feasible, economically competitive. At this point, hydrogen become prominent with its high specific energy content and its abundance in the universe. The amount of energy produced by the combustion of hydrogen is higher than amount of energy when fossil fuels are used.
Developed countries continue to work in this area with the thought that environmentally friendly hydrogen will make an important contribution to sustainable development. The increasing demand for hydrogen has also led to improvements in the conventional methods of obtaining hydrogen. Hydrogen is the common element on the Earth and it is produced with variety of primary sources which are water, coal, natural gas, heavy oil etc. The use of hydrogen is quite high in chemical industries and refineries. Ammonia, methanol, ethanol, dimethyl ether synthesis, rocket fuel, fuel cell applications are current uses area of hydrogen. The steam methane reforming of natural gas, hydrocarbons reforming from refinery off gases, coal gasification, pyrolysis of biomass or water electrolysis are main methods for hydrogen production in the world. However, gases such as carbon dioxide, carbon monoxide, methane and nitrogen also emerge while hydrogen produced by these processes. Therefore, it is necessary to separate and purify the gas mixtures to obtain high purity hydrogen. Pressure swing adsorption, cryogenic distillation and amine based adsorption are common used technology for this purpose.
Adsorption is called as spontaneous phenomenon of attraction between the molecule from a fluid phase when it is close to surface of solid. Gas adsorption occurs when gas molecules interact and attached by solid material. The molecules and atoms attached to the surface is called as an adsorbed substance or adsorbate, and the surface where adsorption occurs is called as an adsorbent. The principle of adsorption is based on different molecules have different interactions with the adsorbent and it supply the separation of them. The unbalanced forces between molecules is balanced by their interactions with other molecules in the adsorption process. Adsorbent is contact with a bulk fluid phase and after a while the equilibrium state is achived in the system. The design and model of adsorption processes are changed according to different species. The time for achieving the equilibrium state is important while the size of molecules and pore are adsorbent are close to each other. The feed stream is interact with adsorbent which is packed in fixed beds. The light component of gas which is less adsorbed break through the coloumn faster than the other components of gas. Before heavy component break through the column, the feed should be stopped to accomplish separation. Also, the adsorbent have to regenerated by desorbing step. It can be provided by changing the process parameters. If the changing of pressure in the process provide regeneration is called as Pressure Swing Adsorption.
The pressure swing adsorption (PSA) process is a technique used for gas separation and purification. The PSA technology is becoming increasingly substantial for petrochemical and fuel cell application to obtaining high purity hydrogen from a gas mixture which is containing % 60-90 mol hydrogen. High purity hydrogen can be obtained from various gas mixtures with recovery rate of % 70-90 by pressure swing adsorption process. The PSA unit needs two main zones between which the total pressure of the system swings. Adsorption is facilitiated by high pressure zone and the regeneration is faciliated by low pressure zone in the PSA unit.
In general, PSA unit include at least two columns and follow the Skarstrom cycle. There are four steps in the Skarstrom cycle which are pressurization, adsorption, blowdown and purge respectively. The gas is fed to first column and adsorption takes place. After the first bed is adequately pressurized, the less adsorbed component break through the column. When the adsorbent packed in first column is saturated with adsorbate, the feed is directed to second column in the system. The saturated adsorbent is regenerated in the second column. In the blowdown step, the adsorbate is desorbed from adsorbent and released at the end of column. The purge step is used for remove the adsorbate from the column. After adsorbate is ready to adsorbe more components the overall pressure of the column should be reloaded. The flow direction is provided with valves opening and closing system. Both column are working as schronized by this way. While the several column are used in the process, several pressure equalization steps can be determined in the system.
In this thesis, the purification of hydrogen is aimed by pressure swing adsorption process. The process were studied with industrial exhaust gas which contains H2/CH4/CO (60/29/11 mol %). Aspen Adsorption software package was used to simulate PSA unit. Firstly, PSA unit is simulated as one column has one layer system and the activated carbon is used as adsorbent. The study was shown that carbon monoxide is the first impurity gas to break through from bed. The accuracy of the parameter which used in the system is examined with literature data and the break through curve was consistent with published paper which has the same system. Also, the pressure swing adsorption process was studied with layered activated carbon / zeolite bed and the break through time of mixture has been longer than the one layered activated carbon bed.
After that, PSA simulation was detailed using Aspen Adsorption software. The purification of hydrogen with two column system was also studied by using same parameters. The system includes two beds, seven tanks, four main valves, six intermediate valves. The cycle organizer was used for determining four steps and manipulated variables in the system. The specifications of valves were stated in manipulated variables section. These specifications explain that valves whether they are open, close or have constant flow rate in the steps. After defining four steps, the simulation results were examined. The break through curve was obtained as oscillating graph and the concentration of hydrogen was decreased in the product stream since the more component of gases couldn't be adsorbed with adsorbent. The number of cycles was increased and the behaviours of curve was continue as symmetrically. It has been observed that the pressure profile of four tank system was swing between high pressure in adsorption and low pressure in regeneration. Also, The CV of product stream valve was decreased and the response time was found as longer than the first case.
In order to examine the effect of adsorbent on the break through chracteristic, the zeolit 5A was used as adsorbent in PSA system. The operating conditions and column parameters were not changed in this case. According to these, it has been observed that the activated carbon provided more purified hydrogen.
Finally, the pressure swing adsorpiton for hydrogen purification from industrial exhaust gas which contains H2/CH4/CO (60/29/11 mol %) were simulated by Aspen Adsorption software for several cases. The hydrogen recovery rate was found as %70, and the purity of hydrogen found as %95 in the two column system.