| Category: เครื่องผลิตก๊าซไนโตรเจน > การแยกก๊าซด้วย Pressure Swing Adsorption (PSA > |
N2 Gas separation by Pressure Swing Adsorption (PSA) with Shirasagi MSC-3K-172
N2 Gas separation by Pressure Swing Adsorption (PSA) with Shirasagi MSC-3K-172
The PSA makes use of a unique adsorption performance of Molecular Sieving Carbon (MSC). The MSC is a functional activated carbon suitable for gas separation. The theory and characteristics of PSA gas separation are explained below.
A swing adsorption and desorption method is used for separation of one or more target components from mixed gases. A cycle of adsorption and desorption is practiced by fluctuating parameters relating to adsorption. The parameters which dominate adsorption performance of activated carbon are temperature, total pressure, partial pressure of composing gases, adsorption velocity, and so forth. For instance, the adsorption and desorption by swinging temperature is called “Thermal Swing Adsorption (TSA).” The pressure is swung in “Pressure Swing Adsorption (PSA)” (See Fig.-1). Generally, the TSA has a longer cycle time. It is because it takes time in heating and cooling to exchange thermal energy, which is required to perform adsorption and desorption. In PSA, on the other hand, a cycle time can be made shorter. Since the condition of adsorption and desorption is created by pressurizing and depressurizing, the PSA does not require the exchange of thermal energy.
2. Characteristic of Pressure Swing Adsorption (PSA)
The basic flow of regenerative PSA by ordinary pressure is shown in Fig.-2. Pressurized raw gas is introduced into vessel A filled with MSC. After some components are easily adsorbed in MSC under pressure (VT1(P1) in Fig.-1) in vessel A, other components that are not adsorbed go out from vessel A as a product gas. During the period, vessel B is maintained at ordinary pressure to decrease the equilibrium adsorption to VT1(P2) in Fig.-1. Adsorbates corresponding to the difference between VT1(P1) and VT1(P2) are desorbed from MSC to be thrown out from vessel B as an exhaust gas. In the cycle, the concentration of target gases is monitored at the outlet of vessel A. When it is confirmed that adsorption of vessel A has reached saturation, vessel B starts adsorption process and vessel A transfers to desorption process. Product gases can be obtained continuously by repeating a set of adsorption and desorption.
PSA has more advantages than the other gas separation methods. For example, the operation of adsorption and desorption in PSA can be made faster than that in TSA. Especially compared to TSA and cryogenic separation, PSA can be operated at ordinary temperature, which results in less energy costs. Also, the process of PSA is simple enough to downsize the units.
3. PSA using Equilibrium Separation
Each adsorbate has its equilibrium adsorption for a certain adsorbent. Equilibrium separation PSA is a gas separation principle that makes use of this characteristic. The hydrogen recovery technology using equilibrium separation MSC is well known as a typical example of this type of separation. As seen in an adsorption isotherm of MSC in Fig.-3, equilibrium adsorption of hydrogen is much smaller than that of methane and carbon dioxide. Moreover, it can be considered that MSC does not adsorb hydrogen. Therefore, when raw gas mixtures rich in hydrogen such as coke oven off-gas and methanol decomposition gas are passed through the unit filled with MSC (shown in Fig-2), concentrated hydrogen is obtained as a product gas. Nowadays, hydrogen gas of more than 99.999% purity can be generated by this type of hydrogen PSA.
4. Pressure Swing Adsorption (PSA) using velocity separation
An adsorbent with a certain average pore diameter has unique adsorption velocity for each gas. Velocity separation PSA is a gas separation principle that makes use of this characteristic. It is most general that this type of PSA uses MSC as an adsorbent. The most famous example is PSA nitrogen generators. As the basic layout of velocity separation PSA is shown in Fig.-2, except small PSA equipment, a standard unit has two or more vessels. The system is operated by switching adsorption and desorption alternately or shifting a phase of each vessel. One of the most famous technologies in nitrogen generation by air separation is cryogenic separation, produces nitrogen gas by utilizing the difference of boiling points between nitrogen and oxygen. On the other hand, velocity separation PSA makes use of the slight difference in their molecular sizes as shown in Fig.-4. This mechanism is achieved by MSC precisely fabricated so as to reflect the difference in adsorption velocity. Fig.-5 shows adsorption velocity curves of a certain MSC for several gases. In case of the separation between oxygen and nitrogen, while oxygen reaches equilibrium adsorption in about five minutes after starting adsorption, the amount of nitrogen adsorbed reaches below 10% of equilibrium adsorption for the same period of time. Until then, only nitrogen gas comes out from the top of the vessel. After changing the cycle from adsorption to desorption, oxygen-rich air comes out from the bottom of the vessel. In this manner, the characteristic of velocity separation PSA is that the difference of adsorption velocity is applied for gas separation.
