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How to calculate the amount of molecular sieve in the hydrogen production system of water electrolysis
High-purity hydrogen has applications in many fields, such as fuel cell vehicles, electronics industry, petrochemical industry, metal smelting, and scientific research. In the water electrolysis system, the method to improve the purity of hydrogen is first to catalytic oxygen removal, that is, to react the oxygen in the hydrogen with hydrogen under the action of a catalyst to achieve the effect of oxygen removal; Then there is adsorption purification, that is, the adsorption and separation principle of molecular sieves is used to remove other impurities (mainly saturated water vapor) in hydrogen.
Today, we will focus on the adsorption and purification in the hydrogen production system of water electrolysis.
1. TSA/PSA?
At present, there are two technologies in the hydrogen purification method, namely Temperature Swing Adsorption (TSA) and Pressure Swing Adsorption (PSA).
In the water electrolysis hydrogen production system, variable temperature adsorption (TSA), that is, a three-tower purification system, the system is equipped with three dryers and their corresponding coolers, the switching cycle is 24 hours, the operation cycle is 72 hours, and each dryer has three states, namely working state, regeneration state, and cooling state.
2. Calculation of adsorbent and dosage of variable temperature adsorption
1. Types of adsorbents
There are three commonly used adsorbents: silica gel, activated alumina and molecular sieve, as follows:
(1) Silicone
Silica gel is an artificial hydrous silica formed by the condensation of a silicic acid solution, and its chemical formula is SiO2·XH2O, the basic structure is a Si-O tetrahedron, which is stacked on top of each other to form a skeleton of silica gel. The structural water in the silica gel is connected to the silicon atom in the form of hydroxyl group (OH) and covers the surface of the silica gel, and the silica gel is retained by the bonding of water molecules and (OH), and the water is released after the temperature rises.
(2) Activated alumina
Alumina is made from the thermal decomposition of various types of aluminum hydroxide. It has the chemical formula Al2O3·nH2O, which is formed by the accumulation of a series of homogeneous heteromers, and the surface of the adsorbent presents the effect of surface acid adsorption of water.
(3) Molecular sieve
The general formula of the chemical composition of molecular sieve is: [M2(I),M(II)]O· Al2O3·nSiO2·mH2O, where M(I) AND M(II) are monovalent and bivalent metals, usually sodium, potassium, calcium, strontium, barium, etc.; n is the silicon-aluminum ratio is general, n=2~10; m=0~9。 Molecular sieve is a type of silicon (aluminum) oxygen tetrahedron (Si4O、Al4O) is composed of the crystals of the cage-shaped pore skeleton, which is very rich in space after dehydration, and has a large internal surface area, which can accommodate a considerable number of adsorbate molecules. At the same time, there is a large electrostatic field and polarity inside the crystal, and there is also a high surface acidity, and the adsorption ability is enhanced.
The molecular sieve in the water electrolysis hydrogen production system generally uses 13X molecular sieve.
2. Calculation of the amount of adsorbent
Before learning the amount of adsorbent, the adsorption capacity and adsorption cycle of the adsorbent should be determined. The adsorption capacity is generally provided by the adsorbent manufacturer, and can also be obtained according to empirical data; The adsorption cycle is generally 24 hours.
After the design adsorption capacity and working cycle are selected, the amount of adsorbent is calculated according to the amount of gas treated, the moisture content of the raw gas and the dryness of the required gas, and the formula is as follows:
(kg)
G1, G0 - the moisture content of raw gas and product gas, generally G. Can be omitted (kg/m3)
V—the flow rate of the gas to be processed (m3/h);
T - duty cycle (hours);
qD – Design adsorption capacity (kg/100 kg or %)
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Doris Zhou
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