上篇我們介紹了沼氣脫硫工藝，本篇介紹沼氣提純工藝。沼氣經脫硫后再脫出二氧化碳、脫水除塵后便制作成了生物天然氣。本篇沼氣提純工藝指的是沼氣脫出二氧化碳的方法，在沼氣行業脫除二氧化碳工藝主要有變壓吸附（PSA）工藝、膜分離工藝、加壓水洗工藝、醇胺吸收工藝。

　　In the previous article, we introduced the biogas desulfurization process, and in this article, we introduce the biogas purification process. After desulfurization, biogas is further dehydrated and dedusted to produce bio natural gas. The biogas purification process in this article refers to the method of removing carbon dioxide from biogas. In the biogas industry, the main processes for removing carbon dioxide include pressure swing adsorption (PSA), membrane separation, pressurized water washing, and alcohol amine absorption.

　　1、不同沼氣凈化工藝介紹（1）變壓吸附工藝?VPSA變壓吸附分離技術是基于吸附劑的兩種特性產生的：一是吸附劑對吸附質有選擇性；二是吸附劑對吸附質的吸附量隨吸附質的分壓的變化而變化，也就是利用氣體組分在吸附劑上的平衡吸附量、吸附速度、吸附力等特性的差異以及吸附量隨壓力的變化而變化的特性，在（相對）高的壓力下進行吸附（吸附原料沼氣中的CO2、殘留的H2S、H2O、VOC以及少量的N2和O2等雜質組分），在（相對）低的壓力下解吸（吸附劑得到再生并將雜質排出），通過周期性的壓力變換實現吸附和解吸過程的交替進行，從而實現混合物分離的化工循環操作過程，同時吸附劑得到循環使用。吸附塔由多個組成，每個吸附塔在一次循環中均需經歷吸附、均壓降、抽真空、均壓升等多個步驟。多個吸附塔在執行程序的安排上相互錯開，構成一個閉路循環，以保證原料沼氣連續輸入和產品氣連續輸出。整個變壓吸附過程由全自動過程控制管理系統來實現，整個操作過程均在常溫環境下進行。

　　1. Introduction to different biogas purification processes (1) Pressure swing adsorption process VPSA pressure swing adsorption separation technology is based on two characteristics of adsorbents: one is that adsorbents have selectivity for adsorbates; The second is that the adsorption capacity of the adsorbent on the adsorbate changes with the partial pressure of the adsorbate, which utilizes the differences in the equilibrium adsorption capacity, adsorption rate, adsorption force and other characteristics of the gas component on the adsorbent, as well as the characteristic of the adsorption capacity changing with pressure. Adsorption is carried out at (relatively) high pressure (adsorbing CO2, residual H2S, H2O, VOC, and a small amount of N2 and O2 impurities in the raw material biogas), and desorption is carried out at (relatively) low pressure (the adsorbent is regenerated and impurities are discharged). Through periodic pressure changes, the adsorption and desorption processes alternate, thereby achieving the chemical cycle operation of mixture separation, and the adsorbent is recycled. The adsorption tower consists of multiple components, and each adsorption tower needs to go through multiple steps such as adsorption, pressure equalization, vacuum pumping, and pressure equalization in one cycle. Multiple adsorption towers are staggered in the arrangement of program execution, forming a closed-loop loop to ensure continuous input of raw biogas and continuous output of product gas. The entire pressure swing adsorption process is implemented by a fully automated process control management system, and the entire operation process is carried out at room temperature. ?

　　（2）膜分離工藝?膜分離技術主要是利用沼氣中氣體與膜材料之間的物理或者化學反應來進行選擇性吸收與分離的技術。其原理主要是使得氣體能快速溶解于吸收液并通過分離膜或吸收膜快速傳遞，從而達到吸收氣體在膜的一側濃度降低，而在另一側富集的目的。膜分離法依靠待分離混合氣體與膜材料之間的化學、物理反應，使一種組分快速溶解并穿過膜材，通過膜材后混合氣體分成滲透氣和滲余氣兩部分，其推動力是膜兩邊的壓差。膜的分離能力取決于膜材料對甲烷和二氧化碳的選擇性和透過性差異。膜材料是膜分離技術的核心。膜材料的滲透系數增加會導致選擇性的下降，反之亦然。因而優質的膜材料應具有較大的氣體滲透系數和較高的甲烷/二氧化碳選擇性，即有較高的分離性能，還要有良好的化學穩定性、物理穩定性、耐微生物侵蝕和耐氧化等性能。根據制備膜的材料的不同，分離膜主要分為三大類：無機膜、有機聚合物膜和混合基質膜。由于具有良好的強度和化學穩定性，對甲烷/二氧化碳有很高的分離性能，有機聚合物膜中的聚酰亞胺（PI）膜是目前在沼氣分離膜中應用最為廣泛的商品膜。

　　(2) Membrane separation technology mainly utilizes the physical or chemical reactions between gases in biogas and membrane materials for selective absorption and separation. The principle is mainly to enable the gas to quickly dissolve in the absorption liquid and be rapidly transferred through the separation membrane or absorption membrane, thereby achieving the purpose of reducing the concentration of the absorbed gas on one side of the membrane and enriching it on the other side. The membrane separation method relies on the chemical and physical reactions between the mixed gas to be separated and the membrane material, allowing a component to quickly dissolve and pass through the membrane material. After passing through the membrane material, the mixed gas is divided into two parts: permeable gas and residual gas, driven by the pressure difference on both sides of the membrane. The separation ability of the membrane depends on the selectivity and permeability differences of the membrane material towards methane and carbon dioxide. Membrane materials are the core of membrane separation technology. An increase in the permeability coefficient of membrane materials will lead to a decrease in selectivity, and vice versa. Therefore, high-quality membrane materials should have a high gas permeability coefficient and high selectivity for methane/carbon dioxide, which means they should have high separation performance, as well as good chemical stability, physical stability, resistance to microbial erosion, and oxidation resistance. According to the different materials used for membrane preparation, separation membranes are mainly divided into three categories: inorganic membranes, organic polymer membranes, and mixed matrix membranes. Due to its excellent strength and chemical stability, it has high separation performance for methane/carbon dioxide. Polyimide (PI) membrane in organic polymer membranes is currently the most widely used commercial membrane in biogas separation. ?

　　（3）加壓水洗工藝?加壓水洗工藝是利用根據亨利定律，氣體組分分壓與液相中氣體組分濃度的關系：不同氣體在相同分壓下，氣體組分的亨利系數越小，其溶解度就越大。在0-30℃條件下，甲烷在水中的亨利系數是二氧化碳的25-30倍，即二氧化碳在水中的溶解度遠高于甲烷的溶解度。加壓水洗工藝利用低溫水作為吸收劑對沼氣中的甲烷和二氧化碳進行分離。加壓越高、溫度越低約有利于氣體吸收和分離。?加壓水洗提純沼氣是在加壓條件下，將CO2溶解于水中而減小其在沼氣中的含量。工藝過程：將沼氣加壓后送入洗滌塔，吸收劑（水）從頂部進入，在洗滌塔內沼氣自下而上與水流逆向接觸，沼氣中的大部分CO2和少量CH4被水溶解，富CH4氣從洗滌塔的上端被引出，進一步干燥后得到生物天然氣；從洗滌塔底部排出的水進入閃蒸塔將溶解在水中的CH4和大部分CO2從水中釋放出來，這部分混合氣體重新與原料氣混合再次參與洗滌分離。從閃蒸塔排出的吸收劑進入解吸塔，利用空氣或惰性氣體進行吹脫再生，之后再作為吸收劑返回洗滌塔。

　　(3) The pressurized water washing process utilizes the relationship between the partial pressure of gas components and the concentration of gas components in the liquid phase according to Henry's law: the smaller the Henry's coefficient of gas components for different gases at the same partial pressure, the greater their solubility. At 0-30 ℃, the Henry's coefficient of methane in water is 25-30 times that of carbon dioxide, indicating that the solubility of carbon dioxide in water is much higher than that of methane. The pressurized water washing process uses low-temperature water as an absorbent to separate methane and carbon dioxide from biogas. The higher the pressure and the lower the temperature, the more favorable it is for gas absorption and separation. Pressure water washing purification of biogas is the process of dissolving CO2 in water under pressure conditions to reduce its content in biogas. Process: Pressurize the biogas and send it into the washing tower. The absorbent (water) enters from the top, and the biogas in the washing tower comes into contact with the water flow from bottom to top. Most of the CO2 and a small amount of CH4 in the biogas are dissolved by the water, and the CH4 rich gas is drawn out from the upper end of the washing tower and further dried to obtain bio natural gas; The water discharged from the bottom of the washing tower enters the flash evaporation tower to release CH4 and most of the CO2 dissolved in the water, and this mixed gas is re mixed with the feed gas to participate in washing and separation again. The absorbent discharged from the flash evaporation tower enters the desorption tower, and is blown and regenerated using air or inert gas before being returned to the washing tower as an absorbent.

　　（4）醇胺吸收法?化學吸收法，指的是采用液相溶液，通過化學反應選擇性地自氣相中脫除易溶于吸收液成分的方法。化學吸收法脫除CO2，實質是利用堿性吸收劑溶液與沼氣中的CO2接觸并發生化學反應，形成不穩定的鹽類，而鹽類在一定的條件下會逆向分解釋放出CO2而再生，從而達到CO2從沼氣中分離脫除。

　　(4) The alcohol amine absorption method and chemical absorption method refer to the method of selectively removing easily soluble components from the gas phase through chemical reactions using a liquid-phase solution. The chemical absorption method for removing CO2 essentially utilizes an alkaline absorbent solution to come into contact with CO2 in biogas and undergo a chemical reaction, forming unstable salts. Under certain conditions, the salts will reverse decompose and release CO2 for regeneration, thereby achieving the separation and removal of CO2 from biogas. ?

　　經過脫硫等預處理后的沼氣，經初步冷卻和增壓后，從吸收塔下部進入，在塔內與由塔頂噴射的吸收劑溶液逆相接觸。沼氣中的CO2與吸收劑發生化學反應而形成弱聯結化合物，脫除了CO2的甲烷從吸收塔上部被排出吸收塔。而吸收了CO2的吸收劑（富CO2吸收液簡稱富液）經富液泵抽離吸收塔，在貧富液熱交換器中與貧液CO2吸收液（簡稱貧液）進行熱交換后，被送入再生塔中解吸再生。富液中結合的CO2經加熱被釋放。再生塔底的貧液在貧液泵作用下，經過貧富液換熱器換熱、貧液冷卻器冷卻到所需的溫度，從吸收塔頂噴入，進行下一次的吸收。

　　After pre-treatment such as desulfurization, the biogas enters from the lower part of the absorption tower after initial cooling and pressurization, and comes into reverse contact with the absorbent solution sprayed from the top of the tower inside the tower. The CO2 in biogas reacts chemically with the absorbent to form weakly bound compounds, and the methane removed from CO2 is discharged from the upper part of the absorption tower. The absorbent that absorbs CO2 (rich CO2 absorption liquid, abbreviated as rich liquid) is pumped out of the absorption tower by the rich liquid pump, and after heat exchange with the lean CO2 absorption liquid (abbreviated as lean liquid) in the lean rich liquid heat exchanger, it is sent to the regeneration tower for desorption and regeneration. The CO2 bound in the rich solution is released by heating. The lean solution at the bottom of the regeneration tower is pumped by the lean solution pump and then cooled to the required temperature through the lean solution heat exchanger and lean solution cooler. It is then sprayed from the top of the absorption tower for the next absorption.

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