Technical comparison and selection
The process of natural gas liquefaction basically includes feed gas pretreatment (purification), liquefaction, refrigerant cycle compression, product storage, loading and auxiliary systems. The main process flow includes feed gas purification and purified gas liquefaction.
Selection of natural gas purification process
Natural gas as feed gas must be thoroughly purified before liquefaction. That is, remove the acid gas, water and impurities in the feed gas, such as H2S, CO2, H2O, Hg and aromatic hydrocarbons, so as to prevent them from freezing at low temperature and blocking and corroding equipment and pipelines.
A) Table 3.1-1 lists the pretreatment standards of feed gas in LNG plant and the maximum content of impurities.
| Impurity | Content limit | Basis |
|
H2O |
＜1ppmV |
A (it is allowed to exceed the dissolution limit without limiting the production) |
|
CO2 |
50～100ppmV |
B (Ultimate solubility) |
|
H2S |
＜4ppmV |
C (product technical requirements) |
|
Total sulfur content |
10～50mg/Nm3 |
C |
|
Hg |
＜0.01μg/Nm3 |
A |
|
Aromatic hydrocarbon |
≤10ppmV |
A or B |
|
Total naphthenic hydrocarbons |
≤10ppmV |
A or B |
From the data of feed gas, the content of carbon dioxide in feed gas exceeds the standard and must be purified.
MDEA amine liquid process is the most suitable process in terms of energy consumption, treatment scale and investment and operation cost. Therefore, MDEA amine liquid process is selected for deacidification gas in this scheme.
B) Dehydration process selection
The existence of water in natural gas often leads to serious consequences: under certain conditions, water and natural gas form hydrate to block the pipeline and affect the cooling liquefaction process; In addition, the existence of water will also cause unnecessary power consumption; Due to the low liquefaction temperature of natural gas and the existence of water, the equipment will be frozen and blocked, so it must be dehydrated.
Natural gas dehydration process generally includes three categories: low temperature dehydration, solid desiccant adsorption and solvent absorption. Freezing separation is mainly used to avoid hydrate when the temperature of natural gas is low. However, the low temperature it allows is limited and can not meet the requirements of natural gas liquefaction; Solvent absorption usually includes concentrated acid (usually organic acid such as concentrated phosphoric acid), glycol (commonly used triethylene glycol), etc., but these methods have low dehydration depth and can not be used in cryogenic units; The common dehydration methods of solid desiccant are silica gel method, molecular sieve method or the combination of the two methods.
Solid adsorption method must be adopted for natural gas liquefaction dehydration. Because molecular sieve has the advantages of strong adsorption selectivity, high adsorption characteristics under low water vapor partial pressure and further removal of residual acid gas, 4A molecular sieve is used as dehydration adsorbent in this scheme.
C) Selection of mercury removal process
At present, there are two main mercury removal processes: hgsiv molecular sieve adsorption method of UOP company in the United States and sulfur impregnated activated carbon to make mercury react with sulfur to generate mercury sulfide and adsorb it on activated carbon. The former has high cost and is suitable for occasions with high mercury content; The latter has low operation cost and is suitable for occasions with low mercury content.
On the one hand, the operating cost of hgsiv molecular sieve is very high; On the other hand, the mercury content in the feed gas of the unit is relatively low. Therefore, the company has successful experience in using sulfur impregnated activated carbon for mercury removal.
Contact:
Sichuan Rongteng Automation Equipment Co., Ltd.
Phone/WhatsApp/Wechat : +86 177 8117 4421 +86 138 8076 0589
Website: www.rtgastreat.com Email: info@rtgastreat.com
Address:No. 8, Section 2 of Tengfei Road, Shigao Subdistrict,Tianfu New Area, Meishan city, Sichuan China 62056
Post time: Sep-11-2022
