Removal of carbon dioxide from high pressure natural gas by adsorption on activated carbons

Al Hanashi, Khalil (2019) Removal of carbon dioxide from high pressure natural gas by adsorption on activated carbons. PhD thesis, University of Nottingham.

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The presence of carbon dioxide (CO2) in raw natural gas is a major challenge for the natural gas industry because it decreases the heating value of the natural gas and promotes corrosion in the piping materials used for transportation. Today, the natural gas industry is dominated by the energy-intensive amine chemical absorption process to remove the CO2, however, pressure swing adsorption process has the potential to provide a sustainable and low energy penalty alternative. This research investigated the feasibility of using activated carbons as solid adsorbents for pressure swing adsorption process to remove CO2 from high pressure and high CO2 natural gas.

Two sets of 14 samples of activated carbons were synthesised from two different precursors: date pits biomass and coal tar pitch. A two-step chemical activation process with potassium hydroxide (KOH) was used to convert the date pits biomass into activated carbons, and a one-step KOH chemical activation process was used to produce activated carbons from the coal tar pitch. Nitrogen adsorption based characterisation revealed that both series of activated carbons were dominated by micropores when prepared with various KOH to carbon ratios (ranging from 1 to 4) and various activation temperatures (600 to 800 oC). However, some mesoporosity was seen at an activation temperature of 900 oC.

All the samples were performance tested for CO2 and methane (CH4) working adsorption capacities by gravimetric measurement. These tests revealed higher CO2 working capacities compared to that of CH4, confirming that the activated carbons have a potential use as solid adsorbents for CO2/CH4 separation. The performance test conditions were based on the process conditions of a South Oman gas field (100 bar pressure, 50 oC temperature and 20% CO2 content). The CO2 uptake and the CH4 uptake by the activated carbons were found to positively correlate with the BET surface area and micropore volume of the activated carbons. However, larger micropores in the activated carbons (>1.6 nm) were found to be less favourable for CO2 adsorption.

The effect of water on CO2 uptake and CH4 uptake of the activated carbons were investigated from two aspects: as a contaminant and as CO2 selectivity enhancement agent. A small amount of water (10% in the vapour phase) was used to investigate water as a contaminant and showed a negative effect on both CO2 and CH4 adsorption capacities. A large amount of pre-adsorbed water (50%) increased the CO2/CH4 working capacity ratios at high pressure indicating CO2/CH4 selectivity enhancement but reduced CO2 uptake capacity.

CO2 and CH4 adsorption isotherms were successfully measured using a high pressure volumetric analyser. The measured isotherms were best fitted with the Dual Site Langmuir (DSL) model compared to the Langmuir or Sips models. The extended DSL model was used to predict the binary mixed gas (CO2 and CH4) adsorption after gravimetric measurements of total adsorption were employed to select the right pair of the extended DSL equation to overcome the issue of site matching. The binary gas adsorption calculations showed favourable adsorption for CO2 over CH4 as CO2 partial pressure in the mixture increased. However, the co-adsorption of CH4 in the activated carbon also increased as CH4 partial pressure increased which would decrease the purity of the removed CO2 in a Pressure Swing Adsorption (PSA) process.

In conclusion, the activated carbons synthesised from low cost precursors by simple chemical activation method showed good potential for separating CO2 from CH4. This potential was demonstrated by CO2/CH4 working capacity ratios higher than one for most samples, and confirmed by DSL adsorption model of mixed gas (CO2 and CH4) adsorption on activated carbons. Future work could include improving the activated carbon mechanical strength, reducing CH4 adsorption in addition to designing a PSA process to demonstrate the expected energy saving.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Liu, Hao
Snape, Colin
Sun, Chenggong
Stevens, Lee A.
Keywords: Adsorption, CO2, Natural gas, Activated Carbon
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 56990
Depositing User: Al Hanashi, Khalil
Date Deposited: 11 Sep 2019 08:16
Last Modified: 06 May 2020 15:16

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