Amine Gas Sweetening & Acid Gas Removal Unit Sizer

Alkanolamines possess at least one hydroxyl group (\(-\text{OH}\)), which reduces volatility and enhances water solubility, and at least one amino nitrogen group (\(-\text{NH}_2\), \(-\text{NHR}\), or \(-\text{NR}_2\)), which provides the basicity necessary to react reversibly with acid gases.

Carbamate Formation vs. Bicarbonate Hydration

The absorption mechanism differs fundamentally between primary/secondary amines and tertiary amines:

• Primary & Secondary Amines (MEA, DEA, DGA): Possess unshared electron pairs on the nitrogen atom alongside hydrogen atoms. \(\text{CO}_2\) reacts rapidly via an intermediate zwitterion to form stable amine carbamates:

  \[ \text{CO}_2 + 2\,\text{R}_1\text{R}_2\text{NH} \rightleftharpoons \text{R}_1\text{R}_2\text{NCOO}^- + \text{R}_1\text{R}_2\text{NH}_2^+ \]

  This reaction is extremely fast and exothermic (\(\Delta H_{rxn} \approx 1930 \text{ kJ/kg CO}_2\)), achieving high \(\text{CO}_2\) mass transfer rates but capping stoichiometric loading at 0.50 mol \(\text{CO}_2\) per mol amine.
• Tertiary Amines (MDEA): Possess no hydrogen atom bonded to nitrogen and cannot form carbamate directly. Instead, MDEA acts as a basic catalyst for the slow hydration of dissolved \(\text{CO}_2\) to bicarbonate:

  \[ \text{CO}_2 + \text{H}_2\text{O} + \text{R}_3\text{N} \rightleftharpoons \text{HCO}_3^- + \text{R}_3\text{NH}^+ \]

  Because bicarbonate hydration is kinetically slow compared to the instantaneous proton transfer of \(\text{H}_2\text{S}\):

  \[ \text{H}_2\text{S} + \text{R}_3\text{N} \rightleftharpoons \text{HS}^- + \text{R}_3\text{NH}^+ \]

  MDEA exhibits kinetic selectivity for \(\text{H}_2\text{S}\) over \(\text{CO}_2\), allowing \(\text{CO}_2\) slip while sweetening gas to < 4 ppmv \(\text{H}_2\text{S}\).
• Promoted aMDEA (Piperazine Activation): Adding 3 to 8 wt% piperazine (a cyclic diamine) to 45 wt% MDEA provides fast carbamate shuttling kinetics, enabling deep \(\text{CO}_2\) removal (< 50 ppmv for LNG) while preserving the low regeneration energy of MDEA.

Regeneration strips acid gases by reversing the exothermic chemical reactions at elevated temperature and reduced pressure (typically 1.8 to 2.2 bar(a), corresponding to a reboiler temperature of 120°C to 126°C).

Reboiler Thermal Duty Breakdown

The regeneration thermal duty represents 70% to 80% of total AGRU utility costs and comprises three distinct thermodynamic components:

1. Sensible Heat Duty (\(Q_{sens}\)): Energy required to heat the rich solution from the lean/rich exchanger exit temperature (\(\approx 95°\text{C} - 100°\text{C}\)) to the reboiler boiling temperature (122°C):

   \[ Q_{sens} = \dot{m}_{soln} \cdot C_{p,soln} \cdot (T_{reb} - T_{feed}) \]

2. Heat of Reaction / Desorption (\(Q_{rxn}\)): The endothermic energy required to break the amine-acid gas chemical bonds, matching the enthalpy of absorption:

   \[ Q_{rxn} = \dot{m}_{CO_2} \cdot \Delta H_{rxn,CO_2} + \dot{m}_{H_2S} \cdot \Delta H_{rxn,H_2S} \]

3. Stripping Steam Reflux Duty (\(Q_{steam}\)): Stripping steam generated in the reboiler sweeps through the regenerator column to lower the acid gas partial pressure in the vapor phase. GPSA design standards prescribe a reflux ratio of 1.0 to 1.5 moles of steam per mole of acid gas overhead:

   \[ Q_{steam} = \dot{n}_{AG} \cdot r_{reflux} \cdot MW_{H_2O} \cdot \Delta H_{vap,H_2O} \]

Low-pressure saturated steam (typically 3.5 barg / 50 psig) supplies this reboiler duty via thermosiphon or kettle reboiler bundles.

Claus Sulfur Production & Environmental Compliance

The concentrated acid gas overhead from the regenerator reflux drum (\(\text{H}_2\text{S}\) and \(\text{CO}_2\)) is routed to sulfur recovery. The stoichiometric sulfur production rate is:

\[ \dot{m}_{S} = \frac{\dot{n}_{H_2S} \cdot 32.065 \cdot 24}{1000} \qquad (\text{metric tonnes / day}) \]

• < 0.1 t/d S: Non-regenerable chemical scavengers (e.g., liquid triazine or solid zinc oxide beds).
• 0.1 to 15 t/d S: Liquid redox processes (e.g., LO-CAT, SulFerox) utilizing chelated iron catalysts for direct single-step conversion of \(\text{H}_2\text{S}\) to elemental sulfur.
• > 15 t/d S: Multi-stage Claus Sulfur Recovery Unit (SRU) (thermal reaction furnace at 1000°C followed by catalytic converter beds) combined with a Tail Gas Treating Unit (TGTU) to achieve > 99.8% sulfur recovery efficiency.