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Mar-2007

Impact of acid gas loading on the heat of absorption and voc and btex solubility in amine sweetening units

In amine sweetening units, the heat of absorption and VOC and BTEX solubility have been found to vary significantly with acid gas loading, as well as with temperature, amine type and amine concentration.

Jerry A Bullin, John C Polasek and Carl W Fitz
Bryan Research & Engineering, Inc
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Article Summary
The heat of absorption declines by up to 20%, while VOC and BTEX solubility can drop by as much as 40 to 50% with loadings up to 0.5 mol/mol for MDEA solutions. VOC and BTEX solubility are also highly dependent on temperature and amine concentration. As a result, amine sweetening units should be operated at the lowest circulation rate possible as limited by corrosion and treating requirements. For example, over circulation of 100 gpm in amine sweetening units can cost about $250 000/yr in additional reboiler fuel, can greatly increase pick-up of VOC and BTEX, and lead to problems with emissions or in downstream sulphur recovery units.

Introduction
Even though amine sweetening of gases and liquids has been used for many decades, the process is still being refined and improved to be more efficient. The cost of fuel to operate sweetening units has increased. Environmental limitations on the allowable emission rates have become more stringent for volatile organic compounds (VOCs) consisting of methane through octane and other light organics and aromatic compounds consisting of benzene, toluene, ethylbenzene and xylenes (collectively referred to as BTEX). As a result, efficient design and operation of amine sweetening units has become more critical.

This paper considers the impact of acid gas loading on two properties that affect the efficient operation of amine sweetening units: the heats of absorption for H2S and CO2, and the solubilities of VOCs and BTEX in amine solutions. Since the heats of absorption and the VOC/BTEX solubility in amine solutions also vary with temperature, amine type and solution concentration, the impact of those parameters is also investigated. A case study is presented to show how the impact of loading, temperature, amine type and solution concentration can be used to improve and optimise the design and operation of amine sweetening units.

Essentially, all absorbed VOC/BTEX compounds exit the amine unit in the rich amine flash gas and stripper overhead acid gas streams. The VOCs and BTEX are a major source of concern for environmental considerations. The Clean Air Act limits the amount of VOCs to 250 tons per year and BTEX are limited to 25 tons per year and 10 tons per year for any individual aromatic compound. In addition, BTEX compounds are quite difficult to combust completely and tend to burn with a sooty flame, which causes a number of problems for sulphur recovery unit operations, such as fouling of catalyst beds.

Background
To facilitate the discussion and to clarify a few points, a brief review is helpful.

Heat of absorption
The process of absorbing CO2 and H2S into amine solutions involves two steps: initial 
physical absorption of the molecular species followed by dissociation of a portion of the molecular species into ionic components. At equilibrium, the degree of dissociation is governed by:
The equilibrium constant, Keq, is a function of temperature only, while the H+ ion concentration is a function of the amount and type of the amine and the temperature.

The heat of absorption for either H2S or CO2 may be expressed in simplified terms as follows:
The heat of physical absorption is relatively small compared to the heat of dissociation. At very low acid gas concentrations, the acid gas would be highly ionised and the concentration of the molecular acid gas would be very low. Thus, the heat of absorption would be highest at very low acid gas concentrations and would decrease as the amine loads up (ie, acid gas partial pressure increases) and the fraction of physically absorbed acid gas increases.

Solubility of VOCs and BTEX
The effect of acid gas loading in amine solutions on the solubility of VOCs and BTEX is less complicated as compared to the heat of absorption. Since VOCs and BTEX are organic compounds that are non-polar or low polarity and do not ionise, their solubility is based only on physical absorption. Water is a polar liquid and organic amines make amine solutions less polar, particularly if there is nothing acidic in the solution such as H2S and CO2, which cause the amines to extensively ionise. Thus, as acid gases are added to the amine solution, they not only contribute their ionisation, but cause the amine to ionise more completely. Since aromatics have a resonance structure based on the alternating single and double bonds, aromatics have greater attractive forces to polar solvents as compared to saturated hydrocarbons that do not have a resonance structure or double bonds. As a result, BTEX compounds tend to be a great deal more soluble in polar amine solutions than normal paraffins.

Fundamental data and process simulation
To properly quantify the above trends requires the collection of fundamental data on the various systems. GPA has long recognised this need for fundamental data and has been one of the most active organisations in the collection of fundamental data in the gas processing field. GPA has sponsored an extensive study, GPA Project 821 by Oscarson et al,1 to measure the heats of absorption for H2S and CO2 in amine solutions as a function of acid gas loading. This is the most complete study of this type that has been undertaken at the present time.

In Project 821, the enthalpy of absorption and partial pressures of CO2 and H2S in DEA, DGA and MDEA at various temperatures, pressures, concentrations and loadings were measured. The technique involved mixing varying amounts of acid gas with a fixed flow of clean amine solution, then using a flow calorimeter to measure the cooling required to exit at the same temperature and pressure as the inlet. A series of tables were compiled listing the “enthalpy of solution” calculated as the total energy required to maintain the inlet conditions divided by the total number of moles of acid gas flowing in the system.
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