NACGOND - Environmental Consequences of the use of Ethylene Glycol in the Processing of Natural Gas in Nigeria
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Environmental Consequences of the use of Ethylene Glycol in the Processing of Natural Gas in Nigeria
Published in Research
16
Mar

Environmental Consequences of the use of Ethylene Glycol in the Processing of Natural Gas in Nigeria Featured

Introduction:

Nigeria’s potential as a major global gas supply and utilisation hub is high. Available statistics show that even without a dedicated gas exploration regime, Nigeria has at present the 7th highest proven gas reserves in the world, with 183 Trillion Cubic Feet. This is gas that has been discovered in the process of exploring for petrol. We also have the potential of realising over 600 TCF, which would place us at 4th position worldwide. 

The FGN believes that in order to realise and sustain this potential, the structure of the gas sector must support continued cost effectiveness in supply of all markets (domestic, regional and export), scalability of capacity and above all, must be fully liberalised and market driven. Hence, the Nigerian Gas Master-plan (NGMP) has been developed and presented to investors since 2008. Among others, this Master plan proposes franchising three major Gas Processing Facilities (CPFs), all located in the Niger Delta region – Western Delta, around the Forcados area; Central Delta, north of Port Harcourt; and South East Delta, between Uyo and Calabar. Each of these Facilities is expected to process at least 1000 MMscf/d (standard cubic feet per day), but in fact 2,500 MMscf/d of raw gas. Largely overlooked, however, is the fact that significant amounts of toxic waste will be generated on a daily basis. 

The purpose of this presentation is to show that from conception to full design, this Master-plan so far fails to take into consideration the safe disposal of, and probable environmental hazards associated with, these toxic by-products and after-effects of the dehydration of gas using chemicals. From all presentations to investors in the industry it has become clear that the logic of economic benefits to be gained predominates, and environmental consequences are not touched on. Apparently there is the fear of scaring off investors when environmental aspects are brought into the equation. For instance, nowhere in it has consideration been given to human persons that might be affected and impacted by this budding gas industry. 

The toxic by-products of this process, as I shall demonstrate, are many and varied, and may bear down directly on the human, animal and aquatic populations in the Niger Delta. My plea is for the inclusion in the overall architecture of the Master plan, and its implementation, a full Life-Cycle Cost (LCC) that indicates not only the initial Capital Expenditure (CAPEX) for building the plants, but also the Operational Expenditure (OPEX) from conception to final removal of these facilities when their lifespan is exhausted. I shall suggest that it makes more economic sense to invest now in available clean technology than be saddled with the burden of disposing toxic wastes later on.

 

 

1. Process of Gas Dehydration 

When extracted from the ground, natural gas usually contains significant quantities of water and other water-based and organic compounds. This is called ‘wet gas”. During production, transportation and processing, changes in pressure and ambient temperature can lead to water condensation, ice and/or gas hydrate formation, or corrosion in the facilities. Dehydration, therefore, is one of the major processes in any gas processing plant, and Ethylene Glycol (EG) has been the chemical of choice for this industrial process since the 1960s. 

EG on its own is a common industrial chemical which has no extreme hazardous properties. Due to its hygroscopic (water-absorbing) properties, its natural affinity for hydroxyl (-O-H-), or water based functional groups, EG is an economical way of stripping the gas of its associated water. Water (H2O), in the chemical structure H=O-H, where the O-H is the chemically active part, will be chemically bonded to the EG. Based on this, the lean EG will absorb water in a counter-current contact with the wet gas and consequently dry it, thanks to its hygroscopy.

 

In a gas processing facility this is done in a so-called glycol contactor. The dry gas rises up and is conducted through pipes for use elsewhere, or is liquefied for easy transportation; the associated water, now heavy with EG is, on the other hand, routed from the bottom of the contactor and removed. The process could well have ended here, but in order to avoid a permanent supply of fresh EG, operators have developed a re-generation process for the ‘used’ or water-heavy EG. The process itself is simple: the used or heavy EG is collected and heated to its boiling point in this system, and in that way the water (plus other associated -OH compounds) is released from the EG (i.e., boiled off). 

This water, now in vapour phase, is vented into the atmosphere via a stack or chimney, and what remains is almost pure EG, which can once again be returned and reused in the process. The entire process has a good efficiency rate, because common practise shows the requirement of fresh EG is limited to 5-10%. So, apparently this 5-10% is lost somewhere in the process, and this happens during each and every sequence, continuously. However, 90-95% of EG is saved, and is re-circulated via the contactor to the MEG-Regeneration Unit all over again. (Note some of the acronyms used in the literature - MEG: mono-ethyl-glycol, DEG: di-ethyl-glycol, TEG: tri-ethyl-glycol, and TETRA EG: Tetra-ethyl-glycol).

 

The problem, however, is that in boiling one adds heat (energy) to the system containing the used EG, which has already bonded with water. But EG also bonds with other components that behave similarly with water: components that also have an active -OH functional group, e.g. phenol, alcohol, etc., and consequently react similarly. These include also organic molecules, small ones like alcohols, but also bigger ones like phenols, etc. 

In other words, any chemical with an end-standing functional –O-H group could possibly react with EG in this process, in a similar way as water does. As energy levels are high due to the heating, the resulting mix will contain many variations, subject to nature’s random occurrence. So a part of the loss, an estimated amount of 80% of it, will leave the regeneration system as a vapour, and a part will precipitate and settle in the system. The vapour part is referred to by process technologists as BTX, referring to Benzene-Toluene-Xylene. These BTX compounds are naturally very toxic, and leave the EG regeneration-system with the water-vapour vented into the atmosphere. The remaining 20% settles down in the MEG-regen-unit and forms a carcinogenic black tar residue, continuously. When the vented vapour (i.e. the 80%) ultimately cools off through ambient temperatures, the gaseous compounds may condensate and/or precipitate, and disappear into the groundwater, which may be in use for human consumption. 

BTX-compounds can go through the human skin, so breathing, bathing and washing will do the damage equally. One does not even need to drink the contaminated water to be affected. Breathing is not a pleasant option either, because the smell one perceives when approaching a MEG-regen-unit is very strong and penetrating, leaving one with the unmistakable impression that this cannot be healthy. The black tar, on the other hand, accumulates in the re-gen-system. At a moment in time, say bi-annually, this has to be removed lest it affect the efficiency of the system. The tar is extremely carcinogenic.

 

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