A Novel Approach To Effluent Treatment In Gas-To-Liquid (GTL)
Authors: Enyi Chimagwu Godpower , Nagib Mohamed , Nasr Ghasem G. 
Published: May 2016

Stringent environmental regulations and higher costs of effluent treatments in oil and gas process industries have necessitated research into ways to improve the operating procedures in effluent treatment plant.

In Gas-to-liquid (GTL) plant, a significant quantity of reaction water is produced and various chemicals are used as intermediate treatment chemicals. The reaction water is contaminated by these chemicals which impair the pH and the related properties of the water. The pH has to be controlled before the water is re-used or released to the environment.

A laboratory-scale effluent neutralization unit for pH control was designed and built to demonstrate the feasibility of utilising produced carbon dioxide (CO2) from reforming reactions in both the synthesis and hydrogen production units in GTL plant for insitu effluent treatment. At the end of the reaction, the total volume of carbon dioxide used was recorded.

This paper presents experimental neutralisation characteristics for different operating conditions. The prime advantage of this process can be thought to be less expensive than other published carbon capture and storage (CCS) processes. Moreover the carbon dioxide does not require further compression, dehydration and storage facilities before usage. Pipeline transportation is also drastically reduced since the captured carbon dioxide is utilised within the plant.

This study demonstrated that, the neutralisation time increased by 3.15 minutes with increase in effluent volume from 40 to 60 litres and by 10.4 minutes as the temperature increased from 20oC to 50oC. The increase in the flow rate of carbon dioxide from 15 litres/min to 35 litres/min decreased the neutralization time from 19.15 minutes to 13.32 minutes. Finally it was estimated that about 64% of the daily carbon dioxide production which would have otherwise been emitted to the atmosphere was used in the treatment process.

Introduction

The fact that the concentrations of carbon dioxide in the atmosphere are increasing is known to environmentalists, researchers and government agencies. The causes of global change lie in the industrial activities of human society and ultimately in the population growth and increase in resource use by man. Human activities have increased carbon dioxide concentrations from approximately 280 to 355ppm since 1800 (Vitousek, 1994). This increase is likely to have climatic consequences on biota in all earth's terrestrial ecosystems.

The need to reduce global climate change due to emission of carbon dioxide (CO2) and other greenhouse gases has led to research in carbon capture and sequestration (CCS). Several relatively small-scale carbon capture and sequestration approaches are currently in development and demonstration stages as highlighted by Hoekman, 2010. Direct injection into the oceans has also been suggested but there are a number of uncertainties over the ecological impact and equilibrium of the gas with the atmosphere.

In the medium term, depleted oil and gas reserves, unmineable coal seams, and deep saline formations are the best options for carbon dioxide storage. Deep saline formations appear to offer the potential to store several hundreds of years' worth of carbon dioxide emissions. This must be validated, and site selection criteria must be developed and shared internationally to identify the most appropriate storage sites. Wider international collaboration and consensus are critically needed to ensure the viability, availability and permanence of carbon dioxide storage.

However carbon capture and sequestration faces both technical and economic challenges. Therefore, there is the need to explore other methods to deal with carbon dioxide emissions. Transformation to chemical feedstock like methanol is a commercially proven technology, however carbon dioxide captured from flue gas from furnaces will not be economical for this purpose because of its low pressure. Recompression is required before the carbon dioxide could be processed to methanol (Ritter et al, 2007). Another well-known process - the Sabatier reaction, converts carbon dioxide to methane as shown in Eq. (1).