INTRODUCTION Research Problem Fischer Tropsch

INTRODUCTION

Research Problem

Fischer Tropsch (FT) process is a process that uses carbon monoxide and hydrogen to produce liquid fuels. During the process wastewater containing fatty acids, alcohols and hydrocarbons are produced with a reported chemical oxygen demand (COD) concentration of up to 30gCOD/l. Depending on the catalyst used and operating conditions employed, the amount of wastewater produced can be in the ratio of 1 tonne of fuel: 2 tonnes of wastewater. At this stage the wastewater needs to be treated to the acceptable legal level of 75mgCOD/l as per Department of Water and Sanitation before it is discharged into water bodies. The focus of this project is to degrade the pollutants (with the emphasis on propionic acid, hexanoic acid and acetone) to less harmful compounds while also reducing the COD level using bacteria and constructed wetlands as biodegradation agents.

Study Objectives

The objectives of the study are to:

Screen and Isolate FT contaminants degrading bacteria with the focus on volatile fatty acids (propionic acid, hexanoic acid) and acetone
Determine the degradation mechanisms of above mentioned contaminants.
Investigate factors affecting degradation of Fischer Tropsch contaminants.
To design wetland system for degradation study
To test the wetlands vegetation for contaminants uptake

Background

In his prognosis for the nation’s water situation in the future, Oberholster (2013) states that South Africa’s fresh water resources are expected to be fully depleted by the year 2030, because they are being drained by the needs of an ever-increasing population, and by industrialization. The situation has been further exacerbated by serious water pollution and environmental problems caused by human activities and factory and mining effluent.

The FT process, a method for the catalytic conversion of synthesis gas to fuels and chemicals, is one of the major contributors to water contamination. The process involves sending feed material to the gasifier, where the carbon contained in the feed undergoes a reaction with water vapour and oxygen to form carbon monoxide (CO) and hydrogen (H2) gas (syngas) (Van Zyl, 2008). When the syngas is fed to the FT reactor, it is polymerized over a catalyst such as iron and cobalt to form long chain hydrocarbons, methane and water. This can be expressed in terms of the equation:

nCO + 2nH2 ? CnH2n + nH2O + CH4 (1.1)

It should be noted that on a molar basis, the FT reaction produces more water than actual product (CnH2n).

In the next step of the process, the methane, the water produced in the FT reaction and the actual product are separated. The amount of wastewater produced can be in the ratio of 1 tonne of fuel: 2 tonnes of wastewater, depending on the catalyst used and the operating conditions (for example temperature and pressure) employed. This process, which is currently employed by SASOL (Secunda), produces up to 29 megalitres of wastewater per day (Van Zyl, 2008).
The wastewater generated in the FT process mainly comprises contaminants such as alcohols (84.9%), volatile fatty acids (VFA) (10.7%), hydrocarbons and ketones (4.5%) (Majone et al. 2010). Volatile fatty acids are present in a high concentration, resulting in a high chemical oxygen demand (COD) of 3002 mgCOD/l, whilst that of acetone is 107mgCOD/l, according to Majone et al. These reported COD values exceed the limit of