Adsorptive Desulfurization
Various desulfurization technologies have been explored either to complement the current hydrodesulphurization (HDS) technology. The main alternative techniques include oxidative desulfurization, extractive desulfurization, biodesulfurization and adsorptive desulfurization (ADS). There are various forms of adsorptive desulfurization which will be discussed in this section and these include reactive adsorption, polar adsorption, selective adsorption, integrated adsorption, π-complexation. The schematic showing different forms of desulfurization is given in Figure 1. ADS is one of the most studied alternative technologies to replace or supplement conventional HDS. In ADS, sulfur compounds are removed via adsorption using a selective adsorbent. This means that the adsorbent plays an important role in the selectivity, capacity, and sustainability such as renewability of adsorbents, of the process.
Sulfur can be removed via two mechanisms, depending on the interaction between the adsorbent and the sulfur compounds:
- 1.
Physical adsorption in which sulfur compounds and adsorbents are mainly bound by van der Waals forces, which is a relatively weak interaction. This enables the adsorbent to be regenerated relatively easily.
- 2.
Chemical adsorption (such as reactive adsorption), which employs chemical bonding between the sulfur compounds and the adsorbents, which may alter the physical and chemical nature of the sulfur compounds.
ADS is a promising technique since it has the potential to be regenerative, cost-effective and environment-friendly while operating under ambient conditions. However, there are some challenges including selectivity and diffusion limitations which are the most important challenges in this type of desulfurization.
Figure 1. Types of adsorptive desulfurization
A reactive adsorption is a form of ADS whereby the sulfur compounds in fuel are removed by chemical interaction between the fuel and the sorbent material. The reaction involves removal or transfer of the sulfur compounds from the fuel followed by attachment of the compounds to the sorbent material which will allow the sulfur-free fuel to be collected in the mainstream. The process uses metal-based sorbent for sulfur removal to form metal sulfide, Figure 2.
Figure 2. Reactive adsorption of benzothiophene (BT) on sorbent material in the presence of hydrogen
Polar adsorption (IRVAD process) is a less expensive method of providing low sulfur gasoline and it was developed and commercialized by Black and Veatch Pritchard and Alcoa Industrial chemicals to address even low concentrations of sulfur compounds in fuels. Activated alumina in particular Alcoa Selexsorb is widely used as the sorbent material for the removal of polar compounds. The adsorption process is performed in a counter-current moving bed with alumina adsorbent in contact with liquid hydrocarbon in a multi-stage adsorber. The adsorbent is normally regenerated using hydrogen at various temperatures due to its heat capacity, thermal conductivity, and its availability. The regenerated adsorbent is then recycled for another round of the reaction. The process operates at lower pressures and does not consume a high amount of hydrogen. The mechanism is based on the polarity of sulfur in gasoline and it is known to reduce sulfur from fuels to as low as 0.5ppmw.
Selective Adsorption for Removal of Sulfur Compounds (SARS) is to remove sulfur compounds from fuels which constitute only less than 1% of the fuels at ambient conditions without hydrogen and to leave behind the remaining hydrocarbon contents of fuels which can be used as ultra-low sulfur fuels. The prime goal of SARS is to design appropriate adsorbents with surface sites having high affinity for sulfur compounds in the presence of aromatics. The adsorbents need to be effective, selective and apt for sulfur removal. The likely adsorption configurations of thiophenic compounds on adsorbents used in SARS process can be explained from the known coordination geometries that thiophene exerted upon contact with organometallic complexes. Examples of coordination geometries are given in Figure 3. It is a known fact that both thiophenic sulfur compounds and non-sulfur aromatic compounds in fuels can interact with metal species by pi-electrons which is why the likely coordinating geometries that will best explain the interaction of sulfur atom of the thiophenic compounds with the adsorbent used in SARS are: ղ1S (sulfur atom of thiophenes and a metal relation) and S-μ3 (sulfur atom of thiophenes and two metal species).
Integrated Process Adsorption is a recent technology that combines selective adsorption with HDS technology using highly efficient catalysts for effective removal of sulfur compounds from fuels. It has added advantages like reduction in cost, better efficiency, faster desulfurization rates and reduction in reactor volumes than using single processes alone.
Figure 3. Coordination geometries of thiophene in organometallic complexes