## 1 Introduction  
In the last decade, Carbon Capture Utilization (CCU) with the aid of Renewable Energy Source (RES) power has led to significant progress in the field of Power to Gas (PtG) and Power to Liquid (PtL) technologies. In order to reduce Greenhouse Gas (GHG) emissions, European Energy policy proposes that the share of renewable energy is 40% by 2020 and 80% by 2050 [1]. This electricity-to-fuel process stabilizes the electrical power grid by converting the fluctuating characteristics of Renewable Energy Sources into storable energy carrier e.g., gaseous hydrocarbons (H\({}_{2}\) or CH\({}_{4}\)) in PtG. This process may also provide liquid fuel for the use of chemicals and transport in PtL technology [2]. In this respect, Power to Liquid is a strong candidate for the transformation of power into chemical electricity as there is no loss during long-term storage; it has wide-ranging applications in the transport sector, due to its high energy density which is compatible with existing infrastructures [3]. The main pathway in this transformation is _Fischer-Tropsch Synthesis_, which offers many advantages in the process design and for optimizing product selectivity. The feedstock of the FT process is attained using generated synthesis gas and steam/co-electrolysis can be employed for generating syngas out of water and carbon dioxide. This enables RES power to be coupled with electrolysis providing a carbon free cycle [4]. Other alternatives for syngas generation include biomass gasification and natural gas reforming. Syngas derived from autothermal reforming or gasification technology with an air separation unit must be employed in large scale FT facilities and those methods have the further benefit of generating a high syngas ratio (H\({}_{2}\)/CO) [5]. However, CO\({}_{2}\) gasification using biomass technology is not able to provide a sufficient syngas ratio for producing liquid fuel. Therefore, steam electrolysis or a water gas shift reactor could be used for adjusting the H\({}_{2}\)/CO ratio so that it is equal to 2 (usage syngas ratio) [6]. In the field of power to fuel technology, flexible analysis of a Low Temperature Fischer-Tropsch (LTFT) reactor under variable operating conditions presents more challenges compared to other value-added processes such as methanation and the Dimethyl ether process. These challenges involve more complexity in FT product selectivity and lower feasibility in the dynamic analysis of an LTFT reactor compared to methanation and the DME synthesis [7]. In this respect, the most suitable FT reactors for analyzing under flexible operation are the Multi-Tubular Fixed Bed Reactor and the Slurry Bubble Column Reactor. In the Fixed Bed Reactor (FBR), dynamic analysis addresses the feasibility of reducing the size of H\({}_{2}\) storage and optimizing the temperature profile along the catalyst bed [8], whereas, in the FT Slurry type, the analysis is focused on improving mass transfer phenomena (gas-to-liquid contact and interfacial mass transfer area), leading to enhanced selectivity and catalyst performance with regard to complex hydrodynamic features and scale up issues [9]. During recent years, the Slurry Bubble Column Reactor has been identified as the best option for Fischer-Tropsch synthesis due to its many advantages compared to the other reactors. These advantages include (1) flexible temperature control and excellent heat transfer; (2) efficient inter-phase contacting which results in higher productivity; (3) low pressure drop leading to reduced compression costs; (4) better use of catalyst surface (fine particles less than 100 \(\upmu\)m) allowing suitable liquid-solid mass transfer [5; 10]. However, this reactor presents several technical challenges in the design of the pilot plant as well as large scale due to following reasons: (1) potential formation of slug regime flow; (2) very little information on mass transfer data; (3) difficulty in scale up due to complex hydrodynamic features; (4) Obstacles to the separation of fine catalysts from the slurry phase.