## 1 Introduction  
Process simulation is an excellent tool for gaining insight into the performance of a chemical plant with many pieces of equipment using a number of different separation techniques and reactive systems. Simulation allows one to determine what might happen depending on operating conditions, including the temperature, pressure, type of feedstock, composition and concentration of components, reaction time, and other technical aspects such as energy and mass integration. In addition, on many occasions, experimentation may be too expensive or even impractical due to limitations in resources related to time, labor, and interferences or disturbances in a running plant. Experimentation cannot be substituted by process simulation, but this makes it possible to reach or complete some challenges where experimentation is too hard, thus promoting or dismissing a certain research route. Additionally, process simulation can even complement experimentation by designing or assessing the more suitable operating arrangement for a process or by planning, in the most economical way, the laboratory and plant experiments required for the design, thus helping establish extreme boundary conditions or obtaining the behavior of a system within and outside the experimental boundaries. Process study under chemical equilibrium is useful because, although many reactors are designed based on kinetics and rates of mass and heat transfer, the most suitable operating conditions can be inspected, determining whether the research of a new process is worthwhile (e.g., by carrying out a techno-economic assessment and a life cycle assessment). Hence, the simulation output can be used as a benchmark and even allows us to propose improvements in a process. Another relevant aspect in chemical process simulation is energy integration, particularly if the process is energy-intensive, such as supercritical water gasification (SCWG), which is a developing technology that is very suitable for dealing with biomass or organic waste containing a large amount of water thanks to the special properties of supercritical water (SCW) [1; 2; 3]. The special properties of supercritical water lead to a high carbon-to-gas conversion and hydrogen yield, which is also predicted by thermodynamic calculations, and this has been achieved effectively with a suitable catalyst [4]. As an emerging technology, many researchers in SCWG strive to develop new concepts for process design or propose improvements by optimizing a previously developed one, which can only be performed by simulation. In this way, techno-economic and life cycle assessments make it possible to check the potential feasibility of the specific process implemented in a complete plant. Energy integration may cause a process to be energy self-sufficient or viable and allows for a more realistic exergy analysis, since the main exergy losses occur in units related to transfer heat, such as heat exchangers or furnaces. Furthermore, considering the high pressure at the reactor, the gaseous product stream leaving the reactor can be expanded, thus generating electrical power and still containing enough enthalpy and temperature to exchange heat with colder streams, such as the feed stream, before entering the reactor. In fact, heat integration aims to optimize energy and exergy efficiencies. Aspen Plus [5] is a powerful chemical process simulation software with a database that includes a large list of components with their properties and suitable thermodynamic methods. However, simulation with fluids is more developed than simulation with solids, so there are feedstocks classified as nonconventional solids without a known chemical formula or structure, such as coal or biomass. This is because of their large number of components or the lack of properties related to this type of solid. Despite this, Aspen Plus is the best software for simulating chemical processes involving solids.