# Towards Semi-Automatic Generation of a Steady State Digital Twin of a Brownfield Process Plant
## 6 Discussion  
The target of the paper was to achieve a POC for the automatic generation of a steady state model. The case process selected for the POC is simple and contains only simple unit operations such as tanks, pumps, and valves that can be modelled in the selected simulation tool with one single symbol. As a result, the rules for a one-to-one mapping from an intermediate graph to a steady state model presented  
Figure 16: Simulations results of the case process.  
in Table 1 are very simple. For chemical processes with more complicated unit operations, such as distillation columns, evaporators or extractors, the rules are longer since a distillation column or a liquid-liquid extractor, for example, are modelled by combining several symbols in series or parallel instead of having only one symbol. Even though the rule is longer, the same approach is applicable as with the simpler rules presented in this paper. However, further research is needed to define a set of rules to cover the most common chemical unit operations.  
If the equipment is not available in the library of the simulation software, it can be modelled by creating a custom combination of equipment available in the library. In the case process, the rules are one-to-one mapping (from one piece of equipment to one symbol in the steady state model). In the said further research on chemical unit operations, several-to-one and/or one-to-several mapping rules are expected. Commercial simulation software may have an emphasis on a certain chemical process technology. For example, the simulation software AspenPlus is powerful for modelling unit operations based on phase separation whereas Balas is designed for modelling and simulating of paper processes. Thus, in AspenPlus there is no single symbol for a headbox of a paper machine and vice versa, in Balas, there is no single symbol for a distillation column. The rule for describing any unit operation is always simulation software specific. In the absence of standardization in the area of steady state simulation tools, one direction for further research would be the development of rules for simulator-to-simulator transfer.  
The methodology presented in this paper assumes as its starting point that the relevant information has been extracted from a P&ID into a graph format. For many brownfield plants, the P&ID is a raster graphics image obtained by scanning a paper diagram. In newer plants a digital P&ID from a CAD tool may be available. The latter scenario applies to our case study. Thus, the methodology is general for all kinds of plants. However, it has been especially designed to work on the limited information at a brownfield plant. In the case of raster graphics P&IDs, the quality of the results obtained by this methodology depend additionally on the quality of the P&ID information extraction solution. The availability of recent publications in this area by several research groups, referenced in bullet 1 of the numbered list in Section 3.1, is an indication that efforts are underway to further advance the quality of information extraction from brownfield design documents.