2.4.1 Mass and Energy Balances
Aspen Plus simulation models for the mass and energy balances of the current processes were developed by Leibbrandt (2010). For each advancement, or desired change, a new scenario would be created by modifying the processes of Leibbrandt (2010) to reflect the change in the mass and energy balance to determine the process needs, outputs and inputs and utilities. Since there were no notable advances on the Gasification-FT model, this was left as is.
The modifications would be based on experimental data from literature. The data that would be implemented includes operating conditions, chemical use and dosages, and the conversions obtained.
Hence, the modifications:
1. Steam Explosion catalysed by sulphur dioxide rather than sulphuric acid. Data from Carrasco et al. (2009) would be used for this advancement.
2. Simultaneous Saccharification and Co-Fermentation. Experimental data from Rudolf et al. (2007) would be used. The data would extend into reconfiguring the yeast cell propagation scheme.
For the modeling purposes in Aspen Plus, the ELECNRTL thermodynamic method will be applied wherever electrolyte solutions occur (Leibbrandt (2010)). For the distillation and energy supply sections, the NRTL method will be used since it describes phase equilibrium of strongly non-ideal solutions very well.
Since the FT model does not involve any electrolytes, the NRTL method will be the underlying thermodynamic model throughout. The mass balance for the distillation-refinery section will be done according to the literature review in section 2.1.4.
2.4.2 Pinch Point Methodology
2.4.2.1 Process Stream and Thermodynamic Data
All process stream data and subsequent thermodynamic data were gathered from the Aspen-Plus Simulation and the thermodynamic database in Aspen Plus. All heat demanding streams were
22
considered for the analysis while only the streams with meaningful excess heats were considered. Thus, heat excesses that were around than 0-2% of the total heat availability were not considered if the temperatures were significantly lower than the pinch temperature (Dias et al. (2010)). This minimum temperature is found iteratively by inspection of a rough composite diagram.
2.4.2.2 Composite Curve Construction and Pinch Point Location
The hot and cold composites were constructed and manipulated to find the pinch temperature. The manipulation was to offset the cold composite curve so that it only touched the hot composite curve at a unique value. This offset could be ascertained using packages such as Aspen Pinch (Grisales et
al. 2005) or by the tabulating method described by Linnhoff et al. (1982) (as cited by Dias et al.
(2010)). The manual for Pinch Point Analysis by Linnhof et al. (1982) is the earlier edition of Kemp (2007).
2.4.2.3 DTmin Optimization for Capital-Utility Trade-off
The table generated by the “Linhoff method” was modified to account for the values of DTmin within the recommended ranges. The modification procedure is also detailed in Kemp (2007). With each new DTmin that was considered, the utility requirements were calculated from the overlaps of the composite curve. In order to have calculated the heat exchange area, the area enclosed by the composite curves and the utility segments was discretised. Each discrete section was treated as a separate heat exchanger for which the area was to be calculated, and after which, the areas were summed up. The net effect of the cost-saving of utility reduction and annualized capital cost was determined using pricing data extracted from Al-Riyami et al. (2001) at each DTmin instance. Thus, an optimized DTmin was to be found in this manner.
2.4.2.4 Heat Exchanger Network Construction
The heuristics detailed in Kemp (2007) and Peters and Timmerhaus (1997) was used to construct the Heat Exchanger Network Construction (HEN) without violating any of the prescribed laws. As mentioned before, the objectives were to design a network that achieved the minimum utility target while minimizing the amount of unit exchangers that are used.
A concerted effort to reach the absolute minimum number of exchanger units for the entire region heat transfer region was not considered as such lengths were not considered to be part of this study. Moreover, as the pinch point splits the entire heat transfer region into two thermally independent
23
regions (Perrins 1994), an absolute minimum is not always achievable. It was imperative though, that the minimum number of units be attained on either side of the pinch.
2.4.3 Comparison of Processes
Processes will be compared in terms of the energy efficiency. The two types of energy efficiencies considered are:
1. Liquid Fuel (Energy) Efficiency (ηliquid fuel)
Equation 1
The liquid energy efficiency measured the ability of a process to convert the thermal energy in the biomass feedstock (HHVfeedstock) into the energy present in the liquid fuel products. This definition for energy efficiency adjusts the energy inputs by subtracting the thermal energy (HHVby-product) of bi- products from the thermal energy in the biomass feedstock (Hamelinck et al. 2005). Since this energy ratio uses a thermal basis, the electricity that is exported is converted to the hypothetical amount of thermal energy that would be used to generate that electricity. The assumed electrical efficiency (ηelec) is 45% (Hamelinck et al. 2005).
2. Overall Energy Efficiency (ηoverall)
Equation 2
The overall energy efficiency describes the performance of the process of the total energetic output, relative to an energy input that is adjusted to account for any additional energy source (Leibbrandt 2010).
24