Product Recovery, and Stillage Evaporation Steps for Fuel Ethanol Production

In a previous work (Grisales et al., 2005), the heat integration approach was uti­lized for the analysis of the fermentation, distillation, and evaporation steps of the fuel ethanol production process from lignocellulosic biomass using azeotropic dis­tillation for ethanol dehydration. Low ethanol concentrations in the culture broth exiting the fermenter increase energy costs in the distillation train and, therefore, in the evaporation train utilized for obtaining concentrated stillage (the first opera­tion of the effluent treatment scheme). For this reason, it is of great importance that the application of energy integration be instituted in order to improve process performance and make it more environmentally friendly (via the reduction in the consumption of external nonrenewable sources of energy).

Process simulator Aspen Plus was employed for calculating mass and energy balances of the analyzed technological configuration. Through a graphical repre­sentation of the energy requirements of the process, the exchanged heat was identi­fied considering the external utilities (steam and cooling water). The process was represented by its hot and cold profiles, which were defined by the corresponding hot and cold composite curves (Figure 11.15). These curves show how much energy could be transferred from hot streams to cold streams within the process. To com­plete the global heat balance, hot utilities (vapor at 2 bar) and cold utilities (cooling water at 10°C) were utilized. For the definition of the required amount of hot and cold utilities, a grand composite curve was built. Consumed energy by hot and cold utilities was determined by simulation. For the design of HEN, a grid diagram was employed in which streams were represented with their respective supply and tar­get temperatures as well as the position of pinch. Through heuristic rules for pinch (Shenoy, 1995), different configurations of HEN were proposed and evaluated in terms of total recovered heat and operation costs. These HENs should ensure the target temperatures of the streams. Heat transfer areas were also calculated in order to define the capital costs. In particular, it was established that the hot stream exit­ing the top of the first distillation column (concentration column) should be split

into two substreams. These two substreams are organized in such a way that they transfer heat to the second effect of evaporation and to the heat exchanger utilized for preheating the evaporated liquid exiting this second effect which is sent to the third effect of evaporation. This configuration contrasts with the base case configu­ration where this stream is condensed and sent as a distillate to the second distilla­tion column (rectification column) without taking advantage of its caloric energy.

Applying the described procedure, the energy saving of the new HEN was determined compared to the original network for the studied process steps. This information allowed quantifying the economic benefits that could be obtained if the defined HENs by means of pinch analysis were implemented. For instance, the external energy supplied to the process by the hot utilities was reduced by 22.8% for a minimum temperature difference of 5°C. The achieved energy recovery is 75.5% of the maximum possible energy recovery calculated in the targeting step (Grisales et al., 2005).