Process modeling and simulation with ProSimPlus
The example shows the model of a biofuel production process from colza oil using alkalin catalyst.
The main areas are the transesterification, methanol separation, water washing, FAME purification, catalyst neutralization and glycerol purification. The equipment used includes in particular reactors, distillation columns, extraction columns and components splitters.
Description of the process follows.
Methanol and Sodium hydroxide feed streams (C101 and C103) are mixed (MIX101) and their pressure is brought to 400 kPa by a centrifugal pump (P101). The pressure of oil feed (stream C105) is also brought to 400 kPa (P103) and its temperature to 60°C by a cooler/heater (E101). These three streams, with the methanol recycle stream (C201A) constitute the feed of the transesterification reactor (R101).
After the transesterification, the mixture is brought in a distillation column with total condenser (T201, from stream C102) in order to separate the methanol from other components.
The distillate that contains almost pure methanol is recycled at the reactor inlet (stream C201), whereas the residue (stream C202) is cooled to 60°C (E201) and its pressure set to 200 kPa (P202). It is then washed with water in an extraction column (T301 from stream C203). The washing separates the biofuel from methanol, glycerol and catalyst.
The overhead flow (stream C301) is sent to a gravity splitter (modeled with a component splitter (X301)) that allows recovering NaOH at the bottom (stream C301B) and the FAME, oil and a fraction of water and methanol at the head (stream C301A). The FAME, oil, water and methanol recovery ratios in the organic phase of the splitter X301 are set to 1.
These components are then separated by a distillation column with partial condenser (T401). This additional separation is required to obtain a biofuel purity that meets the ASTM specifications (American Society for Testing and Materials) and that has to exceed 99.6%. The partial condenser facilitates the FAME and water-methanol separation (at the top of the column).
It should be noted that vacuum operating conditions are required in order to keep the temperature low enough to avoid the biofuel degradation.
The extraction column’s (T301) and the components splitter’s (X301) bottom streams are sent to a reactor in order to neutralize NaOH by adding pure phosphoric acid (stream H3PO4).
The produced Na3PO4 is separated from other components in a components splitter (X302). After recovery of Na3PO4, (stream C306), the stream C305 contains more than 82% mass in glycerol. However, glycerol is considered as a secondary product that must have about 92% purity. Consequently and additional separation is required and the head stream (C305) is sent to a distillation column with total condenser (T501).
Components taken into account in the simulation are taken from the ProSimPlus standard database:
|Name|| Chemical formula || Use in the process|
|Colza oil : triolein or triacylglycerol||C57H104O6||Raw material, main reactant.|
|Sodium hydroxide||NaOH ||Alkaline catalyst.|
|Sulfuric acid||H2SO4 ||Acid catalyst.|
|Methyl oleate (biofuel, FAME)||C19H36O2||Main product.|
|Oleic acid||C18H34O2 ||Impurities in waste vegetable oils.|
|Phosphoric acid||H3PO4||Allows neutralizing NaOH.|
|Sodium phosphate||Na3PO4 ||Product coming from NaOH neutralization.|
|Calcium oxide||CaO||Allows neutralizing H2SO4.|
|Calcium sulfate||CaSO4 ||Product coming from H2SO4 neutralization.|
|Water||H2O||Allows separating FAME from other products, by scrubbing.|
|Hexane||C6H14 ||Allows separating FAME from other products, by scrubbing.|
Triolein properties were estimated with PROPRED, pure components properties estimation software, developed by the CAPEC consortium. The predictive methods implemented in this software (Marrero and Gani, Constantinou and Gani, Joback and Reid, Wilson…) are based on the decomposition of the molecule into functional groups.
The developed chemical formula of triolein is the one that is supplied on the NIST website.
The properties thus obtained were regressed with the regression service available in the themodynamic module of ProSimPlus.
It is to be noted that as electrolytic species involved in the process only intervene in reaction steps as catalysts or catalysts neutralizers, they are considered as non-volatile (Pi0=exp(-30), ΔHvap=0) and the missing thermodynamic properties (liquid density, liquid specific heat…) are assimilated to those of water.
The system contains polar components (such as methanol and glycerol), which implies strong interactions in liquid phase. The operating pressure being low (from 0.1 to 4 bars), the vapor phase behavior can be assimilated to an ideal gas.
From those two considerations, an heterogeneous approach is retained. The equilibrium data for the binary systems not being available, a predictive model, based on group contribution, the Dortmund modified version of UNIFAC model was selected.