Gibbs reactors

In Figure 2.4, data for the equilibrium constants of esterification/hydrolysis and transesterification/glycolysis from different publications [21-24] are compared. In addition, the equilibrium constant data for the reaction TPA + 2EG BHET + 2W, as calculated by a Gibbs reactor model included in the commercial process simulator Chemcad, are also shown. The equilibrium constants for the respective reactions show the same tendency, although the correspondence is not as good as required for a reliable rigorous modelling of the esterification process. The thermodynamic data, as well as the dependency of the equilibrium constants on temperature, indicate that the esterification reactions of the model compounds are moderately endothermic. The transesterification process is a moderately exothermic reaction. 

Figure 2.11 Equilibrium constant for the formation of DEG from EG as a function of temperature, calculated by using the Gibbs Reactor model of the commercial process simulator Chemcad (Chemstations)...

The Gibbs reactor solves the full reaction (and optionally phase) equilibrium of all species in the component list by minimization of the Gibbs free energy, subject to the constraint of the feed mass balance. A Gibbs reactor can be specified with restrictions such as a temperature approach to equilibrium or a fixed conversion of one species. 

The Gibbs reactor is very useful when modeling a system that is known to come to equilibrium, in particular high-temperature processes involving simple molecules. It is less useful when complex molecules are present, as these usually have high Gibbs energy of formation consequently, very low concentrations of these species are predicted unless the number of components in the model is very restricted. 

The problem asks for an equilibrium distribution, so the model should contain either a Gibbs reactor or an equilibrium reactor. 

To specify the feed, we must enter the temperature, pressure, flow rate, and composition. The temperature, pressure, and flow rate are entered in the stream editor window, as illustrated in Figure 4.6. The feed composition can be entered as 100% of any of the G5 paraffin species, for example, normal pentane. The results from a Gibbs reactor would be the same if 100% isopentane were entered. It should be noted, however, that if a mixture of a pentane and a pentene were specified, then the overall ratio of hydrogen to carbon would be different and different results would be obtained. 

To get an initial estimate of the distillation column conditions, the process was first simulated using a shortcut column model, as shown in Figure 4.43. If we assume that no cyclic compounds are formed in the process, then the component list includes all of the available Cg paraffin compounds, i.e., n-hexane, 2-methyl pentane, 3-methyl pentane, 2,3-methyl butane, and 2,2-methyl butane. The reactor achieves complete equilibrium between these species and so can be modeled using a Gibbs reactor. 

Note also that this type of chemical reaction equilibrium calculations is sometimes referred to as a Gibbs reactor simulation. 

Gibbs Reactor Multiphase chemical equilibrium (stoichiometry not required)... 

For instructions on the use of the equilibrium constant and Gibbs reactor models in the process simulators, see the CD-ROM that accompanies this book ASPEN Chemkd Reactors Equilibrium Reactors REQUIL or RGIBBS and HYSYS —> Chemical Reactors... 

In the thermodynamic study of the reaction, one can also use the HYSYS software version 3.1, particularly the Gibbs reactor module, with the thermodynamic package Peng-Robinson and the method of minimization of the Gibbs free energy (G) of the system, given by the following equation ... 

Software HYSYS version 3.1. Gibbs reactor module with package thermodynamic Peng-Robinson. 

For each stream composition, both EREA and GIBBS reactor simulations give similar results but slightly higher values have been found for the latter, indicating that the combination of methanation and water-gas shift only may underestimate CH4 production rate. 

Figure 15.2 shows the flow sheet of the FP-FC system. The fuel forthe system is an aqueous solution of methanol at the molar ratio of methanol to water of 1 2 for the standard case. The fuel is evaporated in the vaporizer (VAP) at 150°C. In the reformer, the vaporized methanol and water react at 250 °C to form a hydrogen-rich gas, which contains also some CO2 and CO. The steam reformer is modeled as a Gibbs reactor assuming chemical equilibrium between the species at the outlet of the reactor. At the reforming temperature of 250 °C, the equilibrium conversion of methanol is almost 100%. The selectivity of methanol to CO2 is about 97% and to CO about 3%. In the mixer (MIX), the hydrogen-rich gas from the reformer is mixed with a small quantity of air, which is needed for the oxidation of CO present in the product gas from the reformer. The selective CO oxidation takes place in the COS reactor at 150 °C. The COS reactor is modeled as a stoichiometric reactor where 50% of the supplied O2 from the air is used for complete oxidation of CO and the remaining 50% of O2 reacts with H2. 

The selection of the reactor depends on the data available. We can use an equilibrium reactor and include the equilibrium constants or a Gibbs equilibrium reaction. For this particular case, we select a Gibbs reactor and we leave the use of the equilibrium reactor as exercise for the reader. We select isothermal operation at the inlet temperature and the reaction will take place in vapor or mixed phase at 50 bar. We can select the presence of inerts in the third tab of the dialogue box in Figure 8.43. 

While the reformer and burner can be considered as Gibbs reactors (delivering thermodynamic equilibrium values), the flow sheet simulation of the overall process requires the implementation of a confirmed stack characteristic. Key figures for the stack are power output, fuel utilization and electrochemical efficiency at the desired operation point. Thus, a Staxera Mk200/ESC4 stack was evaluated in a stack-test-bench with different fuel gas compositions and throughputs. Figure 4 shows the measured U/I-curves, Table 1 summarizes the stack performance data for the different operation points. 

Miliimum Gibbs FVee Energy Reactor This is another common form of the equilibrium reactor. In the Gibbs reactor, the oudet stream conposition is calculated by a free energy minimization technique. Usually data are available from the simulator s databank to do these calculations. The only input data required are the list of conponents that one anticipates in the output from the reactor. In this mode the equilibrium conversion that would occur for an infinite residence time is calculated. Batch Reactor This reactor type is similar to the kinetic reactor (and requires the same kinetics input), except that it is batch. The volume of the reactor is specified. The feeds, oudets flows, and reactor tenperature (or heat duty) are scheduled (i.e., they are specified as time series). 

An important feature of the non-stoichiometric formulation is that no information about the reaction stoichiometry is required. However, the species that the mixture is composed of must be specified. Note also that this type of chemical reaction equilibrium calculations is sometimes referred to as a Gibbs reactor simulation. 

---