Purge Fraction Research Articles

Process Design and Techno-Economic Assessment of biogenic CO2 Hydrogenation-to-Methanol with innovative catalyst

A small-scale 10 ton per day methanol (MeOH) synthesis plant, from CO2 and hydrogen, is designed and simulated with Aspen Plus and a techno-economic analysis is conducted. The e-fuel (MeOH) is produced in a conventional fixed bed reactor featuring an innovative Cu/Zn/Al/Zr catalyst, converting biogenic CO2 from a biogas upgrading plant with H2 produced by a grid powered PEM electrolyzer. The process is thermally autonomous as a result of heat integration and combustion of purged unconverted reactants. A sensitivity analysis is carried out in order to evaluate and compare the impact of the different technical (purge fraction, Gas Hourly Space Velocity and Pressure of the methanol synthesis) and economic parameters (Capital Charge Factor, electricity and H2 cost) on the Levelized Cost Of Methanol (LCOM). Results show that, although the energy efficiency is greater (47.4 % electricity to methanol conversion) in the scenario with “self-sufficiency” in which all the net heat required by the process is provided by off-gas streams, the case with the highest profitability is the one with maximum methanol yield and, therefore, minimum purge and non-zero thermal energy import (provided by a biogas boiler). The best case scenario features a LCOM equal to 1,361 €/tonMeOH, with a GHSV of 7,500 h−1 and synthesis reactor operating at 70 bar, 250 °C. H2 production cost is the key variable and shall be reduced from the base case value of 5.8 €/kgH2 to 1.6 €/kgH2 in order to make the CO2 to methanol plant competitive with a MeOH market price of 550 €/tonMeOH; synthesis reactor operating conditions have more limited impact from a cost perspective, except for the purge fraction that shall be optimized to maximize the amount of MeOH produced.

Stability and Performance Analysis of Bioethanol Production with Delay and Growth Inhibition in a Continuous Bioreactor with Recycle

This paper proposes and analyzes a mathematical model for the production of bioethanol in a continuous bioreactor with recycling. The kinetics correspond to the use of Saccharomyces bayanus for the fermentation of sugars found in wastewater from soft drinks. The proposed model considers product growth latency, which was experimentally found in batch studies of ethanol production. Furthermore, the inhibition effect of ethanol is expressed by a modified version of the classical Andrew’s model for substrate inhibition. The proposed model consists of only three ordinary differential equations containing a minimal number of operating parameters, which include the bioreactor residence time, glucose feed concentration, recycle ratio and the fraction of biomass removed from the reactor by the flow. The positivity and the boundedness of solutions of the model were confirmed under reasonable restrictions of parameters. The stability analysis showed that there is a value of residence time at which an exchange of stability occurs between the trivial washout and non-washout solutions. This critical value depends only on the substrate feed concentration, biomass death rate, recycle ratio and purge fraction. Dynamic simulations of the model were carried out for substrate concentration in the range of 100–250 g/L, commonly used for the production of ethanol. An inverse response due to the inhibition effects of ethanol was observed in the time evolution of substrate and biomass concentrations. Parametric studies showed that ethanol concentration increases with the recycle ratio, with the inverse of residence time and with the inverse of purge fraction. The effect of ethanol latency has, on the other hand, a substantial effect on ethanol concentration. Despite its unstructured nature and the fact that some parameters such as temperature and acidity were not taken into consideration, the proposed model managed to provide useful results on the bioreactor-settler stability and the effect of key parameters on its dynamic behavior, which could pave the way for future optimization studies.

The efficiency of a pulsed detonation combustor–axial turbine integration

The paper presents a detailed numerical investigation of a pulsed detonation combustor (PDC) coupled with a transonic axial turbine stage. The time-resolved numerical analysis includes detailed chemistry to replicate detonation combustion in a stoichiometric hydrogen–air mixture, and it is fully coupled with the turbine stage flow simulation. The PDC–turbine performance and flow variations are analyzed for different power input conditions, by varying the system purge fraction. Such analysis allows for the establishment of cycle averaged performance data and also to identify key unsteady gas dynamic interactions occurring in the system. The results obtained allow for a better insight on the source and effect of different loss mechanisms occurring in the coupled PDC–turbine system. One key aspect arises from the interaction between the non-stationary PDC outflow and the constant rotor blade speed. Such interaction results in pronounced variations of rotor incidence angle, penalizing the turbine efficiency and capability of generating a quasi-steady shaft torque.

Bifurcation and chaotic in a model for activated sludge reactors

A dynamical model of an activated sludge process system is considered and analyzed. Numerical techniques are used to show when the system exhibits chaos. Three choices of bifurcation parameters produce different pictures of solution behavior in the form of limit cycles, two-torus and chaotic behavior. For some range of the reactor residence time the model exhibits chaotic behavior as well. Practical criteria are also derived for the effects of feed conditions and purge fraction on the dynamic characteristics of the bioreactor model.

Time-Resolved Flow Properties in a Turbine Driven by Pulsed Detonations

Theoretically, pressure-gain combustion like that of a pulsed-detonation combustor allows heat addition with reduced entropy loss compared to a typical steady deflagration combustor. The pulsed-detonation combustor is inherently unsteady, and time-resolved measurements are required to identify unsteady flow processes at the turbine inlet and exit. In this study, the radial turbine of a Garrett automotive turbocharger was coupled directly to and driven, full admission, by a pulsed-detonation combustor fueled by hydrogen or ethylene. Full-cycle time-resolved turbine inlet and exit pressures were obtained from pressure transducers mounted to wall static pressure ports. Tunable diode laser absorption spectroscopy was used to obtain full-cycle time-resolved turbine inlet and exit temperatures and velocities. Optical access permitted the use of high-speed video for particle streak velocimetry to obtain time-resolved turbine inlet velocities during blowdown with a 20 Hz ethylene-fueled pulsed-detonation combustor with 0.9 fueled fraction and 0.5 purge fraction. A comparison of laser-based and video-based measurements was made for the blowdown portion of the pulsed-detonation combustor cycle. First-ever full-cycle time histories of turbine inlet and exit velocities, total temperature, total pressure, mass flow, and total enthalpy rate revealed reflected pressure waves and showed momentary reverse flow, and thus unsteady accumulation and expulsion of mass and enthalpy.

Effect of CO and CO 2 impurities on performance of direct hydrogen polymer-electrolyte fuel cells

Mechanisms by which trace amounts of CO and CO 2 impurities in fuel may affect the performance of direct hydrogen polymer-electrolyte fuel cell stacks have been investigated. It is found that the available data on CO-related polarization losses for Pt electrodes could be explained on the basis of CO adsorption on bridge sites, if the CO concentration is less than about 100 ppm, together with electrochemical oxidation of adsorbed CO at high overpotentials. The literature data on voltage degradation due to CO 2 is consistent with CO production by the reverse water–gas shift reaction between the gas phase CO 2 and the H 2 adsorbed on active Pt sites. The effect of oxygen crossover and air bleed in “cleaning” of poisoned sites could be modeled by considering competitive oxidation of adsorbed CO and H by gas phase O 2. A model has been developed to determine the buildup of CO and CO 2 impurities due to anode gas recycle. It indicates that depending on H 2 utilization, oxygen crossover and current density, anode gas recycle can enrich the recirculating gas with CO impurity but recycle always leads to buildup of CO 2 in the anode channels. The buildup of CO and CO 2 impurities can be controlled by purging a fraction of the spent anode gas. There is an optimum purge fraction at which the degradation in the stack efficiency is the smallest. At a purge rate higher than the optimum, the stack efficiency is reduced due to excessive loss of H 2 in purge gas. At a purge rate lower than the optimum, the stack efficiency is reduced due to the decrease in cell voltage caused by the excessive buildup of CO and CO 2. It is shown that the poisoning model can be used to determine the limits of CO and CO 2 impurities in fuel H 2 for a specified maximum acceptable degradation in cell voltage and stack efficiency. The impurity limits are functions of operating conditions, such as pressure and temperature, and stack design parameters, such as catalyst loading and membrane thickness.

System-Level Performance Estimation of a Pulse Detonation Based Hybrid Engine

A key application for a Pulse detonation engine concept is envisioned as a hybrid engine, which replaces the combustor in a conventional gas turbine with a pulse detonation combustor (PDC). A limit-cycle model, based on quasi-unsteady computational fluid dynamics simulations, was developed to estimate the performance of a pressure-rise PDC in a hybrid engine to power a subsonic engine core. The parametric space considered for simulations of the PDC operation includes the mechanical compression or the flight conditions that determine the inlet pressure and the inlet temperature conditions, fill fraction, and purge fraction. The PDC cycle process time scales, including the overall operating frequency, were determined via limit-cycle simulations. The methodology for the estimation of the performance of the PDC considers the unsteady effects of PDC operation. These metrics include a ratio of time-averaged exit total pressure to inlet total pressure and a ratio of mass-averaged exit total enthalpy to inlet total enthalpy. This information can be presented as a performance map for the PDC, which was then integrated into a system-level cycle analysis model, using GATECYCLE, to estimate the propulsive performance of the hybrid engine. Three different analyses were performed. The first was a validation of the model against published data for a specific impulse. The second examined the performance of a PDC versus a traditional Brayton cycle for a fixed combustor exit temperature; the results show an increased efficiency of the PDC relative to the Brayton cycle. The third analysis performed was a detailed parametric study of varying engine conditions to examine the performance of the hybrid engine. The analysis has shown that increasing the purge fraction, which can reduce the overall PDC exit temperature, can simultaneously provide small increases in the overall system efficiency.

Lime enhanced gasification of solid fuels: Examination of a process for simultaneous hydrogen production and CO 2 capture

The lime enhanced gasification (LEGS) process uses CaO as a CO 2 carrier and consists of two coupled reactors: a gasifier in which CO 2 absorption by CaO produces a hydrogen-rich product gas, and a regenerator in which the sorbent is calcined producing a high purity CO 2 gas stream suitable for storage. The LEGS process operates at a pressure of 2.0 MPa and temperatures less than 800 °C and therefore requires a reactive fuel such as brown coal. The brown coal ash and sulfur are purged from the regenerator together with CaO which is replaced by fresh limestone in order to maintain a steady-state CaO carbonation activity ( a ave). Equilibrium calculations show the influence of process conditions and coal sulfur content on the gasifier carbon capture (>95% is possible). Material balance calculations of the core process show that the required solid purge of the sorbent cycle is mainly attributed to the necessary removal of ash and CaSO 4 if the solid purge is used as a pre-calcined feedstock for cement production. The decay in the CaO capture capacity over many calcination–carbonation cycles demands a high sorbent circulation ratio but does not dictate the purge fraction. A thermodynamic analysis of a LEGS-based combined power and cement production process, where the LEGS purge is directly used in the cement industry, results in an electric efficiency of 42% using a state of the art combined cycle.

Periodic and nonperiodic oscillatory behavior in a model for activated sludge reactors

A dynamic model for an activated sludge process is proposed to investigate the stability and bifurcation characteristics of this industrially important unit. The model is structured upon two processes: an intermediate participate product formation and active biomass synthesis processes. The growth kinetics expressions are based on substrate inhibition and noncompetitive inhibition of the intermediate product. The bifurcation analysis of the process model shows static as well as periodic behavior over a wide range of model parameters. The model also exhibits other interesting stability characteristics, including bistability and transition from periodic to nonperiodic behavior through period doubling and torus bifurcations. For some range of the reactor residence time the model exhibits chaotic behavior as well. Practical criteria are also derived for the effects of feed conditions and purge fraction on the dynamic characteristics of the bioreactor model.

Decomposed algorithmic synthesis of reactor-separation-recycle systems

A two-level algorithmic approach to synthesis of reaction-separation-recycle systems is presented. On the upper level, reactor conversion and purge fraction are optimized using a simple direct search method, and the flows among subsystems such as the reactor system and separation system are calculated. On the lower level, individual subsystems are optimized in every iteration of the upper level loop. On this level, the structure and operating conditions of the subsystems are determined, and the cost is returned to the upper level. This decomposed approach makes the synthesis of the overall system less complex and more robust than approaches addressing the entire problem simultaneously. The proposed approach is demonstrated on the well-known HDA process. The results are comparable to previously published results using the Hierarchical Decomposition Procedure, and the optimal results are demonstrated to be insensitive to initial estimates.

Decomposed algorithmic synthesis of reactor-separation-recycle systems

More robust methods are needed in the synthesis of reaction-separation-recycle systems. Previously reported approaches using mathematical programming have attempted to solve the reactor-separator-recycle synthesis problem simultaneously, modeling it as a complex MINLP problem (e.g. Floudas, 1990). We propose to take advantage of the inherent hierarchical structure of this class of problems: When the reactor conversion and the recycle purge fraction are fixed, it becomes substantially easier to synthesize the separation system. The reactor synthesis problem, the separation problem and the recycle problem may each be posed as Mixed Integer Programming problems, but each of these will be smaller and less complex than a corresponding formulation for the entire system; — this is illustrated in figure 1. Taking advantage of the fact that the two most important decision variables in the system — conversion and purge fraction — are continuous variables, we do therefore advocate the use of a two-level optimization approach where a response surface method (Simplex) is used as the upper coordinating level and the actual structure of the individual subsystems are determined on the lower level. This algorithmic procedure we have called REREDOS — REactor-RE-cycle-Distillation Optimization System. To illustrate the approach in terms of the onion diagram described by Smith and Linnhoff (1988), the layers of the onion will have to be redefined as shown in fig. 2. In the inner layer of the onion the recycle rate and the total conversion are chosen. These basic variables will dictate the synthesis of the recycle structure, and thereafter the reactor network and the heat integrated separation system. After having synthesized the full system, we may go back to the inner layer of the onion and make new guesses for the basic variables. The guess of the new basic variables arc based on the cost of the total system for the previously chosen basic variables. The procedure will terminate when the basic variables are sufficiently close to an optimal point. The proposed approach will be demonstrated by applying it to the synthesis of the HDA process ( Douglas, 1988).