(617d) Design and Control of Stand-Alone Autothermal Reforming Systems Using Methanol

Abstract

The conceptual design of compact, stand-alone autothermal reforming systems using methanol is presented. Based on specific conditions of maximum waste heat recovery, the optimization algorithm for maximizing hydrogen yield is obtained. Through the closed-loop control tests for the Aspen Dynamics model, the feasible manipulation by adjusting preheated water flow is established and the stable output regulation of nonlinear systems in the presence of unknown inlet perturbations is guaranteed.

Introduction

     Methanol has certain advantages compared to other hydrocarbons due to the low reforming temperature, ease of handling and a relatively high hydrogen-carbon ratio. Similarly, ethanol is also a potentially attractive feedstock because of its availability, nontoxicity and handling safety. From the aspect of kinetics, the reforming processes using different catalysts could affect the hydrogen yield, conversion efficiency and selectivity of undesired products. Hung et al.¹ provided an optimal operating conditions by adjusting H₂O/ethanol and O₂/ethanol ratios to improve catalytic activity. In terms of catalyst utilization, Tang et al.² showed that the autothermal reformer has superior performance over steam reformer. The stand-alone fuel cell power system associated with fuel processing units for stationary and potentially mobile applications was recently investigated. Wang and Wang⁹ showed that the autothermal reforming system is optimized through thermodynamic equilibrium and exergetic analyses. Stamps and Gatzke³ showed that a PEMFC stack coupled to methanol reformer with aid of a specific auxiliary power could effectively meet desired power targets. Degliuomini et al.⁴showed the dynamic manipulation and control of a heat integrated PEMFC power system connected to ethanol fueled processing units.

     In this work, stand-alone autothermal reforming systems using methanol is evaluated according to hydrogen yield, cost of hydrogen production and energy consumption. As for reducing fuel consumption as well as increasing heat recovery capability, the optimization algorithm for maximizing hydrogen yield of autothermal MeOH-to-H₂ processor subject to the condition of maximum waste heat recovery is performed. Moreover, the dynamic manipulation and control implementation is deployed in Aspen Dynamics environment. According to the closed-loop autotune variation (ATV) tests of the whole dynamics system in the face of unknown inlet perturbations, the PID control method can ensure the no offset and stable output regulation performance. Autothermal MeOH-to-H₂processor

Autothermal alcohols-to-H₂processor    

    In Fig. 1, a so-called autothermal alcohols-to-H₂ processor is considered as another stand-alone hydrogen production process, where an autothermal reformer (ATR) is an adiabatic plug flow reactor. It is noted that the flue gas from the burner is directly used to adjust the inlet temperature of the ATR, . Regarding the heat recovery design for autothermal alcohols-to-hydrogen processors in Fig. 1, the water flow is preheated by the outlet stream of the WGS reactor using a heat exchanger (E-102), and the feed flow is preheated by the high-temperature flue gas from the burner using anther heat exchanger (E-101). Notably, the heating jacket is removed and the inlet temperature of the ATR, T_ATR,in, can be adjusted.

                                                       Fig. 1 Autothermal alcohols-to-H₂processor

Optimization

     To address the maximum waste heat recovery, the following optimization algorithm for maximizing T_ATR,in .  Assumed that T_ATR,in is adjusted by changing the heat capacity of exchanger (UA), and UA_urepresents an ultimate bound of UA. If the maximum inlet temperature of the ATR is achieved, i.e. , then the corresponding  would be close to the minimum value. Furthermore, the optimization algorithm for maximizing the hydrogen  yield. Fig. 2 shows that the maximum hydrogen yield is obtained while the feed ratio (S/C) and  are specified at 0.75 and 833.9K, respectively.

Figure 2 Sensitivity analysis

     Based on above optimization algorithms, the steady-state simulation under optimal operating conditions is shown in Fig. 3. Notably, it can ensure a lowest flue gas temperature, e.g. T_flue,out=321K.

Figure 3   Optimization and steady-state simulation

Process control

     Aspen Dynamics are employed to model the system dynamics with prescribed steady-state initials. To address a feasible dynamic manipulation, if the initial temperature of T_ATR,inis 799.4 K and the corresponding UA of heat exchanger (E-101) is fixed at 463.2 K/W, then the open-loop tests of the system with respect to  step changes of water flow are shown in Fig. 4.

Figure 4 Open-loop tests of system with  step changes of water flowrate

     It shows that the manipulation of water flow not only dominates the hydrogen yield but also it affects the waste heat recovery capability. Moreover, a closed-loop ATV test is used to determine the ultimate period and gain when the hydrogen production rate and water flow rate are treated as the controlled output and manipulated input, respectively. The default value of the relay output amplitude is 5%, which is usually good. Fig. 5 shows that ATV test is finished after several (4–6) cycles. Fig. 6 shows that the PID controller settings are determined by Ziegler–Nichols tuning rule.

Figure 5 Closed-loop ATV test

 Figure 6 PID controller settings

To check the disturbance rejection capabilities, Fig. 7 shows that the stable and no offset output regulation is achieved while  step changes of inlet temperature of water flow appear.

Figure 7  perturbations in the inlet temperature of water flow

By steady state situation, the higher inlet temperature of water flow can induce the higher T_ATR,in but T_flue,outis not affected.

Conclusions

     With respect to hydrogen yield, cost of hydrogen production and energy consumption, the simulation shows that the autothermal MeOH-to-H₂ processor can effectively maximize hydrogen yield as well as minimize heat loss. To address feasible dynamic manipulation, the PID control design using closed-loop ATV tests is employed. Although the autothermal MeOH-to-H₂processor is a typically nonlinear system due to complex kinetics and specific heat recovery conditions, the single-input and single-output (SISO) control structure is successfully established by Aspen Dynamics. Simulation shows that the disturbance rejection by manipulating a preheated water flow can be guaranteed.

References

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