Power Density Assessment of Variable Volume Batch Reactors for Hydrogen Production with Dynamically Modulated Liquid Fuel Introduction

Abstract

A new concept of a dynamically controlled reactor, which combines the variable volume operation of CHAMP (CO2/H2 Active Membrane Piston) with direct injection of liquid fuel of DDIR (Direct Droplet Impingement Reactor), is introduced and analyzed with the primary goal of identifying conditions for the highest volumetric power (hydrogen yield) density. In the proposed CHAMP-DDIR, a liquid fuel mixture is pulsed-injected onto the heated catalyst surface for rapid flash volatilization and on-the-spot reaction, and a hydrogen selective membrane is collocated with the catalyst to reduce the diffusion distance for hydrogen transport from the reaction zone to the separation site. Uniquely, CHAMP-DDIR allows dynamic variation of the reactor volume to optimally control the residence time and reactor conditions (pressure and temperature), thus improving both the reaction and separation processes. Idealized CHAMP-DDIR simulations, without heat and mass transfer limitations in the reactor volume, are used to determine the theoretical limits on power density for various operational conditions. An enhanced CHAMP-DDIR model, which accounts for the effects of mass transport limitations and bulk temperature changes in time, is employed to evaluate possible performance improvement through combining time-modulated fuel introduction and the active change of reactor volume. Analysis reveals that significant improvement in the volumetric power density can be achieved primarily as a result of two factors: time-modulated fuel injections enable higher reaction/permeation rates by preventing the large temperature drop that accompanies a single batch liquid fuel injection and volume modulation during a batch cycle allows for reduction in required reactor volume under a constraint of maximum operating pressure.

Experimental investigation of hydrogen production by variable volume membrane batch reactors with modulated liquid fuel introduction

“…From the findings in the previous study which classified CHAMP-DDIR's operational regimes with theoretical analysis [23], the hydrogen yield is limited by permeation through the membrane under the operating conditions of these experiments. The driving force for permeation of hydrogen through the membrane is the difference in partial pressure of hydrogen across the hydrogen selective membrane.…”

“…For a given fuel amount the magnitude of heat input required for evaporation and reaction are of a similar magnitude (DH Water,evap ¼ 40.66 kJ/mol, DH Methanol,evap ¼ 32.84 kJ/ mol and DH MSR ¼ 49.2 kJ/mol), but the time scale predicted for evaporation is 3 orders of magnitude smaller than that for reaction time scale [23]. As a result, the catalyst temperature drops immediately after the fuel sprayed on to it, as the catalyst provides the majority of energy input for evaporation [21].…”

“…This pulse-modulated injection benefits reactor performance by promoting efficient evaporation of fuel, maintaining the elevated temperature for enhanced reaction/permeation. It further allows for a smaller reactor volume to accommodate a given fuel amount without exceeding pressure limits [23]. (2) Flash volatilization of the injected fuel occurs on the heated catalyst surface.…”

“…In CHAMP-DDIR (Figs. 1 and 2), a liquid fuel mixture is pulse-injected onto the heated catalyst surface for rapid flash volatilization and on-the-spot reaction, and a hydrogen selective membrane is collocated with the catalyst to reduce the diffusion distance for hydrogen transport from the reaction zone to the separation site [23]. In addition, CHAMP-DDIR exploits dynamic variation of the reactor volume to control the residence time and reactor conditions, thus improving both the reaction and separation processes, as well as enabling on-demand dynamic variation in hydrogen throughput without sacrificing fuel conversion.…”

“…In a previous study for CHAMP-DDIR, the coupled reactiontransport-permeation reactor models were developed using state-of-the-art, reduced-order analytical framework of increasing complexity and were used to investigate the optimal design and operation of CHAMP-DDIR [23]. The idealized theoretical analysis of the model revealed that significant improvement in the volumetric power density under a constraint of maximum operating pressure can be achieved by combining time-modulated fuel injections with volume modulation during a batch cycle.…”