(532a) Economic Analysis of Alternative Flowsheets for the Hybrid Chlorine Cycle

This paper will report the results of the most complete study conducted to date on the hybrid chlorine hydrogen production cycle. Three different flow sheets were developed to produce 200 million kg of hydrogen per year at 25oC and 20 barg. Each process was modeled using Aspen Plus 12.1, supplemented where necessary by custom Excel spreadsheets. Major equipment was sized, and feasible alternatives were identified for all materials of construction. Fixed capital investment, annual operating costs, cost per kg of hydrogen produced, and energy efficiency were determined for each of the three alternatives (Final results on costs and efficiency are not yet available. The economic analysis will be completed by July 31, 2007.)

The hybrid chlorine cycle consists of two reactions. The reverse Deacon reaction is

Cl2(g) + H2O(g) <=> 2HCl(g) + 0.5O2(g)

This reaction is endothermic and reversible with an equilibrium constant of 6.8 at 850oC, but only 0.0001 at 130oC.

The second reaction involves the decomposition of hydrogen chloride, which is accomplished electrochemically.

2HCl (gas or aqueous) => Cl2(g) + H2(g).

The three alternative flow sheets differ in how they conduct the reverse Deacon reaction and separate the product gases and whether the electrolysis step is conducted in the liquid or gas phase.

Aqueous quench. In this cycle the reverse Deacon reaction proceeds to near equilibrium at 850oC in a conventional tubular reactor. The gas leaving the reactor is quenched rapidly with an aqueous stream recycled from the electrolyzers to absorb the HCl and prevent reversion of products to reactants. The basic concepts behind running the reactor in this way were demonstrated by Carty et al. (1981). Oxygen and chlorine leave the quench tower in the gas phase and are separated by cryogenic distillation. The chlorine is recycled, and oxygen can be sold as a byproduct. The aqueous HCl stream flows from the quench tower to electrolysis where approximately 25% is converted to hydrogen and chlorine. The chlorine is recycled to the reverse Deacon reactor, and the more dilute aqueous HCl back to the quench tower. Energy integration allows some reduction of heat requirements in this process, but the electrolysis step dominates energy consumption as well as capital investment.

Low temperature reverse Deacon shift via HCl sequestration. This alternative uses a concept suggested by Amendola (2005). A solid sorbent is used to sequester HCl as it is produced at lower temperature in one of many parallel reactors. The reverse Deacon reaction proceeds to completion with only oxygen leaving the reactor as a saleable byproduct until the sorbent is saturated with HCl. Valves then switch the reactor to the desorb mode, and the temperature is raised to recover the HCl. We have demonstrated proof of concept of this scheme in bench-scale experiments. The reverse Deacon reaction produced oxygen and HCl at 105oC over a zeolite. Desorption was conducted at 140oC, liberating HCl gas. HCl can be converted to hydrogen and chlorine electrochemically in the gas phase at lower voltage than what can be achieved using aqueous electrolysis (Motupally et al. 1998). Reducing the temperature of the reverse Deacon reaction and the energy input to the electrolysis step means that this alternative offers lower capital investment and higher efficiency compared to the aqueous quench option.

Reverse Deacon shift by permeation of oxygen through a dense ceramic membrane. Balachandran and Ma (2006) and others have demonstrated the ability of dense ceramic membranes to permeate virtually pure oxygen from gas mixtures at high temperatures. In this hybrid chlorine alternative the reverse Deacon reaction proceeds to near equilibrium at 850oC in a conventional tubular reactor. The gases then pass into a second reactor made of dense ceramic membrane tubes. The membrane walls permeate the oxygen into a sweep gas (probably nitrogen) and allow the reaction to continue to completion. The oxygen can be recovered via cryogenic distillation and sold as a byproduct. The sweep gas can be recycled. HCl gas leaving the membrane reactor is converted to hydrogen and chlorine electrochemically in the gas phase. Compared to HCl sequestration, this option does not allow the reverse Deacon reaction to be conducted at low temperature because dense ceramic membranes require higher temperatures for effective use. But opportunities for effective heat integration should result in comparable costs and energy efficiency.

References cited.

Amendola, S., ?Thermochemical hydrogen produced from a vanadium decomposition cycle,? U. S. Patent Application Publication US 2005/0013771 A1, (January 20, 2005).

Balachandran, U., and B. Ma, ?Mixed-conducting dense ceramic membranes for air separation and natural gas conversion,? J. Solid State Electrochem, 10, 617-624 (2006).

Carty, R.., M. Mazumder, J. Schreider, and J. Panborn, ?Thermochemical Hydrogen Production, Vol. 3, Appendix B., Discussion of Experimental and Design Work,? Gas Research Institute for the Institute of Gas Technology, GRI-80/0023, Chicago, IL 60616 (1981).

Motupally, S. D. Mah, F. Freire, and J. Weidner, ?Recycling chlorine from hydrogen chloride,? The Electrochemical Society Interface, (Fall 1998).