Producing Hydrogen More Efficiently

Those of us with teenage kids, or memories of their kids as teenagers, or even what we were like as teenagers may recall the question, “Why do I have to learn ____ if I’ll never have to use it?” This is very fresh in my mind because I had that very conversation the other night. The blank in this case was chemistry. My point was that you really have no way of knowing what you’ll need to know so you might as well learn it.

Well today, I’m reading an article from the August edition of Hydrocarbon Processing magazine written by Air Product and Chemical’s Win Hoglen and Emerson’s Julie Valentine, a member of the Micro Motion business. The article, Coriolis flowmeters improve hydrogen production describes how accurate steam-to-carbon ratio control improves efficiency in a reforming hydrogen plant located within a refinery. The article explores the chemistry in the reforming process converting the light hydrocarbons (methane, ethane, propane, butane) and water (in a superheated steam state) into hydrogen and carbon monoxide. A shift reaction then converts the carbon monoxide and water into carbon dioxide and hydrogen.

For those not in the refining industry, this hydrogen is needed to scrub the sulfur out of gasoline and diesel to meet the clean fuels regulations that countries around the globe have adopted. The sulfur reacts with the hydrogen to make hydrogen sulfide and then it is further processed into elemental sulfur.

The thrust of the article is not the chemistry lesson I just described, but the challenges to most efficiently produce this hydrogen. A major challenge is the chemical composition of the natural gas since:

…the amount of steam required for the reforming reaction can vary widely depending upon the number of carbon atoms per molecule of the gas (i.e., one molecule of steam is required for each carbon atom, but there can be from one to four atoms).

Traditionally, volumetric flow measurements were used which usually involved differential pressure measurement and gas chromatograph or mass spectrometer analysis. Calculations determine the actual mass flow (carbon mole flow.) Errors in the carbon mole flow result from errors in the volumetric flow when the composition changes. Also, this analytical equipment requires regular maintenance and steam flow must be increased to handle any spikes in carbon mole flow during this maintenance period.

There are problems with both too much and too little steam flow. Too little reduces catalyst life, and production instability that may lead to a costly plant shutdown. Too much steam wastes energy and may require additional capital investments for more steam capacity. The measurement and control challenge is maintaining a constant steam-to-carbon ratio.

Coriolis flowmeters, through the Coriolis effect, measure actual mass flow very accurately and require less maintenance. The drawback is that the mass flow measurement cannot distinguish impurities like nitrogen and carbon dioxide in the natural gas supply.

The article describes testing done where methane concentration ranged between 78% and 89% and ethane between 7% and 15%. Maximum variation in the steam-to-carbon ratio was 0.02 units of steam, much better than the 0.2 in the traditional measurement method. The percentages of nitrogen and carbon dioxide were relatively stable.

From the testing done at various Air Products and Chemical facilities, Micro Motion Coriolis flowmeters are well suited for a natural gas stream that has relatively fixed percentages of inert gases or nitrogen concentrations that do not vary outside of 3% less than design.

A final note, I forwarded this article on to my teenagers to demonstrate the point that one never knows when one might need to know something learned in one’s past.

Posted Monday, October 8th, 2007 under Measurement, Refining.

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