As demand for industrial-grade hydrogen increases, more facilities are recovering H2 from offgas and selling it. Several recovery technologies are available, each with its own strengths and limitations.
Hydrogen is an integral part of many industrial processes, and its consumption is expected to increase as government mandates and consumer preferences drive demand for cleaner fuels. Although most industrialgrade hydrogen is produced intentionally, hydrogen can also be recovered from chemical-, olefin-, and gas-processing streams. In some cases, combustion is used to recover hydrogen’s fuel value, downgrading the value of the hydrogen offgas stream. However, this approach is not always economical, because the thermal energy supplied by hydrogen can cost about 11 to 15 times more than the equivalent amount delivered by natural gas.
A more profitable and efficient way to capture hydrogen’s full value is by capturing and purifying hydrogen from offgas and repurposing it for industrial use. Several recovery technologies are available to do this.
Growing demand makes hydrogen recovery a more attractive option. Industry trends, including fuel desulfurization, lower quality crude sources, and projected growth in the use of fuel-cell-powered vehicles, will increase the need for hydrogen and for more profitable sources of the gas.
Today, many industrial warehouses use hydrogen for fuel-cell-powered forklifts, which take less time to charge and run longer than traditional combustion- or battery-powered vehicles. Also contributing to increased hydrogen demand is growth in important chemical manufacturing and industrial markets, such as electronics chemicals, glass manufacturing, and annealing atmospheres for metals processing. All of these markets can be served by sources of merchant hydrogen that are either deliberately produced or recovered as a byproduct of other processes.
Various processes generate hydrogen-rich offgases, among them ethane steam cracking, propane and butane dehydrogenation, chlor-alkali processing, and catalytic reforming. This article examines different techniques that are available for recovering and purifying hydrogen from these streams and reviews the economic drivers behind these techniques. It also examines two cases of hydrogen recovery and identifies feasible technologies based on criteria that include hydrogen purity, volume, geography, demand, and supply mode.
The true value of hydrogen
To quantify the value lost by burning hydrogen as fuel, consider the cost of natural gas and hydrogen relative to the lower heating value (LHV), or net calorific value, of each. For example, a pipeline-supplied hydrogen consumer on the U.S. Gulf Coast can pay three to four times more for hydrogen, on a volumetric basis, than for natural gas. However, the LHV of hydrogen, at 274 Btu/scf, is roughly one-third that of natural gas. This disparity puts the real cost of hydrogen used as fuel gas at 11–15 times that of natural gas used for the same purpose.
Depending on the quantity and purity of hydrogen offgas streams, recovered hydrogen can be recycled to the front end of the process or, in some cases, sold to a third party. Significant cost savings and productivity improvements can be realized by installing a hydrogen-recovery system.
To maximize their value, hydrogen-rich offgas streams must first be cleaned of process-specific impurities. Membrane separation, pressure swing adsorption (PSA), and cryogenic distillation are the predominant technologies available to clean offgas hydrogen streams and likely to be the most financially beneficial (Table 1).
|Table 1. Hydrogen-purification techniques are available to meet a wide range of process conditions.|
|Parameter||Membrane Separation||Pressure-Swing Adsorption||Cryogenic Distillation|
|H2 Product Pressure||< Feed pressure||Feed pressure||Feed/Low pressure|
|Feed Pressure||300–2,300 psig||150–600 psig||>75–1,100 psig|
|H2 Feed Content||>25–50%||>40%||>10%|
|H2 Capacity||1–50+ MM scfd||1–200 MM scfd||10–75+ MM scfd|
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