Hydrogen Purification Qualitative Explanation

Hydrogen can be produced from different industries and processes such as Steam Methane Reforming (SMR), Partial Oxidation, Gasification, Refineries, Petrochemicals, Electrolysis, etc. The hydrogen stream produced is usually not fully pure and contains at least one impurity that needs to be removed or minimized following some standards and requirements. The method of production of interest on HyJack is electrolysis (green hydrogen). The impurities present in the hydrogen product from electrolysis are mainly water (H2O) and oxygen gas (O2). Hydrogen purification and separation can be fulfilled by different methods such as: Pressure Swing Adsorption (PSA), Temperature Swing Adsorption (TSA), Cryogenic Separation, Metal Membrane Separation, Polymer Membrane Separation, Metal Hydride Separation and Catalytic Recombination (Deoxidizer).

The most common and used technique to purify H2 after electrolysis is by using a deoxidizer followed by a PSA accompanied by several heat exchangers, compressors, flash drums and other units. The deoxidizer's role is to combine the O2 present in small amounts with the H2 to form H2O aiming to get rid of the O2. The product from the deoxidizer contains mainly H2 with some H2O and little amounts of O2. The product is cooled down (the catalytic recombination is exothermic) and treated in a flash separator to decrease the water content. The decrease in the water content is limited to the freezing temperature of water. Thus, further drying using PSA is required if very low (ppm level) of water is needed.

The purity and recovery of H2 from the deoxidizer are usually 99+ % and up to 99 % respectively. The purity and recovery of H2 from the PSA are usually 99+ % and up to 85 % respectively. The recovery of H2 in PSA is relatively low due to the loss of some H2 in the purging step of the PSA. This recovery value can be increased by about 10 % by using VPSA (Vacuum) which also decreases the energy consumption compared to the PSA.
The Hydrogen Purification Section on HyJack offers a calculator for the deoxidizer, flash separator and PSA.

HyJack Deoxidizer

The deoxidizer model aims to:
• Develop a relationship between inlet total mass flow rate, composition of H2, O2 and H2O in the feed and product, conversion, dimensions (volume, length, diameter) and temperature
• Develop a first level estimation of the deoxidizer cost

Oxygen is removed by performing a reaction between hydrogen and oxygen gases to produce water according to the following reaction:

A typical feed composition to the deoxidizer coming from an alkaline electrolyzer contains 99.5% H2, 0.25 % O2 and 0.25 % H2O. For a Proton Membrane Electrolyzer, a typical feed contains 99.99% H2. For a Solid Oxide Electrolyzer, a typical feed contains 99.9 % H2. These streams need to be treated in several steps in order to reach the limit of 5 ppm O2 and 5 ppm H2O according to the standard of fuel cell ISO 14687:2019 so that the Hydrogen product can be used in fuel cells.

The deoxidizer reaction is usually performed in a Plug Flow Reactor (PFR). The reaction is exothermic, adiabatic and isobaric. The catalyst used is usually of Pd type (0.5wt%) on alumina support. The reaction is usually performed under a Temperature between 25 and 85 ⁰ C (less than 100 ⁰ C and a Pressure of about 1 bar.
The user should just input the total flow rate, H2 purity (%) and O2 composition (%) in the inlet. The H2O composition (%) in the feed is automatically calculated by subtracting 100 from the sum of the H2 and O2 compositions in the feed (%).
The first step is to calculate the conversion of the limiting reactant in this reaction which is the oxygen O2. The targeted O2 composition leaving the deoxidizer is taken by default as 4 ppm. This value can be edited from the customizable parameters. The conversion is calculated from the following formula:

The amounts of H2 reacted and H2O produced are calculated from the following equation:

Where:

n represents the molar flow rate (kmol/hr)

The H2 purity (%) in the outlet is estimated by calculating first the total mass of hydrogen leaving the deoxidizer.

The H2 purity in the outlet (%) is calculated by dividing (100*mH2,outlet) (kg/hr) by the total flow rate (kg/hr).
The O2 composition in the outlet (%) is calculated by divided the targeted O2 ppm level in the product by 10,000.
The H2O composition (%) in the outlet is calculated by subtracting 100 from the sum of H2 and O2 compositions in the outlet.

Once the conversion is calculated, the temperature at which the reaction takes place is estimated by a formula developed relating the conversion with the temperature:

The next step is to estimate the temperature leaving the reactor since the reaction is exothermic. This is done by using the following formula:

Where:

-∆H_rxn is the enthalpy of the reaction and is – 244.9 kJ/mol -Cp is the specific heat capacity of the component -X is the conversion fraction

and

Where:

y_i,0 is the mass fraction of the component “i” in the feed stream

The specific heat capacities for the components are estimated by the following relations:

Where:

To avoid extremely high temperatures leaving the reactor, a maximum 150 ⁰C value is implemented. If the temperature leaving the reactor is higher than this value, cooling is performed to decrease the temperature to 150 ⁰C.

Then, the initial concentration of oxygen CA,0 and the concentration of hydrogen at the reactor outlet CB are calculated from the following formulas:

Where:

Where:

Where:

Where:

-P is the pressure in Pascal -T is the inlet temperature in Kelvin -R is the universal gas law constant = 8.314 m3.PaK.mol -M_total is the Molar Mass of the total stream entering the deoxidizer and is estimated by the following relation:

Where:

-y_i is the individual mass fraction for every component in the feed to the deoxidizer
-M_i is individual Molar Mass for every component in the feed to the deoxidizer

Once the concentration of the hydrogen is estimated, the reaction rate showing the rate of disappearance of hydrogen is calculated by the following formula:

The next step is to calculate the volume of the tubular PFR reactor by the following formula:

This integral is estimated by the Simpson's One-Third Rule which simplifies the formula to:

Where:

-F_A,0 is the flow rate of oxygen at the inlet of the deoxidizer in kmol/hr

To estimate the length and the diameter of the deoxidizer reactor, the length is first assumed and then the diameter is calculated. The length of the tublar PFR is taken by default equal to 1m when the total mass flow rate is lower than 100 kg/hr (exclusive), 2 m when the mass flow rate is between 100 and 1000 kg/hr and 3 m when the mass flow rate is higher or equal to 1000 kg/hr . The length can also be chosen by the user from the customizable parameters.
The diameter is calculated by assuming a tubular shape of the reactor:

HyJack's Deoxidizer cost estimation equation

Last update 15/09/2021

Where:

-CV is the Empty Vessel Cost and the catalyst cost
-FM is the material Factor = 1 for Carbon Steel
-CPL is added costs

Where CV is estimated from the following equation:

With:

-W is the weight of the empty vessel in lbm and is estimated from the following equation:

Where:

-D is the Diameter of reactor in inches
-L is the Length of reactor in inches
-p _C.S. is the density of carbon steel in lbm/inch3 = 0.284 lbm/inch3
-t_s is the reactor thickness in inches and is estimated from the following relation:

Where:

-P _d is the Design Reactor Pressure in psi
-S is the maximum allowable stress at the design temperature in psi = 13,750 psi
-E is the weld efficiency = 0.85
-Diameter in inches

P _d is estimated from the following equation:

Where:

-P _o is the reactor pressure in psi

The catalyst chosen is 0.5 wt % Pd on Alumina support.

The cost of the catalyst is calculated from the following formula:

The cost of catalyst per kg is found to be 325 $/kg.
The mass of the catalyst is evaluated by using the following relation:

The density of the catalyst is found to be 12,020 kg/m3 .

The volume of the catalyst is calculated by assuming that the catalyst is fully loaded inside the tubular reactor and by using the following relation:

Where:

-L _Catalyst is equal to the length of the reactor used minus 10% of the length of the reactor which is used to account for additional length needed for instrumentation, etc.
-D_Catalyst is calculated using the following formula:

Where:

-D _reactor is the diameter of the reactor vessel -The thickness of the vessel is set to 6.0 mm

Finally, C_PL is estimated from the following formula:

Where:

-D is the diameter of the reactor in feet (ft)

It is worth mentioning that the material of construction of the reactor is chosen to be Carbon Steel at 6.0 mm Corrosion Allowance since the hydrogen gas's partial pressure is low, the temperature is not above 200 ⁰C and there is water in the system (corrosive). Also, the conversion rate €/$ is taken to be 1.17.

Results accuracy

A grade is added to each calculation to reflect the accuracy and precision of the results.

A grade of “Accurate Estimation” (3/3) is given if the compressors are used in the recommended ranges. For this technology and sizing, the data available is sufficient to consider the costing used for the pre-feasibility evaluations.

A grade of “Projected Estimation” (2/3) is given to reflect that the costing computed might rely on insufficient sources for this technology and sizing. The lack of data may reflect either that the technology is still under development and thus the results are projections of reachable costs in the future or the sizing computed doesn't correspond to a configuration where this technology is usually deployed.

A grade of “Enhanceable Estimation” (1/3) is given to reflect that the costing computing might rely on an insufficient number of sources for this technology and sizing. To enhance the costing for this configuration, your feedback is welcome.

For the deoxidizer, an accuracy level of “Improvable” (Accuracy Index of 1/3) is given.