Hydrogen Production and Management for Petroleum Processing

Abstract

As refinery product specifications become more stringent to meet environmental requirements, refinery demand for hydrogen has continually increased to supply the required hydroprocessing units. Additional improvements in burning qualities, like cetane, also require more hydrogen. This chapter addresses the processes used to make and/or recover hydrogen for petroleum processing applications. The processes described here include naphtha catalytic reforming, steam-methane reforming, hydrogen recovery, partial oxidation, gasification, olefins cracking, and electrolysis as they relate to hydrogen. Some basic methods for overall refinery hydrogen optimization and management are also described.

Steven A. Treese has retired from Phillips 66.

Appendix Example SMR Hydrogen Plant Material Balance and Monitoring Parameters Development

To calculate the key monitoring parameters for an SMR plant, consider the following example that is typical of the sort of calculations needed. This example is for an SMR feeding natural gas plus refinery saturated off-gas that has been scrubbed free of sulfur (for simplicity). Hydrogen purification is by PSA unit. The plant only has high-temperature shift.

The objectives are to calculate:

• Material balance around the reforming furnace, shift converter, and PSA

• Approach to equilibrium in reformer

• Approach to equilibrium in shift converter

• PSA recovery efficiency

• Shift condensate rate

• Normalized reformer pressure drop

• Actual steam/carbon ratio

The following data are given:

• Flow rates

  • Sweet feed gas = 6.87 MMscfd

  • Product hydrogen (99.9999 % H2) = 20.6 MMscfd

  • Raw hydrogen to the PSA = 31.8 MMscfd

  • Process steam rate = 66,500 lbs/h

• Temperatures

  • Reformer outlet temperature = 1,500 °F

  • HTSC outlet temperature = 810 °F

• Pressures

  • Reformer inlet = 315 psig

  • Reformer outlet = 290 psig

  • Shift converter outlet = 280 psig

  • Product hydrogen from PSA = 260 psig

• Dry gas analyses, vol % by GC

  Component	Feed gas	PSA inlet
  Hydrogen	9.3	69.5
  Methane	79.7	2.3
  Ethane	3.4	0.0
  Propane	1.9	0.0
  Mixed butanes	1.6	0.0
  Mixed pentanes	1.3	0.0
  Hexane and heavier	0.2	0.0
  CO	0.3	3.0
  CO2	0.6	15.7
  N2	0.7	7.3
  O2	0.0	1.9

• Solution

  • 1. Normalize the dry gas analyses to 100 %. Notice that the analysis of the raw gas to the PSA inlet contains a lot of oxygen and nitrogen. This indicates air contamination and is typical of GC results. The air has to be backed out of the analysis. So remove the 1.9 % oxygen and 7.1 % nitrogen (79/21 × 1.9). This leaves 0.2 % nitrogen in the PSA inlet. Normalize the PSA inlet gas to 100 % without the air. The final normalized PSA Inlet gas is:

    • Hydrogen = 76.7 %

    • Methane = 2.5 %

    • CO = 3.3 %

    • CO₂ = 17.3 %

    • N₂ = 0.2 %

  • 2. Set up a material balance spreadsheet for the composition, molar flow rate, mass flow rate, pressure, and temperature at each key location in the unit:

    • Feed gas

    • Reformer inlet feed gas

    • Process steam

    • Reformer outlet

    • HTSC outlet

    • Process condensate (shift condensate)

    • PSA inlet gas

    • PSA product hydrogen

    • PSA off-gas

  • 3. Assume the feed gas flow rate is correct and use the feed gas analysis to calculate the moles of carbon and hydrogen per hour in the feed. Do not forget to include any CO and CO₂ in the carbon. We are assuming no moisture in the feed. Use these values to calculate alpha (α) for use in Eq. 20. In this case, we have 816.2 mols of carbon per hour and 3,105.5 mols hydrogen per hour:

    $$ 2\upalpha =3,105.5/816.2=3.80\upalpha =3.80/2=1.90 $$

  • 4. Calculate the steam/carbon ratio (X) at the reformer inlet by converting the steam rate to moles per hour and dividing it by the moles per hour of carbon:

    • 66,500 lb/h steam/18 = 3694.4 mols/h steam

    • Steam/Carbon = 3694.4/816.2 = 4.53 S/C Ratio = X

  • 5. Calculate the wet shift converter outlet composition using the following equation and knowing α and X from above. The methane, CO, and CO₂ proportions (1-a-b, a, and b) come from the PSA inlet dry gas. The steam portion is calculated from the equation. Hydrogen falls out in the calculation. Remember that the nitrogen will come through as an inert:

    $$ {\mathrm{CH}}_2\upalpha +{\mathrm{XH}}_2\mathrm{O}\leftarrow \to \mathrm{a}\mathrm{C}\mathrm{O}+{\mathrm{bCO}}_2+\left(1-\mathrm{a}-\mathrm{b}\right){\mathrm{CH}}_4+\left(\mathrm{X}-\mathrm{a}-2\mathrm{b}\right){\mathrm{H}}_2\mathrm{O} + \left(3\mathrm{a}+4\mathrm{b}+\upalpha -2\right){\mathrm{H}}_2 $$

    (31)

    • where

    • α = factor based on feed C/H molar ratio

    • X = steam/carbon molar ratio

    • a, b = coefficients for CO and CO₂ concentrations, respectively, at any point in the process

  • 6. The shift or process condensate should be the residual water from the reaction. Use this as the process condensate flow rate. You can include dissolved CO₂, but the amount of dissolved CO₂ is not significant in the overall process.

  • 7. Now check the overall balance in the unit by adding the feeds (feed gas plus steam plus any recycle hydrogen) and comparing to the HTSC outlet (PSA inlet gas plus calculated process condensate). Check the carbon and hydrogen balances also. Make adjustments to flow rates until you closed the balance. This sometimes means compromising on rates or slight adjustments to compositions. There are many sources of possible error in these calculations so flexibility is needed. If the balance can be closed to +/−2 %, that is good enough for the analysis.

  • 8. Now estimate the reformer outlet composition.

    • Assume the HTSC outlet (in moles) is the correct methane slip from the reforming furnace. Here the slip is 2.5 v% of the dry gas or about 87 mols/h.

    • Calculate the CO and CO₂ proportions by assuming the gas is at water-gas shift equilibrium at the reformer outlet. The CO and CO₂ proportions can be derived by iteration until the calculated K_WGS at the reformer outlet matches the K_WGS at the reformer outlet temperature of 1,500 °F. Use the definition:

      • $$ {\mathrm{K}}_{\mathrm{WGS}}=\left.1\left({\mathrm{P}}_{{\mathrm{H}}_2} \times {\mathrm{P}}_{{\mathrm{CO}}_2}\right)/{\mathrm{P}}_{{\mathrm{H}}_2\mathrm{O}}\times {\mathrm{P}}_{\mathrm{CO}}\right) $$

      • where the partial pressures are in atmospheres in the wet gas.

      • Use the equation for K_WGS for the SMR process discussion.

      An easier approach with sufficient accuracy is to use the equilibrium charts provided in the SMR monitoring discussion for methane to estimate the % CO and CO₂ in the dry gas at the reformer outlet. From the chart, the CO at the reformer outlet should be about 11.5 v% and CO₂ would also be about 11.5 v%.

      We brought in 816.2 mols/h carbon. 87 mols/h stayed in methane as methane slip. The remaining 729 mols per hour is roughly split evenly between CO and CO₂ at the reformer outlet (about 365 mols/h each). Iterative calculations give values of about 359 mols/h for CO and 370 mols/h for CO₂.

      Check to be sure the reformer outlet mass balances, including C and H.

  • 9. Calculate the partial pressures of methane, CO, CO₂, hydrogen, and water in the wet gases at the outlet of the reforming furnace and shift converter. The partial pressures must be in atmospheres absolute.

  • 10. Calculate K_SMR at the reformer outlet and K_WGS at the shift converter outlet using the definitions in the SMR process discussion.

    Reformer outlet actual K_SMR = ~298 (Table is reciprocal of this)

    Shift converter outlet actual K_WGS = ~6.23

  • 11. Calculate the equivalent equilibrium temperatures using the equations in the SMR process discussion or use the charts in that section.

    Reformer actual equilibrium temperature = ~1, 515 °F

    Shift converter actual equilibrium temperature = 878 °F

  • 12. Finish the material balance streams and check the overall, carbon, and hydrogen balances.

  • 13. The key monitoring parameters can now be calculated:

    • Overall material balance was derived as part of the calculations. See Table 15 for the basic material balance.

    • Approach to equilibrium (ATE) in the reformer

      $\mathrm{Reformer}\ \mathrm{ATE}=\mathrm{actual}\hbox{--} \mathrm{calculated}=1,500\hbox{--} 1,515{}^{\circ}\mathrm{F}=-15{}^{\circ}\mathrm{F}$

    • Approach to equilibrium (ATE) in the shift converter

      $\mathrm{Shift}\ \mathrm{ATE}=\mathrm{actual}\hbox{--} \mathrm{calculated}=878\hbox{--} 810\ {}^{\circ}\mathrm{F}=+68\ {}^{\circ}\mathrm{F}$

    • PSA recovery efficiency

      • $\mathrm{Recovery}=\left(\mathrm{mols}\ {\mathrm{H}}_2\mathrm{in}\ \mathrm{product}*100\%\right)/\mathrm{mols}\ {\mathrm{H}}_2\mathrm{in}\ \mathrm{feed}$

      • $\mathrm{Product}\ {\mathrm{H}}_2=20.6\ \mathrm{MMscfd}\to\ 2,262.0\ \mathrm{mols}/\mathrm{h}$

      • $\mathrm{P}\mathrm{S}\mathrm{A}\ \mathrm{inlet}\ {\mathrm{H}}_2=76.8\%\ \mathrm{of}\ 31.8\ \mathrm{MMscfd}\to 2,721\ \mathrm{mols}\ {\mathrm{H}}_2/\mathrm{h}$

      • $\mathrm{Recovery}=100\%*2,262/2,721=83.1\%$

    • Shift condensate rate

      This falls out of the material balances as 2, 378 mols/h (leftover process steam) = 42.3 Mlb/h or ~85 gpm.

    • Actual steam/carbon ratio

      This also fell out of the calculations as X = 4.53.

    • Normalized pressure drop

      For normalizing the pressure drop, prior history and a normalization basis (reference conditions) are required. Here, from prior development work, the equation for normalization of dP was found to be

      $$ {\mathrm{dP}}_{\mathrm{norm}}={\mathrm{dP}}_{\mathrm{meas}}\times {\left[{\left({\mathrm{Q}}_{\mathrm{feed}}+{\mathrm{Q}}_{\mathrm{steam}}\right)}_{\mathrm{norm}}/{\left({\mathrm{Q}}_{\mathrm{feed}} + {\mathrm{Q}}_{\mathrm{steam}}\right)}_{\mathrm{meas}}\right]}^{1.6} $$

      • where:

      • dP_norm = normalized dP at reference conditions

      • dP_meas = measured dP at actual conditions

      • Q_feed = inlet volumetric flow of feed, reference or actual

      • Q_steam = inlet volumetric flow of steam, reference or actual

      Here:

      • ${\mathrm{dP}}_{\mathrm{meas}}=315\hbox{--} 290\ \mathrm{psig}=25\ \mathrm{psig}$

      • ${\mathrm{Q}}_{\mathrm{feed},\mathrm{meas}}=6.87\ \mathrm{MMscfd}$

      • ${\mathrm{Q}}_{\mathrm{steam}, \mathrm{meas}}=66,500\ \mathrm{lb}/\mathrm{h} \times 24\ \mathrm{h}/\mathrm{day} \times 379.45\ \mathrm{s}\mathrm{c}\mathrm{f}/18\ \mathrm{l}\mathrm{b}\mathrm{s}\to 33.6\ \mathrm{MMscfd}$

      • ${\left({\mathrm{Q}}_{\mathrm{feed}} + {\mathrm{Q}}_{\mathrm{steam}}\right)}_{\mathrm{norm}}=51.9\ \mathrm{MMscfd}\ \left(\mathrm{f}\mathrm{o}\mathrm{r}\ \mathrm{normalization}\ \mathrm{r}\mathrm{eference}\ \mathrm{case}\right)$

      So

      $$ {\mathrm{dP}}_{\mathrm{norm}}=25\times {\left[51.9/\left(6.9+33.6\right)\right]}^{1.6}=37\ \mathrm{psi} $$

      Table 15 Example SMR hydrogen plant material balance development

  • 14. From the parameters monitored, we can observe that:

    • Depending on how much adjustment was needed to get a good material balance, flow meter, instrument, or analytical issues may be indicated that would bear some evaluation.

    • The reformer approach to equilibrium is fairly good. Trending the calculated value would provide an indication of deactivation rate.

    • The shift converter ATE should also be trended over time for complete analysis. An ATE of 68 °F is a little high and may indicate the shift converter catalyst has deactivated some or the temperature is not optimum. It could also indicate bad data.

    • The PSA recovery efficiency of 83 % is lower than you should expect. Recovery of at least 85 % should be achievable. This would bear further investigation.

    • The shift condensate rate is informational primarily. It may be needed for loading evaluation of the degasifier on occasion.

    • The normalized pressure drop of 37 psi is somewhat high for most reformers and may indicate some catalyst crushing or coking from age or an incident. 37 psi is not particularly alarming. This value should be trended over time.

    • The steam carbon ratio at 4.53 is a little high for most PSA-type units, but would be common in older units using solvent CO₂ cleanup. It could probably be lowered.

  • 15. These calculations can be built into a spreadsheet, including the trial-and-error, iterative solution to the reformer outlet composition and material balance closure. This would save a significant amount of time and enable more detailed operations evaluation to be completed more often.