Oct-2012

Simulating and monitoring H2 plant operations

A computer program was developed to enable a refinery to monitor its hydrogen production daily to foresee potential operating problems

GENE YEH, Saudi Aramco
PRABHAS MANDAL, ABDULALI SIDDIQUI and FAHAD ALHEMIDDA, Saudi Aramco Riyadh refinery

Article Summary

Daily monitoring of H2 plant operation on-site is essential so that we can spot potential operating problems in time. Therefore, there is a need to develop a user-friendly Visual Basic computer program in-house for refinery process engineers to monitor catalyst activity and important operating parameters, as well as simulate H2 plant operation. This article describes the development of such a computer program for the Riyadh refinery’s conventional and modern H2 plants, and cites actual examples to show how this program has been used to simulate and monitor H2 plant operations. 

A hydrogen plant includes a hydrogenator, desulphuriser, steam reformer, high and low temperature shift converter, as well as a CO2 removal unit and methanator for a traditional H2 plant (see Figure 1), or a pressure swing adsorber (PSA) for a modern H2 plant (see Figure 2). Monitoring critical operating parameters including the steam-to-carbon ratio (S/C), methane and CO slips, as well as catalyst activity is essential for smooth hydrogen plant operation.

Our current practice is that refineries send operating data to the catalyst vendor periodically for evaluation of catalyst performance. The catalyst vendor, after evaluation, sends the report back to the refineries. This practice takes time and the feedback from the vendor could not provide in-time warning. 

To facilitate H2 plant operation, a Visual Basic program should calculate steam reforming equilibrium composition, steam reforming catalyst activity in terms of approach to equilibrium (ATE), high temperature shift (HTS) and low temperature shift (LTS) reaction equilibrium composition, HTS and LTS catalyst activities in terms of rate constants, ATE for HTS and LTS reactions, and hydrogen product flow and purity for conventional and modern H2 plants. 

The computer program should provide a scientific method to monitor H2 plant operation and check catalyst performance quantitatively. The catalyst replacement can be scheduled based on the trend of catalyst activity. This program should allow an inexperienced engineer to monitor and simulate H2 plant operation and can be used to identify dubious analytical data and conduct operational variable studies. 

Visual Basic program development
The programs are developed using an Excel spreadsheet for easy input and output for the Riyadh refinery’s modern H2 plant and conventional H2 plant, respectively. To increase accuracy, a non-ideal gas mixture is assumed and the fugacity coefficient is calculated using the Soave-Redlich-Kwong equation of state for each component (CH4, CO, CO2, H2, H2O, N2 and NH3). The fugacity coefficient is then used in calculating the equilibrium composition. The equilibrium composition for the steam reformer is calculated by solving material balance equations for each element (C, H, O and N) and equilibrium equations for steam methane reforming (Equation 1), water gas shift (Equation 2) and ammonia synthesis (Equation 3). Nevertheless, the equilibrium composition for a shift converter is calculated by solving material balance equations of each element and the equilibrium equation of the water gas shift reaction only (Equation 2):

Methane steam reforming
CH4  +  H2O ⇔ CO  + 3 H2     ΔH298K = 49 kcal/mol        (1)

Water gas shift
CO  + H2O ⇔ CO2 + H2     ΔH298K = -10 kcal/mol             (2)

Ammonia synthesis
1/2 N2 + 3/2 H2 ⇔ NH3      ΔH298K = -10.9 kcal/mol           (3)

ATE is an indicator of the steam reforming catalyst activity, which is equal to Tout- Teq for the endothermic steam reforming reaction. Nevertheless, ATE equals to Teq -Tout for the exothermic water gas shift reaction. (Tout is outlet temperature and Teq is the temperature at which the outlet gas is in equilibrium with respect to the particular chemical reaction.)

To facilitate monitoring of the HTS and LTS catalyst performance, catalyst activities in terms of rate constants for shift reactions are calculated according to Equation 4. The plot of catalyst activity can facilitate the planning of shift catalyst replacement: 
where:   
k0: Forward rate constant for HTS or LTS reaction
GHSV: Gas hourly space velocity
K: Equilibrium constant for HTS or LTS reaction
CO0: CO composition at shift
converter inlet in %
H2O0: H2O composition at shift
converter inlet in %
CO2 0: CO2 composition at shift
converter inlet in %
H2 0: H2 composition at shift
converter inlet in %

a = K-1

b = - K (1+ H2O0) - CO2 0 - H2 0
            CO0         CO0       CO0

c = K H2O0 - CO2 0 * H2 0
                 CO0     CO0       CO0