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Jun-2017

Upgrading CO2 removal systems

Simple modifications to three carbon dioxide removal systems raised their efficiencies with short payback periods

V K ARORA
Kinetics Process Improvements
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Article Summary
Acid gas removal is an important step in petrochemical plants, refineries and syngas production. This article describes experience in cost effectively upgrading CO2 removal systems in three ammonia plants with an attractive payback of just a few months. One of the plants uses a MDEA system and the other two use Benfield systems. A similar approach can be used in acid gas removal systems in petrochemicals plants and refineries. 

MDEA based CO2 removal system
The existing single stage MDEA CO2 removal system scheme is shown in Figure 1. This conversion of an old MEA based system was implemented as a part of the overall ammonia plant capacity revamp from the original nameplate capacity of 600 t/d to about 1100 t/d. The original absorber and stripper columns were used, with trays replaced with packings and other internals. The current operating capacity is 1140 t/d to 1170 t/d, depending on seasonal variation. This plant was stretched to its design limits and beyond.

A holistic review of the reference CO2 removal system was carried out by KPI to identify all the potential bottlenecks contributing to a shortfall in performance. To support this, the following steps were taken:
• Gamma scan of the columns to determine any maldistribution
• Representative operating data corresponding to maximum operating capacity
• Reconciliation of the operating data
• Simulation of the existing scheme to match the reconciled operating data
• Evaluation of potential bottlenecks at the current operating conditions:
    ν Mass transfer limits of the existing packing type and height
    ν Adequacy/limitations of liquid distributor
    ν Adequacy/limitations of feed vapour distributor
    ν Hydraulic adequacy/limitations of the solvent circulation loop
    ν Solvent and activator concentration for optimal performance.

Figures 2-5 represent the base operating performance at 1140 t/d as modelled and reconciled with actual operating performance. A gamma scan of the absorber indicates the liquid density variation profile in Figure 2, with a variation between 8 and 15 units indicating maldistribution. The absorber is operating at about 85% flood while the stripper has enough hydraulic capacity available (see Figure 5). The absorber temperature profile in Figure 3 seems reasonable while the CO2 concentration profile in Figure 4 indicates about 2600 ppmv of CO2 slip.

Potential causes of high CO2 slip
Based on an initial evaluation, the absorber column indicated major limitations, resulting in a shortfall in performance. The potential causes identified in the absorber system were:
• Liquid maldistribution determined through gamma scan
• Under-sized liquid distributor in the absorber, leading to maldistribution
• High momentum through the vapour distributor in the absorber, leading to maldistribution
• Mass transfer limitations due to short packing height and incorrect loading 
• Hydraulics and mass transfer limitations of the existing packing. 

The stripper column did not indicate any hydraulic or mass transfer limitations or any performance issues.

Options to reduce CO2 slip
As the next step, several options were evaluated with relevant inputs gathered from vendors. The following options were further simulated and reviewed for improved performance, including cost-benefit analysis:
• New efficient packing configurations with improved mass transfer and hydraulics
• Increase in packing height, as noted later for different options
• New liquid distributor
• New feed vapour distributor
• Increase in circulation rate
• Optimise solution concentration.
      
New liquid distributor
The existing trough type V-notch liquid distributors were inadequate and considered less efficient for the service conditions. They were replaced with new efficient orifice deck distributors rated with sufficient design margin over the new service conditions for current and future operating cases. Most importantly, the new distributors were designed for installation and removal through the existing 17in manways to facilitate correct loading of packing.  

New feed vapour distributor
The existing feed vapour distributor was also found to be inadequate, with a much higher momentum than recommended and also insufficient coverage of the cross section. It was replaced with a T-type lateral distributor rated with sufficient design margin over the new service conditions for both the current and future operating cases. Most importantly, the new distributors were designed for installation and removal through the existing 17in manways.  

Increase in circulation and hydraulics adequacy
Increasing the solvent circulation rate was reviewed along with a complete hydraulics evaluation of the lean circuit and the lean MDEA pumps, with a clear premise not to replace any of the existing pumps and drivers. Interestingly, a marginal increase in circulation rate was possible with replacement of the existing impellers at the maximum possible size, well within the maximum design rating of the existing drivers. Further, the impact of the higher circulation rate was also evaluated for both absorber and stripper columns with new packing type, size and different bed configurations.

New efficient packing
To improve the limitations of both mass transfer and hydraulics in the absorber, new and efficient packings from two suppliers were evaluated with extensive in-house modelling for their quantitative impact on performance. The improved hydraulics with the selected new efficient packing with increased packing height (127% of the existing height) is shown in Figure 6 and compared with the hydraulics of the existing packing for both base and future capacities (1140 t/d and 1250 t/d, respectively). The hydraulic capacity of the absorber indicates a substantial improvement with the new efficient packing.

New packing configurations
The latest and most efficient proven packings from two suppliers were reviewed and modelled to evaluate their impact on CO2 slip and hydraulics. A combination of split bed with two different packing sizes – with and without liquid redistributors – was also reviewed. Based on the detailed evaluation and modelled performance, it was decided to go ahead with only one deeper bed for the most value.
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