March 2018

Process Optimization

Cost-effective revamp of CO2 removal systems

The cost-effective revamp experiences of carbon dioxide (CO2) removal systems in three different ammonia plants, and the resulting attractive payback of just a few months are detailed here.

Arora, V. K., Kinetics Process Improvements, Inc.

The cost-effective revamp experiences of carbon dioxide (CO2) removal systems in three different ammonia plants, and the resulting attractive payback of just a few months are detailed here. One of the plants uses a methyldiethanolamine/piperazine (MDEA/PZ) system, and the other two use Benfield systems. Similar approaches can be used in acid gas removal systems in various petrochemical plants and refineries.

Ammonia Plant 1 with the MDEA-based CO2 removal system was earlier revamped from its original nameplate capacity of 600 tpd to approximately 1.1 Mtpd (thousands of tons per day), using the original MEA-trayed absorber and stripping columns replaced with packing and new internals. The plant experienced excessive CO2 slip (up to 3,000 ppmv) at increased rates, resulting in reduced plant efficiency. A review of the complete CO2 removal system was conducted, along with field measurements to identify the key bottlenecks. Following this, several cost-effective and practical options were identified to reduce CO2 slippage to a target value of less than 300 ppmv at the present capacity, along with its maximum capacity of 1,170 tpd and future capacity of 1,250 tpd.

A combination of new efficient packing and new distributors, along with an increase in circulation, were insufficient to meet this target due to mass transfer limitations. To support the performance targets, the absorber column was closely reviewed to increase the packing height with nominal modifications with different configurations. The studied options were jointly reviewed with the customer’s operations, engineering and construction groups to select the most suitable practical option to meet the target performance with a 27% increase in the packing height. The selected option is now in the implementation phase and the hardware has already been ordered.

A similar issue of high CO2 slip and corrosion in a two-stage MDEA/PZ system in another ammonia plant is now being studied.

The other two ammonia plants, Plant 2 and Plant 3, each maintained 2 Mtpd of capacity using Benfield ACT-1 CO2 removal systems operated at 108% of nameplate capacity. Both plants consistently experienced significant carryover from the absorber, resulting in pressure-drop buildup across the downstream methanator feed-effluent exchanger. Based on plant historical data, the system segment pressure drop increased from 20 psi to 30 psi in about 3 mos, resulting in a gradual increase in front-end pressure and a gradual reduction in ammonia production and efficiency. This forced operators to briefly shut down the plant every three mos to clean the exchanger, and also resulted in an additional loss of ammonia production of nearly 10 hr, reducing plant reliability. This problem continued despite new liquid distributors and demisters.

Following this, an independent process technology consulting and engineering company was engaged to study and review the potential deficiencies, and to recommend cost-effective improvements to minimize or eliminate the carryover in the Benfield systems of both plants.

Following the review of all studied options with recommended modifications, the consulting and engineering company was advised to further carry out the engineering and supply of all necessary hardware for both plants. The hardware was successfully installed in 2009 in both ammonia plants, which saw better than expected performance without any loss of ammonia production or plant shutdown until the next turnaround in 2013.

AMMONIA PLANT 1

MDEA-based CO2 removal system

The existing single-stage MDEA CO2 removal system scheme is shown in FIG. 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 tpd to 1,100 tpd. The original absorber and stripper columns were used with trays replaced with packings and other internals. The present operating capacity is 1,140 tpd–1,170 tpd, depending on the seasonal variation. This plant was well stretched to its design limits and beyond.

FIG. 1. MDEA/PZ CO<sub>2</sub> removal scheme.
FIG. 1. MDEA/PZ CO2 removal scheme.

 
A holistic review of the reference CO2 removal system was conducted to identify all potential bottlenecks that might have been 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 present operating conditions:

o  Mass transfer limits of the existing packing type and height

o  Adequacy/limitations of the liquid distributor

o  Adequacy/limitations of the feed vapor distributor

o  Hydraulic adequacy/limitations of the solvent circulation loop

o  Solvent and activator concentration for optimal performance.

FIGS. 2, 3, 4 and 5 represent the base operating performance at 1,140 tpd, as modeled and reconciled with the actual operating performance. A gamma scan of the absorber indicated the liquid density variation profile in FIG. 2, with a variation of 8–15 units indicating maldistribution. The absorber operated at 85% flood, while the stripper had sufficient hydraulic capacity, as shown in FIG. 5. The absorber temperature profile in FIG. 3 seems reasonable, while CO2 concentration profiles in FIG. 4 indicate 2,600 ppmv of CO2 slip.

FIG. 2. Absorber liquid density profile.
FIG. 2. Absorber liquid density profile.

  

FIG. 3. Absorber temperature profile.
FIG. 3. Absorber temperature profile.

 

FIG. 4. Absorber vapor CO<sub>2</sub> concentration profile.
FIG. 4. Absorber vapor CO2 concentration profile.

  

FIG. 5. Flood %—absorber and stripper.
FIG. 5. Flood %—absorber and stripper.

 

Potential causes of high CO2 slip

Based on the initial evaluation, the absorber column indicated the major limitations resulting in performance shortfall. The potential causes identified in the absorber system were:

  • Liquid maldistribution determined through gamma scan
  • An undersized liquid distributor in the absorber, leading to maldistribution
  • High momentum through the vapor 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

In the next step, several options were evaluated, and relevant inputs were 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
  • An increase in packing height, as noted later for different options
  • New liquid distributor
  • New feed vapor distributor
  • An increase in circulation rate
  • Optimized 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 that were rated with sufficient design margin over the new service conditions for both present and future operating cases. Most importantly, the new distributors were designed for installation and removal through the existing 17-in. manways to facilitate correct loading of packing.

The existing feed vapor distributor was also found to be inadequate, with a much higher momentum than recommended and insufficient coverage of the cross section. It was replaced with a T-type lateral distributor that was rated with sufficient design margin over the new service conditions for both present and future operating cases. Most importantly, the new distributors were designed for installation and removal through the existing 17-in. 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 or their drivers. Interestingly, a marginal increase in circulation rate was possible by replacing the existing impellers with the maximum possible size 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 the new packing type and size, and different bed configurations, as covered under the new packing.

Efficient packing and configurations

To improve the limitations of both mass transfer and hydraulics in the absorber, new packings from two reputed suppliers were evaluated with extensive in-house modeling for their quantitative impact on performance. The improved hydraulics with the newly selected packing with increased packing height (127% of the existing packing height) are shown in FIG. 6 and compared with the hydraulics of the existing packing for both base and future capacities (1,140 tpd and 1,250 tpd, respectively). The hydraulic capacity of the absorber indicates a substantial improvement with the new packing.

FIG. 6. Flood %—absorber, new and old packing.
FIG. 6. Flood %—absorber, new and old packing.

 

The latest and most efficient proven packings from two suppliers were reviewed and modeled to evaluate their impacts on CO2 slip and hydraulics. A combination of split-bed configurations with two different packing sizes—with and without liquid redistributors—were also reviewed. Based on the detailed evaluation and modeled performance, it was decided to pursue only one deeper bed for the most value, as discussed here.

Incremental packing height and practical constraints

The existing packing height was determined to be a limiting factor to achieve the target CO2 slip, despite the changes with the packing, the vapor-liquid distributors and the optimized solution concentration. Several options to maximize the packing bed height were closely investigated (TABLE 1) with all practical constraints for this old column.

 
Based on a thorough review of all options with the customer’s operations, construction and engineering teams, as well as the facility’s inspection history, it was decided to pursue the third maximum height option, with some hot work within the absorber column.

Estimated performance improvements

The new performance of the CO2 removal was estimated using the new packing, new vapor and liquid distributors, and an optimized solution concentration. The performance with the new internals/packing with optimized solvent was further compared for two capacity cases (base and future) using the modified packing height (127% of the existing packing height) in the existing absorber to provide the most value for the lowest cost.

The additional packing height provides a significant reduction in CO2 slip, achieving well below 300 ppmv for the base capacity and below 500 ppmv for the future capacity, as shown in FIG. 7.

FIG. 7. Performance estimate with modifications.
FIG. 7. Performance estimate with modifications.

 
Reducing CO2 slip benefits ammonia plant efficiency with a proportionate increase in ammonia production for the same amount of feed gas used with high CO2 slip. Incremental ammonia production with the improved performance of the CO2 removal system for the base and future operating capacities are estimated and shown in FIG. 8.

FIG. 8. Incremental ammonia production with reduced CO<sub>2</sub> slip.
FIG. 8. Incremental ammonia production with reduced CO2 slip.

 

Economics

Based on the modifications being carried out and the expected performance improvements, the payback periods for the base and future capacities are estimates to be less than 8 mos and 4 mos, respectively (FIG. 9). The basis of this estimate is the incremental ammonia production relative to the base ammonia production, corresponding to high CO2 slip for the two capacity cases using the median netback on ammonia.

FIG. 9. Payback estimate of modifications.
FIG. 9. Payback estimate of modifications.

 

Another MDEA-based, two-stage CO2 removal system is under review for high CO2 slip and corrosion related issues (FIG. 10).

FIG. 10. MDEA/PZ scheme, two-stage.
FIG. 10. MDEA/PZ scheme, two-stage.

 

AMMONIA PLANTS 2 AND 3

The existing Benfield process scheme for CO2 removal in Ammonia Plant 2 and Plant 3 is shown in FIG. 11. Each ammonia plant operated at approximately 108% of nameplate capacity of 2,000 tpd and consistently experienced a significant carryover from the absorber, resulting in pressure-drop buildup across the downstream methanator feed/effluent exchanger. Based on plant historical data, the system segment pressure drop increased from 20 psi to 30 psi in about three mos, resulting in a gradual reduction in ammonia production and plant efficiency. This situation forced operators to briefly shut down the plant every three mos to clean up the exchanger, which also resulted in additional loss of ammonia production for nearly 10 hr, further reducing plant reliability. This problem continued despite a replacement with new liquid distributors and demisters in both absorbers and syngas knockout (KO) drums. Following this equipment replacement, the consulting and engineering company was engaged to study and review the potential deficiencies, and to recommend suitable cost-effective improvements to minimize or eliminate the carryover.

FIG. 11. Benfield process schematic for ammonia Plants 2 and 3.
FIG. 11. Benfield process schematic for ammonia Plants 2 and 3.

 

The following potential causes of carryover were identified:

  • There was a significant fraction of smaller droplets (< 10 microns) in the carryover. Recently replaced separation devices were considered inadequate to efficiently capture the smaller droplets below 10 microns.
  • Insufficient vapor disengagement space in absorbers and syngas KO drums was leading to channeling with inefficient vapor-liquid separation.
  • Makeup water quality with the carryover of any undissolved solids was eventually deposited in the downstream methanator feed/effluent exchangers.
  • Excessive foaming could potentially result in carryover.
  • Lower velocities with carryover, coupled with higher localized temperature in the downstream methanator feed/effluent exchanger could promote fouling rates.

Findings and recommendations

Based on the adequacy check and further analysis of the absorber overhead system, the following recommendations were made:

  • The vapor-liquid disengagement space in the syngas KO drum was found to be inadequate. This was considered to be a significant cause of uneven flow distribution and channeling, resulting in poor separation efficiency and potential carryover. It was recommended to replace the existing slotted pipe feed distributor with an even flow distributor to overcome this limitation (FIG. 12).
  • The recently replaced new demister pads in the absorbers and syngas KO drums of both plants were also found to be inadequate to efficiently capture the smaller liquid droplets, potentially resulting in carryover. It was recommended to replace the pads with a new design using a combination of co-knit polymer with metal, as shown in FIG. 13.
  • Syngas velocities in the shell side of the feed/effluent exchangers caused concern initially, but no modification was warranted, as the intent was to simply minimize or eliminate the carryover as opposed to pushing the carryover through higher exchanger velocities into the downstream catalyst beds. Therefore, no change in the downstream exchanger was recommended.
  • A Phase 2 recommendation was made for an in-situ spray system for the syngas KO drums, in the event that the above recommended modifications fail to yield the expected performance.
FIG. 12. Even flow distributor.
FIG. 12. Even flow distributor.

 

FIG. 13. Special demister with co-knit polymer.
FIG. 13. Special demister with co-knit polymer.

 

Modifications and performance improvements

Based on these findings and recommendations, the following modifications were engineered and supplied for both plants:

  • Special co-knit polymer demisters for absorbers and syngas KO drums for both ammonia plants
  • Even flow distributors were engineered to be supported within the existing vessels without any hot work on the vessel shells.

FIG. 14 charts an ∆P trend of more than 450 d of performance before and after the modifications, and clearly indicates a fairly steady pressure drop. No plant shutdown or any loss of ammonia production was experienced for the next 4 yr before the turnaround for this lingering carryover problem in both ammonia plants. The simple modifications were successful and were carried out within a day. Further, the Phase 2 recommendation to include the spray system was not required during this period.

FIG. 14. <i>ΔP</i> trend, before and after the modifications.
FIG. 14. ΔP trend, before and after the modifications.

 
The modifications implemented were simple, and were engineered and supplied within a month by the consulting and engineering company. They were installed quickly within a day shift by the customer. Based on the reclaim of the lost production following the modifications, the real payout was less than three mos.

Key learnings

High CO2 slippage is a common problem experienced in ammonia plants and other acid gas removal systems in petrochemical plants and refineries, particularly when plant capacities are stretched by common limiting factors, including:

  • Limiting mass transfer due to inadequate vapor/liquid distribution, inefficient packing, and packing height or stage limitations
  • Heat transfer limits (cooling, reboiling)
  • Insufficient circulation due to limiting pump capacities
  • Less than optimal solution concentration. HP

 

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