Lower Cost Operation with Low Pressure Drop Support Grids in HTS

and LTS Shift Reactors In June 2014, Agrium's ForI Saskatchewan, Alberta Nitrogen facility took an outage to perform

mabrtenance on the ammonia plant. This maintenance included the replacement of the high and low temperature shift catalysts. Upon removal of the spent catalysts and support balls, new catalyst

support grids supplied by Haldor Topsoe were safely installed inside the reactors. The reactors were then reloaded with Haldor Topsoe's HTS and LTS catalysts. The plant was restarted, the catalysts

were reduced and HTS and LTS put online. This paper describes the modifications made to the catalyst support system, the resulting decrease in pressure drop and the reduction in operating cost.

Christopher Biegel, P.Eng Agrium Inc.

Dan Morton, P .E. Haldor Topsoe

Introduction

Reducing front-end pressure drop (6.P) in ammonia plants is well known to reduce operating costs by decreasing energy consumption and increasing production.

One specific area of interest involves reducing pressure drop across catalyst containing vessels. There are several technologies aimed at reduc­ing the 6.P across these vessels such as altering internals or by choosing to load low pressure drop catalysts and support ballslhigh voidage fraction inerts. This paper discusses the vessel modifications made to the Agrium Fort Sas­katchewan Nitrogen facilities' high temperature shift (HTS) and low temperature shift (L TS) re­actors, the resulting reduction in 6 P, and the

2015 71

economic benefits realized by implementing this particular technology.

Background

Agrium's Fort Saskatchewan Nitrogen Opera­tion (FNO) operates a M. W. Kellogg designed ammonia plant with a nameplate capacity of 1000 MTPD ( 1100 STPD). Present day produc­tion of the ammonia plant is 1300 MTPD (1430 STPD) due to several small debottlenecks and by maximizing existing equipment capacity. To achieve further incremental increases in ammo­nia production and for better energy efficiency FNO teamed up with Haldor Topsoe (HT) engi-

AMMONIA TECHNICAL MANUAL

neers to design and supervise the safe installa­tion of the catalyst support grids in the HTS and L TS reactors.

Original Configuration and Performance

The HTS reactor at FNO had an original internal configuration as shown in Figure I :

____ ~=-~ .!l~.!-____ _

High Void Fraction Inert

HTS Catalyst

14ft ----01

••••• l -Suppo!I BaIll •• -.-". _.'

Figure 1. Original HTS Catalyst and Support Ball Configuration

This configuration had been utilized in the HTS reactor since plant inception and only changes to catalyst and support balls have been made in order to reduce pressure drop. The typical vol­ume of catalyst loaded in FNO's HTS reactor is approximately 42.47 m3 (1500 ft3) and is changed-out every 4 to 5 years in order to corre­spond with maintenance outages_ The typical 6.P of the HTS over the course of its service life can be seen below in Figure 2_

AMMONIA TECHNICAL MANUAL 72

r HTS Pressure Drop· Ori,inal Confilluntion (60 Monthsl .• ••

, .• t • I .

I • • __ ~L...,~~

. . - - - - - - - - ---_. Figure 2. Typical Pressure Drop of HTS Reac­tor - 60 Month Service Life

The original configuration in the HTS provided a relatively low pressure drop of approximately 23 kPa (3.33 psi) at start-of-run.

The original internal configuration of the L TS reactor is shown in Figure 3.

----I'.::.~~---­Catalyst Guard

LIS Catalyst

,4ft ---I

::::I£:§@~:::: y .. SuppoII 8aII

• '. '. 1- Suppo<I Balls •• ,' • ". ..'

Figure 3. Original LTS Catalyst and Support Ball Configuration

This configuration had also been utilized since plant inception and changes to catalyst and sup­port balls had also been made in order to reduce pressure drop. The L TS at FNO is typically

2015

loaded with 50.97 m3 (1800 f(3) of catalyst but is capable of holding up to 62.30 m3 (2200 f(3).

The "short" load of L TS catalyst better aligns with FNO's maintenance outages and not only reduces pressure drop across the catalyst bed but also reduces the amount of active catalyst wast­ed.

The typical ~p of the L TS reactor with the con­figuration described above over the course of its service life can be seen below in Figure 4.

lTS Pressure Drop · Ori,inal Configuration ISO Months)

• •

"

" "

"'" "'" ..... ,JIll "e .. _.-Figure 4. Typical Pressure Drop of LTS Reactor - 50 Month Service Life

This particular charge of L TS catalyst had a pressure drop of 40 kPa (5.80 psi) at start-of­run.

Support Grid Design Advantages

Overall Support Grid Design

The catalyst support grid consists of a modified out let collector, outlet brackets. outlet connector rods, skirt section and a mesh grid section, all of which floats in the bottom of the vessel of inter­est. A side and top view of the grid is shown in Figure 5 and Figure 6 respectively .

Catalyst Suppon Gridin Vnul

"tssd Outlet !><oult

Figure 5. Side View a/Support Grid in Vessel

2015 73 AMMONIA TECHNICAL MANUAL

Figure 6. Top View of Support Grid

Prior to purchase and installation FNO identi­fied several advantageous features with the HT catalyst support grid design which aided in the decision to complete the investment project.

Fabrication Time

One advantage that influenced FNO's decision to install the HT catalyst support grids was the relatively short fabrication time. HT was able to fabricate both grids in a 16 week window (standard supply time) and claims that as little as 6 week fabrication is possible if required.

Flexible Mechanical Sealing

HT has des igned the catalyst support grid with­out the need to use packing rope or insulation to seal to the vessel wall . A good seal between the support grid and the vessel walls which per­fonns well over thennal cycles is critical in pre­venting support ball or catalyst migration over the li fe of the grid through many shutdowns.

Figure 7. Support Grid Sklrtmg Seal to Vessel Wall

AMMONIA TECHNICAL MANUAL 74

Top Manway Hatch

One change from previous HT catalyst support grid designs includes the addition of an inspec­tion hatch located at the top of the outlet collec­tor. This allows access to the outlet collector and outlet piping for inspection during mainte­nance outages without having to disassemble and remove the entire catalyst support grid.

Figure 8. Top Manway Hatch

No Weld Installation

A no weld installation results in no approval re­quirements from local pressure vesse l authori­ties for modifying equipment. This also results in less installation time required and the catalyst support grids are easily retrofitted to existing vessels. The estimated time required to install both HTS and L TS grids easily fit within FNO 's turnaround schedule. The outle t collector is fi t­ted with three tabs which prevent the grid from shifting horizontally which would result in the grid-vessel seal to be inadequate.

Due to the no weld design the installation could be completed by FNO's catalyst removal and loading contractor. The contractor employees are well trained and equipped for confined space entry and have experience with numerous cata­lyst containing vessels. FNO believed the no weld des ign along with the contractor's skill set would reduce the risk of incurring an injury dur­ing installation of the grids.

2015

Predicted Pressure Drop Reduction

Prior to installation of the catalyst support grids in the HTS and LTS reactors, HT presented the Computational Fluid Dynamics (CFD) modeling on their support grid design in order to help jus­tify the project to Agrium FNO. Two excerpts from a recent CFD model report for the catalyst support grid compares the original elephant stool outlet design with the new support grid.

Figure 10. CFD Model oj Original Elephant Stool

2015 75

Figure J J. CFD Model of Catalyst Support Grid

As observed in the CFD model, a large part of the new support grid pressure differential is caused by the exit losses through the outlet pipe and the difference in volume and placement of the support balls. The total pressure loss of the new reactor bottom outlet design was estimated to be about 60% lower than with the elephant stool type design.

New Configuration and Performance

HTS Reactor

The new grid was installed in the HTS reactor without any impact to schedule or employee wellbeing. After installation it was topped with a small layer of 19 mm (34 inch) support balls and a layer of 13 mm (~ inch) support balls. This was followed by Haldor Topsoe's SK-201-2 HTS catalyst and topped with a layer of TK-20 (high void fraction inert material) and 19 mm (% inch) support balls. The new configuration is shown in Figure 12.

AMMONIA TECHNICAL MANUAL

1--- '" ---I

Figure 12. New HTS Catalyst and Support Ball Configuration

After a successful start-up of the ammonia plant the t.P perfonnaoce of the HTS decreased by approximately 36% or 8 kPa (1.16 psi) at the start-of-run. The trend of the HTS reactor L1P since the installation of the catalyst support grid is shown in Figure 13 as well as the original 6.P for comparison. At the time this paper was writ­ten the new charge of catalyst had been in ser­vice for 10 months.

HTS Pressure Drop · Grid Confiluration (10 Months)

• •

~ ,.. , .. _--_ .... _----_.----------_ .. --1" ______ . _______ //

l.

• 'OIl " • ---Figure J 3. Pressure Drop of HTS - New Grid Configuration 10 Month Service Life

LTS Reactor

The LTS reactor was also installed wi thout inju­ry and met schedule requirements. On top of the

AMMONIA TECHNICAL MANUAL 76

newly installed grid a layer of 19 mm (:X inch) support balls and a layer of 13 mm (Y, inch) support balls were loaded. On top of this was Haldor Topsoe LK-821-2 and LSK catalyst. Fi­nally, the catalyst volume was topped with 19 mm (% inch) support balls. The new configura­tion is shown in Figure 14.

LTS Catalyst

1--- '" ---<I

--~ --~

Figure 14. New LTS Catalyst and Support Ball Configuration

After start-up the L TS experienced a relatively signi ficant decrease in pressure drop - an ap­proximate decrease of 50% - 23 kPa (3.33 psi) less than previously seen at FNO. Since start-up the pressure drop across the L TS has exhibited the behavior illustrated in Figure 15.

• ,. f . I.

.. • .

lTS Pressure Drop _ Grid Conflluratlon 110 Months)

---.... -- - -

Figure 15. Pressure Drop oj LTS - New Grid Configuration 10 Month Service Life

2015

Economic Benefits Realized

Support Material Savings

The amount of support material required for the new grid configuration is less than what was re­quired for the original configuration. FNO de­tennined that the savings from the reduction in material for the bottom section of the HTS and L TS reactors was approximately $32,200 USD and $8,620 usn respectively. These costs are broken down in Table I and 2 and are based on material pricing and volumes obtained for FNO's 2014 maintenance outage.

Original HTS Co nfigu ration HTS Grid Configuration

M aterial Cost ($USD) M ateria l Cost ($USD)

19mm (3/4") $ 10,800.00 13mm (1/2" ) $ 14,800.00

25mm (1") S 12,100.00 19mm (3/4" ) $ 24,600.00

50mm (2") S 48,700.00

Subtotal {AI $ 71,600.00 Subtotal (B) $ 39,400.00 Savings - Subtota l (A) - Subtota l (8) = $32,200

Table 1. HTS Support Matenal Cost Savings

Table 2 LTS Support Material Cost Savings Origin a l L TS Co nfigurat ion lTS Grid Config ura t io n

Ma teria l Cost ($USO) Mate ria l Cost ($USO)

13mm (1/2") $ 2,450.00 13mm (1/2" ) $ 2,940.00

19mm (3/4") $ 2,300.00 19mm (3/4" ) $ 4,600.00

25mm (1") $ 2,530.00

50mm (2") $ 8,880.00

Subto tal (A) $ 16,160.00 Subtotal (B ) $ 7,540.00

Savings = Subtota l (A) - Subtota l (8) = $8,620.00

For future catalyst replacements in either the HTS or the L TS reactor, these material savings will be realized; however, this will change de­pending on current market pricing of support materials.

Unloading and Loading Labor Savings

In addition to the materi al cost savings a reduc­tion of support material also reduces the amount of time necessary to unload and load the HTS and L TS reactors. Impacting the unloading and loadi ng times the most are the large support

2015 77

bans, 50 mm (2"), due to their weight and the power required to vacuum them out of the ves­sel. Table 3 and 4 outline the labor costs associ­ated with the materials li sted in Tables I and 2 for the HTS and L TS

Original HTS Configuration HTS Grid Configuration Unloading Cost ($USOj Unloading Cost ($ USDj 19mm (3/4") S 1,050.00 13mm (1/2") S 1,050.00 25mm (1") S 1,050.00 19mm (3/4") $ 2,100.00 SOmm (2") S 14,700.00

Subtotal (A) $ 16,800.00 Subtotal (6) S 3,150.00

Load ing Cost ($USO) Loading Cost ($USO)

19mm (3/4") $ 1,050.00 13mm (1/2") $ 1,050.00 2Smm (1") $ 1,050.00 19mm (3/4") $ 2,100.00 SOmm (2") S 14,700.00

Subtotal (e) S 16,800.00 Subtotal (D) S 3,150.00

Savings = Subtota l [(A) + (ell - Subtot a l ((8) + {OJ] -

$27 300.00

Table 3. HTS Unloading and Loadmg Labor Savings

OriginallTS Configu ration LTS Grid Co nfiguration

Unloading Cost ($USOj Unloading Cost ($USDJ 13mm (1/2") S 1,050.00 13mm (1/2") S 1,050.00 19mm (3/4") $ 1,050.00 19mm (3/4") $ 2,100.00 25mm (1") S 1,050.00 SOmm (2") S 14,700.00

Subtotal (A) $ 17,850.00 Subtotal (B) $ 3,150.00

l oad ing Cost ($USOJ l oading Cost ($USDJ 13mm (1/2") $ 1,050.00 13mm (1/2") S 1,050.00 19mm (3/4") $ 1,050.00 19mm (3/4") $ 2,100.00 2Smm (1") $ 1,050.00 SOmm (2") $ 14,700.00

Subtotal (e) $ 17,850.00 Subtotal (D) $ 3,150.00

Savings = Su btota l [(AI + (C)] - Subtota l (( 8) + (Oil = $29,400.00

Table 4. LTS Unloading and Loadmg Labor Savings

Production Increase

Calculating the p roduction increase due to a re­duction in pressure drop for an ammonia plant can be complex and is often unclear. The ap­proach used for this project was simpli fied by focusing on the effect of pressure drop of FNO 's synthesis gas compressor's capacity. Firstly, a simulator was used to determine the power fac­tor specific to FNO's synthes is gas compressor

AMMON IA TECHNICAL MANUAL

which assumed constant flow rate and discharge pressure, the results of which are shown in Fig­ure 16.

Effed of Synthesis G;n Compressor Suction PnlS4.lre on Power

. . '" ,-

J i ~

,-,--,-- - -

Req ... lr ements

-

. . " '. '.

'.

'. --~

. . • . . - -

Figure 16. FNO Synthesis Gas Compressor Power Factor Determination

Using this power/.6.P relationship it was deter­mined that the HTS and LTS catalyst support grids resulted in a power reduction of 0.78% or 96.4 kW ( 129.3 hpj for the synthesis gas com­pressor. One can use this infonnation to calcu­late the energy savings associated with a reduc­tion in the steam required to power the turbine , however; this is only realized if ammonia plant rate remains constant. FNO has instead chosen to operate the compressor with the same power input and has increased the flow rate through the machine, ultimately increasing production and decreasing the energy input per tonne of ammo­nia.

To capture the increase in ammonia production the affinity laws (or fan laws) for centrifugal compressors were used. The affinity laws state that power (P) is proportional to the capacity (Q) cubed and can be used as fo llows:

(;:) = (~: ) 3 ............ (Eq.lj

The results of using (Eq.1) and the infonnation from Figure 16 are summarized in the following tab le.

AMMONIA TECHNICAL MANUAL 78

Value Units

Power Reduction 96.39 kW P1/P2 0.992 unitless

Q1/Q2 (calculated) 0.997 unitless

Table 5. DeterminIng Compressor Flow Ratio using the Affinity Laws

Lastly, with a flow increase of 0.26%, as calcu­lated using the above flow ratio, thi s equates to an increase in ammonia production of approxi­mately 3.28 MTPD (3.62 STPDj.

Support Gr ids Payback Determination

Considering all the savings described above the payback for both catalyst support grids was ap­proximately 7.3 months which is summarized in Table 6 below.

Item Cost Unit

Grid Cost (Ma terial & Labor) $250,000.00 Total

HTS Savings (Material & Labor) $ 23,300.00 Total

LTS Savings (Material & Labor) $ 45,500.00 Total

Product ion Increase $ 821.31 per day

Pay back Du ration 220.6 days

7.3 montl'lS

Table 6. Support Grids Payback DeterminatIOn

Conclusion

The grid installation was considered a success in all areas of the project - safety, process im­provement and economic return. Payback for the grid installation wi ll be dependent on many factors such as those associated with operating costs and the market price of ammonia. Alt­hough the grids did not quite achieve the pre­dicted pressure drop reduction of 60%, they have still had a significant positive impact on the performance of the ammonia plant at FNO.

References

1. Agrium Technical Memo, "Quantifying Pressure Drop in Gas Processes", Eddy Cooper (p.Engj. March 27, 2012

2015

2015

2. Agrium Technical Memo, "Catalyst Support Grids for HTS and L TS", Chris Biegel (P.Eng) & Michael Dunlop, (P.Eng) October 10, 2013

3. "GPSA Engineering Data Book" Vol­ume I Section 13 "Compressors and Ex­panders", by the Gas Processors Suppli­ers Association, Tenth Edition 1994

4. Haldor Topsoe CFD Model Report

79 AMMONIA TECHNICAL MANUAL

AMMONIA TECHNICAL MANUAL 80 2015