UPGRADING A SULPHURIC ACID PLANT: PROJECT EXECUTION ... · NORAM Engineering and Constructors Ltd....

NORAM Engineering and Constructors Ltd. June 10-11, 2016

AIChE Clearwater Conference, 2016. 40th International Phosphate Fertilizer & Sulfuric

Acid Technology Conference.

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UPGRADING A SULPHURIC ACID PLANT: PROJECT

EXECUTION STRATEGY AND PERFORMANCE

EVALUATION

Andrés Mahecha-Botero*, Brad Morrison, Brian Ferris,

Hongtao Lu, J.P. Sandhu, C. Guy Cooper, Igor Aksenov, Nestor Chan.

NORAM Engineering and Constructors

200 Granville Street, Suite 1800. V6C 1S4

Vancouver, BC, Canada.

http://www.noram-eng.com/

&

Grace Juzenas, Tom Hamilton

AXTON Inc.

441 Derwent Place, Annacis Business Park. V3M 5Y9

Delta, BC, Canada.

www.axton.ca

Prepared for:

AIChE Clearwater Conference, 2016

40th International Phosphate Fertilizer and Sulfuric Acid Technology Conference

* Corresponding author: [email protected], [email protected]

http://www.noram-eng.com/

http://www.axton.ca/

mailto:[email protected]

mailto:[email protected]




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ABSTRACT

Defining the scope and schedule of a major plant upgrade is a challenging task. Several

factors must be taken into consideration including: budgetary constraints, turnaround planning,

duration of plant shutdown, lost production, technical risk, mechanical conditions of existing

equipment, space availability, space constraints, logistic considerations, as well as the time

required for fabrication and installation of major equipment.

The focus of this paper is a medium-size sulphuric acid regeneration plant that utilizes

single absorption technology followed by a tail-gas scrubber. The plant is located in the USA.

Equipment was replaced in three separate shutdowns over a period of over 5 years (the final stage

of the project was completed in November, 2015). The plant required the replacement/upgrade of

essentially all the gas-side equipment, including: a four-bed catalytic converter, three gas-to-gas

heat exchangers (hot, intermediate and cold), a preheater gas-to-gas heat exchanger, most of the

gas ducting, acid tower packing, instrumentation, SO2 conversion catalyst, as well as other

ancillary equipment.

This paper discusses the upgrade strategy implemented, engineering challenges

encountered, discusses fabrication and logistics strategies, and provides insights on the

performance of the upgraded plant based on recent plant data.

Keywords: Plant upgrades, Gas processing equipment, Project execution, Plant turnaround, Case

study, Sulphuric acid regeneration.




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1. INTRODUCTION

NORAM worked with a client located in the USA to develop a project execution strategy

to modernize a sulphuric acid plant. The plant had a number of pieces of equipment approaching

their end of life and suffered from some technical issues, such as thermal expansion issues and gas

leaks that caused unscheduled shutdowns.

The plant is a medium-size sulphuric acid regeneration plant that utilizes single absorption

technology followed by a tail-gas scrubber. Gas handling and conversion equipment were replaced

in three separate shutdowns over a period of over 5 years (the final Step of the project was

completed in November, 2015).

The plant required the replacement/upgrade of a four-bed catalytic converter, three gas-to-

gas heat exchangers (hot, intermediate and cold), , a preheater gas-to-gas heat exchanger, most of

the gas ducting, acid tower packing, instrumentation, SO2 conversion catalyst, as well as other

ancillary equipment. Essentially all the gas handling equipment was replaced by modern

equipment designs that provide higher reliability, lower pressure drop and lower SO2 emissions.

2. UPGRADE STRATEGY

The performance of the acid plant and the mechanical conditions of key pieces of

equipment were carefully evaluated. A process study was performed and mechanical integrity

reports were reviewed, to identify plant issues and to suggest solutions. A plant upgrade strategy

was then developed based on the findings of the process study, taking into account the mechanical

conditions of the existing equipment. The equipment that was in the worst mechanical conditions

was replaced first. The following was considered as the basis for the upgrade:

1. Specific dimensional limits had to be maintained to comply with client requests.

2. Specific tie-points to be matched.

3. New equipment to allow for lower emissions of SO2.

4. New equipment to have lower pressure drop, thus saving energy consumption of the main

blower.

5. New equipment to be fabricated utilizing better materials than existing.

6. New equipment to be safer and more ergonomic than existing.

7. New equipment to provide better reliability, higher on stream time and lower maintenance

requirements than existing. Elimination of gas leaks is also important.




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8. New equipment to match the existing inlet/outlet ducting in all beds. This was possible for

all ducts with the exception of the gas inlet to bed one, which required modifications to

reduce the gas velocity head and to increase the room above bed 1.

9. Converter mechanical design to allow for a maximum pressure drop per bed of 85’’ WC,

an extreme case (for limited time).

10. Design for high seismic conditions.

Satisfying the design constraints given above was a challenging task. It was particularly

challenging to fit all features of the converter within the allowable height. With all the

considerations above, and taking into account the conditions of the existing equipment, shut-down

time and maintenance budgets, the following staged approach was implemented:

Step 1 (2010)

Installation of one SF™ Split Flow Preheat Heat Exchanger and ancillaries.

Installation of one RF™ Radial Flow Hot Heat Exchanger

Installation of all S.S. Ducting To/From Converter

Step 2 (2013)

Installation of one RF™ Radial Flow Cold Heat Exchanger

Installation of one RF™ Radial Flow Intermediate Heat Exchanger

Installation of all S.S. Ducting Between Heat Exchangers

Step 3 (2015)

Installation of Stainless Steel Converter with new catalyst and improved instrumentation.

Installation of Converter Preheat Ducting System

Installation of HP Packing in the dry tower.

3. ENGINEERING

NORAM provided the basic design, detailed engineering, site services, and fabrication

advisory services for all the equipment replacement projects. Some aspects studied during the

engineering stages are discussed below:

3.1. Upgrade Planning

Process flow diagrams and material and energy balances were developed for the entire acid

plant. Based on the required process conditions, individual pieces of equipment were designed and

integrated in the existing physical space. Figure 1 shows the equipment replacement strategy

implemented in the different Steps.




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Figure 1. 3-D Models of the acid plant.

Top: 3-D Model of the upgrade strategy for Steps 1 and 2.

Bottom: 3-D Model of the upgrade strategy for Step 3.




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3.2. Gas-to-Gas Heat Exchangers

To achieve the design objectives, it was necessary to implement low pressure drop gas

exchangers that use a radial flow arrangement. NORAM’s designs also allow for control of the

metal temperature of the heat exchanger tubesheets. Control of the metal temperature was used for

the preheat heat exchanger by using a Cold Sweep design. This Cold Sweep allowed for increased

temperatures at the preheating furnace and higher preheating efficiencies. Figure 2 shows typical

NORAM designs.

Figure 2. NORAM radial flow exchangers with metal temperature control.

Left: Cold sweep. Center: Hot sweep. Right: SO3 cooler.

3.3. Catalytic Converter

The existing converter has a carbon steel shell and cast iron posts and grids. The first bed

from the top of the catalyst to the division grids is brick lined. The brick lining is required to protect

the carbon steel shell from the high temperature generated in the first bed. The grids are cast iron

plates shaped as equilateral triangles supported by a cast iron post at each corner. This post and

grid construction has well-known weaknesses such as mechanical integrity issues and inter-bed

gas leaks.

It was determined that the existing carbon steel converter is at the end of its service life.

NORAM’s converter is constructed from 304 H stainless steel with dished plates to allow for

thermal expansion by flexing as the converter heats or cools. The new converter was designed

without any internal posts or core, thus maximizing the area available for catalyst loading. The

stainless steel converter can be heated and cooled faster than carbon steel converters because the

stainless steel converter design is much lighter, therefore having lower thermal mass. The catenary




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plates (bed support and division plates) are flexible, membrane-like plates which minimize thermal

stresses on the various parts of the converter.

More information about equipment considerations is given in the references section.

3.4. Catalyst Preheating System

The plant preheating system was re-engineered, specifically to improve the preheating of

the catalytic converter. The capacity of the preheater was reviewed, and hot gas lines were directed

to each of the catalyst beds to ensure adequate preheating is achieved in all parts of the converter.

The equipment was designed and a dynamic simulation was developed to identify the

improvements obtained. Based on the results of the simulation (Figure 3), the following is

concluded:

Case 1 (Existing Design): If all the hot gas from the preheater is fed to bed # 1 only:

Based on the simulation results, achieving catalyst temperature of 800 °F is not possible

for bed # 3 and # 4 for the existing design.

Case 2 (New Design): Preheating all four beds simultaneously:

All four beds heat-up evenly, and reach 800+°F in approximately 10 hours.

The required hot preheat gas flow distribution between converter beds is as follows:

Preheating Gas Distribution

To preheater To bed # 1 To bed # 2 To bed # 3 To bed # 4

Fraction of Gas Flow

100% 24.0% 30.4% 32.7% 12.9%

The simulation shows how the plant can be heated-up faster to the target temperatures. This

also provides flexibility in case the client prefers to increase the heat-up rate of a certain bed. Tight

shut-off valves and a spectacle blinds are provided to get back to the original design conditions

(The four preheater isolation valves are triple offset high-temperature valves). The increase in bed

temperature after preheating (for beds 2, 3 and 4) provides higher margin over the sulphuric acid

dew point and reduces the chances of acid condensation, corrosion and fouling.




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Figure 3. NORAM dynamic simulation of converter preheating.

Top: Catalyst heat-up curves with 100% of the hot gas fed to bed #1. Bottom: Catalyst heat-up

curves with heating of all four beds simultaneously.

Graph # 1

Catalyst heat up time (no preheat duct flows)

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

time, hrs

Tem

pera

ture

, °C

32

212

392

572

752

932

1112

Tem

pera

ture

, °F

bed 1 cat T bed 2 cat T bed 3 cat T bed 4 cat T Gas T from B-103

Graph # 2

Catalyst heat up time (optimized preheat duct flows)

0.000

100.000

200.000

300.000

400.000

500.000

600.000

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

time, hrs

Te

mp

era

ture

, °C

32

212

392

572

752

932

1112

Te

mp

era

ture

, °F

bed 1 cat T bed 2 cat T bed 3 cat T bed 4 cat T Gas T from B-103




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3.5. Design of Inter-Bed Gas Mixing Device

The plant utilizes a cold air quench in between beds 3 and 4 to cool the process gas.

NORAM modified the mixing strategy to simplify the equipment design. NORAM developed

Computational Fluid Dynamics (CFD) models to design an adequate gas mixing device. This

device is important to ensure that the inlet gas to bed 4 has a uniform temperature, and uniform O2

and SO2 concentration. This allows for adequate conversion of SO2. This is important during start-

up as well as normal operation.

ANSYS Software R15.0 was utilized to carry out the CFD simulations. The fluid is

modelled as 3-Dimensional, non-isothermal flow. The following parameters are evaluated for each

CFD case to identify the best inter-bed quench system design:

Inlet temperature to converter bed 4 (uniform temperature is preferred).

Inlet gas velocity to converter bed 4 (uniform velocity is preferred).

Process gas pressure drop (low pressure drop is preferred).

Inlet concentration profile of O2 and SO2 to converter bed (uniform concentrations are

preferred).

Overall conversion of converter bed (maximum conversion is preferred).

Figure 4 shows the CFD results. The performance of the existing design was compared against the

new design:

Existing Design: The simulation results of bed temperature above converter bed #4 are

shown in Figure 4 (left). It is clear that the gas mixing for this geometry is not complete.

For this reason, the temperature above bed #4 is not uniform and varies from 752 to 866°F

(400 to 463°C). The deviation in gas temperature was 114°F (63°C). In this case the

conversion of bed #4 would be affected due to the variation in temperature.

Improved Gas Mixing: The gas streamlines and the bed temperature above bed #4 are

shown in Figure 4 (right). In this case, the temperature above bed #4 is much more uniform.

The temperature varies between 826 and 840°F (441 to 449°C). It is important to note that

the target temperature (as per NORAM’s converter design) for this particular bed is 832°F

(444°C). All parts of bed#4 would receive gas within 10°F (5°C) of the target temperature

(This was validated in Section 6, below).




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Figure 4. NORAM CFD simulation for gas mixing.

Left: Existing design. Right: Upgraded design.

See numeric values on left side of each plot for temperature predictions.

3.6. Catalyst Evaluation

The client asked NORAM to review the technical and commercial quotations from three

major catalyst vendors. Best efforts were made to ensure that all vendors worked using the same

assumptions to be able to make a balanced assessment of the different proposals. A technical

evaluation was made based on the following performance indicators:




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Conversion per bed.

Outlet temperature per bed.

Pressure drop per bed (gives pressure drop estimate of grids and ceramics separately).

Emissions in ppm.

Also, a commercial evaluation was made based on the following:

Catalyst price.

2-year warranty on conversion.

Pressure drop warranty at start-up and after 6 months.

Cost of blower power required to overcome the catalyst pressure drop over a period of 10

years.

It was found that the catalyst with the highest conversion, and the lowest operating pressure

drop had the lowest cost to the client over a period of 10 years (on a net present value basis). It

was noted that pressure drop was an important factor in the comparison of the three proposals. The

final selection, purchase and installation of the catalyst was made by the client, and they decided

to source the catalyst from a vendor based on other considerations.

3.7. Dynamic Simulations for Gas Temperature Control

NORAM developed a dynamic simulator using Aspen Dynamics software to confirm the

operability of the gas temperature controllers. Based on the dynamic simulations, specific

recommendations were given to implement automatic temperature control for converter beds 3 and

4. The controlled variable is the bed inlet temperature at the top of the catalyst for each bed. The

auxiliary quench air blower discharge pressure and vent to atmosphere can be kept on manual

control for periodic adjustment.




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4. FABRICATION

The converter, cold heat exchanger, intermediate heat exchanger, ducting, expansion joints,

preheating lines, converter platform, and grillages were fabricated by NORAM’s own fabrication

shop (AXTON, Canada). Some photos of the fabrication process in AXTON are given in Figures

5 and 6. The balance of the equipment was fabricated in a local shop.

Figure 5. Gas exchanger at fabrication shop (Step 2).




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Figure 6. Converter at AXTON-NORAM shop (Step 3).

Top left: Catalyst support plate. Top center: Converter walls. Top right: converter bottom.

Bottom: Complete converter at fabrication shop.




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In addition to carbon steel and stainless steel, AXTON manufactures equipment from

diversified types of alloys including chrome moly, duplex stainless, Inconel, titanium, zirconium

and various clad combinations. AXTON’s fabrication facilities include:

12,000 sq. ft. material preparation bay with state of the art plasma CNC cutting center

and plate rolling capacity up 1 1/4” thick.

Four fabrication bays totaling 35,500 sq. ft. Fifty tons lifting capability. Dedicated bays

for alloy fabrication (over 16,000 sq. ft.).

Welding processes — GTAW/ GMAW/SMAW/FCAW/SAW

Approved welding procedures — NORAM SXTM/CS/CrMo/300 series

SS/Duplex/Nickel Alloys/Titanium/Zirconium.

Transportation by road, rail, and ocean (via barge or thru Vancouver seaports).

NORAM provided fabrication shop inspections and advisory services, which helped to

ensure that the intention of the design specifications were met. AXTON developed an inspection

and testing plan, and trial-fitted in the shop key pieces of equipment such as the preheating ducting.

The converter was made of all-welded stainless steel 304H with catenary plates (no posts).

The walls of the converter are ½ inch thick. The new converter was crafted to match the existing

tie-points. Figure 6 shows some photos of the fabrication of the converter.

5. LOGISTICS AND INSTALLATION

All major pieces of equipment where fully fabricated in a shop and shipped to site in one piece.

The gas exchangers, grillages, ducting and ancillaries where shipped on low-bed trucks.

Installation of the equipment was done by lifting onto the equipment foundations. The equipment

grillages and supports were re-designed to adequately distribute the loads from the new equipment.

The converter was lifted at the fabrication shop and rolled onto a barge at AXTON’s dock. The

barge was then towed by a tug boat to the main port. The converter was lifted by the on-board

cranes of a cargo ship. Figure 7 shows the maneuvers required to transport and install key vessels.




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Figure 7. Transportation and erection of the converter. Top Left: Loading at dock. Top Right:

Barge towing. Bottom Left: Vessel lift into a cargo ship. Bottom Right: Site erection.

6. RESULTS AND PERFORMANCE EVALUATION

The performance of the plant was evaluated after the introduction of the upgrades. All

pieces of equipment were installed during regular plant turnarounds. The following improvements

are noted:

6.1. Pressure Drop:

The plant pressure drop decreased significantly. The total plant pressure drop was reduced

by about 40 in WC. The plant pressure drop was estimated at a nominal gas flowrate under similar

operating conditions.

Some of the reasons for improvement include:




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Gas-to-gas heat exchangers utilize radial flow designs that provide lower pressure drop

than older designs.

Ducting sizes were reviewed for lower gas pressure drop.

Replacement of fouled equipment.

Converter and catalyst replacement.

High performance (HP) packing reduce tower gas pressure drop to half of the existing.

This reduction in pressure drop is translated into large savings in energy consumption by

the main blower. Since the blower is driven by steam, the net steam production of the plant is

increased. The base costs of steam for blower power were provided by the client. The Net Present

Value of the steam savings amounts to close to $1.4 Million USD over 10 years.

6.2. SO2 Conversion

The SO2 conversion improved as well. Although the SO2 emissions from the converter

have not been measured under identical feed conditions, it is estimated that the overall chemical

conversion increased by at least 1% based on indirect measurements (this corresponds to a larger

percent reduction in total SO2 emissions on a tons per year basis). Some reasons for this

improvement include:

The catalyst loadings of the converter were increased by 14%.

All the old catalyst in the converter was replaced by new catalyst.

Some Cesium promoted catalyst was added to the converter to increase the SO2 conversion

at low temperatures.

The converter preheater was improved to allow for faster preheating at higher temperatures.

A multi-bed preheating system was installed to allow for the reduction of start-up time and

emissions.

All these lower emissions have a side benefit of better operability of an existing tail gas

scrubber.

6.2. Ergonomics

Ergonomic features include:

The new converter design improved ergonomics whenever possible.

Significant improvement was achieved in the headroom of converter bed 1, which requires

most frequent catalyst screening.

More significant improvement in the ergonomics of other converter beds could not be

achieved, due to the design constraints given by the client, in particular with respect to total

height available (see above).

When possible, the new equipment allowed for better access.

6.3. Mechanical Design

Some of the features are:

The new converter has an all-welded design with catenary plates that are more robust than

the existing converter design.

The converter shell metal plate is thicker than usual to accommodate for a higher-than-

usual bed pressure drop. This maximum pressure drop was specified by the client.




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Fully welded catenary plate design. The catenary plates do not require any internal support

structures, are structurally simple, and minimize thermal stresses on the various parts of

the converter.

The existing converter’s mechanical design is prone to leakage and bypassing between the

beds. The new design is fully welded and no leakage is expected. The emissions of the new

design will be much lower due to increased catalyst volume and tighter mechanical design.

6.4. Gas Mixing

The results of the CFD simulation were checked when the plant started production. The

converter temperature measurements on the opposite sides of Bed 4 were almost identical (about

1°F difference). This measurement proves that the mixing device works well.

6.6. Materials of Construction

The new equipment is made of more robust material in terms of corrosion and high-

temperature strength than the materials of the existing equipment.

Gas handling equipment was built utilizing stainless steel alloys such as SS 304H, which

provide higher corrosion resistance and strength than carbon steel.

Replacement acid tower packing made of slip casted ceramics, which is mechanically

stronger than conventional packing.

6.7. Reliability

The new designs are intended to achieve better reliability, higher on stream time and lower

maintenance requirements than existing. Some features include:

All pieces of equipment utilized modern designs.

All welded equipment used to eliminate leak points.

Thermal expansion strategies reviewed to eliminate trouble areas.

Split-flow gas-to-gas heat exchangers utilized for improved equipment reliability.

7. CONCLUSIONS

The present project highlights NORAM’s ability to meet and exceed the client’s

expectations to modernize the complete gas-side of a sulphuric acid plant. In this case the project

was successfully executed in three Steps, achieving improved reliability, lower pressure drop,

improved ergonomics, and lower emissions.

8. REFERENCES

Other references about sulphuric acid technologies and NORAM equipment can

be found here:

1. Louie, D.K. Handbook of sulfuric acid manufacturing. DKL Engineering. Richmond Hill, Canada, (2008).




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2. Kim Nikolaisen, Andrés Mahecha-Botero, C. Guy Cooper. “Strategies for reducing start-up emissions from sulfuric

acid plants”. in American Institute of Chemical Engineers (AIChE) Conference, Chapter of the 39th International

Phosphate Fertilizer & Sulfuric Acid Technology Conference, (2015). www.aiche-cf.org. Oral presentation in:

Clearwater, USA. June 5-6, (2015).

3. Kim Nikolaisen, Igor Aksenov, Andrés Mahecha-Botero, C. Guy Cooper. “Improved performance with split-flow gas-

gas heat exchangers”. in Sulphur 2014, CRU British Sulphur: Sulphur, Sulphuric Acid and Sulphur Dioxide, pages

389-401, (2014). Oral presentation in: Paris, France. Nov. 3-6, (2014).

4. Andrés Mahecha-Botero, C. Guy Cooper, Igor Aksenov, Kim Nikolaisen. “Debottlenecking metallurgical and

sulphur-burning sulphuric acid plants to increase capacity and reduce emissions”. in Sulphur 2013, CRU British

Sulphur: Sulphur, Sulphuric Acid and Sulphur Dioxide, pages 157-174, (2013). Oral presentation in: Miami, USA.

Nov. 4-7, (2013).

5. http://noram-eng.com/groups/sulfuric-acid-group-technologies.html

http://noram-eng.com/groups/sulfuric-acid-group-technologies.html

UPGRADING A SULPHURIC ACID PLANT: PROJECT EXECUTION ... · NORAM Engineering and Constructors Ltd....

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