Small Ammonia Plants Redesigned - KANDLA

Small Ammonia Plants Redesigned

A capacity of 600 MTPD or below is defined as a small capacity ammonia plant. There are several remote locations of the world where small ammonia plants make economic sense. These remote locations have natural gas and water available, and there is a ready market nearby. For example, ammonia is further processed to dynamite in some of these locations for the local mining industry. Without a local market, the transportation cost of ammonia could be higher by about 100 USD/ST, and rail and truck safety concerns always remain. For reasons like this some ammonia producers have shown a continued interest in small ammonia plants. The last 600 MTPD ammonia plant designed by KBR was for Zepu, China in 1998. There have been other small ammonia plant proposals made for locations like New Zealand and Myanmar, but none of these projects have gone forward. Recently there has been some renewed interest in small ammonia plants, and KBR opted to re-evaluate the design to lower installed cost and upgraded technology. A redesigned ammonia plant of 300 MTPD capacity is presented in this article. Lower capital cost was a primary goal while energy savings was a secondary target of this evaluation. A comparison of the conventional and the new design is discussed.

Mahesh Gandhi and Shamik Bhattacharya KBR

ighty percent of anhydrous ammonia is used as fertilizer, either by direct injection to the soil or in the form of nitrogen

fertilizers such as urea. Other direct consumers of anhydrous ammonia are mining industries, nitric acid and ammonium nitrate plants, nylon intermediate manufacturers, some waste water treatment facilities, refrigeration plants and power plant pollution control systems. Many of these have a relatively small consumption and import their ammonia feedstock by railcar or truck. The important point to be taken into consideration here is that the ammonia produced by small plants on-site is not meant for sale but is supposed to serve as intermediate for other non-fertilizer products such as Tri-nitro-toluene for dynamite in mining industry. It thus acts as a substitute for ammonia that otherwise would have to be purchased from other production

sites and would have to be transported to the respective site. Thereby not only the ammonia production costs but also transportation costs have to be taken into account for the cost analysis. The key issues or factors for ammonia plant design in general includes (but not limited to) energy savings, reliability and economics. The right balance between project economics and performance objectives is considered to be better for large capacity ammonia plants compared to smaller capacities. The consumers for small ammonia plants generally intend to focus on lower total installed cost (TIC) and reliability rather than energy savings. So the challenge remains in designing a reliable but economically viable ammonia plant to meet a client’s requirements. The focus of this paper is to demonstrate the benefits of building a small ammonia plant (300

E

2932017 AMMONIA TECHNICAL MANUAL

MTPD) to cater to the industries (nonagricultural users) that are located in remote areas and require appreciably small but steady supply of ammonia product onsite. In general, the plant design options discussed in the paper includes the following considerations: 1. Use of available natural gas pressure for

feed gas. The supply pressure (670 psig / 46 Bar G) was estimated based on that used in many ammonia plants.

2. Avoid high capital per ton costs by eliminating the conventional reformer and convection section by utilizing a fired process heater and an Auto Thermal Reformer (ATR).

3. Steam generation at Medium Pressure (700

PSIG/ 48 Bar G).

4. Maximize use of electric motor drivers for rotating equipment.

5. Use of centrifugal compressors.

6. Reduce the cost of high pressure equipment by minimizing synthesis loop equipment. Use of unitized chiller and horizontal converter helps to reduce construction cost.

WHY BUILD A SMALL AMMONIA PLANT?

A significant amount of ammonia used as fertilizer or intermediate product is transported by railcar or trucks. Several accidents have occurred in the past during transportation. General safety and security considerations have led to transport restrictions and high transportation and insurance costs. The U.S. Department of Transportation categorizes anhydrous ammonia as a toxic inhalation hazard (TIH). Exposure to high concentrations can cause blindness along with

other severe or fatal injuries and ammonia at 5,000 to 10,000 ppmv is rapidly fatal to humans. Due to the increased liabilities and insurance costs associated with the transport risk, the railroad companies have been trying to avoid transporting TIH’s. In addition, railroad carriers have sought to raise their rates as a precaution for covering the risk exposure. The best solution is to eliminate transportation risk. This could be achieved by co-locating production and consumption facilities. The idea of having a small-scale ammonia plant owes its genesis to minimizing risk and cost for transport of ammonia. Many of the small users have an ammonia demand of 300 to 600 tons per day. In those cases a smaller ammonia plant is the most suitable option. Another reason for building a small ammonia plant could be government regulations (does not apply to North America) against having liquid ammonia storage tanks and/or unloading facilities. This is primarily due to ammonia being considered as a hazard (as described in above paragraphs in this section). There has been interest from clients in the Middle East regarding small ammonia plants for tactical reasons – like no liquid ammonia storage allowed.

SMALL AMMONIA PLANT PROCESS DEVELOPMENT

KBR’s main goal for a small ammonia plant process design was to have a low cost plant for North American market (or for locations with low cost natural gas). Following are considered in developing low cost small ammonia plant: Eliminate Radiant Section of the Primary Reformer: Some unit operations don’t seem to prove out economics on flowsheets for small plants. The number of reformer tubes scale

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linearly with plant capacity but the other associated equipment such as furnace box, the inlet and outlet manifold, the burner system, the waste gas recovery and the combustion/ flue systems do not scale linearly with capacity and thus the capital cost per ton of ammonia. Reformer for a smaller plant is relatively higher compared to that for a larger plant. The issue is resolved by using KBR Reforming Exchanger System (KRESTM) in combination with Auto Thermal Reformer (ATR), without associated waste heat recovery or combustion air/flue gas system. Mechanical Drivers: Steam turbines are the most common choices for the compressor drivers in ammonia plants. However, for small ammonia plants, the turbines do not seem to be viable due to lower efficiency and high cost of turbines and associated accessories. Most of the rotating equipment including some compressors would thereby be driven with electric motors in the options discussed in the following paragraphs.

Compressor Selection: Centrifugal compressors have been used for synthesis gas compression since 1960s. As plant capacity is lowered the volumetric flow through the compressor decreases sharply due to internal leakage. In one of the process scheme options prepared by KBR, the synthesis loop pressure was reduced to bring up the volumetric flow rate and thereby allow the use of a centrifugal compressor. Steam Generation: To keep the cost low, medium pressure (MP) steam is being generated from the process heat recovery. High pressure (HP) steam is normally generated for the larger ammonia plants thus leading to higher energy efficiency. Also the piping metallurgy becomes expensive for HP steam system. Generating MP steam (48 Bar G / 700 PSIG) reduces equipment and piping cost.

PROCESS DESIGN OPTIONS

Two flowsheet options were looked at for process design of the small ammonia plant. Both of these utilize excess air for combustion in ATR. The general process schemes are discussed in the following paragraphs with differences pointed out as required.

The process schemes are shown in Figs. 1 and 2. Both options include KBR proprietary equipment like KRESTM. Option 1 uses KBR’s PurifierTM and conventional CO2 Removal with a solvent based system. Option 2 also uses KRESTM but also uses a Pressure Swing Adsorption (PSA) unit and does not need a PurifierTM.

NG

DESULPHURIZATION

KRESTM

STEAM

COMPRESSED PROCESS AIR

(EXCESS)

SHIFT AND CO2 REMOVAL

ATR

METHANATION AND DRYERS

PURIFIERTM

SYNGAS COMPRESSION

AMMONIA SYNTHESIS

NH3

WASTE GAS TO FUEL

CO2

Figure: 1 – KRESTM with PurifierTM - Option 1


NG

DESULPHURIZATION

KRESTM

STEAM

COMPRESSED PROCESS AIR

(EXCESS)SHIFT

ATR

METHANATION

PRESSURE SWING ADSORPTION (PSA)

SYNGAS COMPRESSION

AMMONIA SYNTHESIS

NH3

WASTE GAS TO FUEL+

CO2

Figure: 2 – KRESTM with Pressure Swing Adsorption (PSA)- Option 2

In both of the options cited above, the first step involves removal of sulfur from natural gas. The desulfurized gas is split into two streams, one going to KRESTM exchanger and the other going to ATR after being preheated by a natural gas fired heater. Steam is mixed with the above two streams prior to preheating The preheated natural gas and steam mixture enters the KRESTM and flows through catalyst filled tubes in which the reforming reaction takes place. The hot effluent from the secondary reformer enters

the KRESTM shell and provides heat for endothermic reforming reaction in tubes. The ATR is a refractory lined, water jacketed vessel with a catalyst bed - similar to that used in conventional ammonia process. The reformed raw synthesis gas (syngas) with a high Carbon Monoxide (CO) content then enters the Shift conversion section comprising of high and low temperature shift reactors (stacked in single column with intermediate cooling). The syngas from the shift section contains mostly hydrogen (H2), nitrogen (N2) and carbon dioxide (CO2) and also carbon monoxide (CO), methane (CH4) and argon (Ar) in small quantities. CO and CO2 need to be removed as they poison the Ammonia Synthesis catalyst. The surplus N2 has to be removed to adjust the stoichiometric ratio of H2 and N2 in the make-up gas going to the synthesis loop.

For Option 1 (Scheme with PurifierTM, Figure 1), the syngas from shift section is passed through BASF OASETM CO2 Removal system to bring down CO2 content in the syngas to less than 500 ppmv. However the CO content still remains high and needs to be converted back to CH4 in methanator. The methanator outlet gas is cooled and passed through a mole sieve dryer to get rid of the residual moisture before it enters the PurifierTM. The PurifierTM off gas containing mostly CH4 and N2 is sent to process heater fuel. The make-up gas containing H2 and N2 in ratio of ~ 3:1 is compressed to about 105 Bar (A) in a one case (plus recycle) centrifugal compressor. The gas is then preheated in a loop exchanger before entering KBR designed horizontal, two bed intercooled exchanger where ammonia is synthesized. The hot gas at exit of the Ammonia Converter is used to raise Medium Pressure (MP) steam (at 700 psig / 48 Bar G) in a waste heat boiler, and then cooled in cooling water exchanger and three stage Unitized Chiller to -16.6 ⁰F (-27 ⁰C). The condensed liquid ammonia is knocked out in the atmospheric stage of the Unitized Chiller and sent to either consumer or storage in an atmospheric tank (at -27 ⁰F / -33 ⁰C). KBR PurifierTM provides syngas with


traces of inerts in the first place thus enabling reduction in synthesis loop pressure. However, in order to control the inert level in the loop at ~ 3.5%, some of the recycle gas is purged. This high pressure purge gas along with low pressure purge gas from ammonia letdown drum and ammonia receiver is sent to ammonia recovery section. The recovered ammonia is recycled to the discharge of refrigeration compressor and the waste gas is sent to fuel.

For Option 2 (Scheme with PSA), the front end up to shift section is similar to Option 1 (Refer to Table 1 which provides comparison of main features for Options 1 and 2).

In Option 2, the raw synthesis gas coming out of the shift section is passed through the Pressure Swing Adsorption (PSA) unit in which molecular sieves are used to split the synthesis gas into a high pressure product stream with high H2 and N2 content, and a low pressure off-gas stream with high CO2 content. Most of the H2 is recovered and approximately half of the N2 is separated out. The PSA off-gas with high CO2 content is sent to fuel. The syngas leaving the PSA unit has H2 and N2 in ratio of ~ 3:1 as well as high CO content. The CO along with residual CO2 in the gas is converted to CH4 in the Methanator. The gas is then cooled and fed to the Syngas compressor in the loop. The compressed gas is further cooled down by a cooling water exchanger and the KBR three stage Ammonia Unitized Chiller to -16.6 ⁰F (-27 ⁰C). This gas is passed through the primary separator from which the overhead vapor is recycled into the loop. The gas is heated up subsequently in the hot side of Unitized Chiller and a loop exchanger and passed through a KBR designed horizontal, two bed intercooled exchanger where Ammonia is synthesized. The gas is then recycled back to the Syngas compressor less a small purge required for maintaining the inert level in the loop at 7.5%. Similar to Option 1, the condensed liquid ammonia is knocked out in the atmospheric stage of the Unitized Chiller and sent to either

consumer or storage at atmospheric pressure (-27 ⁰F / -33 ⁰C).

Table -1: Features of Options 1 and 2 in comparison to conventional scheme of producing

ammonia

Option 1 Option 2 Conventional Scheme

Primary Reforming

No Primary Reformer. Reforming supplemented by KRESTM Process Heater

No Primary Reformer. Reforming supplemented by KRESTM Process Heater

Externally fired Primary Reformer and elaborate waste heat recovery system

Secondary

Reforming

Operated with excess air. No air separation unit (ASU) required

Operated with excess air. No air separation unit (ASU) required

Operated with stoichiometric amount of air

CO Shift

HTS and LTS stacked in one reactor column



CO2 Removal

Liquid solvent with associated pumps and columns CO2 available as product

Pressure Swing Adsorption CO2 not available as product

Liquid solvent with associated pumps and columns CO2 available as product

Synthesis Pressure

1500 psig (100 Bar G). Clean gas from KBR PurifierTM contains very low inerts thus allowing the loop to work at lower pressure Dry Loop Configuration

2030 psig (140 Bar G) Wet Loop Configuration

2030 psig (140 Bar G)

Steam System

Medium Pressure Steam, 700 psig (48 Bar G) Maximize use of Electric Motor Drives

Medium Pressure Steam, 700 psig (48 Bar G) Maximize use of Electric Motor Drives

High Pressure Steam, 1800 psig (125 Bar G).

Ammonia and

Hydrogen Recovery

(HRU)

Ammonia Recovery required, HRU not required.

Ammonia Recovery required, HRU not required.

Ammonia Recovery required, Membrane Unit for HRU.


GENERAL DESCRIPTION OF KBR PROPRIETARY EQUIPMENT ENVISIONED

KBR Reforming Exchanger (KRESTM)

Refer Figure 3: KRES - reforming exchanger is optimally integrated into this ammonia flowsheet to provide a plant which is in steam balance. KRESTM is added in parallel to the ATR in this case. The steam generation in the ammonia plant is balanced to provide steam for process, running key compressors and to generate power for Inside Battery Limits (ISBL) and Out Side Battery Limits (OSBL) use. The key principles behind integration of KRES are: 1. Utilize the high grade waste heat exit ATR

for generating synthesis gas rather than generating MP steam which is a lower grade carrier of energy.

2. Maximize recovery of lower grade heat elsewhere in the plant to minimize reduction in production of MP steam due to incorporation of KRESTM.

3. Pressure drop through the front-end is minimized since the flow through is in parallel to the secondary reformer.

4. Waste Heat boiler duty is reduced as part of the heat from the secondary reformer effluent is used to reform gas in KRESTM. By doing this the steam export is reduced to have a balanced plant with no urea plant downstream.

About 25% of desulfurized feed gas is mixed with process steam keeping an optimum molar steam to carbon ratio. This feed stream is preheated in a coil placed in the convection section of the Process Heater. The preheated feed flows through tubes of KRESTM (filled with conventional Ni catalyst based but smaller rings) for reforming and shift reactions to take place. The reformed gas exit KRESTM tubes gets mixed with the secondary reformer outlet gas and flows through the shell-side of KRESTM supplying necessary heat for reforming reaction in KRESTM.

The effluent from the KRES shell side passes to Secondary Reformer Waste Heat Boiler, which is a natural-circulation design and produces medium pressure steam. The gas then passes to Natural Gas Heater Steam Superheat Coil.

Figure: 3 – KBR Reforming Exchanger (right corner in front of conventional reformer).[Courtesy CFCL, India]

KBR PURIFIERTM KBR Purifier™ is included in this small plant evaluation because it offers CO2 as product and has lower specific energy consumption. The KBR Purifier™ - Figure 4 ammonia process is a well-proven technology. Purifier™ technology has accumulated over 300 plant-years of experience, and has demonstrated operating, maintenance and reliability advantages. The key process features of KBR Purifier™ process are: feed gas flexibility, mild primary reforming (as applicable), secondary reforming with excess air, mild reformed gas waste heat boiler conditions, cryogenic removal of excess nitrogen and inerts from the syngas, efficient ammonia synthesis scheme in a single horizontal converter and unitized chiller. The Purifier process is an inherently low energy consumption process due to its unique process


parameters and heat integration. All process features in the proposed design have been commercially demonstrated in numerous KBR Purifier ammonia plants. A couple of these features are described in the following paragraphs. Secondary Reforming With Excess Air In a conventional secondary reformer, the quantity of air is limited to the amount that is required to produce a 3:1 ratio of hydrogen to nitrogen in the synthesis gas. With the Purifier process, the hydrogen to nitrogen ratio is controlled at the inlet to the Purifier (1.7:1 in this case) so that excess air can be used in the secondary reformer. Operationally, unlike that in the conventional process, the process air flow control does not need to be rigid as the hydrogen to nitrogen ratio in the syngas is now controlled by the Purifier. In addition, because of the downstream Purifier, the allowable methane slip from the reforming section is much higher than in conventional plants. This results in much milder reforming conditions that are advantageous in terms of steady and reliable operation and longer equipment life. The unreacted methane, together with excess nitrogen and most of the argon are removed and sent to the primary reformer furnace as fuel.

Cryogenic Purification Unit The cryogenic purification unit is the heart of the KBR Purifier process. In this unit, essentially all the unreacted methane and about 60 percent of the argon in the raw synthesis gas are removed together with the excess nitrogen as waste gas. This waste gas is returned to the Natural Gas Fired Heater as fuel after it has been used to regenerate the molecular sieve driers. The product from the Purifier is highly pure, dry syngas, low in inerts (<0.3%) with a hydrogen to nitrogen ratio of 3:1.

Figure: 4 – KBR PurifierTM

Most of the net cooling required by the Purifier is provided by a gas expander, which causes a modest pressure drop in the synthesis gas stream. Further cooling is provided by low-pressure vaporization of the waste gas described above. The Purifier has two main controls. The energy balance is controlled by the amount of work removed from the system by the expander. The material balance is controlled by a Joule-Thomson valve in the bottoms liquid stream from the rectifying column. This valve is controlled by a hydrogen analyzer on the purified process gas exit the cryogenic system. The highly pure and dry synthesis gas produced by the Purifier permits the operation of the synthesis loop at lower pressure, thereby saving syngas compressor power. This also leads to a very efficient ammonia synthesis process scheme with savings in both syngas and refrigeration compressor power. The cryogenic step allows a higher concentration of unreacted methane to slip in the raw synthesis gas leaving the secondary reformer as methane which removed from the syngas in the Purifier and used as fuel. Thus, the reformer outlet temperatures are typically lower by about 180 °F (100 °C) than in a conventional


ammonia plant. Also, as higher methane content is allowed exit the reforming section, a lower steam to carbon ratio is used in the primary reformer. The lower steam to carbon ratio improves the energy efficiency of the process. Complete removal of all traces of water and carbon oxides lengthens the life of the ammonia synthesis catalyst. Another key advantage of the Purifier process is that it stabilizes the operation of the plant by separating the front end of the plant from the back end. First, it permits setting the hydrogen to nitrogen ratio in the synthesis loop independently of the secondary reformer operation. Second, it effectively compensates for any problems or upsets in the front end.

KBR Synthesis Converter – Figure 5 The proposed synthesis converter is KBR’s well-proven horizontal design. This design has been used in numerous KBR plants over the past several years. The converter is intercooled and has two equilibrium stages. The converter has been designed to achieve a high conversion of ammonia with very low pressure drop. The converter contains a removable catalyst basket within the pressure shell. Integral with the basket is the intercooler. An annular space exists between the cylindrical catalyst basket and the high pressure shell to provide a path for feed gas cooling the shell.

Figure: 5 – KBR Horizontal Ammonia Converter

The converter primarily uses 1.5 to 3 mm size promoted-magnetite based catalyst. The catalyst volume is chosen for end of the life conditions (typical 18 years). While there are only two equilibrium stages, there are three catalyst beds as the second stage is split into two sequential beds. Each catalyst bed is supported on Profile Wire Screens. The process flow is downwards through each bed. All of the feed gas passes through each of the catalyst beds. The preheated feed gas passes through the shell/basket annulus and is heated in the intercooler. A small part of the feed bypasses the intercooler. The re-combined feed flows to the first catalyst bed at approximately 355ºC (EOR). The ammonia reaction is exothermic; equilibrium governed, and proceeds with a significant temperature rise.

Upon leaving the first bed, the partially reacted gas passes through the grid supporting the catalyst and into the space between the bottom of the bed and the basket wall. From here it is routed to the intercooler and is cooled to the proper feed temperature for the second catalyst bed. The hot effluent from the second bed is similarly passed to the third catalyst bed. There is no heat exchange between the two sections of the third bed. Hot converter effluent from the last catalyst bed exits the converter via a special connection between the basket and the pressure shell. KBR Unitized Chiller – Figure 6 The Unitized Chiller is a specially designed, multi-stream heat exchanger that cools the effluent from the ammonia synthesis converter with recycle gas and with boiling ammonia refrigerant at several temperatures. In doing so, the Unitized Chiller combines several heat exchangers, compressor knock-out drums, and expensive high-pressure piping and fittings into one piece of equipment. This design saves pressure drop in the synthesis loop and reduces capital cost.


Figure: 6 – KBR Unitized Chiller

The basic concept of the Unitized Chiller is the use of concentric tubes and a compartmentalized shell to replace several equipment items with one. The converter effluent flows through the annuli of the concentric tubes, and the recycle gas flows through the inner tubes. Refrigeration ammonia at various temperatures boils on the shell side in several compartments. Thus the converter effluent is simultaneously cooled by two media, the recycle gas and ammonia refrigerant. KBR has used the Unitized Chiller in numerous ammonia plants since 1978.

ECONOMICS AND LOGISTICS

The design features mentioned in the options above, particularly Option 2, targets reduction in disadvantages in the economics of scale for smaller plants (300 to 600 MTPD) in comparison to the larger plants.

The CAPEX is typically estimated for a North American location for a payback period of 10 years based on 5 percent interest rate. An attempt has been made to reduce CAPEX for small plant in Option 2, but it works in favor of larger plants due to economics of scale. The OPEX is also marginally higher for smaller plants compared to larger plants as specific energy consumption is not the primary objective here. The small plant process design however

makes sense for logistics factor and lower capital requirement.

There may be additional factors which would make small plant a requirement. This could include government policies (in Middle East) against having ammonia storage or unloading facilities to minimize safety hazards thus leading to integration of ammonia plant with ammonia consuming facility.

CONCLUSIONS

Anhydrous ammonia is a key feedstock for many industrial and farming applications. A significant quantity of ammonia has to be transported by rail which has led to several accidents in the past with serious effect on people and environment. This has resulted in increase in freight rates for ammonia transport which could be mitigated by product substitution and co-location of ammonia production and consumption sites. Another consideration would be to utilize small captive natural gas.

Option 2 with PSA Unit is recommended as it offers lower Total Installed Cost. Please note CO2 is not available in this scheme and natural gas requirement is higher than Option 1.

The above issue would lead to increase in interest for building small ammonia plants. The small scale production would be of interest to mining industries, countries with government regulations on storage and unloading of ammonia as well as sites with stranded gas of medium size.

In future if required Option 2 could be customized to produce CO2 as by-product by adding a liquid based CO2 Removal System downstream of the PSA unit. The unit would be of smaller size compared to Option 1 because of the following parameters for the feed gas to the system: a) lesser volumetric flow rate and b) higher concentration of CO2.


In recent projects the cost and schedule of construction proved to be much higher than estimated. This was true for all contractors who handle construction. KBR’s focus is on modularizing the design to reduce construction costs at site. Modularization of the small plant is more feasible than larger plant. Stacking of vessels, like HTS and LTS etc. is given, but further modularization of pipe rack (building pipe rack offsite and bring to site in sections); valve skids etc. to be included.

REFERENCES

(1) B. Keil; K. Noelker; J. Pach: “Small scale ammonia plants to reduce transport risk and cost”, Nitrogen + Syngas 2016, Paris, Berlin.

(2) A. Malhotra; P. Kramer; S. Singh: “Revamp of Liaohe’s Ammonia Plant with KRES technology to reduce natural gas usage”, AIChE, Ammonia Plant Safety, Vol 44, 2004


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Small Ammonia Plants Redesigned - KANDLA