Two ammonia plants revamped to improve urea production

Saskferco Products Inc. expanded its ammonia production by 490 metric tons/day, or 26%, in 1997. The tall towers in the front are the CO₂ strippers, and the gray building to the left is the reformer (Fig. 1).

Two ammonia plants, revamped in the past 2-3 years, illustrate two different routes to increased capacity in response to rising demand for ammonia products in North America and India.

In 1997, Saskferco Products Inc. revamped its Belle Plaine, Sask., ammonia plant to meet increasing urea demand in North America (Fig. 1).

Rashtriya Chemicals & Fertilizers Ltd. (RCF) completed a revamp of its Thal, India, ammonia plant in September 1997. RCF expanded its plant to make effective use of its natural gas feedstocks and to effectively use its urea capacity.

The Nitrogen 99 Conference, held in Caracas from Feb. 28-Mar. 2, 1999, covered these two revamps in detail.

Saskferco project

In 1996, the Board of Saskferco decided to increase its granular urea capacity from 2,000 metric tons/day (mtd) to 2,850 mtd. To accomplish this increased urea production, it had to increase its ammonia plant capacity from 1,500 mtd to 1,890 mtd.

Today, both ammonia and urea plants have demonstrated their abilities to run above design capacity.

Of the 1,890 mtd of anhydrous ammonia production, 1,625 mtd of ammonia is used for the production of 2,850 mtd of urea.

The ammonia revamp consisted of two major steps:

In 1996, Saskferco added syngas driers.
In 1997, the company added a prereformer, chillers for the syngas compressors, a supplementary compressor, and a new package boiler. Also, Saskferco modified the process air pigtails to the secondary reformer.

Orphanides Consultants (Orphanco), Vouliagmeni, Greece, was Saskferco's technology consultant on this revamp. It assisted Saskferco in basic engineering and expedited and inspected critical equipment. Orphanco was also involved during the construction and commissioning of the plant.

Uhde GmbH (now Krupp Uhde GmbH), Dortmund, Germany, was the contractor for engineering, procurement, and construction (EPC) as well as the licensor of the existing ammonia-plant process.

Syngas driers

Pan Orphanides, consultant for Orphanco, presented the details of the Saskferco ammonia plant expansion at the Caracas conference.

Saskferco installed a new synthesis-gas (syngas) drier on the second-stage suction of the syngas compressor. Fig. 2 [31,684 bytes] shows the process flow of the synthesis loop design.

The synthesis loop includes a two-bed, radial-flow converter with an interchanger followed by a one-bed radial flow converter. The process gas is chilled to 10° C. to reduce adsorber mass and reduce compressor-power requirements.

According to Orphanides, the dry syngas reduced the ammonia content of the recycled gas to the converter from 4.5 to 2.8%. This reduction allowed higher per-pass conversion and greater circulation in the converters.

The syngas driers increased the capacity of the plant by 200 mtd.

Further changes

A team of Saskferco, Orphanco, and Krupp Uhde worked together to determine the scope of the second stage of the project. Based on the team's recommendations to Saskferco's Belle Plaine ammonia plant staff, the second stage of the project, which was completed in September 1997, included the following tasks:

A new prereformer
A new secondary reformer-burner design
A new CO₂-booster compressor
Two new syngas-compressor chillers
A new compressor for the ammonia-refrigeration system
A new package boiler.

The new prereformer, a Haldor Topsoe AS design, does 10% of the total reforming. It was installed to reduce the duty on the existing reformers. The prereformer is expected to increased the life of the low temperature shift (LTS) catalyst as well as that for the primary reformer.

The existing secondary reformer pigtails had a high delta pressure and thus were too small for the increased ammonia capacity. Unde designed larger pigtails that reduced the delta pressure at the inlet of the secondary reformer by 2.2 bar. The pressure drop increased the suction pressure of the syngas compressor and thus reduced the power needed to drive the compressor.

The increased plant capacity increased the CO₂ load in the lean solution and thus reduced the overall absorption capacity of the CO₂ removal system.

Installing a booster compressor at the suction of the CO₂ compressor increased suction pressure, which in turn increased the flow capacity of the CO₂ compressor. The new booster compressor also reduced the desorption pressure which provided additional CO₂ capacity.

Although the new pigtails reduced some power requirements for the syngas compressor, more power-requirement reduction was necessary. Thus, Saskferco added two ammonia chillers to reduce the suction temperatures below 10° C. Lower temperatures allowed the syngas compressor and turbine to handle higher syngas flow.

In addition, Saskferco added a fourth compressor to the ammonia refrigeration system to accommodate the three new chillers (one for gas drying and two for lowering the syngas compressor-suction temperature). Refrigeration capacity increased by 30% with the new compressor.

Finally, the new plant needed more steam. Therefore, a new 60 ton/hr, 120 bar/520° C. package boiler was added to provide more steam as well as more reliability. Jumpovers were made in the steam headers to give the new urea plant an additional 12 tons/hour of steam.

RCF project

Rashtriya Chemicals & Fertilisers Ltd. is headquartered in Mumbai, India, and owned by the government of India.

Before the revamp, its Thal complex has two Haldor Topsoe-designed ammonia plants, each with a capacity of 1,350 mtd and three Snamprogetti-designed urea plants, each with a capacity of 1,500 mtd.

In 1997, RCF increased the capacity of each ammonia plant to 1,500 mtd (Phase I) and the company plans to further increase each plant to 1,600 mtd (Phase II) by June 1999 to address the following concerns:

The existing urea plants could operate at higher capacities
A shortfall in natural gas supply
Medium-pressure steam consumption in various turbines and compressors were higher than normal
Flow from the syngas compressor was lower than normal.

Phase I is expected to reduce the specific energy consumption in the plant by 0.25 x 109 calories/metric ton (Gcal/mt) of ammonia, and Phase II is expected to decrease specific energy consumption by 0.15 Gcal/mt of ammonia.

M.B. Desai, who authored a paper about the project with S.D. Pandare and G.S. Lele, presented RCF's project experience at the Nitrogen 99 conference.

Phase I scope

In Phase I, RCF:

Installed a new hydrogen-gas recovery unit (HGRU) and new process-air compressor unit.
Replaced reformer tubes, old packing in the absorber and regenerator, condensate stripper trays, and several back-pressure turbines
Installed a new cooling tower and pumps
Converted steam-generation boilers, auxiliary boilers and auxiliary steam superheaters to dual fuel (both naphtha and natural gas) equipment.

Basic engineering for Phase I was awarded to Haldor Topsoe, and detailed engineering was awarded to Projects & Developments India Ltd. (PDIL). Phase I cost about Rs880 million and took about 30 months.

New equipment. Although the original Thal plant recovered ammonia from purge gases, it did not recover hydrogen. Before the new HGRU, the hydrogen was consumed as fuel in the reformer.

According to Desai, recovering this hydrogen resulted in an increase of about 4% in ammonia production. The hydrogen that was once in the fuel gas was replaced with natural gas.

In the reformer, RCF replaced tubes of IN-519 material with tubes of Manurite 36XM, or its equivalent. This material allows higher tube-wall temperature without creep-void formation.

Because the new tube material allowed a thinner tube-wall thickness, it allowed an 18% higher catalyst volume and a reduced pressure drop.

In another replacement project, the CO₂-removal unit had to be upgraded to allow for the increased load from the reformer. Replacing the pall-ring packing with Intalox saddles in four towers allowed the absorber to handle process gases equivalent to 1,500 mtd with a CO₂ slip of less than 800 ppm (vs. 1,350 mtd with a CO₂ slip of 1,000 ppm before the new packing).

In the process condensate stripper, the existing trays were replaced with Sulzer Industries trays. The downcomer area was increased by about 9%, which resulted in a better mass-transfer coefficient and increased capacity.

The existing two process-air compressors could handle 2,700 mtd of ammonia with help from the instrument-air compressor grid. A new third air compressor was necessary to produce the 3,200 mtd of ammonia expected of Phase II.

Plant performance. After the revamp, the yearly energy consumption was reduced to the lowest in years-that is, to 9.40 Gcal/mt for the period between 1997-98. In the 1996-97 period, the consumption was 9.75 Gcal/mt, and in 1995-96, the consumption was 9.56 Gcal/mt. The energy consumption was high in 1996-1997 as a result of longer shutdowns required for revamp jobs.

Fig. 3 [77,859 bytes] compares the energy consumption on a day-to-day basis before and after the revamp.

Phase II scope

In Phase II, RCF planned three projects to increase the production level of each ammonia plant to 1,600 mtd. Desai expects Phase II to be complete in June 1999. That includes:

Modification of the syngas compressor
Replacement of the problematic reformed-gas and converted-gas boilers
Installation of additional separators, a suction chiller, and a new refrigeration system
Modification of a Benfield circulation pump
Installation of a suction chiller for the syngas compressor.

The internals of the syngas compressor barrels and the steam-turbine inlet-nozzle box are being redesigned to handle the extra load.

As a result of severe thermal shocks from power outages during the past runs, the existing boilers are weak. Thus, they are being replaced. The steam drum is also being replaced to handle the increased steam load.

To accommodate the two new chillers, RCF is adding another refrigeration system to handle the additional refrigeration load.

The Benfield circulation pumps will get new impellers to meet the higher loads. Drive turbines were already modified in Phase I.

Finally, to improve syngas-compressor efficiency and to handle the increased load, Phase II will include a suction chiller to the exit of the methanator.