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| Stock Number: |
600087 |
| Capacity: |
760,000,000 LBS/Y |
| Year Built: |
1977 |
| Year Closed: |
2005 |
| Status: |
Cleaned and Available |
| Technology: |
BP catalyst license |
| Raw Material: |
Propylene, Ammonia, Sulphuric Acid, Air |
| Documentation: |
Excellent - Available in both paper and Electronic. |
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| Products Produced: |
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| Acrylonitrile (AN), Acetonitrile (CAN), Hydrogen Cyanide (HCN) |
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| Utilities Consumed: |
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| Electricity 0.067 KWH/lb AN, Steam (Exported ) 3500 BTU/LBS AN |
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| Upgrades: |
| 1977 Expanded 1989 with and addition of a third reactor increased capacity by 50%. $15MM invested on improvements in recent years. |
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| Brief Overview: |
| Started-up in 1977 and expanded to its current 760 MM lb/yr capacity in 1989 by the addition of a 3rd reactor. Plant is very well maintained and has a large amount of stainless steel equipment, especially in the finishing end of the plant. The current owners spent about $15MM on improvements in recent years.
Excellent site services and amenities. Deep water port, two rail companies, six propylene pipelines, 35k ton Ammonia tank, Co-gen. |
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| Process Description: |
| OBJECTIVES: Provide a description of the Acrylonitrile (AN) process at Sterling Chemicals, Inc. (SCI)
GENERAL:
AN is produced at SCI by a Sohio-licensed process (now British Petroleum Chemicals, Inc.). The rated capacity of the manufacturing unit is 740 million pounds of AN per year, 260 M from the 'A’ and ‘B’-train reactors and 240 M from 'C'-train.
REACTOR TRAINS:
Sterling has three similar AN Reactor 'Trains'. Each train includes the following dedicated equipment:
Propylene Vaporizer, Dry Drum, and Superheater (1 common spare)
Ammonia Vaporizer, Dry Drum, and Superheater (1 common spare)
Air Compressor with support equipment
Start-up Heater
Reactor
Effluent Cooler
Quench Column with Coolers and Pumps
Absorber Column with Pumps
Waste Oxidation Boiler with associated equipment
CATALYST:
Catalysts used in the AN reactors are silicon-based powders with particle size distributed largely between 22 microns and 88 microns. The catalysts differ in active metals used by the catalyst manufacturers.
The 51R1-1&2 catalyst is based on Sohio-technology that was purchased by British Petroleum. 51R1-3 contains catalyst manufactured by Nitto Catalyst Industries Limited in Japan.
REACTION:
AN is produced at Sterling by the Sohio ammoxidation process. The AN reaction takes place in fluidized bed reactors. The feed streams are Air (to supply oxygen for the reaction), Propylene, and Ammonia. The catalyst is maintained in a turbulent state by the agitation provided by the air feed. The three feeds bubble up through the catalyst and mix with it intimately.
Air is drawn from the environment through filters by turbine-driven air compressors and is compressed to a pressure of 30 psig. Air is fed through the start-up heater (fired for reactor start-up only) into the cone-shaped reactor bottom and distributed evenly through a perforated dished plate (air grid), which separates the catalyst from the cone.
Liquid Propylene (C3H6) is supplied via pipelines, with intermediate storage in four bullets. The Propylene is chemical grade, or ~93% purity. Liquid Ammonia (NH3) is supplied by pipeline directly from an on-site Ammonia Storage Tank. Both feeds are vaporized individually with Absorber Water in vaporizers (51E4s for Propylene, 51E5s for Ammonia), which have 15 minutes surge capacity on the shell sides. They combine to feed the reactor through a hydrocarbon sparger, which evenly distributes the feeds across the reactor.
For optimum reactor performance and catalyst life, precise ratios among the three feeds are maintained via a process computer and online analyzer that controls the molar ratios of Air to Propylene and Ammonia to propylene.
Heat from the exothermic reaction is removed from the Reactor by boiler feed water from the Reactor Coolant Drum (51S1) circulating through sets of internal coolant coils. Saturated nominal 600 psig steam is produced and reenters the Coolant Drum, where water is removed. Other coils superheat the steam for uses inside the unit or for export to the plant header. Coil arrangement inside the reactor is controlled to provide uniform temperature distribution across the catalyst bed.
The reaction gases pass through 12 sets of multi-stage cyclones inside the reactor head to separate the bulk of the catalyst particles, which are returned to the bed. The gases, along with any un-removed catalyst, pass out the top of the Reactor and are partially cooled in the Effluent Cooler (51E1) and fed to the sump of the Quench Column.
QUENCH COLUMNS:
The Quench column (51Q1) cools the Reactor effluent further by contact with circulating water streams. Most of the water vapor and small amounts of organics are condensed. The Quench bottoms stream is split, with most of it cooled in the Quench Column Coolers (51E2s - 3 per train) and circulated to the top of the column. Sulfuric Acid (H2SO4) is injected to neutralize unreacted Ammonia. Excess water in the Quench is fed to the waste water column (51D1), where AN and Hydrogen Cyanide (HCN) are recovered and returned to the quench.
The A&B-train spray quench design is different from the trayed design in the C-train. However, the circulating water functions the same in both designs.
ABSORBER COLUMNS:
Quench overhead vapor is fed to the Absorber Column (51D2). Chilled absorber water, which has been used to vaporize Ammonia and Propylene, is used to absorb AN and other organics. The upper section, called the humidification section, recovers the heat value from the cold low-organic gases by cooling Lean Water. Absorber overhead vapor containing non-condensable gases from the reactor is burned in the Waste Oxidation Boiler (WOB).
RECOVERY:
Absorber bottoms streams from all three trains join to become Rich Water. The stream is heated in Interchange, a complex system of heat exchangers, before being fed to the Recovery Column (51D3).
AN and HCN are taken overhead in the Recovery Column. Separation of AN from Acetonitrile (ACN) is accomplished in the Recovery Column by extractive distillation. Solvent Water from the Stripper is used as the extractive solvent. Reboil heat is provided by side draw vapor from the Stripper.
Water and heavy organics are rejected to the Stripper Column (51D4). It separates ACN, HCN, and other organics overhead. This stream is fed to the HCN Stripping Column (51D8), which removes HCN and adds it to the Heads Column feed. The ACN is sent for purification or burned in the A-train WOB.
Lean Water for the Absorber Columns is drawn from the Stripper, along with Solvent Water for the Recovery Column. Stripper bottoms removes excess water and heavy organics to waste treatment, where it is combined with Waste Water Column (51D1) bottoms for injection to the deep wells.
Reboil to the Stripper is normally provided by two thermosiphon reboilers (51E12-3,4), supplemented with live steam injection, with two forced-circulation reboilers (51E12-1,2) in reserve.
PURIFICATION:
Crude AN from the Recovery Column overhead is decanted to break the water-AN azeotrope, and the water phase is recycled back to the column. The organic phase can be stored in the HCN-bearing Crude storage tank, 51T15-2. The purification section of the unit can be operated when the reaction and recovery sections are shutdown.
Crude AN is fed to the Heads Column (51D5) for HCN+ removal. Acetic acid and sulfur dioxide are added to inhibit HCN polymerization in the liquid and vapor phases, respectively. HCN is taken overhead and condensed (51E13) to feed the cyanide users. Any excess HCN above their needs is burned in the #1 WOB. HCN-bearing crude AN can be wholly or partially diverted to 51T15-2 for later rerun.
Heads Column bottoms is HCN-free Crude AN, which is fed to the Drying Column (51D6) by way of a decanter or stored in 51T15-3. Prior to decanting, it mixes with a total liquid draw from the mid-section of the Drying Column, and the combined stream is chilled. The water phase is sent back to the Recovery Column, and the AN-rich organic phase is fed to the Drying Column below the draw tray. Water and light organics are further stripped, and a small organic stream is taken overhead to the Recovery Column to recycle light ends.
Drying Column bottoms are fed to the Product Column (51D7). Product AN is withdrawn from a side-draw near the top of the column. Light ends are concentrated overhead and also sent back to the Recovery Column. Heavy ends are rejected out the bottoms and recycled to the Quench for disposal with Quench bottoms.
The Drying and Product Columns both operate under a vacuum to reduce temperatures and minimize polymerization. MeHQ and condensate are added to product AN after it is cooled to inhibit polymerization in storage. On-line analyzers ensure that the product is on-spec.
PRODUCT STORAGE:
Product storage consists of 6 API-type storage vessels of between 1 to 2.5 million gallons each. Off-spec material can be stored in 2 smaller rundown tanks.
WASTE TREATMENT:
AN process waste water and storm water is collected, treated, and deep well-injected in the AN unit.
Surface water run-off is collected and pumped either to the Waste Equalization Tanks (530T10-1,2,3) or to the clarifier (530S14). The three equalization tanks store up to three million gallons of excess waste water prior to treatment for disposal down the injection wells (deep wells).
Process waste streams are neutralized by ammonia, 20% caustic, and sulfuric acid. A coagulant is added and the stream is fed through an in-line mixer to the Clarifier (530S14). Water flows to Sand filters, which remove additional suspended solids and protects the deep wells. Sludge from the Clarifier bottoms is processed in one of two Centrifuges (530F7s),.
STEAM GENERATION SYSTEMS:
The AN unit is a large producer and consumer of steam. The reactors generate superheated 600 psig steam. Most of this steam drives the air compressor turbines. The WOB’s also produce 600 psig steam, which goes to either drive other turbines or is exported to the plant header.
Boiler feed water from the Deaerator (51T28) supplies the Reactor Coolant Drum and a portion of the WOB steam generation facilities' requirements. High-pressure boiler feed water from Power 2 supplements T28’s supply. Makeup water to the deaerator can be any combination of Zeolite water, import condensate, or condensate from the Recovery and Purification reboilers. The water is treated with a chelant/caustic solution to prevent sludge and scale formation and maintain pH in the water-end of steam generation equipment. Sodium sulfite is added to the deaerator to remove residual oxygen from the boiler feed water, preventing corrosion in equipment.
Other steam systems in the unit operate at 130 psig, 35-50 psig, 30 psig, and 15 psig. 130 psig steam is used on vacuum ejectors on the Drying and Product Columns and for atomizing waste fuels in the WOBs. 75 psig steam is used for WOB flashback protection, on the Flare tip, and for a few small turbines. 35-50 psig steam is consumed by Stripper reboilers, and 30 psig steam is used by Purification reboilers. 15 psig steam is used on the Refrigeration units.
Condensate from reboilers in Recovery and Purification is pumped to the Deaerator. Condensate from the Refrigeration Units, the low pressure condensate header, and the north area condensate tank (51S20), and a portion of pure condensate (for level control) are collected in the Condensate Receiver (51S13). Condensate is pumped to the steam desuperheating stations. Excess condensate is exported to the plant header.
COOLING TOWER:
The cooling water system circulates water (87°F design) throughout the unit for cooling and condensing . The Cooling Tower (89909CT-9) has eleven cells, each with an induced-draft fan to draw air across the tower. Two turbine-driven and three electric pumps, are available to circulate cooled water.
Cooling water is treated with sulfuric acid to maintain pH. Sodium Hypochlorite is added to restrain algae and bacteria growth.
REFRIGERATION:
Six refrigeration units (51Z2-1 thru 6) supply chilled water. The units are York steam absorption type chillers, charged with lithium bromide, with the design capacity to chill cooling water to 52°F. The chilled water is stored in an insulated tank (51T43), from which it is pumped to exchangers throughout the unit.
FLARE SYSTEM:
Relief valve discharges and scrubber vents from various locations discharge into the Flare Headers and up the 300 ft. Flare. The Flare is fired with natural gas and provided with 75 psig steam to prevent smoking. |
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