Polyols Process Description
The Polyols Plant consists of two separate streams, each capable of producing the full range of polyols. Common raw material storages feed the two streams and each product has its own finished product storage. The capacity of the plant is 20,000 tons per year of polyols (10,000 tons per year per stream).
Each production stream consists of a reaction section and a work up section. In the reaction section the condesation reaction of glycerolwit propylene oxide and ethylene oxide takes place, catalyzed by potassium hydroxide. In the work up section the catalyst is neutralized with adipic acid and the resulting potassium adipate precipitate is removed by filtration.
Metered quantities of glycerol and 50% KOH are pumped into the initiator solution vessel. The mixture is agitated and the water of reaction is removed by heating under a vacuum. This leaves the basic initiator solution. The mixture is then blown by nitrogen into reactor 1.
Propylene oxide is pumped into the propylene oxide break vessel and a preset quantity of the propylene oxide is fed at a controlled rate into reactor 1. The temperature of the reactor 1 mixture is controlled by pumping the mixture through an external heat exchanger. After the preset quantity of propylene oxide has been fed, the reaction is allowed to come to completion. The polyether is then pumped to reaction 2.
A preset quantity of propylene oxide is metered into reactor 2 at a rate which is controlled by the pressure within the vessel. Upon completion of the reaction, either:
a) the polyether is transferred to the dehydration panb) ethylene oxide from the ethylene oxide break vessel is fed to reactor 2c) propylene oxide and ethylene oxide are fed concurrently into reactor 2. They are fed under flow-ratio-controlled conditions to give the correct ratio of propylene oxide to ethylene oxide in the final product
The temperature of the polyether is automatically controlled by means of an internal cooling coil. When the oxide charge is terminated, the reaction is allowed to come to completion before transferring the polyether to the dehydration pan.
Traces of residual oxide in the polyether batch are removed under a vacuum in the dehydration pan. The polyether is then treated with an aqueous solution of adipic acid, Topanol OC and phenotiazione that is prepared in the slurry vessel. The adipic acid neutralizes the basic catalyst and produces a potassium adipate precipitate. The Topanol OC and the phenithiazine act as polyether antioxidants and as scorch inhibitors during foam manufacture. The polyether batch is then dehydrated by heating and by the application of a vacuum. The polyether batch is now ready for filtration. A precoat (filtercel) slurry is prepared in the precoat vessel. It is applied to the product filter prior to filtration. The polyether/potassium adipate slurry is then pumped from the dehydration pan through the filter. A guard filter or polishing filter prevents the breakthrough of fine material. The filtered polyether then enters the buffer product storage, where it is analyzed for quality before being pumped to either the final product storage or the bulk storage.
The solids retained in the filter occlude polyether which is recovered by the circulation of an aqueous solution of sodium sulphate. The solution is prepared in the sodium sulphate solution vessel. After displacing the residual polyether the mixed liquor is allowed to settle in the separator.
The polyether is returned to the process, via the recovered product storage, at the dehydration pan.
The product filter residues are then flushed to the effluent system by water jets that are fitted internally.
The various polyethers are pumped through product coolers to the final product storage or the bulk storage.
Propylene Oxide Process Description
This chlorohydrin based plant includes titanium reactors and vessels along with two nice propylene storage spheres. There is also a waste water pre-treatment plant incorporated in the PO facility.
Lime is delivered to the plant by standard railcar. It is unloaded into the lime storage silo and transferred to two slaker vessels where the lime is dissolved in hot water. The lime slurry is then pumped to the slaked lime storage tank located in the PO plant. The lime process equipment is located in another section of the facility.
Propylene is delivered to the plant by standard railcar. This process can operate on propylene with as low as 90% purity as long as the main impurity is propane. It is unloaded with pumps rated for 40 m3/hr each and sent to two spherical storage vessels; each holding 1,220 cubic meters and operating at about 14 bar.
Liquid propylene is pumped from the storage spheres with two pumps rated for 5.0 m3/hr each at 10.3 bar. The pumps are occasionally not necessary depending on the propylene pressure in the storage spheres. The propylene pumped to the vaporizer can contain up to approximately 10% (weight) propane.
Propylene is vaporized with steam in the vaporizer. The steam controls the liquid level in the vessel, which houses 14 square meters of U-tubes for heating. The propylene gas is then superheated with steam from 30°C to 80°C in a 5 square meter superheater operating at less than 11 bar. The heated propylene gas then mixes with the recycle reactor gas stream in a small vessel. The recycle stream contains propylene, propane, propylene dichloride (PDC), ethylene, ethane, and water.
Chlorine gas at 3.5 bar line pressure is injected into the bottom of the chlorohydrin reactor. This important and intricate part of the reactor is explained later.
Process water for cooling (12°C to 20°C) is pumped into the reactor for temperature control of the exothermic reaction. The two pumps are rated for 30 m3/hr each.
In the chlorohydrin reactor, chlorine is dissolved in hot water to form hypochloric acid (HOCl) and hydrochloric acid (HCl). The HOCl then reacts with the propylene to form propylene chlorohydrin and propylene dichloride (PDC) byproduct. Chlorine is injected in the reactor with four horizontal tubes with Kynar jets and Teflon nozzles. The water is also injected in the bottom of the reactor, just below the chlorine. The reactor is packed to a height of 1.8 meters with ceramic saddle packing. The propylene injection nozzle, which is in the form of two concentric rings, is located just above the ceramic packing. About 10% excess propylene is used in this reaction to minimize any residual chlorine. Antifoam additive is injected to reduce foaming in the reactor which can occur with poor water quality.
The chlorohydrin reactor is 2.6 meters diameter at the top and 1.7 meters diameter in the lower section. It is 15.8 meters tall and is constructed of titanium. The lower section uses palladium stabilized (0.15% Pd) titanium for added corrosion resistance. The top of the reactor is designed for the release of the reaction gases. The gases exiting the reactor are routed to a chlorohydrin scrubber to remove any propylene chlorohydrin from the PDC with warm water. The scrubber is constructed of titanium and is filled with ceramic saddle packing.
The recycle reactor gases are then washed in the PDC scrubber with cold water to remove the PDC byproduct. Recycling of PDC with the reactor gases reduces the overall plant yield. The PDC scrubber is 0.9 meters diameter by 13.7 meters tall and is packed with ceramic saddles. It is constructed of carbon steel, but the bottom section is lined with acid brick due to the corrosiveness of this process. Wastewater is pumped from the bottom of the PDC scrubber.
The large diameter piping from the reactor to the scrubbers is constructed of titanium.
The recycle gas is transferred with two blowers which take the pressure from near atmospheric to 2.5 bar. The blowers are Nash model H-7 units.
The recycle gas then flows through a separator for water removal. The separator is 3.7 meters diameter by 1.3 meters tall and is made of carbon steel with stainless steel internal plates. The overheads from this separator flow to another smaller separator where propane is removed overhead and sent to the flare. Recycle gas analyzers are included to analyze the amount of excess propylene in the system for proper reactor control.
In the second step of the reaction, lime is added to the propylene chlorohydrin to produce propylene oxide. Slaked lime slurry is stored in a 150 cubic meter tank with redundant pumps and pipelines in case of plugging. The lime reacts with the propylene chlorohydrin, and it also neutralizes the HCl in the system. A small amount of propylene glycol is produced as a byproduct which leaves with the waste water. The first 2.5 meters of the pipe reactor is constructed out of titanium and the remainder is carbon steel.
The vacuum stripper column concentrates the propylene oxide (PO) from about 2% to 89%. The carbon steel column is 1.4 meters in diameter and 16.2 meters tall with 17 baffle trays. The column overheads, containing the crude PO, are condensed and used as reflux in the column.
Vacuum is pulled on the vacuum stripper column with two Howden Godfrey model MK2/H255 compressors. The gearboxes are Howden model YH8122 with 1470 rpm input and 7078 rpm output. They are driven with 200 kW electric motors. The crude PO at this point contains89% PO, 5.8% water, and 2.8% PDC and other impurities. The stream is then cooled from 90°C to 45°C in a cooling exchanger.
The crude PO stream then enters the dechlorination column which is 0.6 meters diameter by 15.4 meters tall. It has three packed sections of ceramic saddles and is constructed of carbon steel. The product is then cooled and pumped to the 50 cubic meter intermediate storage tank. Crude product is pumped from the intermediate storage tank to the purification column which is 1.7 meters diameter by 47 meters tall. The column is constructed of carbon steel with an acid brick lining in the bottom section. It has 100 sieve trays and a lower packed section with ceramic saddle packing. This column purifies the PO to less than 300 ppm water and only traces of impurities. The PO overheads from the column are cooled to 30°C and some of the liquid is used as reflux on the column. The majority of the condensed PO is sent to storage. Water is removed from the bottom of the column.
PO is stored in three 50 cubic meter carbon steel storage bullets operating at about one bar. There are two pumps used to transfer the PO to the Polyol Plant. The EO comes in by rail and is stored in three larger vessels for use in the Polyol Plant.
There is a large quantity of wastewater generated in the chlorohydrin process for making PO. For this reason, there is a waste water pre-treatment facility integrated into the PO plant. This unit uses distillation and clarification for pre-treating the PO wastewater so it can be sent to a conventional biological treatment plant. The distillation column is packed with ceramic saddle-type packing. The waste water treatment plant (WWTP) is rated for about 200 cubic meters perhour
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