This page describes the process operations which took place at Octel Amlwch. The site had a number of different reactors and buildings which were interlinked and important for efficient and safe process operations. The process is shown in the diagram below.
The dilute acid tanks (DAT) are geographically located in the centre of the site. They also have a central role to play in all site process operations. The byproducts of many processes are fed to the tank and become part of the feed stock of other processes on the site. Many plants are interlinked via the chemicals and processes which make up part of the tank’s operation. The contents of the DAT is made up of waste streams from the SOT , Br2 drying, chlorine off loading and HBr and BLR processes. Most of these streams contain acid which can be re-used to adjust the pH of the sea water at the BOTs. The strength of this acid will depend on the exact operational conditions of all the processes which have a recycle stream entering the tank. However it is normally necessary to add some sulphuric acid from the strong acid stock tanks to achieve a sufficiently strong acid to be effective at the BOTs.
The biggest input stream to the DAT comes from the SOT. This contains recycled PAL from which the bromide has been converted to Br2. The operation of one SOT at around 16 to 20 tph PAL feed will normally provide sufficient acid value to allow one BOT to be operated on three pumps. Higher SOT feed rates normally require a second BOT to be operational. The acid steams entering the DAT also contain residual amounts of un-recovered halogens. The SOT is the main source and the effluent eH must be monitored continuously and maintained within acceptable limits. The BLR and bromine dryer plants can also be a significant source of halogen for recycle.
The maximum temperature of the materials in the DAT must be kept below 60 0C. This is achieved by limiting the SOT feed rate if required. Sea water is also added to the DAT both to help reduce the temperature in the tank and to control the strength of the acid. If the temperature is exceeded problems can be encountered with cavitation of the pumps supplying the dilute acid to the BOTs.
If the SOT and BLR plants are off line, strong sulphuric acid from the stock tanks can be added directly to the DAT to maintain the pH control at the BOTs. If this occurs the strong acid stock will quickly fall. However some of this stock will be recovered when the PAL produced is later processed via the SOT. As a rough guide 10 tonnes of PAL is equivalent to 1 tonne of strong sulphuric acid.
The DAT tank is fitted with a sea water and caustic scrubber system. Both these systems must be operational before chlorine loading can take place. There are three dilute acid pumps on the outlet of the DAT. One of these pumps is normally on recirculation around the tank and helps to control the feed pressure to the BOT. One pump is used to supply the BOT and the other is a standby by spare.
Loss of Dilute acid flow to the BOT will have a very sudden effect on sea water main pH and will prevent the reaction between chlorine and bromide taking place in the sea water main. If the flow of acid is lost or reduced consideration should be given to immediately take chlorine out of the BOTs using the remote operated valves outside the chlorine house. If acid flow cannot be restarted quickly consideration will also have to be given to taking the SOT and BLR plants off line as their acidic effluent will quickly fill up the DAT.
Once at the Blowing out tower (BOT) the dilute acid is used to adjust the pH of the material in the main. For optimum extraction efficiency the pH should be maintained between 3.2 and 3.4. At higher pH, extraction efficiency is lost, at lower pH more acid is being used for no additional extraction benefit. After pH adjustment Chlorine is added to the main to convert bromide to bromine. SO2 gas, from the acid plant sulphur burners, is then added to convert the bromine back to hydrogen bromide which is then dissolved in fresh water to make Primary Acid Liquor (PAL).
The extraction efficiency of Bromine from sea water is very dependent on sea water temperature.
The BOTs each have a fresh water circulation system. BOT1 has a series of “trickle bars” which traverse across the top of the packing in the absorber area. The trickle bars irrigate the packing with fresh water which flows downwards. The water picks up acid mist as it flows down the packing and produces the PAL product which flows to a stock tank from where it is transferred under level control to one of number 1 to 3 PAL stock tanks. PAL density is controlled by adjusting the flow of water to the trickle bars.
On BOT2 a PAL recirculation system is used with recirculating PAL being sprayed over the surface of the packing via a series of static spray heads. PAL hence flows down over the packing, absorbs acid forming stronger PAL. This PAL is collected in a circulating stock tank. PAL density is controlled by adjusting the flow of water to the stock tank. Material in the stock tank overflows to a transfer tank from where it is pumped to one of number 3 to 6 PAL stock tanks.
Changes to the flow of seawater on BOTs will affect the amount of PAL made. It is normal to keep a minimum PAL stock of around 350 te. If PAL stocks increase above 1000 te reduction in BOT make may be required. Maximum PAL stock holding is 1250 te. Three pump operation of a BOT will produce around 16 tph PAL.
Each of the three pumps on each BOT will pump up to 60 000 gallons per minute of sea water using around 1.1 MW of electricity per pump. The number of pumps operational and the flow rate of each BOT is varied to maintain a maximum last drop cost of Br2 produced. This last drop cost is based on the amount of chemicals and electrical power used to produce the bromine. The maximum allowable last drop cost, must be below the mean sale price of bromine minus the processing costs for converting PAL to dried bromine in containers delivered to the customer , to ensure that we make a profit on the bromine. The actual figure is reviewed on an annual basis.
The amount of electrical power used is based on two main factors. The first of these is the tide height. Lower tide levels mean larger head losses have to be overcome to lift the sea water into the ponds. This requires high pump power usage. The second factor is the price that we have to pay for the electricity. This varies from season to season and also on the time of day. The required sea water flow is adjusted to make maximum use of the cheaper rate electricity periods.
PAL produced in the BOT is stored in 6 stock tanks. This provides a buffer capacity for Steaming Out Tower (SOT) operation and allows changes to Bromine and DBE sales to be catered for. The SOT has a steady flow of PAL at between 16 and 36 tph. Normally only one SOT is operational however two can be run at times of high PAL stocks. The limit on two SOT operation is normal DAT temperature or low chlorine pressure to the SOT.
In the SOT the bromide in the PAL is converted to Bromine by the addition of more Chlorine and then using steam distillation to evaporate and then cool a Bromine and water mixture. This mixture is allowed to separate and the aqueous effluent layer is returned to the DAT and used to control BOT sea water pH as above. The remaining water in the bromine is dried using a counter current of 98% sulphuric acid to produce dry bromine.
The corrosive nature of Bromine mean that a lot of the plant in this area was initially constructed from glass. However where possible this was later changed to Polyvinyldifloride (PVDF)
The sulphur burning plants produce both SO2 and steam. The SO2 plant produces gas exclusively for use in the BOTs. The acid plant’s SO2 gas is mainly converted to H2SO4 via a contact process, however the tail gasses from the converter can be made to bypassing the converter to provide more SO2 to the BOTs. The H2SO4 is used for pH adjustment at the BOTs and for drying the bromine recovered in the SOT.
To produce SO2 and sulphuric acid, molten sulphur is sprayed into a furnace in a blast of dry air at around 1300K. The sulphur is converted to SO2 and emits a blue flame. The exit gas is around 12% sulphur dioxide and 10% oxygen and this mixture is cooled to around 700K using a water heat exchanger which in turn raises steam for the rest of the plant.
Some of the SO2 is directly used in the BOT as described above. The rest is converted to sulphuric acid using a catalyst in the “contact process”. This is achieved by passing the hot gases through a fixed bed reactor containing up to 4 separate beds of Vanadium Oxide on silica pellets with a caesium sulphate promotor which ensures the catalyst is molten at 700 K. This allows for 99.5% conversion of the sulphur dioxide to sulphur trioxide.
2SO2 + O2 == 2SO3
The direct reaction of sulphur trioxide with water would be too energetic and produce acid mist. The final stage is the reaction of sulphur trioxide with water remaining in 98% sulphuric acid to produce more sulphuric acid. The concentration of which is adjusted by adding more water.
SO3 + H2O == H2SO4
Each of the sulphur burners will rapidly lose temperature when shut down. After a shutdown Sulphur should only be re-introduced if the burner temperature is greater than 420 0C. The SAP boiler will cool to below this temp in around 3 hours. The SO2 plant burner can be down for up to 12 hours before dropping below the minimum temp. If a burner drops below 420 0C oil will have to be used to increase the burner temperature before sulphur can be reintroduced, ignited and burnt.
Two Hamworthty auxiliary oil burners are used to produce steam made around the site and for sulphur burner warm ups. If the auxiliary burners cannot maintain the steam pressure the MPBF,DBM and HBr plants may have to shut down.
The chlorine plant supplies both the BOT and SOT. Normally each plant is feed by its own vaporiser and buffer system. If required the two systems can be cross connected to have a common feed from the chlorine house.
The ethylene plant only supplies gas to the DBE reactors. Low ethylene gas flows or stock will reduce DBE make. This may have a knock on effect to the SOT operation, PAL stocks and perhaps even BOT operation.
The main product of the SOT is bromine which is dried in three driers. Each drier has a capacity of 1.5 tph bromine produced. The driers use strong sulphuric acid from the stock tank to dry the bromine. This is supplied by one of two “labour” electrically driven pumps. The used acid is returned to the DAT. The dried bromine is either sold to customers or used in the HBr or MPBF plants to make downstream products.
The bromine was also reacted with ethylene to produce Dibromoethane ( DBE) which was sent to Octel Elesmere port where it was used to make lead Anti Knock Compounds.
The HBr plant is supplied with bromine from the bromine stock tanks and has a small effluent stream which goes back to the DAT. Bromine and Hydrogen and burnt together in the burner to produce HBr gas. The Hbr gas has three uses:
- used as HBr feedstock for reaction in downstream plants (DBM and MPBF)
- dissolved in distlilled water to produce 48% or 62% hydrobromic acid.
- refridgerated to produce liquid hydrogen bromide which is stored in cylinder or drums for sale to customers.
The DBM plant takes HBr gas and mixes it with Dichloromethane is a glass lined reactor. Al
The DBM plant takes HBr gas and mixes it with Dichloromethane is a glass lined reactor. Solid Aluminium Chloride is added to act as a catalyst. The reaction results in a typical substitution reaction and a mixture of Dibromochloromethane (DBM), Bromochloromethane ( BCM) and the remaining Dichlromethane (DCM) is evaporated from the reactor.
This is scrubbed with water to produce a water / organic mix. The mix is separated to produce a DCM/Water mixture. This is further separated and dried in the DCM distillation column before the DCM is returned to the reactor for further processing.
The organic layer is further distilled to separate off the BCM and DBM products which are sold in polythene lined drums.
The Multi Product Bromination Facility (MPBF) plant takes its gas from the HBr plant or bromine from an isomodule. It can be affected by loss of sea water for fire hydrants, steam or nitrogen. The plant is based on a batch processing model and can make a range of products. One such product is 1,3 bromochloropropane (BCP) (BrCH2CH2CH2CL) It is manufactured through free-radical addition of anhydrous hydrogen bromide to allyl chloride.
Allyl Chloride + Hydrogen Bromide = Bromochloropropane
CH2CHCH2Cl + HBr = BrCH2CHBrCH2Cl
Also n propyl bromide
n-Propanol + Hydrogen Bromide = n -propyl bromide
CH3CH2CH2OH + HBr = CH3CH2CH2Br
Another MPBF product was Calcium Bromide which was made by the reaction of hydrogen bromide gas and calcium carbonate. This material was used as an oil well drilling fluid.
A third product was Methyl Bromide made by the reaction of hydrogen bromide gas and methanol using sulphuric acid as the catalyst.
The MPBF also made methyl bromo butyrate (MBB) and methyl bromo myristate (MBM).