The project brief required that we undertake a research and comparative study into the alternative routes of Anthraquinone production based on the vapour phase oxidation of Anthracene.
A list of factors were taken into consideration prior to the selection of the method of production and also at the process design stage. These included analysis of the initial capital costs associated with each method, the commercial and technical risk involved, safety and environmental issues, and the evaluation of continuous vs. batch applications.
Proposed design
A continuous liquid phase production process of 2502 tonnes Anthraquinone of purity 99.12% through the oxidation of Anthracene with Nitric Acid has been proposed. An estimated capital cost of the installed plant has been valued at £#######. The Anthraquinone plant forecasts an estimated annual turnover of £##### with a payback period of ##### years. The costing data is an approximation limited to our resources, pricing reference and necessary approximations
Introduction
The manufacture of anthraquinone based on the liquid phase oxidation of anthracene using nitric acid currently operates at Lubrachem, one of many products manufacture at Lubrachem. The existing batch process has three main problems; it is aging and needs to be modernised, it has inadequate reaction capacity to meet at present and future requirement, and it has numerous technical problems.
The demand for anthraquinone substance has been predicted to grow steadily worldwide. It has substantial commercial importance known for its recognition as a synthetic dyestuff precursor, digester additive in papermaking and in large industrial production of hydrogen peroxides. In small scale production
An investigation into the design and construction of a new plant of 2500 te/annum production of anthraquinone has been proposed to replace the existing plant. In which case, other alternative routes into the manufacture of anthraquinone have been considered, predominantly the vapour phase oxidation of anthracene with air (oxygen). The new plant designed should be a significant improvement to the existing liquid phase process in which there is a greater commercial advantage in the chemical industry.
Block Diagrams
The synthesis of 9,10-anthraquinone, abbreviated to Anthraquinone is realised through the selective oxidation of the anthracene central ring. Alternative methods of synthesis do exist, however the aforementioned method is predominant in Industrial scale manufacture.
Liquid Phase Oxidation of Anthracene
Referring to Block Diagram - Liquid Phase Oxidation of Anthracene with Nitric Acid:
Equation 1:
C14H10 (soln) + 2HNO3 (aq. Soln) C14H8O2 (soln/suspn) + 2NO(g) + 2H2O(g)
Anthracene Nitric Acid Anthraquinone Nitric Oxide Water
The nitric acid is assumed to react rapidly on addition, with a holding time of 45 minutes after the total addition. In this case, the nitrobenzene is recovered and recycled back for further reaction and used for washing. The excess nitric acid is partly retained in the reaction mass and is boiled off. It is also assumed that plant occupation of 75% overall.
Anthracene is suspended in nitrobenzene, an organic solvent serving as a suitable medium for the solid Anthracene. Nitric acid in excess is added to the well-mixed anthracene suspension. A suspended solution of anthraquinone is formed with nitric oxide gas and water vapour to be treated.
After the conversion in the reactor the aqueous solution is further stirred and cooled to room temperature (20C) at which the different it is crystallised out and recovered by filtration, washed by nitrobenzene recycled and removed by the steam distillation. Under the operation conditions of the process, approximately equal weights of water and nitrobenzene and distilled off. The anthraquinone formed is further washed using hot water and dried.
In the current liquid phase batch process, there are 2 agitated reactors in parallel each with initial charges of 2825 kg of Nitrobenzene and 2234 kg of Anthracene with the reaction carried out at 147C. The mixer volume accounts for both the initial charges of Nitrobenzene and Anthracene with the addition of 2770 litres of nitric acid charge (of 47% of nitric acid).
Vapour Phase Oxidation
Referring to Block Diagram - Vapour Phase Oxidation of Anthracene with Air:
Equation 2:
C14H10 + 3/2 O2 C14H8O2 + H2O
Anthracene Oxygen (Air) Anthraquinone Water
From our research into the vapour phase oxidation of Anthracene with air, the inlets of air and water are mixed and heated. The Anthracene of solid form is evaporated with the preheated air-water vapour mixture (adjusted to reaction specifics). The vapour current is mixed carefully more with air and led into the catalytic reactor, from the bottom at 325C.
Iron (III) vanadate catalyst is added into the reactor, the lower part of the furnace, where the reaction takes place, is heated to 390C and the upper part is heated to 339C.
The air flow rate going into the catalytic reactor is 2150m3/hr where each cubic meter contains 20g of 94% Anthracene. The gas leaving the catalytic furnace first goes through heat exchangers and then through cooling towers, cooling chambers, and dust filters.
The Anthraquinone is formed with water vapour to be treated and has an average purity of 99.6%. The conversion of anthracene is nearly quantitative. Phthalic anhydride C6H4(CO)2O is a by-product is small quantity which is separated easily. After a period of time the activity of the catalyst falls off, resulting in a decline in the yield of anthraquinone and an increase in phthalic anhydride.
The advantage of using this procedure is that it produces anthraquinone of 99% purity.
[mixture of Ken and Marc's work]
Process Comparison
Criteria
Reactor sizing for liquid phase batch process
The required production rate of anthraquinone is 2500 te/yr. Assuming operation time of the plant being 75% of the year and each batch producing 2300 kg of anthraquinone, the required number of batches per day using two reactors is going to be:
No.of batches required per day= AQ needed per year(AQ produced per batch x Days in a year x Operational time per year)
No.of batches required per day= 2500000 kg(2300 kg x 365 days x 0.75)
No.of batches required per day= 3.97 ˜4
No.of batches required with two reactors=2
Table x - Reactor inlet amounts
Table y - Amounts in reactor after reaction
From table x it can be seen that the minimum reactor volume has to be the total inlet volume of all the reactants so that it can accommodate all the chemicals. Total Volume = 6.913 m3 ˜ 7 m3.
Due to the nitric oxide and water vapour being purged, the total volume occupied is reduced considerably. So the minimum reactor volume remains the same as the reaction process doesn't require more volume than what enters the reactor.
The general cost analysis of the liquid phase batch production of anthraquinone includes the cost for the mixers, reactors, storage vessels and other equipment.
The majority of the costs were obtained from a specific website where the equipment and its size was input and the cost of the equipment was output. The total cost for this method of production was £697423.44 for the capital cost only.
The sizing for most of the equipment was based on the reactor volume which was found out earlier in this report. It was assumed as the combined volume of the main reactor was the largest volume, the rest of the equipment would be equal to or smaller, so the volumes were kept roughly the same.
The complex part of this cost analysis was to work out the cost of the cooling tower. Using the specific heat capacities and densities of both nitrobenzene and anthraquinone, and the information on the block diagram, the total energy required to cool down these chemicals was found.
Table z (per reactor)
Using Table z,
Energy required to cool all NB=Mass cooled Ã- Specific heat capacity Ã- ? Temp
Energy required to cool all NB=222384 kg Ã-1.4 Jg .K Ã-127 K
Energy required to cool all NB= 39539.88 kJ
And,
Energy required to cool all AQ= Mass cooled Ã-Specific heat capacity Ã- ? Temp
Energy required to cool all AQ=2300000 KG Ã-1.3 Jg .K Ã-127 K
Energy required to cool all AQ=379730 kJ
Total energy required to cool all the components down, and taking into account there are two reactors, the energy equals to 838539.76 kJ.
For a whole batch to go from start to finish it takes nine hours with each stage of the process taking roughly one hour, this gives a mass flow rate through the cooler of,
Mass flow rate through cooler=Mass of NB+Mass of AQTime taken to pass through cooler Ã-No.of reactors
Mass flow rate through cooler= 222.38 kg+2300 kg1 hour Ã-2
Mass flow rate through cooler=5044.768 kg/hr
Therefore it takes 838539.76 kJ of energy per hour which corresponds to 232.93 kW which is, using a conversion tool, the same as 0.795 million BTU.
Reactor Sizing for vapour phase
The production of anthraquinone for this continuous vapour phase is through a tubular reactor. Using information such as the flow rate of the feed, the tube dimensions and the required production of anthraquinone, a specific volume for the reactor can be acquired.
Fig x - Diagram of one tube in the reactor
Feed = 2 mol % Anthracene, 98 mol % Air.
Conversion of anthracene= 68% anthraquinone, 23% Phthalic Anhydride, 9% Carbon Monoxide
Anthraquinone required per year = 2500 te/yr
The information above shows the only unknown Q, which, when worked out, will give enough information to work out the mass of anthracene entering the tube per second which can then, using the conversion, be used to work out the amount of anthraquinone produced per second.
The above shows the volumetric flowrate through one tube, but isn't enough information to work out the amount of anthracene flowing thorugh it as the volume is the total volume of anthracene and air.
Therefore the mass of the anthraquinone produced by one tube operating all year,
Mass of AQ produced=Volume Fraction of AC in air Ã-Flowrate through tube Ã-Mass of AC Ã-Conversion into AQ Ã-Seconds in a year
Mass of AQ produced= 0.0002850.000285+2.3585 Ã-8.82Ã-10-5 Ã-356.46 Ã-0.68 Ã-31536000
Mass of AQ produced=81.46 grams
And the number of tubes required is just the required anthraquinone per year divided by the amount produced per tube which is around 31000 tubes, and using the dimensions of the tube the total volume required for the reactor is 17.4 m3.
Material Costs
Material Cost for Liquid Phase
Material Cost for Vapour Phase
Technical Risk
Commercial Risk
Safety
Vapour Phase
One of the main concerns that come with using the vapour phase process is the slight danger of explosion that is present in the stage where Anthracene is evaporated in more air (having been evaporated with a heated air-water vapour mixture beforehand). As Anthracene has a lower flammable limit of 0.6%, caution must be taken when mixing it with air, and as it may operate above its LFL, sources of ignition should be eliminated.
Also, this process utilises high temperatures (as high as 390°C) and high pressures and this would require careful planning in design of the plant to ensure a very low risk of failure for components/piping etc.
Liquid Phase
The Nitrobenzene used in this process as a solvent is very toxic and a carcinogenic and so attention would have to be paid to the exposure limits for workers and health and safety rules tightened to avoid any spillages of material. Nitric Acid is also used in this process which would affect the materials proposed for components in the design of the plant as it reacts with common metals and can corrode steel (and obviously very corrosive or dangerous if split or come into contact with).
Environment
There are a few environmental aspects to consider during the production of anthraquinone via both the liquid and vapour phase. The common chemicals in these processes are anthracene and anthraquinone even though they are at different temperatures and in different phases, therefore the environmental issues relating to these chemicals are going to be common. Nitric acid on the other hand is used only in the liquid phase where as nitrobenzene and phthalic anhydride are used and produced in the vapour phase respectively.
Anthracene does not have a major effect on the environment rather than on the living organisms. The chemical is readily absorbed through the skins of humans and targets mainly the kidneys and liver. Symptoms include headaches, nausea, swelling and inflammation amongst other things. It is also known to have an effect on the reproductive organs and immune symptoms of animals.
Anthraquinone on the other hand has an effect on the environment, mainly on the aquatic environment. It can affect the aquatic organisms and may cause long term effect to the environment. The chemical can also be used as a bird repellent if used intentionally or may unintentionally affect the bird population around the plant. On humans the chemical tends to have laxative effects and prolonged exposure may cause melanosis coli.
Nitric acid has no dangerous effect on the environment but it can enhance the effects of some of the other chemicals in the process. It is used in the liquid phase where the temperature reaches temperatures as high as 147 °C and causes an increase in pressure which can increase chances of explosions. If an explosion does occur, the nitric acid produces toxic fumes which cause irritation and even a short term exposure can cause damage to the skin, respiratory system and may cause lung oedema.
The secondary reactant in the vapour phase is nitrobenzene which is very toxic and has a harmful effect on the aquatic environment similar to anthracene. It has a very strong effect on living organisms as only a small amount needs to be inhaled to cause nausea, weakness, can render someone unconscious and in some cases can even be fatal. The vapour phase also has a by product which is phthalic anhydride. This chemical is relatively safe compared to others used in this process. Inhalation may cause redness, soreness and abdominal pain. There is no proof that it has any major effect on the environment.
Batch & Continuous
The production of anthraquinone from the reaction between anthracene and other reactants can be carried out under both batch and continuous process conditions. This mode of operation is dependant on many factors such as the costs, the running time of the operation and also importantly the rate of production of anthraquinone that is required, in this case 2500 te/year.
Producing the anthraquinone in a batch process means that the required initial cost will be relatively low as a single production line or system can be used to produce the several products. If the demand for anthraquinone falls, the process as a whole will incur small losses as the process can be adapted and changed to accommodate the production of other chemicals or can even be used for storage. Batch reactions are also ideal to use for slow chemical reactions which makes it suitable for the production of anthraquinone as the conversion from anthracene to anthraquinone takes approximately one hour. Batch operation processes also provide high conversion per unit volume for one pass and are easy to clean and maintain.
Though batch processes have a lower start up cost, the overall cost can be quite high due to the high operating costs. The quality of the products will also vary as the conditions under which the reaction takes place may differ slightly for each batch. Apart from the inconsistency of the product quality, this disadvantage could have a further negative effect, that is, if the quality is a lot lower than required the whole batch may have to be disposed off adding to the losses. Batch processes are also generally used for small scale productions which would make it unsuitable for this process.
Economically the continuous production of anthraquinone is better than the batch production as both the manufacturing and production costs are lower. In addition to this, the production is largely controlled by control loops and feedbacks which eliminate the need to have manual labour reducing the costs. The construction of a continuous process is also quite simple making it cheaper to construct and like the batch reactor is easy to maintain and clean. The most important advantage is that process is run continuously meaning there is no down time and therefore this extra time increases profits as a larger volume of anthraquinone is produced. Conversion of anthracene to anthraquinone takes place at 147 °C, as this is a fairly specific temperature, a continuous reactor is well suited as it is fairly easy to control the temperature.
Unlike the high conversion per unit volume under batch conditions, the conversion using a continuous process is low. Due to the fact the process is run continuously, if a part of the process breaks down, the whole process has to be shut down resulting in a large loss of revenue. Even though the amount of personnel required to control this process is low, the personnel that are required need to be highly trained. If the contents in the reactor aren't well agitated, then channelling and by-passing may occur.
For the design of this plant and the amount of anthraquinone that is required, the preferred mode of operation is the continuous process. This conclusion is reached by assuming constant and continued demand for anthraquinone in the future. The lower manufacturing costs and increased efficiency of the process in addition with the other mentioned advantages makes this the more attractive choice.
Recommendation
PFD - Process Description
In the Continuous Liquid Phase Process that was decided upon, the Anthracene is fed into a mixer in solid form via a Screw Feeder 2. Also flowing into this mixer is Nitrobenzene through a fresh feed 1 and through the Nitrobenzene recycle 12 both of these streams are heated before entry to the Mixer in order to ensure that the all the Anthracene dissolves before being fed into the Agitated reactor 3. Nitric Acid is also fed into the reactor through both the fresh feed 4 and the Nitric Acid recycle 30, the fresh feed is adjusted according to the recycle to ensure that the appropriate excess of Nitric Acid is always present. The reactor is continuously stirred to ensure that the reactants all experience an equal amount of contact time with each other and therefore, due to the Nitric Acid being in excess, a complete reaction is assumed. The reaction vessel will be maintained at a constant temperature of 147C, this is done through the use of a steam filled jacket. As a result of the reaction taking place at this temperature there will be a large amount of material leaving as vapour which will leave through the top of the vessel 5 where it will then be partially condensed in order to recycle the useful material.
As a result of the reaction occurring, Anthraquinone is formed along with Water and Nitric Oxide, most of the Water and the Nitric Oxide leaves as vapour, leaving the liquid stream 6 consisting of mostly Nitrobenzene and Anthraquinone. So as to maximize economic efficiency this Nitrobenzene will be separated and recycled. Having left the Reactor the Mixture is passed through a cooler 8 in order to precipitate out the Anthraquinone thus making it possible to be filtered through a centrifuge. Before entering the Centrifuge the Anthraquinone Rich stream is first mixed in a mixer with a recycle from both the Volatile Products 22 and the Dryer 19. Due to the temperatures of the recycles coming into this mixer, the stream to the centrifuge from the mixer needs to be cooled to 20 degrees to ensure that a minimum amount of Anthraquinone is left still dissolved. The centrifuge separates the solid Anthraquinone from any material in the Liquid Phase. It is however assumed that 5% of the Anthraquinone is lost in the Filtrate 10 and that the filter cake exiting the centrifuge 13 consists of 10% liquid. The Filtrate is sent to a decanter 10 where the Nitrobenzene is Separated and sent back as a recycle 12 to the feed mixer tank. The filter cake is then sent through a washer 13 where it is washed with water in order to lessen the concentration of Nitrobenzene in the liquid phase of the filter cake. The cake is then dried in a heated conveyor dryer, out of which the heated vapours are removed and sent through a cooler in order to condense them 18 before being sent as a recycle to the mixer before the centrifuge. Also from the dryer comes the dry Anthraquinone product 17 which consists of 98.5% pure solid Anthraquinone.
The volatile products leaving the reactor as vapour 5 are treated appropriately to separate the material to be recycled. They are first passed through a cooler where they are partially condensed. The condensed liquid leaves the condenser 20 and is sent to a gravity separator. This stream consists mainly of Nitrobenzene, Water and Nitric Acid. The Nitrobenzene is separated from the bottom of the separator along with what little Anthraquinone left the reactor in the volatile product stream and this is sent as a recycle 22 to the mixer located before the Centrifuge. From the lighter phase of the separator, dilute Nitric Acid is taken and sent to a mixer 23 where it is mixed with the uncondensed vapour that exits the Partial Condenser 21. The stream exiting this mixer 24 leaves at 85C and therefore contains a large vapour fraction of Nitric Acid, in order to ensure that a large amount is recycled, it is cooled to 40C so that the vapour fraction of Nitric Acid is much lower. At this temperature the Vapours consist largely of Nitric Oxide. Once cooled the stream enters a separator which separates the Vapours (mostly Nitric Oxide) which are disposed of 27, The liquid that is separated is sent to a distillation column 26 in order to separate the Nitric Oxide from the Water. The distillation column concentrates the Nitric Acid from about 36% to 60%. This Nitric Acid is sent from the top of the column via a recycle back to the reactor. The waste water emerges from the bottom of the column 28 and is disposed of.
P&ID
Safety Review
The types of safety to be taken into consideration in the design of this plant include both personnel safety and plant/equipment safety.
Personnel
A number of alarms have been put into place around most of the components including high and low level alarms on the main reactors, mixers and separators. Pressure and temperature alarms are also present on the separators and the reactor. These alarms consist of both a visual and audible component to make the personnel aware of a significant system disturbance.
Should the alarms put in place fail, manual operation, or shut down, of the plant may be required. In event of this, measures have been taken such as providing the personnel with protective clothing including breathing apparatus, safety goggles and full body clothing to protect against the harsh conditions.
Plant/Equipment
Relief valves serve the purpose to protect against overpressure within the main reactor vessel and the specified mixers. The relief valves discharge to flare (away from the plant) to meet safety and emission standards.
Walls are employed around the main reactor and the high pressure mixers which act as both a blast walls and a containment field for any leaked chemicals. Water curtains are also in place to protect the main reactor in the event of a fire or explosion and to reduce personnel incident heat radiation in the case of an evacuation.
The feed tanks for the inlet components have been earthed to prevent an explosion from the static build up as the material passes through the tank.
Cost Analysis
Capital Costing
Through analysing the material balances around each major component in the plant it was possible to obtain a rough estimate of their capital cost. The two dependent factors which affected the cost of each component were the element material and dimensional requirements.
The material balances around each component enabled the calculation of the sizing requirements. In order to ensure that the plant operated safely, all components with volumetric requirements were scaled up by 50% as an overflow prevention measurement in case of a system failure. The component sizing calculations can be found in Appendix (?). A basis price for the individual components was obtained from three independent sources. Reference
The type of materials chosen depended on the physical process conditions to which the particular component is subject. In the case of certain components it was necessary to employ a construction metal that was resistant to corrosive material exposure (e.g CR-101- Stainless Steel). The choice of material was dependent upon the through flow composition and heating requirements. In the case of the heat exchangers, it was not cost effective to use stainless steel and the presence of corrosive materials did not warrant carbon steel. In this case aluminium was a suitable choice since it was cheap and resistant to corrosion. The costing of pipe, vand control loops are qualified by the P&ID design.
The capital cost estimates required an adjustment on the basis of annual inflation, as many of our original reference prices were outdated. It was assumed that inflation was constant at 2% per year and that this was the only economical factor which affected cost. After determination of the total capital cost, a Lang Factor was introduced in order to estimate the cost of installation. The Lang Factor for plant involving liquids/solids has a value of 3.7 reference. Through multiplying the capital cost of the components by the Lang factor value, an estimate for the total capital cost of the plant is realised.
Operating Costs
Operating costs consist of many different factors such as energy usage, labour, materials costs. However labour and energy costs were difficult to calculate thus certain assumptions had to be made. These assumptions will be discussed in their relevant sections.
Energy Costs:
It was assumed that energy was only used for heating purposes and that this amount of energy could be modelled as electrical energy and priced accordingly. The energy requirements of operating mechanical devices such as agitators, mixers and centrifuges were ignored.
Labour Costs:
Labour costs are more difficult to calculate since determining the number of workers required to operate the plant depends on how capable workers are. However, for the purposes of this report it will be assumed that every major plant item requires one operator who will be paid a salary of £30,000 per annum. It shall also be assumed that one plant manager will be required at a salary of £40,000 per annum. The table below shows the estimated annual cost of labour.
Material Costs:
The material costs of the process include the cost of all the raw materials used as well as the cost of treating waste water. Akin to the material costs it was necessary to calculate the turnover obtained from selling the raw anthraquinone. A table containing the costs of all the material flowrates and costs is shown below.
Profit:
The table above shows the theoretical profit that should be obtained assuming the costs calculated in the previous sections. It was also assumed that the demand rate stayed constant at 2500 tonnes thus implying that all of the product produced could be sold. The payback period calculated refers to the amount of time required for all of the capital cost to be paid back. This value is equal to half a year, therefore after half a year of running the plant, the profit will no longer have to be spent on paying back the capital loan.
