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Production Of Anthraquinone Plant Design Engineering Essay

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Published: Mon, 5 Dec 2016

The manufacture of anthraquinone based on the liquid phase oxidation of anthracene using nitric acid currently operates at Lubrachem. The existing batch process is aging and needs to be updated since it currently has inadequate product capacity in order to meet current and future demands.

The demand for anthraquinone substance has been predicted to grow steadily worldwide. It has substantial commercial importance in the production of dyes, paper and in large industrial production of hydrogen peroxides. REFERENCE

In this report an investigation into the design and construction of a new plant with a production capacity of 2500 te/annum of anthraquinone will be undertaken. Alternative routes into the manufacture of anthraquinone will be considered, specifically the vapour phase catalytic oxidation of anthracene with air and a newly designed liquid phase process involving the reaction of anthracene with nitric acid. This report aims to determine which method of production will be the most effective, taking into account, product quality, safety, environmental hazards and process economics.

Organic Chemistry REFERENCE FIGURES

The synthesis of Anthraquinone is achieved through the selective oxidation of the anthracene central ring. Alternative methods of synthesis do exist, however the method mentioned is predominant in Industrial scale manufacture.

Liquid Phase Oxidation of Anthracene

The block diagram for the liquid phase oxidation of anthracene can be found in appendix A.

Eq. 1: REFERENCE

C14H10 (soln) + 2HNO3 (aq. Soln) C14H8O2 (soln/suspn) + 2NO(g) + 2H2O(g)

Anthracene Nitric Acid Anthraquinone Nitric Oxide Water

The liquid phase oxidation of anthracene takes place by reacting nitric acid with anthracene which has been dissolved in nitrobenzene.

The current batch liquid phase process, consists of two 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.

Anthracene is dissolved in nitrobenzene, an organic solvent serving as a suitable medium for the solid Anthracene. The anthracene takes approximately one hour to dissolve in the nitrobenzene after which the mixture is split into two equal streams put into two reactors. A solution of 47% nitric acid is then added in 10% excess to the reactor. The excess nitric acid is required in order to drive the reaction to complete completion during the 45 minutes reaction time. The operating temperature of the reactors is maintained at 147°C by means of a heated jacket.

The reacted material is then sent to a cooler where it is cooled to 20°C upon which it is sent to a crystalliser in order to precipitate the anthraquinone product from the nitrobenzene solvent. The solid product is then filtered out and washed using nitrobenzene to remove impurities. The slurry is then sent to a steam distillation column in order to remove the majority of the nitrobenzene so that it can be recycled. The refined slurry is the sent to a hot water wash and then a dryer to give a product with a minimum purity of 99%.

Vapour Phase Oxidation

The block diagram for the vapour phase oxidation of anthracene can be found in appendix A.

Equation 2:

C14H10 + 3/2 O2 C14H8O2 + H2O

Anthracene Oxygen (Air) Anthraquinone Water

There are many different methods of oxidizing anthracene in the vapour phase however the process considered involves the reaction of anthracene with air in the presence of a catalyst. The process begins by evaporating water and mixing it with evaporated anthracene. This mixture is then sent to a mixer in order to introduce the oxygen required for the reaction. The vapour then enters the reactor at 325°C with an anthracene concentration of 2mol%

The catalyst used comprises of 9.4% V2O5 and SO3/K2O in a 1.6 ratio. The inside of the reactor consist of reactor 5 foot reactor tubes with ¾” diameters. Within the tubes is a packed bed of catalyst through which the anthracene/air mixture is passed at a linear velocity of 1 foot/s. The lower part of the furnace, where the reaction takes place, is heated to 390°C and the upper part is heated to 339°C.

Once the vapour has passed through the reactor it is then cooled to precipitate the anthraquinone. The mixture must then be washed in order to remove the by-product Phitalic Anhydride. The liquid mixture is then sent through a dust filter in order to remove any unreached anthracene. The anthracene is recycled but the liquid mixture containing the anthraquinone is sent through a series of cooling operations before the excess water is evaporated in the drier. This process gives a purity of 99.6% but a yield of only 68%. The other two products of the reaction are Phitalic Anhydride of which the product contains 23% and carbon monoxide and dioxide which constitutes the remaining 9%.

Over time the activity of the catalyst decreases resulting in increased production of Phitalic Anhydride and less anthraquinone.

Comparison of Processes

Introduction

In order to assess the advantages and disadvantages of each process various criteria were considered. In this section each process will be compared in each category in order to determine which would be the most preferable process.

Cost Analysis

Cost analysis can involve a detailed breakdown of the process, but for the purposes of this preliminary cost analysis, only the major capital and material costs will be considered.

Liquid Phase

Capital Cost:

In order to determine the capital cost of the items in batch liquid phase process, it was necessary to determine the sizes of each component. In order to do this certain assumptions had to be made. In order to size reactors, reaction kinetics must be known but since these values were unknown the reactor sizing was done by using the following equation;

= The mass of component ‘i’ (kg)

= The density each component ‘i’ (kg/m3)

= The sum of all volumes of the separate components (m3)

VT = Total Volume of all components (m3)

The equation calculates the volume occupied by each component and sums the values in order to give the total volume necessary to run the process. Due to the nature of a batch process an assumption was made that production only occurred for 75% of the year while the other 25% was used for the preparation of new batches. Thus, each component was calculated to have a larger capacity in order to accommodate down-time.

Most components required a volume in order to cost them but other components require surface areas, or cooling loads. In some cases this called for estimations but it other cases it was possible to calculate the values. All the calculations for each component can be found in Appendix B. The table below shows the approximate specifications and cost of each major component in the liquid phase batch process.

Table 1: Liquid Phase Capital Costs

Equipment

Specifications

Material

Cost (£)

Notes

Reactor x 2

8.4 m3

Cast Iron

£75,233.02

Jacketed and agitated

Mixer

16 m3

Carbon Steel

£142,676.43

Cooler

Cooling load

844045 kJ

Carbon Steel

£53,462.05

Forced draft

Crystallizer

16 m3

Carbon Steel

£65,046.61

Batch, Atmospheric

Dryer

Surface area 18.58 m2

Carbon Steel

£14,247.67

Tray, atmospheric

Washer

1.09m Diameter

Carbon Steel

£119,041.28

Centrifugal separator, horizontal,

Steam Distillation

£216,731.03

Cost of a boiler and separator together as distillation column price could not be found

Storage Tanks x 4

13 m3

Carbon Steel

£8,788.28

Horizontal storage tank

Recovery vessel

13 m3

Carbon Steel

£2,197.07

Horizontal storage tank

Total: £697,423.44

According to Table 1 the expected capital cost for this process is equal to £697,423.44. This value however has not been adjusted for inflation which is important since most sources contain data from at least three years ago. The United Kingdom aims to keep inflation as a constant 2% per annum therefore this value will be assumed. The capital cost when taking inflation into account can be calculated using the following equation.

Where

Ci is the capital cost taking into account inflation

Cc is the capital cost not taking inflation into account

t is the number of years elapsed since the component was priced

The source used for the pricing is three years old, thus the actual capital cost should be

Raw Material Cost:

The raw material cost for the batch liquid phase is simple to calculate since the initial charges and output are known from previously used processes REFERENCE. The table below shows the estimated cost of the materials per annum.

Table 2: Liquid Phase Material Costs

Material

Mass per Batch (kg)

Cost per Tonne (£)

Cost/Batch (£)

Batches/Day

Cost Per Annum (£)

Anthracene

4468

1100

4914.80

4

£5381706.00

Nitrobenzene

5650

480

2712.00

4

£2969640.00

Nitric Acid

3437.75

80

275.02

4

£301147.00

Total

£8652493.00

The material cost shown is the value necessary to produce 2500 te/year and can therefore be used in a direct comparison with the Vapour phase material cost shown in Table 4. In the case of material costing it is not possible to determine the effects of inflation since the date of the reference is not known.

Vapour Phase

Capital Cost:

Determining the capital cost of the vapour phase process was more difficult because the process was much more complicated than the liquid phase process. In order to accurately determine the cost of a component at least one specification needed to be known but it was only possible to definitively determine the catalytic reactor volume. The table below shows the estimated specifications and costs of all the major components in the vapour phase process. The calculation of the catalytic reactor volume can be found in Appendix B.

Table 3: Vapour Phase Capital Costs

Equipment

Size

Material

Cost (£)

Notes

Catalytic Reactor

5.61 m3

Stainless Steel

£385,000.00

Jacketed and

non-agitated

Mixer

6 m3

Carbon Steel

£91,639.02

Evaporator

(Heat Exchanger 1)

Surface area 18.58 m2

Aluminum

£55,813.65

Horizontal Tube

Condenser

(Heat Exchanger 2)

Surface area 18.58 m2

Aluminum

£21,436.16

Air Cooler

(Heat Exchanger 3)

Surface area 18.58 m2

Aluminum

£17,518.25

Dust Filter

Surface area 18.58 m2

Carbon Steel

£13,072.33

Cartridge type filter

Dryer

Surface area 18.58 m2

Carbon Steel

£14,247.67

Tray, atmospheric

Washer

Diameter

1.09 m

Carbon Steel

£59,520.64

Centrifugal separator, horizontal,

Storage Tanks x 6

Volume

20 m3

Carbon Steel

£17,520.00

Horizontal storage tank

Separator

Diameter

0.61 m

£19,973.46

Ceramic Lined

Cooling Tower x 2

£121,831.46

Cooling Load:

1055056 kJ/hr

Total

£817572.64

When the total amount is adjusted for inflation the total capital cost for the vapour phase is equal to

Material Cost:

The vapour phase oxidation process is in difficult to calculate accurately because of the catalyst. The catalyst is so specific that it was not possible to find a price. However, the catalyst used in the vapour phase process is a specific combination of three catalysts impregnated on silica beads. Thus it can be assumed that the catalyst will be expensive and will bear weight during the decision process. However the annual costs for all the other materials used in the process is shown in the table below.

Table 4:

Material

Mass Required per Day (kg)

Cost per Tonne (£)

Cost per Annum (£)

Anthracene

10350

1100

£4255525

Air

1778.43

0.08

£51930.156

Total

£4307455.12

Cost Analysis Conclusion:

In conclusion, the capital cost of the vapour phase is approximately £100,000 greater than the liquid phase process however the material costs of the liquid phase process exceed that of the vapour phase by about £400,000. However, as discussed previously the cost of the specialized catalyst could not be determined therefore this difference does not necessarily warrant the approval of the vapour phase process. The two key aspects two consider are the cost of the catalyst and its activity. As discussed before, the catalyst deactivates after a certain period of time. The lack of information on the price of the vapour phase catalyst results in a possibility of the cost of the process escalating.

Technical and Commercial Risk

The economic risks undertaken by a prospective investor for each process vary for a variety of different reasons. In the case of the reaction occurring in the liquid phase, the nature of the process means that the various components are more versatile in that not only might they be used for anthracene, but also for other processes that run in a similar way. The sequence of components used in the liquid phase process is relatively common in the process industry. As a result of this, should anthraquinone no longer be so highly demanded, the plant could relatively easily be converted to work for another product, thus greatly reducing the risk posed to any potential investors.

Contrary to this, the vapour phase process is more specialised in its design, meaning that should the demand for anthraquinone fall, it would be more difficult to modify it to produce a different, more desirable product. This increases the risk involved.

In the past the liquid phase oxidation of anthracene has been the more commonly used process, one example being Huddersfield where the process was operated up until 1997 REFERENCE. This would suggest that the process is more likely to work correctly, and as planned.

In contrast to the liquid phase process, during the research undertaken for this report, no examples of the vapour phase process being used were found. It was however suggested by Ullmanns encyclopaedia that it is the preferred production method in Europe REFERENCE.

Earlier it was stated that it was not possible to calculate the price of the catalyst used in the vapour phase process. This lack of knowledge regarding the cost presents a substantial risk to anyone who might invest because the price of the process effectively has no known limits.

In conclusion it was decided that the liquid phase process was seen to have the lowest potential risk to any investors as it does not rely so heavily on the demand for anthraquinone remaining constant, the cost analysis conducted was seen to be more reliable and it has also been used extensively in the past, therefore the probability of the process failing is small.

Safety

Shared:

Anthracene is readily absorbed through the skin of humans and targets mainly the kidneys and liver. Exposure can cause have many physical side-effects including damage to the immune system.

On humans the chemical tends to have laxative effects and prolonged exposure may cause illness.

Liquid Phase

The Nitrobenzene used in this process is toxic and classed as a class 3 carcinogen. Safety measures will have to be implemented in order to ensure that exposure limits fall under the maximum level. This could be in the form of Personal Protective Equipment for workers operating near any components containing nitrobenzene and regular maintenance on all pipes and vessels so that leaks are detected and dealt with accordingly. Nitric Acid is also another consideration since it affects the materials that can be used. Since Nitric Acid is heated in the process, the rate at which is corrodes will increase and pose more of a hazard. When very concentrated it is a hazard to humans but this is not a major concern. However, Nitric acid fumes can cause irritation and even a short term exposure can cause damage to the skin, and the respiratory system.

Vapour Phase

One of the main concerns that comes with using the vapour phase process is the danger of explosion during the stage when 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 REFERENCES. This can be achieved by grounding tanks to avoid static build-up and using anti-spark tools when performing any maintenance.

The process utilises temperatures as high as 390°C and high pressures. Therefore the consequences of a component failing are very high thus the probability of a failure must be made to be very low in order to minimize risk.

Environment

Shared dangers

Materials that are common to both processes include anthracene and anthraquinone. Anthracene mainly affects aquatic organisms. This effect is magnified by the fact that it is not biodegradable and therefore accumulates in the exposed ecosystem. Waste from both processes must be purged of anthracene before being released into the environment. REFERENCE

Anthraquinone presents little risk to the environment. No toxicity to marine life nor animals has been previously noted and thus no has no cumulative effects. REFERENCE http://www.epa.gov/pesticides/biopesticides/ingredients/factsheets/factsheet_122701.htm

Liquid Phase

The two chemicals used in the liquid phase process that are not used in the vapour phase process are nitrobenzene and nitric acid. Nitrobenzene, due to its biodegradable nature is not expected to accumulate in the environment. It is not known to be toxic to aquatic life in low concentrations however as a volatile organic compound it may react with other air pollutants to cause the formation of ozone which may be damaging to plants and animals. REFERENCE http://www.environment-agency.gov.uk/business/topics/pollution/201.aspx

The other potentially damaging chemical used in the reaction is Nitric Acid, if entered into the environment it can cause acid rain which has corrosive effects as well as being harmful to living things.

Vapour Phase

In the vapour phase, the only chemical specific to that process which is a cause for concern is Phitalic Anhydride

The only product involved only in the vapour phase that could present any potential harm to the environment is Phitalic Anhydride. It is relatively safe compared to anthracene and does not have any known effects on the environment.

Process Decision:

Having taken into account a large range of factors the process decided upon is the liquid phase oxidation of anthracene. This was a result of the cost analysis revealing that there would be an increased commercial risk in choosing the vapour phase and even though the vapour phase was approximately £300,000 less expensive the lack of data present about the catalyst, its activity, and the use of the vapour phase process by other major chemical companies warranted the choice of the liquid phase process. Furthermore, the environmental and safety issues were in favour of the liquid phase. Although the vapour phase utilised chemicals which had fewer safety and environmental risks than the liquid phase process, the fact that anthracene needed to be oxidized in air at a temperature of 390°C at a high pressure posed a high risk of explosion which could potentially be catastrophic and would require expensive controls and high grade equipment in order to minimize risk. For these reasons, the liquid phase process has been chosen.

Liquid Phase Process

Mode of Operation: Batch or Continuous

The production of anthraquinone from the reaction between anthracene and nitric acid can be carried out under both batch and continuous process conditions. This mode of operation is dependent on many factors such as cost, running time and also 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 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. 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 45 minutes. Batch operation processes also provide high conversion per unit volume for one pass and are easy to clean and maintain.

Though batch processes are versatile, their overall cost can be quite high due to their 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. As a result, batch processes need to be monitored closely in order to keep reaction condition identical. Apart from the inconsistency of the product quality, this disadvantage could have a further negative effect, that is, if the quality is 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 more cost effective 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 labor, thus reducing the costs. 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. Alternatively, to produce the same amount, smaller reactor volumes are needed. Specifically, the batch process has an estimated down-time of 25% while the continuous process has a down-time of only 10%.

Conversion of anthracene to anthraquinone takes place at 147 °C, as this is a fairly specific temperature, a continuous reactor is well suited to maintaining this temperature as the variations in temperature will be small as opposed to a batch process where the temperature will be continually oscillating as the reactants are put in and taken out. A continuous reactor allows for the installation of sensitive temperature sensors calibrated only for a small range which is ideal for steady-state processes.

A continuous process has the down-side that should something go wrong, 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 are not well agitated, then channeling 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.

Continuous Liquid Phase Process

Introduction

Since a decision has been made on the specific type of process recommended for the production of 2500 te/year of Anthraquinone, this report will now investigate further the finer details of the process. Specifically, a detailed description relating to a P&ID, mass and energy balances over the entire process, a detailed cost analysis and further considerations of safety and environmental precautions.

Process Description

Letters in bold depict stream numbers on the PFD.

The PFD and more detailed P&ID can be found in Appendix C for referral purposes.

In the Continuous Liquid Phase Process that was decided upon, the Anthracene is fed into a mixer (MX-101) in solid form via a Screw Feeder shown as stream 2. Also flowing into MX-101 is Nitrobenzene through a fresh feed 1 and through the Nitrobenzene recycle 12, both of these streams are heated by E-101 and E-107 respectively before entry to MX-101 in order to ensure that the all the Anthracene dissolves before being fed into the agitated reactor (CR-101) in stream 3. Nitric Acid is also fed into CR-101 through both the fresh feed 4 and the Nitric Acid recycle 30, the fresh feed is adjusted according to the recycle 30 to ensure that the appropriate excess of Nitric Acid is always present. CR-101 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. CR-101 will be maintained at a constant temperature of 147C, this is done through the use of a 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 shown in stream 5 where it will then be partially condensed in order to recycle the useful material.

As products of the reaction, 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. Following leaving CR-101 and before entering the Centrifuge (C-101) the Anthraquinone Rich stream 6 is first mixed in a mixer (MX-103) with a recycle from both the Volatile Products 22 and the Dryer 19. Should MX-103 ever become full, the material from stream 6 can be stored in a hold up tank (T-101). Due to the temperatures of the recycles coming into MX-103, the stream to C-101 from MX-103 needs to be cooled by a heat exchange (E-106) to 20 degrees to ensure that a minimum amount of Anthraquinone is left still dissolved. C-101 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 C-101 in stream 13 consists of 10% liquid. The Filtrate is sent to a decanter (D-102) via stream 10 where the Nitrobenzene is Separated and sent back as a recycle 12 to MX-101. The filter cake is then sent through a washer (W-101) via stream 13 where it is washed with water in order to


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