Brief details of the process

You are required to manufacture 32000 kg h-1 styrene, using catalytic dehydrogenation of EB:

Pure EB and pure steam are initially at 150°C and 4 atm pressure. Pure EB is mixed with ca. 10% of the steam and then the mixture passed through Heat Exchanger 2 (HE2) in which the EB is vaporised. The vapor mixture of steam/EB leaving the HE2 passed through Heat Exchanger 3 (HE3) to be heated to 530°C.

The remainder of the steam is superheated first to ca. 350°C in Heat Exchanger 1 (HE1) then to ca. 750-800°C in a gas-fired superheater. This superheated steam is then mixed with the steam/EB mixture from HE3 immediately before it enters the reactor. The steam/EB ratio can be determined from energy balance across the reactor. All the Heat Exchangers 1, 2 and 3 are supplied by the product stream from the reactor. Why is this feed-heating strategy used?

The combined flow of EB and steam enters the reactor at 650°C. The steam does not react with the EB, but it helps to provide the necessary heat of reaction (by cooling itself). It also reduces the partial pressure of styrene and improves the conversion of EB to styrene. A pressure of 40 kPa (~6 psia) is maintained in the reactor.

The Relative Molecular Masses (in g mol-1) for the compounds are:

Benzene = 78 Methane = 16

Ethane = 30 Methylbenzene (Toluene) = 92

Ethylbenzene = 106 Styrene = 104

Hydrogen = 2 Steam or Water = 18

1. The Reactor

Complete conversion of EB to styrene is not achieved under normal operating conditions. For the purpose of this exercise, assume that 65 mol% of the EB entering the reactor is converted to products and by- products. 96.4 mol% of the EB that is converted goes through the styrene synthesis reaction (1):

C6H5CH2CH3    C6H5CH=CH2 + H2 (1)

(EB) (Styrene)

Some of the hydrogen formed in reaction (1) reacts with EB to produce unwanted by-products and also some of the unconverted EB can polymerize to form tars, which have very high Relative Molecular  Masses (RMM). The most important side reactions (which will generate the by-products) can be summarized as follows:

a) 1 mol% of the EB converted reacts with hydrogen from reaction (1) to produce benzene and ethane as shown in (2) below:

C6H5CH2CH3  + H2   ®  C6H6  + C2H6 (2)

(EB) (Benzene) (Ethane)

b) 2 mol% of the EB converted reacts with some of the hydrogen produced in reaction (1) to form toluene and methane as shown in (3) below:

C6H5CH2CH3  + H2   ®  C6H5CH3 +  CH4 (3)

(EB) (Toluene) (Methane)

c) 0.6 mol% of the EB converted is polymerised to form tars.

C6H5CH2CH3   ®  Tar (4)

(EB)

NOTE: The RMM of tar is not known - make the assumption that 1.0 kg of tar is formed from 1.0 kg of EB that is polymerised.

The product mixture leaving the reactor therefore contains styrene, methylbenzene (toluene), benzene, ethane, methane, hydrogen, steam, tar and the un-reacted EB. This mixture leaves the reactor at 560°C, and much of its heat is recovered as it passes through the heat exchangers used to preheat the reactor feed as explained before. The temperature of this product stream at the end of the preheating section is about 180°C. Remember, though, that the reactor contents were maintained at reduced pressure. At some stage the pressure must be raised up to atmospheric again.

2. The Condenser and the Organics Separator

The reactor product must be quenched and the temperature reduced to ca. 95°C using water as the service fluid to condense all of the steam in the outflow; the PFD provided uses a quench tower but you can use other methods if justified. It should then be cooled to ca. 40°C using an air-cooled heat exchanger.

How will you now separate the condensed steam from the organic liquids (which include the tar)?

It may be useful to know that the solubility of the organics in water is negligible (less than 0.01% by mass).

Separation via gravity in a 3-phase separator is suggested on the PFD. However, the organic vapours from the liquid phase are considerable at the temperature and pressure of separation, and it can be assumed that 15.1 wt% of the benzene and 4.2 wt% of the methylbenzene escape in the gas phase along with small amounts (2.2 wt% of each) of the ethylbenzene and styrene. Therefore, the gas stream will contain all the ethane, methane and hydrogen, and also some organic vapours (benzene, methylbenzene, ethylbenzene and styrene). Can you see any use for any/all parts of this gas stream?

3. The distillation train

The organic liquids are separated into individual components using a series of distillation columns (called a train) each operating at different temperature and pressure. Unfortunately, during the distillation some of the styrene is polymerized to form polymers. These will also have very high molecular weight compounds of unknown RMM. However, these and the tars formed in the reactor may be considered to be non-volatile so that they always remain in the bottom product of each column. Assume that 0.21wt% of the styrene that enters any column is converted to polymers. NOTE: Assume that 1 kg of styrene that polymerises will produce 1 kg of polymer.

Think what must be achieved in this distillation train. You must remove, as far as it is possible, all the components that are more volatile than the product styrene and also all the components that are less volatile, including the tars and polymers. The styrene monomer product should be free of tars and polymers but may contain very low concentrations of minor contaminants such as unconverted ethylbenzene and volatile by-products. It is desirable that you recover as much of the unconverted ethylbenzene as possible since you should then be able to consider whether it may be worthwhile putting this back into the process. Where would you feed such a recycle stream back into the process?

You will not achieve this in one single distillation column, so you must think about the minimum number of columns in the train that would enable you to achieve your objectives. Think where the components will go (i.e. to top or to bottom product streams in each column), not forgetting to consider all the contaminating by-products and unconverted raw material as well as the desired styrene product. As a rule of thumb at this stage you just need to assume that the purity of the product streams (top or bottom) will increase as the height (and therefore number of stages) of the column increases.

What temperature and pressure might you need to operate at in each column? Hint: Some or all of the columns may operate at pressures below atmospheric. Why? Consider the challenges that this may also present.

Basic physical data (boiling points, etc.) for many of the components of the streams in this process can be found in:

Perry R H and Green D W, Perry’s Chemical Engineers’ Handbook Seventh Edition, McGraw-Hill, New York, 1997.

For websites, try the IChemE Resources page (http://ed.icheme.org/edlinks.html) and the really excellent NIST websites (especially http://webbook.nist.gov).

What do you have to do? – The brief is:

You have been brought in as external consultants to investigate the possibility of building a plant that would produce approximately 32000 kg h^-1 of styrene. The project is still in the preliminary phase and the engineers of the organisation have suggested a process that it is attached.

This assignment is an individual assignment. Your report should include: a brief introduction, answers to the following 6 questions, and a list of references.

You will need to:

1. Complete a process-wide material balance to identify raw material quantities required and product rates. 

2. Complete an energy balance over the reactor. 

3. Select one of the heat exchangers marked H101, H102, or H103, and calculate the area required. Would it be feasible to consolidate all three into a single heat exchanger?

4. Choose an item of plant upstream of the organics separator, and discuss how one or more physical variable may be monitored and controlled. 

5. The distillation train is situated 430 m across the plant from the separator and will be connected via pipework with internal diameter of 22 cm. The delivery pipe needs to enter the train a vertical height 18 m above the exit from the separator. Discuss how feed may be pumped through this section most efficiently, using calculations and highlighting any assumptions you make.

6. The hydrogenation reaction requires a catalyst. Research and discuss the catalyst used, including its composition and physical form. Give references.

You should be able to complete relatively detailed material and energy balance calculations, although complex calculations within each unit are not expected. You should take the types of calculations introduced in the module as the basis of what is expected. If you require additional data, you should state the values you have found and used, along with references.

The objective is to get you to have a realistic ‘feel’ for the quantities and sizes involved without resorting to hugely detailed calculations.

Also, you will learn to make the assumptions needed to get started and think about the points in bold in the “Brief details of the process” which should help you to get on.

Please note that these rhetorical questions in bold do not necessarily need to be answered explicitly in your report. They are there to prompt you to think about these factors, some may help you in performing calculations, and some may help you in answering more ‘discussion’-type parts of the report．

Each student will submit a single report

You should be guided by the steps below:

1. You are at the first stages of a Design. You are assessing the feasibility of a proposed process and reporting the results of your work to a prospective client. So, you must set out a process that you believe can achieve the production objective. Present this process and discuss it in your report.

2. In setting out your recommended process you must show the bases for your proposal and support it by reasoned argument. What assumptions have you had to make to get to where you are? Give references for your conclusions, especially numerical data.

3. The report should include:

· A brief introduction

· Answers to points 1-6 above.

· A reference list