Scale$-$Down of Phthalic Anhydride Production at STW$\text{'}$s

Unit $700-$ Reaction Section

Background

You have recently joined the STW Chemical Corporation. One of STW$\text{'}$s major businesses

has always been production of phthalic anhydride from naphthalene. Phthalic anhydride

production is integrated as part of a large chemical plant, in which naphthalene is produced and in

which phthalic anhydride is immediately used to make polyester resins. In recent years, there have

been some problems. Some end users have complained about the quality of the resins produced,

and have taken their business to other companies which produce phthalic anhydride from o$-$

xylene. Therefore, our plant, which had been designed to produce $100,000$ metric tons$/$year of

phthalic anhydride from naphthalene was scaled back to about $80,000$ metric tons$/$year several

years ago. We are now forced to scale$-$down production once again, due to the loss of another

large customer. Marketing informs us that we may lose additional customers. Research is

working on development of catalysts for the o$-$xylene reaction, but their results are not expected

for up to a year. There is an immediate need to determine how to scale$-$down operation of our

plant to $50\%$ of current capacity $(40,000$ metric tons$/$year$).$ We would like to accomplish this

without a shut$-$down, since one is not scheduled for a few months. If $50\%$ scale$-$down cannot be

achieved without a shut$-$down, we need to know how much scale$-$down is possible immediately.

Specifically, you are to determine the maximum possible scale$-$down, up to $50\%$ of current

operating conditions, and you are to define these operating conditions. Furthermore, you are to

determine how the plant can be scaled$-$down to $50\%$ of current capacity, what operating

conditions are required, and what capital modifications, if any, are needed. Any suggestions for

plant improvements which can be made during shut$-$down are encouraged. You must clearly

define the consequences of any changes that you recommend for this process and the

consequences of these changes on other processes which might be affected. It should be noted

that one possible scenario is to operate the plant at design capacity for $6$ months of the year and

shut$-$down the plant for the remaining $6$ months. Although this solution might work, we are

reluctant to lay$-$off our operators for half the year and also to purchase additional storage in order

to store enough phthalic anhydride to supply our customers for the $6$ months that the plant is

down. You should not consider this option any further.

For your first assignment, you are to address the issues described above for the portion of the

process before the switch condensers, see Figure $1.$

Phthalic Anhydride Production

Unit $700$ now produces about $80,000$ metric tons$/$yr of phthalic anhydride. The feeds are

essentially pure naphthalene and excess air. These are pressurized, heated and vaporized

$($naphthalene$)$ and reacted in a fluidized bed with a vanadium oxide on silica gel catalyst. The

reactions are: