Chemical plant design:

Toluene disproportionation reaction to produce benzene and xylenes is of considerable importance due to the increasing demand of benzene and xylenes in chemical industries.

The demand of toluene is found to be the lowest, whereas, the para isomer of xylene is of greater significance due to its use in the manufacture of polyester fiber and many petrochemicals. In industrial practice, heavy aromatics stream, so$-$called A$9+$ stream

$($aromatics with carbon number higher than nine$),$ is used as the blending feed of transalkylation. $C9+$ aromatic stream contains main components including trimethylbenzene $($TMB$)$ and methylethylbenzene $($MEB$)$ and some minor components.

Main Reactions:

$2C_7{H}_8=C_6{H}_6+C_8{H}_{10}$

$C_7{H}_8+C_9{H}_{12}=2C_8{H}_{10}$

Beside Benzene, xylene heavy aromatics $A10+$ are produced.

In addition, there are a lot of $A9$ cracking reactions to light gases.

Process Description:

The fresh $C9+$ aromatic feed is combined with $($i$)$ fresh toluene, $($ii$)$ recycled toluene, $($iii$)$ recycled $C9$ aromatics, $($iv$)$ compressed makeup $H_2$ gas and $($v$)H_2-$rich recycle gas.

The mixture is preheated in a combined feed exchanger $($E $101)$ and then heated in a fired heater $($H $101).$ The hot feed vapors go to an isothermal fixed bed reactor $($R$-101).$ The reactions take place on zeolite catalyst at $35$ bar and $440C.$ The input is $50-50wt\%$ ratio of toluene and $C9$ aromatic and small amount of make up $H_2.$

The yield of benzene and xylene is around $95\%.$ The reactor effluent $,$ light hydrocarbons, benzene, toluene, mixed xylene, and $C9$ and $C10+$ aromatic$)$ is cooled using two heat exchangers: the combined feed exchanger $($E$-101)$ and E$-102$ and then sent to a flash separator V$-101).$ Hydrogen$-$rich gas is taken off the top of the separator, and recycled through a compressor $($C$-101)$ to the reactor $($R$-101)($along with long with the makeup hydrogen gas stream$).$ A small portion

of the recycle gas is purged to avoid accumulation of gases in the reactor and exported to the fuel gas system. Liquid from the bottom of the flash separator $($V$-101)($light hydrocarbons, Benzene, toluene, mixed xylene and $C9$ and $C10+$ aromatic$)$ is sent to a stripper column $($T$-101).$ The stabilizer overhead gas $($mainly light hydrocarbon ends$)$ is cooled down in a partial condenser and exported to the fuel gas system. The bottom stream of T$-101$ is cooled down in a heat exchanger $($E$-104)$ and sent to clay treatment columns

$T-102$ A$/$B$.$ The exit stream of the clay treaters is sent to benzene distillation column $($T$-103)$ in which benzene $($the lighter component$)$ is recovered as distillate. The bottom product is sent to Toluene column $($T$-104)$ in which toluene $($the lighter component$)$ is recovered as distillate and recycled. The bottom product of the toluene tower is sent to xylene tower $($T$-105)$ in which xylene $($the lighter component$)$ is recovered as distillate while the bottom product $(C9$ and $C10+$ heavy aromatic products$)$ is sent to the heavy aromatic column $($T$-106)$ in which the heavy $C10+$ aromatic product is collected and the

A$9$ aromatic is recycled for extinction conversion.

$($i$)$ Use the principles $($i$)$ onion models and $($ii$)$ Approach of Douglas, $($hierarchical approach$),$ to draw the BFD and PFD for the process.

$($ii$)$ What is the purpose of using two clay treatment columns.

$($iii$)$ Justify the addition of $H_2$ to the process.

$($iv$)$ Justify the use of reflux streams in T$-101,$ T$-103,$ T$-104,$ and T$-105$