Coal the solid fuel one of the energy source known to us. When heated at high temperatures in a closed system by resisting the contact with air or oxygen it turns into a substance called coke. During the heating process, valuable chemical compounds escape from coal as vapour and eventually, solid coke is formed. Benzene, toluene and xylene are some of the components present in the gases. The generated vapour/gas technically known as coke oven gas. Here we discuss how it can be treated to recover these BTX (Benzene-Toluene-Xylene) chemicals before it is used as fuel in steel and other process industries. Other chemicals compounds like tar, ammonia and valuable heavy and inorganic compounds are removed initially from coke oven. Finally, the unit process becomes simple to extract BTX compounds by extractive distillation technology.
Extractive distillation technology
Benzene, Toluene and Xylene (BTX) are the compounds have boiling points 80.1oC, 110.6oC, 144oC (at atm pressure) respectively. The mixture of these components recovered from coke oven gas in benzol section of a coal chemical process plant. Extractive distillation technology provides effective separation of these compounds in pure liquid form.
Initially, xylene is separated from the mixture by distillation than the left out benzene and toluene mixture is subjected to extractive distillation. Solvent called N-formylmoropholine (NFM) act as the azeotrope breaker to separate benzene and toluene. The process requires the following distillation and separation unit operation.
- Pressure distillation
- Extractive distillation
- Solvent recovery column
- Aromatic stripper
- BT separation column
- Xylene solvent column
The BTX solvents raffinate is pumped to the feed surge drum and from it, the mixture continuously pumped to pressure distillation column through four heat exchangers. General distillation column used in most industries designed with 50 bubble cap trays is sufficient to perform the distillation operation. At its 25th trays feed in injected and utilized as feed tray, its operating pressure maintained at 15 kg/cm2. These conditions may vary based on the regular optimization experiments. Hot oil is used as a heating medium to the column reboiler and passes through its shell side. The benzene-toluene vapours from the top of the column are collected in reflux drum before which they are condensed. Benzene-Toluene fraction continuously accumulated in the reflux drum. Some of it is used as reflux to the column and remaining fraction is passed to preheater.
Extractive distillation column:
The BT excess is pumped from the vessel via heat exchangers and fed to the extractive distillation column. The engineering model separation column has 60 trays and feed is introduced in the middle of the column. For extraction, the solvent N-formylmoropholine (NFM) is introduced on to the top tray of the extractive distillation column with a flow ratio of 56 kg NFM per kg of feed at 92оC. The solvent NFM temperature directly affects the separation of aromatics from non-aromatics.
Extractive distillation column makes it possible to separate non-aromatics contained in the feed, which is not possible under normal distillation conditions. The solvent changes the boiling points of non-aromatics from higher to lower and makes it easy to withdraw at the top of the extractive distillation column. Aromatics dissolve in the NFM solvent and removed from the bottom of the extractive distillation column.
The LP Steam reboiler and partially vapour heated reboiler supplies heat to the extractive distillation column. Engineering design of column model
- ED column trays 60-bubble cap
- Feed plate 31 tray
- Top temperature 110оC
- Bottom temperature 150оC
- Top pressure 0.8kg/cm2
- Bottom pressure 0.4kg/cm2
Solvent recovery column:
To separate non-aromatics and residual carried over solvent contents, solvent recovery column is used. Top vapours of the extractive distillation column injected to the location below pall rings packing of the solvent recovery column. NFM recovered at the bottom of the column is returned to the extractive distillation column. The purpose of the solvent recovery column is to eliminate the loss of NFM losses. The difference between normal distillation and extractive distillation column is based on phase separation in the separator at the bottom section. Non-aromatic compounds are separated to the bottom of the column with a two-phase product.
- Packing Pall rings
- Bottom pressure 0.25 kg/cm2
- Top pressure 0.2 kg/cm2
- Top temperature 100оC
- Bottom temperature 125оC
NFM and the dissolved aromatic solution obtained from the bottom of the extractive distillation column are fed to stripping column to recovery NFM for recycling. The column is operated along with aromatic column which operates at a vacuum, pure aromatics are separated from the top and the NFM from the bottom. The heat of NFM is used to for the preheating the stream in heat exchanger system and returned to the extractive distillation column.
Equipment’s through which hot NFM is circulated:
- Center reboiler on ED column
- Reboiler on solvent recovery column.
- NFM reboiler on the stripper column
NFM is cooled by the trim cooler using solvent stream. Continuous reboilers are provided heat to the stripper column with BT vapours as the heating medium. The solvent is removed by the reflux in the columns stripping section.
- Total trays 30
- Feed tray 5th
- Top temperature 56оC
- Bottom temperature 119оC
- Pressure 0.36 kg/cm3 vacuum
Benzene toluene separation column:
Benzene-Toluene separator is binary distillation column produces pure aromatics. Low-pressure steam is used as heating media for the reboiler. Pure benzene is obtained from the top of the column and pure toluene from its bottom section. General column specification used in existing plant and may change to present optimum modification.
- Total trays 65 Bubble cap
- Feed tray 30th
- Pressure 1.2 bars
- Benzene purity 99.97%
- Toluene purity 99.95%