Handbook of Solvents - George Wypych - ChemTech - Ventech!

1552 Isao Kimura
• Difficult in decreasing outlet temperature - The short cycle in adsorption and
desorption makes the cooling process difficult to synchronize.
• Expensive - ACF are more expensive than PAC, thus the unit modification cost is
more expensive.
22.2.5.2 Tableting process of pharmaceutical products
In a process which produces tablets, a mixture of solvents, containing MDC and methanol
and binders is used. Solvents are emitted from the tableting unit. The following process is
used to recover the emitted solvent.
Feed gas at 50°C from the tableting process is pre-treated by a filter to remove dusts
and by a rotary wet scrubber to absorb mainly methanol. During the scrubbing MDC also
dissolves in water, although at low concentration. This water is treated in two steps, by a degassing
unit at room temperature and by an activated carbon filter so that the concentration
of residual solvents is decreased below that of the environmental standard. The gas from the
degassing unit is joined to the feed gas at the outlet of the demister of the MDC recovery
unit. The gas, the pretreated feed, and gas from the degassing are cooled down to 30 - 40°C
before feeding to the MDC recovery unit which consists of at least two columns for adsorption
and desorption cycles. After processing the gas is released to the atmosphere.
Because little methanol is vaporized at room temperature, the water stream is fed to
another degassing unit heated to 65°C to vaporize methanol. For safety purposes, the degassing
air flow rate is controlled at 10 m 3 /min. The methanol laden gas from the heated degassing
unit is fed to a catalytic oxidation unit where methanol is decomposed before
exhausting the gas stream to the atmosphere. Table 22.2.8 shows the process performance.
Table 22.2.8. Performance of the solvent recovery in the tableting process
Item Performance
Flow rate at rotary scrubber 1,000 l/min
Feed concentration
MeOH 10,000 ppm
MDC 2,000 ppm
Exhaust concentration
MeOH
MDC
150 ppm
3 ppm
Catalytic oxidation flow rate 10,000 l/min
Feed concentration
MeOH 8,500 ppm
MDC 1 ppm
Exhaust concentration MeOH 10 ppm
This is a unique example of a process in which the recovery and the catalytic oxidation
steps are combined. In addition to its use in the pharmaceutical industry, the adsorption is a
suitable method for the separation of chlorine-containing solvents. Figure 22.2.11 shows
the process flow diagram.