Polyisocyanate production system and gas treatment apparatus

1. A gas treatment apparatus for treating a gas by bringing the gas into contact with a treatment liquid, the gas treatment apparatus comprising:

a plurality of treatment tanks,

the plurality of treatment tanks each comprising:

a gas-liquid contact chamber for a gas-liquid contact of the gas with the treatment liquid,

a storage chamber, located over the gas-liquid contact chamber, for storing the treatment liquid, and

a storage chamber bottom plate dividing the interior space of the treatment tank for defining the gas-liquid contact chamber and the storage chamber is provided in the treatment tank,

wherein the gas-liquid contact chamber is defined as an interior space of the treatment tank located below the storage chamber bottom plate,

the storage chamber is defined as an interior space of the treatment tank located above the storage chamber bottom plate,

a gas passage path for passing the gas in series through the respective gas-liquid contact chambers is further provided,

at least one treatment tank located at a most downstream side with respect to a flowing direction of the gas flowing along the gas passage path is provided with a treatment liquid supply pipe that supplies the treatment liquid to the gas-liquid contact chamber from the storage chamber,

the plurality of storage chambers are connected with each other via a storage chamber communication pipe, and

one end of the storage chamber communication pipe at a most downstream side is connected to the storage chamber located upstream of the storage chamber at a most downstream side, and the other end of the storage chamber communication pipe at a most downstream side is connected to the treatment liquid supply pipe.

2. The gas treatment apparatus according to claim 1 , which further comprises a treatment liquid back-flow unit for returning the treatment liquid discharged from the gas-liquid contact chamber to the storage chamber.

3. The gas treatment apparatus according to claim 1 , wherein the gas is carbonyl-chloride-containing gas, and the treatment liquid is an alkaline.

4. The gas treatment apparatus according to claim 1 ,

wherein one end of the treatment liquid supply pipe is connected to a side wall at a lower portion of the storage chamber, and the other end of the treatment liquid supply pipe is inserted in the gas-liquid contact chamber from a side wall thereof, whereby the treatment liquid supply pipe supplies the treatment liquid in the storage chamber to an inside of the gas-liquid contact chamber with a gravity-drop,

a restriction orifice is interposed in the treatment liquid supply pipe for restricting a flow-rate of the treatment liquid flowing in the treatment liquid supply pipe, and

the other end of the storage chamber communication pipe at a most downstream side is connected upstream of the restriction orifice in the treatment liquid supply pipe.