In the case of benzene as cargo, this vessel can transfer about 1000 metric tons in each transfer.
2.2 Transfer Cycle
Fig.2 shows benzene transfer cycle. First operation of the cycle is loading operation. The liquid benzene is loaded at marine terminal of refinery producing liquid benzene. The benzene gas evaporating on the surface of liquid phase in cargo tank is emitted from vent post located at the center on weather deck. Second operation is navigation. When the pressure of ullage space in cargo tank exceeds the pressure set point, pressure vacuum valve opens to release the gas until the pressure reduces below its set point. Third operation is unloading operation. The liquid benzene is unloaded by cargo pump integrated in chemical tanker. Air is drawn into cargo tanks during this operation. Therefore, if there is no leakage of cargo pump, manhole and ullage hatch then airborne concentration of benzene should be low value. Fourth operation is tank cleaning. After unloading operation, cargo tanks are cleaned by air from blower to avoid contamination with next cargoes. The largest amount of benzene gas in the transfer cycle is emitted during this operation. The source of benzene gas is the residues of liquid benzene adhering tank walls, bottom and ceiling due to its surface tension and viscosity.
Since the amount of emission depends on the kind of operation, we carried out onboard measurement for each operation described above.
3. ONBOARD MEASUREMENT
We investigated the concentration of benzene gas emitted from chemical tanker during transfer operation. Canister was used to collect air samples on chemical tankers during each operation. Canister in Photo. 1 is made of stainless and its volume is 20(L). The canister is decompressed before measurement, and air surrounding canister is drawn by the difference of pressure between canister and atmosphere. The valve that has the capability to control sampling flow rate was opened when each operation started and was closed at the end of operation. The benzene concentration of the collected air was analyzed by gas chromatograph with FID (Flame Ionization Detector). Therefore the result of the analyzed data gives airborne concentration for the period of time over which the measurement taken. Three typical working places in Fig.1 were selected for sampling gas. One of them was cargo pump room. The cargo pump installed in chemical tanker works for unloading operation and liquid benzene passes through in cargo pump room. Second workplace was vent post on weather deck. If there is no leakage, the source of benzene gas should be only vent post. Third place was mess room in accommodation. It was expected that crew spend most of the time in accommodation except for loading and unloading operation.
Table. 1 Result of Onboard Measurement
