Method for removing sludge in crude oil tank and recovering oil therefrom

A method for removing bottom sludge in a crude oil storage tank and recovering oil therefrom, the bottom sludge having been accumulated during the storage of untreated and unrefined crude oil, comprising the steps of:

• (a) introducing thermal oil from a thermal oil storage tank into a the crude oil storage tank at a volume ratio of thermal oil to bottom sludge of 3:1 to 20:1 under operation of a pump and a heat exchanger, said thermal oil being selected from the group consisting of atmospheric residue, vacuum residue and vacuum gas oil, wherein if the thermal oil stored in the thermal oil storage tank is 50° C. or higher, the thermal oil is directly introduced into the crude oil storage tank under operation of the pump and the heat exchanger, while if the thermal oil is less than 50° C., the thermal oil is circulated to a separation tank connected to the crude oil storage tank under the pump and the heat exchanger until reaching 50 to 130° C. and then introduced into said crude oil storage tank, and wherein a sludge level indicator including a plural number of thermocouple tips located at regular intervals is mounted in the crude oil storage tank;
• (b) melting the sludge at 50 to 130° by use of the introduced thermal oil with or without stirring until the temperature of the bottom tip in the sludge level indicator accords with that of the top tip, the molten mixture of the thermal oil and the sludge being circulated for maintaining said temperature range under operation of the pump and the heat exchanger;
• (c) transferring the molten mixture of the thermal oil and the sludge into the separation tank connected to said crude oil storage tank;
• (d) allowing the molten mixture of the thermal oil and the sludge to stand at 60 to 130° C. in the separation tank in order to settle inorganics contained therein;
• and
• (e) recovering oil components separated from the inorganics by the settlement,
• wherein the bottom sludge removal in the crude oil storage tank is carried out under a closed system.