5. Sections of a two-stage crude distillation unit

Vocabulary to be remembered:

sulphur - сера

sulphurous - сернистый

hydrochloric - солянокислый

fraction - фракция

battery - батарея

primary tower - первая колонна

vapour - пар

submerged coil-type condenser and cooler - конденсатор-холодильник

погружного типа

coil - змеевик

reflux - орошение

fired tube heater - трубчатый подогреватель

lower portion - нижняя часть

overhead vapour of the secondary tower - пары в верхней части второй

колонны

intermediate (circulating) reflux - промежуточное (циркуляционное)

орошение

stripping - отпарка

top (live) reflux - верхнее (острое) орошение

live steam - острый водяной пар

stripping tower - отпарная колонна

side cut - боковой погон

two-stage ejector - двухступенчатый эжектор

non-condensable gases - неконденсирующиеся газы

simple reciprocating pump - поршневой одноцилиндровый насос

preheater - подогреватель

narrow boiling range cut узкая фракция

stabilizer - стабилизатор

a. ATMOSPHERIC SECTION

A total light distillate yield within 50 and 53%, including motor gasoline, kerosene and diesel fuel, can be obtained in the atmospheric section from desalted sulphurous crudes.

Special charge pumps deliver the desalted crude in two parallel streams into a heat exchanger battery. Caustic solution is injected into the charge pump suction line to prevent hydrochloric corrosion. The heat exchangers preheat the crude to 200⁰ or 220⁰ C. The preheated crude flows in a single stream into the primary (gasoline) tower where it loses its light fractions. The vapour leaving the tower top with a temperature of 102⁰ C is directed into a submerged coil-type condenser and cooler, whereafter it arrives in a rundown tank. Part of this product returns into the primary tower as a reflux.

The bottom product of the primary tower, having a temperature of 200⁰ C, is pumped in two parallel streams into the atmospheric fired tube heater where its temperature is brought up to 320⁰ C. Then it is directed into the lower portion of a secondary tower.

Part of the hot oil returns to the primary tower to maintain there the required temperature. The overhead vapour of the secondary tower passes a submerged coil-type condenser and cooler after which it is delivered into a rundown tank. A pump returns some of the overhead condensate into the secondary tower top as a reflux fluid. The remainder flows into the stabilization section of the unit. Intermediate(circulating) reflux is also used in the secondary tower apart from the top (live) reflux.

Three side streams are obtained from the secondary tower through a three-sectional stripping tower. Live steam is here used for stripping the bottoms and side cuts of lighter hydrocarbons. The bottom product of the secondary tower, which has a temperature of 310⁰ C, is pumped in two streams through the fired tube heater of the vacuum section, whereafter it continues its flow into the vacuum tower.

The side stream of the second tower, having passed the three sections of the stripping tower, run first through heat exchangers, then through cooling coils, into the treating section tanks to be washed with caustic.

b. VACUUM SECTION

Four lube oil fractions can be obtained in the unit from the secondary fractionating tower bottoms (reduced crude). About 28-30% of the crude charge leave the distillation section in the form of residium. A wide boiling range fraction can be obtained, if desired, which can be effectively used as feed stock for catalytic cracking units. It can be seen from the flow diagram that secondary tower bottoms are pumped in two parallel streams through the fired heater, where their temperature is raised to 420⁰ C after which they enter the vacuum tower. A vacuum of 60 mm mercury absolute is maintained here. A constant temperature of 385⁰ C is kept at the tower bottom, and 125⁰ C at the top. Steam is supplied into the tower bottom end. Overhead pipes conduct the steam decomposition gases, air and small quantities of petroleum vapours into the barometric condenser. Two-stage ejectors remove the non-condensable gases from the barometric condenser.

Side streams produced in the vacuum tower if the unit is set up for lube oil processing, leave the tower and are sent through heat exchangers and condensing and cooling coils to tanks.

The vacuum tower product (heavy residium) is sent through a heat exchanger and cooler into the tankage by a simple reciprocating pump.

c. STABILIZATION AND GASOLINE

RE-RUN SECTION

To produce finished gasoline , the overhead distillate of the primary fractionating tower is pumped into a stabilizer working under a pressure of 10 kg per sq. cm. The stabilizer bottom temperature (160⁰ C) required for stabilization process is maintained by means of a preheater in which the product is heated by steam. Stabilized gasoline leaving the preheater flows through heat exchangers and through a cooler to the caustic treatment section, whereafter it is directed into the finished product storage tanks.

The stabilizer overhead vapours are condensed and cooled in tubular condensers and coolers, respectively, and land in a run-down tank, from which some of the liquid product is used for reflux purposes, the remainder being collected in liquid petroleum gas storage tanks.

If narrow boiling range cuts are to be produced, the stabilizer bottoms can be re-run in a special small fractionating tower.

Caustic treatment of the distillation products is effected in horizontal settling tanks designed to work under pressure. An injector is used for circulation of the caustic. The caustic content in the tanks may be brought up to 30% of the total tank volume.