Vacuum Distillation Column

Overview

A vacuum distillation column separates the heavy residue left over from atmospheric crude distillation into more valuable gas oil fractions and a final vacuum residue. The residue from the atmospheric column contains heavy molecules that would have to be heated above about 700 F to boil at normal pressure, and at those temperatures they crack and form coke instead of distilling cleanly. The solution is to distill under deep vacuum, which lowers the boiling points so the heavy fractions vaporize at a temperature low enough to avoid thermal damage. The column recovers vacuum gas oil that feeds the FCC unit and the hydrocracker, leaving a vacuum residue that goes to the coker, the asphalt plant, or fuel oil blending.

The column is among the largest vessels in a refinery, very wide so vapor can move at the high volumetric flow that deep vacuum creates without excessive pressure drop. The Column Shell is built for external vacuum load rather than internal pressure, the Column Internals use low-pressure-drop packing, and the Vacuum System pulls and holds the vacuum that makes the whole separation possible.

How it works

Reduced crude from the atmospheric column is heated in a vacuum heater and enters through the Feed and Flash Zone. As the hot feed passes through the Transfer Line and the Feed Nozzle into the low-pressure column, the pressure drop makes much of it flash instantly to vapor. The Feed Inlet Device spreads this flashing mixture gently and separates the liquid that does not vaporize, which falls to the bottom as the start of the residue. The Flash Zone Baffle knocks entrained residue droplets out of the rising vapor.

The flashed vapor rises through the Column Internals. Each Structured Packing bed gives vapor and liquid intimate contact at very low pressure drop, which is essential because every fraction of a millimeter of mercury lost across the internals must be pulled back by the vacuum system. A Liquid Distributor spreads liquid evenly over each bed, and a Collector Tray below draws off the condensed product. Side draws through the Side-Draw Nozzle nozzles take light and heavy vacuum gas oil at successive levels. Just above the flash zone, a Wash Bed washes any entrained residue out of the rising vapor so the heaviest gas oil draw stays clean and low in metals and carbon.

Pumparounds and heat recovery

A vacuum column has no overhead reflux drum returning liquid in the usual way, because the overhead is tiny. Instead, internal liquid traffic and condensation are created by pumparounds. The Pumparound System draws hot liquid from a collector tray, sends it through the Pumparound Pump and the Pumparound Cooler, and returns the cooled liquid through a Pumparound Return Distributor distributor higher up. The cool returning liquid condenses rising vapor, which both sets the product draw rates and recovers large amounts of heat. That recovered heat is usually used to preheat incoming crude, which is one of the biggest energy savings in the whole refinery. Two to four pumparound loops control the temperature profile up the column and therefore the cut points between products.

The vacuum system

The deep vacuum is produced by the Vacuum System, a chain of steam jet ejectors. The First-Stage Ejector sits on the column overhead and pulls the deepest vacuum by entraining the non-condensable gas and remaining vapor in a high-velocity steam jet. Because each ejector can only compress over a limited ratio, several stages are used in series, with a Intercondenser between stages condensing the motive steam so the next stage handles a smaller load. The Third-Stage Ejector and the Aftercondenser finally discharge the non-condensables near atmospheric pressure. A modern dry vacuum unit reaches flash zone pressures of 15 to 40 mmHg absolute, which is what lets the column reach cut points equivalent to over 1,000 F without cracking the oil. Air leaks are the enemy of vacuum, so the column and all connections are built and maintained tight.

Materials and the residue zone

The shell is carbon steel, thin compared to a pressure vessel but reinforced by external Stiffening Ring rings that keep the wide thin wall from collapsing under the external vacuum load. The hottest zones, the flash zone and wash bed, are clad with stainless Corrosion Cladding because hot heavy oil with sulfur and naphthenic acids attacks carbon steel. The Bottoms System handles the hottest, most fouling-prone stream: the Quench Injection sprays cooled residue back into the base to suppress cracking and coking of the standing residue, and Stripping Steam Injection lowers the hydrocarbon partial pressure to recover the last gas oil from the residue. The Bottoms Pump is a demanding high-temperature service moving viscous residue out to the Bottoms Cooler.

Control and operation

The Instrument Package holds the column on its targets. Vacuum transmitters watch flash zone and overhead pressure, the most important variable, since vacuum sets the achievable cut point. Section temperature elements and the pumparound control valves set the cut points between products, and the Bottoms Level Control Valve holds the base level. The Control Panel runs the loops. Operators run the column to maximize gas oil recovery while keeping the heaviest draw clean enough for downstream conversion units and keeping the flash zone temperature below the cracking threshold. Sized for the refinery's residue rate and matched to its conversion units, the vacuum column sets how much of the heavy barrel becomes valuable product rather than low-value residue.