Q & A > Question Details
1. What is the purpose/function of Steam Ejector in Vacuum distillation column and how it works?
2. Why it is placed at the top of column and why not the bottom in refinery? please explain the barometric concept regarding this installation
20/07/2012 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Ejectors have chambers that convert pressure energy into velocity energy. Steam (or motive fluid) that acts as motive fluid is introduced into the chamber with an reducer type arrangement and followed immediately by reducer and expander arrangement (Please check the typical sketch of ejectors) This arrangement increases the velocity of motive fluid as it enters the chamber. As the motive fluid exits the ejector the empty space left in the chamber is filled with the process fluid (liquid / vapor) and is carried out along with the motive fluid. This is the basic phenomena that is known as the vacuum.
Location of steam ejectors at the top of the systems is to ensure lesser energy consumption during the vacuum production. Any vapor will condense in the piping and will build a liquid head in the upstream of ejectors and will need more energy to create vacuum and subsequent process. Unless there is space created for the incoming material in the down stream of the vacuum production equipment, energy costs of vacuum production will be expensive. The location at top will allow lot of vertical space and time for the evacuation of material drawn out of the columns. Also the system under vacuum is ready to take in air in case of any leak. Air ingress could be dangerous for systems with Hydrocarbon. So it is better that the areas of possible leaks (under vacuum) to as minimum as possible. The system in the down stream of vacuum production equipment will be on higher pressure side and will not allow air ingress even if it leaks. So probably it is a combination of safety and energy optimisation that governs the Ejectors location.
26/06/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The ejectors, sometimes called eductors, along with any vacuum pump in the process, create the low pressure (vacuum) at the inlet of the first ejector. A head of 34 feet of water (less at higher altitudes) can be used to remove the condensed steam from the system. Normally, at the bottom of the water column, the water is pumped from a drum as the water column is held suspended by the pressure from the atmosphere. This water column dictates the elevation of each component of the system. Crude vacuum units are designed this way.
A water column of 34 feet is not essential for all applications, however. A vacuum operated glycol regenerator, for example, may have simply an overhead condensing system with an educator exhausting to the atmosphere or flare.