CHEMICAL PROCESS TECHNOLOGY
Module name: Chemical Process Technology III
Module code: GPT3102
Name of student: Mosiya Anggalo Bonolo
Student number: 216098653
Lecturer: Dr S Gouws
Title of assignment: Assignment 2: Distillation
Date of submission: 13 April 2018
Distillation is one of the widely separation method used in the industry to separate mixtures, normally liquid-liquid mixtures. This method separates a component from the mixture by its boiling point. Separation in distillation occurs because from the mixture the lighter components will vaporize easily than the heavier components, because they have the lowest boiling point and higher vapor pressure. The vaporized component is then recovered as a distillate while the heavier component is withdrawn from the column as a residue. This principle is normally the same for all types of distillation1.
For the mixture to separate the components must have different boiling points. Distillation process can either be batch, continuous batch and modern fractional distillation. There are four common types of distillation: simple, steam, fractional and vacuum distillation. The type of distillation to be used for the separation of a certain mixture is dependent on the conditions of the separation. Simple distillation is used for mixtures where components have significantly different boiling points; for heat sensitive components, steam distillation is normally used. When the boiling points of the components are close to each other fractional distillation will be used. Components with very high boiling points require their pressure to be lowered so that the process can be faster hence in such cases vacuum distillation is used.4
Vacuum distillation will be further discussed.
As mentioned vacuum distillation is used in the industry for the separation of components that have very high boiling points and thus their vapor pressures needs to be reduced so that they can boil at low temperatures. Vapor molecules are allowed to escape at low temperatures. it is commonly used by chemical processing industry. Any pressure below the atmospheric pressure is considered a vacuum. In situations where large, heavy hydrocarbons are broken down at the bottom of the column vacuum pressure can be very helpful. By lowering the boiling point of a component, this enhances the molecular escape of molecules from the surface of liquid, reduce energy cost, reduce molecular damage by overheating and reduce equipment damage4.
An example of a mixture distilled with vacuum distillation is atmospheric distillation residue in crude oil distillation. Before the crude oil can be fed in the atmospheric distillation column it is first heated and desalted, after which it is again heated and fed into the distillation column where the separation takes place. Vapor rises through the column, contacting refluxed liquid to separate the crude oil into the specific boiling range streams.
A typical vacuum distillation process steps
? The atmospheric distillation residue is preheated until the desired temperature (350°C – 390°C) is reached.
? The residue is flashed to separate to desired fractions. Light vapors rise to the top and heavier hydrocarbon liquids fall to the bottom.
? Separation of lighter components is improved by steam injection at the bottom of the column.
? Vacuum pumps are used to maintain temperature in the vacuum column
? Light vapour gases are removed at the top of the tower, condensed and recycled back to the column as reflux. Light Naphtha is drawn off and excess gases sent to flare.
? Vacuum gas oil (VGO) and lubricating oils are drawn off and routed for further treatment in Hydrotreating units
? Vacuum residue from the bottom is sent to intermediate storage or typically to be further processed in a FCC or delayed coking unit.5
Every distillation process includes a feed system, a preheat system, the distillation column, the overhead system and the bottom system. The term system is normally used in the industry to describe the various distillation equipment arrangements. The feed system as the name implies it is basically designed for raw feedstock storage, blending and transportation. The system consists of tanks, pipes, valves, instrumentation and pumps. Preheating the feed before it is routed to the column is very important as this will help conserve energy for the distillation. The temperature of the feed is normally brought close to the operating temperature of the tray, this is to ensure that it takes optimum time and heat and with this energy will be conserved. Usually a heat exchanger or a fired heater is used to preheat the feed.
In vacuum distillation a vacuum fired heater is used to preheat the feed. The residue is routed to the furnace through several passes which each of them has its own flow control valve so that the flow and heat distribution throughout the furnace can be easily controlled. The furnace will then heat the feed to the desired temperature. The furnace is normally fired up with either fuel gas or fuel oil. The feed will first be routed through the convection section tube rows of the furnace, where hot flue gases from the radiant section provide the first part of heating. It is then routed through the radiant section of the furnace where the tubes are exposed to the flames and final heating occurs.
The separation of the residue takes place inside a distillation column. The column can either be packed or plate-type. Packed materials are filled with packing materials which enhances vapor-liquid contact while plate column have number of trays stacked on top of each other. The column allows the lighter components from the mixture to vaporize or rise to the top of the column while the heavier components stays put on the bottom of the column. In vacuum distillation the packed columns are the ones widely used because they ensure the pressure drop and increase the capacity for the high vapor volume traffic. Fuels or lubes vacuum column can be used. Fuels vacuum column is normally packed with 3 or 4 beds and produces 2 or 3 side draw VGOs while lubes vacuum column contains 6 or 7 packed beds and produces multiple streams.
The bottom system of the distillation consists of a section in the column designed to allow liquids to boil and roll freely. This system has a reboiler connected t it to maintain and add heat energy into the liquid. To control the level of the liquid a level control loop. Liquid level, temperature must be carefully controlled. On the top of the column there is an overhead system which is used to cool off the hot overhead vapors. The condensed or cooled liquid is ether sent to reflux if further distillation is needed on it routed to the tank farm if it meets specification. Reflux is used for further quality control and tower top temperature. For packed columns a feed distributor is used to evenly disperse the liquid1. Below is a typical industry vacuum distillation figure.
Figure: Vacuum Distillation Column6
It is very important to know the risks, safety and environmental implications of your process. This will help in the operations and make it easier to find solutions if any problems arise. To maintain the overhead or bottom composition at a specified value product quality control is introduced. Process variables like level must be controlled to maintain maximum and minimum limits of the materials, temperature and pressure are also controlled to ensure that flooding doesn’t occur. It is very important to control your flow. The above mentioned will be further discussed.
There can either be no flow, less flow or more flow to the column. These cases can be due to some causes. These causes might be there is blockage somewhere in the line, or perhaps a control valve shut, or a valve either fail open or fail close, or a pump failure. This can lead to operations not taking place, the column will dry out if there is no flow, the quality product might change due to change in flow. If more flow goes in the column might flood, the temperature will increase and there will be rise in bottom.
It is always very important to know the operating limits of the process. The temperature of the feed can easily increase to that of the heating medium if vacuum is lost. Vacuum loss can be due to blockage in the pipeline and this could lead to pressure increase. Vacuum pump failure can also have the same effect as it is normally sealed. Heating mediums normally have furnaces if they are operated at temperatures higher than the normal operating limits, the tubes carrying the process fluid can be ruptured and explosions can occur in the radiant section. To avoid flooding the distillation column must allow effective separation. An undersized column will easily flood, which may result in high pressures and therefore higher temperature in the still.
Flooding occurs in the column when its level increases beyond operating limits. The operator can notice flooding by the drop in differential pressure between the top and bottom of the column, as the levels on trays are increasing. The temperature at the top of the column will also increase as the product is becoming contaminated with the residue. There might be pressure surge if the flooding worsens and the levels at the top and bottom will surge. to correct the flooding in the column, the vapor temperature controller must be in manual mode, the flow of the heating medium to the reboiler must be reduced, both the flow rate of the feed and reflux must be reduced, and the column must be allowed to stabilize before continuing with operations.
Crude oil composition can be toxic to human life as it can be highly carcinogenic and benzene which can be one of the products of crude oil can cause leukaemia. The product can lower white blood cells and lead to drop in immunity and a higher susceptibility to diseases. Since crude oil cannot be used in its natural state and thus requires that it is processed this leads to release of toxins which causes air pollution. The burning of the oil usually releases CO2 and thus contributes to high amount of greenhouse gases. Soot which is one of the product of burning the oil can coat human lungs which can cause cancer or heart diseases. Below is a figure showing toxins released to the atmosphere.
Figure 2: release of toxins9
Safety is the number one priority in the industry hence safety assessment must be done before any kind of operation can be done. Thus, every work done requires a work permit, be it hot work, cold work, confined space or heights. It is required for the personnel to have a work permit in place to ensure that a proper risk assessment has been performed beforehand. Distillation column have pressure relief systems to release pressure when needs be. This is to ensure that pressure does not build up and lead to rupture or explosions. Check valves are installed to prevent any backflow and they have too be regularly inspected. To ensure safety in the workplace routine inspections and precautions must be carried out.
The operator must know process trends and keep ahead of the process to ensure that there is safe operation. Ensure that all controllers are functioning well and that process variables match their set points. Ensure that the column levels are within the control range. Because if there are high levels in the overhead it can cause increase in pressure, and loss of level can cause hot or volatile vapors route to the low-pressure tanks which can upset the process. all pipelines, pumps and vessels integrity ensured. Inspect for any leaks, malfunctioning of pumps, valves and rotating equipment.