ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
CYROGENIC NATURAL GAS FRACTIONATION
This article will discuss natural gas and how it is changed from the ground to your hot water heater or furnace. The process to do this is called Fractionation. It comes from the word fraction – which means a part of the whole. In fractionation, the natural gas will be broken into its components (or fractions). In order for this process to happen, many processes and many pieces of equipment are required.
The natural gas is composed of the following elements: methane (95%), ethane (3.2%), propane (0.2%), iso-Butane (0.03%), normal-Butane (0.03%), iso-Pentane (0.01%), normal-Pentane (.01%). These are all average values as they can change somewhat from the location of the well.
These natural gas liquids (NGL) are hydrocarbons—in the same family of molecules as natural gas and crude oil, composed exclusively of carbon and hydrogen. Ethane, propane, butane, iso-Butane, and pentane are all NGLs (see table below). There are many uses for NGLs, spanning nearly all sectors of the economy. NGLs are used as inputs for petrochemical plants, burned for space heat and cooking, and blended into vehicle fuel.
The chemical composition of these hydrocarbons is similar, yet their applications vary widely. Ethane occupies the largest share of NGL field production. It is used almost exclusively to produce ethylene, which is then turned into plastics. Much of the propane, by contrast, is burned for heating, although a substantial amount is used as petrochemical feedstock. A blend of propane and butane, sometimes referred to as "autogas," is a popular fuel in some parts of Europe, Turkey, and Australia. Natural gasoline (pentanes plus) can be blended into various kinds of fuel for combustion engines and is useful in energy recovery from wells and oil sands. All these different components have different boiling points which will help explain the operation of the equipment.
Following are the boiling points for some of the Natural Gas Liquids NGL) above:
Initially, the natural gas (rich gas) will flow from a well, through a pipeline to the Gas Fractionation Plant. After removal of particulate matter, water, and other contaminates, it will enter the Demethanizer column.
The methane will depart the demethanizer column as dry methane gas and will either enter a pipeline headed for your home for use or be liquefied (compressed and chilled) and be stored and shipped as Liquefied Natural Gas (LNG). The remaining NGLs will then be processe through the following equipment:
- Deethanizer – this column will operate at the ethane boiling point (-89 °C, -127 °F) where it will be distilled or fractionated from the remainder of the liquids, to pure ethane and will be stored in tanks for later use. The remainder of the NGLs (less ethane) will go to the next column.
- Depropanizer – this column will operate at the boiling point of propane (-42 °C, -43 °F) where it will be distilled or fractionated from the remainder of the liquids, to pure propane and will be stored in tanks for later use. The remainder of the NGLs (less ethane and propane) will go to the next column.
- Deisobutanizer - this column will operate at the boiling point of iso-Butane (-9 °C, 8-16 °F) where it will be distilled or fractionated from the remainder of the liquids, to pure iso-Butane and will be stored in tanks for later use. The remainder of the NGLs (less ethane, propane and iso-Butane) will go to the next column.
- Debutanizer - this column will operate at the boiling point of butane (-1 °C, 30-34 °F) where it will be distilled or fractionated from the remainder of the liquids, to pure butane and will be stored in tanks for later use. The remainder of the NGLs (less ethane, propane, iso-butane and butane) will go to the next column.
As you can see, the columns and process is performed from the least boiling point to the highest boiling point. That makes it easy to pull off each of the products in order. Now that we have described the process, let’s look at the equipment required for this process to work. We will talk about the big towers first in this session.
DEMETHANIZER – this fractionation column is stainless steel and carbon steel. It is designed for approximately -150 (top)150oF (bottom) @ 560 psi. They are typically over 100 feet long.
The biggest fraction of natural gas is the methane. It is about 95% of the total volume. Therefore, the largest column is usually the demethanizer. This is a tall, two diameter column with trays for internals. The natural gas is chilled usually with a turbo expander down to -120oF. The cold natural gas enters the demethanizer at approximately -120oF where all the NGLs are liquid but the methane is still in gas form. The liquid mixture of mixed hydrocarbons (the propane, ethane, butane and methane, etc.) falls by gravity down thru the demethanizer distillation column.
Some of the liquid (about 10%) at -132oF is diverted from the column and is run thru the "cold box", which is just a counter current heat exchanger. The -132oF cold of this liquid circulates thru the cold box and precools the fresh gas mixture coming into the plant. But precooling the gas makes the -132oF liquid warm up to 32oF gas.
This 32oF gas is one source of heat to the demethanizer. It enters the column in the bottom 1/3 and bubbles up thru the falling -132oF liquid (the remaining 90%).
This heat boils the easiest component of the mixture to boil out first. That component is the methane. The propane, ethane and butane remain a liquid. The methane bubbles out the top as a gas/ liquid at -132oF and runs thru the cold box to give up its cold to the cold box and then eventually run to the customer pipeline.
The liquids remaining (propane, butane, ethane etc) fall to the bottom of the column and are skimmed off below the entering stream of 32oF gas. The warm 32oF gas warms up the hydrocarbon leftovers liquid at 36oF. The 36oF mixture gases are run thru the cold box and warmed up to 54oF and reentered into the bottom of the column. This is the second source of heat. Any remaining methane gas immediately bubbles out of the hydrocarbon liquids and the rest sinks to the very bottom and is pumped out as the mix minus the methane. The remaining liquids are sent to the next column – the Deethanizer.
DEETHANIZER – this fractionation column will typically be all carbon steel and designed to 450 psi @ 275oF with the MDMT of -20oF. It is typically over 100 feet high with trays for product separation.
The Deethanizer will take these liquids and pressurize and cool the liquids to around -132oF and is warmed by the stream of 32oF gas from the reboiler. This stream of gas will heat the liquid ethane to its boiling point of -127oF where it will be removed from the top of the column and the remainder of the liquids will fall to the bottom of the column.
The ethane will be chilled and compressed and sent to a dedicated storage tank. The remainder of the NGLs will be pumped to the next stage of processing.
DEPROPANIZER – this fractionation column will remove propane from the remaining NGL stream. This column is typically designed at 350 psi @ 325oF and a MDMT of -20oF. It is usually larger in diameter than the two previous columns (10-12 feet ID) and less than 100 feet high. Propane will boil off in this tower and be removed from the top. It contains trays for distilling the remaining NGLs which will be collected at the bottom of the column.
DEBUTANIZER – this column is typically the smallest of the fractionation columns. It is typically 8-10 feet in diameter and less than 100 feet high. It has trays also for distilling the pentanes (gasoline) and other heavier hydrocarbons. These would include naphtha as well as others. The debutanizer is typically carbon steel in construction with stainless steel trays and internals. The design is typically 150 psi @ 300oF. The butanes are boiled off at their boiling point and the remainder of the NGLs are collected and stored for sale or further processing.
Each of the major fractionation columns are mini process areas. Each has the fractionation column, a reboiler for furnishing heat for the process; a reflux drum (for reprocessing unprocessed liquids), and a condenser for condensing NGLs. These are continual process flows and the process continues until there is not enough heat or feed stream for the process to continue, or too much heat or process flow which will push the process reactions out of control.
In the next edition we will talk about liquefaction of natural gas (LNG) and the metallurgy and equipment required. If you have any comments or questions, please contact us at Boardman.
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Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers. With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers. Boardman also does ASME Section VIII Division 2 design and fabrication.
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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick. BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.
BOARDMAN INC. is available for shop tours and Pressure Vessel and Static Equipment Fabrication Classes.