Separation in ethylene plants
Separation technology is not complex, but separators deliver a range of benefits in ethylene cracking
Lee Siang Hua and Yang Quan
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Ethylene plants use several types of separators, especially in the quench and hot fractionation sections (see Figures 4-6). This article discusses in particular the separation processes involving the immiscible fluid phase with varying densities. The three principles used to achieve physical separation of gases, liquids or solids are momentum, gravity settling and coalescing. Separators may employ one or more of these principles. There are four types of physical separation device currently applied to crackers: liquid-liquid separation, three-phase separation, gas-liquid separation and particulate removal (filtration).
Two types of liquid-liquid separation device are described here. The first uses a chevron baffle in a water quench tower to separate raw pyrolysis gasoline from quench water. This operation is based on gravity separation. The second type applies coalescing separation in the oil-water coalescer to separate dissolved hydrocarbon in quench water or to remove water from raw pyrolysis gasoline.
Three-phase separation is applied when water, liquid hydrocarbons and hydrocarbon gases are present in the inlet stream. Two types of vessel, vertical and horizontal, are described here. A vertical vessel is mainly applied when the ratio of gas to liquid is high. Horizontal vessels are most efficient where large amounts of dissolved gas are present within the liquid.
The most commonly used gas-liquid separator is the flash drum. It is normally used to separate the gas evolved from liquid flash from a higher pressure to a lower pressure; for example, the hydrogen drum separating hydrogen from methane. Besides this, steam drums are used to separate saturated steam from water.
Particulate removal is common to all crackers. The most common approach to particulate removal is a pipe strainer or filter using metal perforations with wire mesh lining in a T-type, Y-type or basket type, depending on pipe routing, flow capacity, pressure drop or cleaning frequency. The important parameter in particulate removal is the monitoring of pressure drop. A clogged cartridge needs a change-out, while a strainer requires cleaning when the pressure drop increases. This job requires little manpower. However, if it is not carried out, a clogged filter might trip the compressor and subsequently shut down the whole plant.
Liquid-liquid separation can be divided into two broad categories of operation. The first is defined as gravity separation, where two immiscible liquid phases separate within the vessel by virtue of the difference in the densities of the liquids. Sufficient retention time must be provided in the separator to allow for gravity separation to take place. The settling of raw pyrolysis gasoline in a quench water tower is the most representative gravity separation process. The cracked gas from the furnace must be cooled down as fast as possible to avoid over-cracking. This happens in three steps: the selective linear exchanger (SLE), the quench oil tower and the quench water tower. In the quench water tower, cracked gas is cooled by direct contact with circulating quench water, whereby the gas temperature will be brought down to 40°C. Some hydrocarbon will be condensed and needs to be separated with the water in the bottom of the tower. The bottom of the quench water tower contains a series of chevron-type plate baffles for the settling out of the water and hydrocarbon phases. Figure 1 shows a chevron plate baffle. The V-shaped baffles provide sufficient retention time to enable gravity settling of an oil-water mixture in the tower. Water will then be collected in the bottom of the conical portion, while oil will float and be collected inside the solid plate. This internal separator is efficient, with a relatively low requirement for maintenance. The tower need only be opened up for cleaning during a plant shutdown every three to five years.
The second category is defined as coalescing separation. This applies when small particles of one liquid phase must be separated or removed from a large quantity of another liquid phase. The application is common in the quench, compression and hot fractionation sections. A separator applying coalescing separation can be described as:
• A horizontal coalescer with a cartridge element at the water stripper feed to reduce the hydrocarbon content in the dispersed phase from 2.5 vol% to less than 30 ppm. This coalescer is important to ensure oil-free water can be fed to the steam generator
• A horizontal coalescer with a cartridge element used to remove water from a continuous hydrocarbon phase containing a disperse phase of water droplets. This unit is located upstream of a liquid hydrocarbon dehydrator. It will ensure that no free water is carried over to damage the molecular sieve in the dehydrator
• A horizontal vessel with a wire mesh coalescer pad to remove water from a continuous hydrocarbon phase containing a disperse phase of water droplets. This unit is located upstream of the pyrolysis gasoline (RPG) hydrogenation reactor. The vessel slants at about 2 degrees to ensure water flow into the collecting boot.
Horizontal vessels have an advantage over vertical ones with respect to the larger interface area available in the horizontal design and the shorter distance that particles must travel to coalesce.
A three-phase separator is required when the stream contains water, liquid hydrocarbons and hydrocarbon gases. These separators are usually characterised as vertical or horizontal. Regardless of shape, the separation vessel contains four main sections:
• The primary separation section is used to separate the main portion of free liquid in the inlet stream
• The secondary or gravity section is designed to utilise the force of gravity to enhance separation of entrained droplets
• The coalescing section utilises a coalescer or mist extractor; for instance, a knitted wire mesh pad on top of the vessel
• The sump or liquid collection section acts as the receiver for all liquid removed from gas in the primary, secondary and coalescing sections.
The following considers a three-phase separator as a knockout drum for a five-stage cracker gas compressor, to provide adequate forward flow to downstream fractionation units. In between each stage, there is a knockout drum, whose function is to knock out condensed hydrocarbon and water. These separators are important to ensure a liquid-free gas feed to the compressor. The knockout drum for the second stage is a three-phase separator. It is a vertical vessel with two compartments in the bottom portion and a mist eliminator on top of the vessel (see Figure 2).
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