Hi, I’m Dwain Waguespack, senior account director at Rosemount Analytical. One of the most widely used building blocks of the petrochemical industry is ethylene, which is used to make such common chemicals as polyethylene, polystyrene, and alpha-olefins. A typical ethylene plant also makes a number of other important building block chemicals such as propylene, butadiene, and an aromatics-rich pyrolysis gasoline. In the ethylene plant fractionation train in a refinery, the use of process gas chromatographs is critical for improving process operations.
The typical ethylene plant is divided into two basic sections: the cracking furnaces (hot side) and the fractionation train (cold side). The fractionation train takes the effluent from the furnaces and separates it into the wide range of chemical products. The separation of the furnace effluent into various products is done through a series of fractionator towers that selectively separate one chemical group at a time. The effluent stream moves from one fractionator tower to the next with the remainder being sold as a pyrolysis gasoline.
Improving Unit Performance with Process Gas Chromatographs
With the large number of chemical separations being performed in the ethylene plant fractionation train, process gas chromatographs play an important role in maintaining efficient operation. The first process gas chromatograph (AX #1 in Figure 1) monitors the overhead streams of the demethanizer to minimize the loss of ethylene into this stream. Two more process gas chromatographs (AX #2 and #3 in Figure 1) monitor the separation of this stream into the hydrogen-rich tail gas stream and the methane-rich tail gas.
A fourth process gas chromatograph (AX #4 in Figure 1) monitors the bottom streams of the demethanizer to minimize the light gases such as C1 and CO2. Any gases that get to this point ultimately end up in the final ethylene product stream as impurities so the amount needs to be controlled. This is done by controlling the C1 to C2 = ratio. The next series of chromatographs are used to control the purity of the ethylene product. This starts with the measurement (AX #5 in Figure 1) of the deethanizer overhead to minimize the amount of C3 in the stream while still maximizing the recovery of the C2 =. To monitor the removal of the acetylene, one or more gas chromatographs (AX #6 in Figure 1) measure the effluent from the acetylene reactors to insure that the acetylene levels will meet final ethylene product specifications. At the C2 splitter, a gas chromatograph (AX #7 in Figure 1) monitors the ethylene product stream for impurities while another gas chromatograph (AX #8 in Figure 1) measures the bottom streams to minimize any loss of ethylene in the ethane that is being recycled.
A similar series of chromatographs are used to control the purity of the propylene product starting with the measurement (AX #9 in Figure 1) of the stream leaving the bottom of the deethanizer. Any C2 components present would end up in the propylene product stream so they need to be controlled by maintaining the optimum C2 to C3 = ratio. The depropanizer overhead stream is monitored (AX #10 in Figure 1) for C4s to minimize their presence in the propylene product. To monitor the removal of MA and PD, measure at the exit of the MA/PD reactor (AX #11 in Figure 1). Finally, two gas chromatographs monitor the propylene product for purity (AX #12 in Figure 1) and the propane stream (AX#13 in Figure 1) to minimize the loss of propylene.
The final series of chromatographs monitor the final separations of the furnace effluent beginning with the measurement of the depropanizer bottom streams (AX #14 in Figure 1). This analyzer monitors the C3 to C4 = ratio to control the C3 impurity levels in the mixed olefins product stream. Then two more analyzers monitor the purity of the mixed olefins (AX #15 in Figure 1) and minimize the loss of C4 olefins in the gasoline stream (AX #16 in Figure 1).
Process gas chromatographs are critical for maintaining efficient operation throughout the chemical separation processes in the ethylene plant fractionation train.