Gasoline water settling

Sea water and corrosion products were delivered with premium gasoline by barge to the fuel distribution terminal storage tanks. The water settled to the bottom of the terminal tank and was drawn off with fuel and delivered to service station tanks. Customers fueling at these service stations pumped premium gasoline containing sea water and corrosion products into their automobiles. 

Figure 11.4-2 shows process flows for an HF alkylation unit. The three sections are 1) reaction, 2). settling and 3) fractionation. In the reaction section isobutane feed is mixed with the olefin feed (usually propylene and butylene) in approximately a 10 or 15 to 1 ratio. In the presence of the HF acid catalyst the olefins react to form alkylate for gasoline blending. The exothermic reaction requires water cooling. The hydrocarbon/HF mixture goes to the settling... 

Another important factor is the size of the overhead product liquid accumulator. Ten minutes liquid residence time based on net product rate is common however, aromatic gasoline fractions may require 5 to 8 min settling time for clarification because water vapor tends to condense in small droplets. The required accumulator size is also affected by the type of process equipment downstream. 

Observe the time for the two phases to separate. The height of the sample divided by this time is called the settling velocity. Typically, for well-mixed water-gasoline systems, this velocity is one-half of the ft/min. 

Butanol can increase the water tolerance of gasoline-ethanol blends (Bolt 1980 Noon 1982). The water tolerance of the gasoline-ethanol blends increases with the ethanol concentration and temperature. At room temperature, a 25 % ethanol blend with gasoline can tolerate about 1 % of water. If 2 % of water is present in the blend, most of the ethanol will separate from gasoline in a few seconds and settle to the bottom, which is known as phase separation. Phase separation renders the ethanol-gasoline blend unfit as an automobile fuel (Bolt 1980). Addition of butanol to the blend increases the water tolerance to prevent phase separation (Bolt 1980 Noon 1982). 

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