Petroleum Refining and Petrochemical
Trace Moisture In Liquid Hydrocarbons
TIM 019 07.97
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Introduction:
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Many products we use every day begin with crude oil as the raw material. Refining and separating the various fractions of crude oil produces the building blocks for synthetic fibers, plastics, agricultural chemicals, and pharmaceutical products. Most of these chemical reactions take place under tightly controlled conditions where moisture is a contaminant or needs to be controlled in some way.
The process engineer and the chemist work to optimize production efficiencies and to lower the cost of raw materials. In addition, the process must be monitored to reduce the chance of producing an unacceptable product; this is achieved by continually tightening the process control. Therefore, consistency and repeatability provided by an on-line moisture measurement system can be very helpful. Once the process runs the same way repeatedly, controls can be tightened up to fine-tune the process. On the other hand, if the process is unstable and requires intense supervision, resources are not available for fine-tuning.
If unwanted moisture is in the process, the capability to measure that moisture is the first step in its elimination or control. Quality problems in chemical reactions are critical and must be eliminated from the process, but moisture measurement can be difficult. In complex petrochemical processes, quality control may dictate that the stream must be manually sampled at various critical points. This is done by drawing off a sample and taking it to the lab for analysis. Without due care, however, this procedure may introduce moisture contamination into the sample and create errors.
The sample quality is greatly improved by making an on-line measurement. Also, if operators get information continuously, they can find the source of problems as they arise-not after the fact.
Terms:
The following definitions may help to better understand the behavior of moisture (water) in liquid hydrocarbons.
Feedstocks - In this discussion, feedstocks are liquid hydrocarbons used in various reactions that produce another finished product. Feedstocks can originate internally, from somewhere else in the plant, or from an external source (a supplier or vendor).
In solution - this term refers to a mixture of two fluids, usually of different chemistry, and herein implies that the sample is homogeneous and cannot be separated by allowing the solution to stand at the same temperature.
Miscibility: Think of this term as mix-ability. For example, a fluid that is miscible with water means that water can be added to the hydrocarbon in an infinite ratio, 0-100%, and be completely dissolved. There would be no evidence of an interface, an emulsion, or individual droplets of water remaining. The water would be in solution and would not separate out in any way; it is in solution. An example of a hydrocarbon that is totally miscible with water is ethanol. These totally miscible fluids are generally not good candidates for measurement using aluminum oxide sensors.
Miscibility, however, is a go-no-go issue for the purpose of this discussion, and though the techniques presented may be used on some miscible fluids, the procedures to follow refer only to immiscible hydrocarbons, or those which have a saturation point for water. Even though a hydrocarbon is immiscible with water does not mean that water cannot be dissolved in it. It simply has a finite capacity for carrying water. An example of a hydrocarbon that is immiscible with water is benzene that can carry up to 639 ppmw of water in solution at 20C.
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Saturation - This is the point where the quantity of water in solution is at the maximum. If any more water were added, it would NOT go in solution at that temperature but would be free water and would be the beginning of an emulsion or an interface.
Free water: This term describes the condition after a fluid is saturated and is past the point where water is in solution. An example is water in oil. If water is poured into a cup of oil, the water sinks to the bottom and the oil rises to the top. The visible horizontal line at the boundary between the two elements is called an interface. Another example of free water is an emulsion. Emulsions form when enough mechanical agitation acts on the fluid that the free water forms a cloudy mixture of water and the hydrocarbon. The mechanical shearing action creates very small water droplets that have too much surface tension to join and form an interface. This is still free water, it is not in solution, but it does not create the interface boundary. A common example, in reverse, is homogenized milk. Raw milk separates with the butterfat rising to the top, but mechanical homogenizing breaks up the hydrocarbon and fat and suspends it in the water-based milk.
Cs : This is an abbreviation of concentration at saturation or the concentration of water when the fluid is carrying as much water as it can possibly hold in solution. If any more water is added to this sample at these conditions, a free-water condition would result. When Cs is given, a corresponding temperature also is given because saturation is different at different temperatures.
SC or Solubility Constant - This constant is the saturation concentration in relation to the saturation vapor pressure at the same temperature.
The saturation vapor pressure can be found in steam tables. The term "SC" is simply an easy form to use for calculations. An example of the solubility constant for benzene at 20C where the Cs at that temperature is 639 ppmw. The saturation vapor pressure at that temperature is 17.54 mm Hg. Substituting in the above formula:
Ppmw or parts per million by weight - This unit is popular with chemists as a method of expressing moisture in a liquid phase stream of hydrocarbons.
Properties of H2O in Liquid Phase Hydrocarbons
Moisture behaves very similarly in liquids and gases, but there are significant differences that need attention. For example, Cs is analogous to a saturation condition in a gas. Typically, saturation conditions in gases are described with terms like 100 percent relative humidity, or that the dew point temperature equals the dry bulb temperature or that the wet bulb and dry bulb temperatures are equal. All of these describe the fact that the gas is carrying all the moisture that it can. Similarly, if a liquid stream is carrying all the water it too is said to be saturated and at that point, the concentration of water is Cs.
Temperature effects - In liquid-phase measurements, temperature effects on moisture are very similar to temperature effects when making gas-phase measurements. More moisture can be carried at higher temperatures. As temperature increases, the amount of moisture that can be carried in solution increases.
Chemical effects - Whatever may attack a sensor in a gaseous phase measurement also may attack the same sensor in a liquid-phase measurement, but liquid-chemical effects have a different twist than they do as gases. In gaseous methane at nominal pressures, for example, we apply the ideal gas laws. In a liquid phase, moisture in liquid natural gas behaves as only that chemical combination will behave. If compared to another liquid (for example, hexane or heptane), the two carry totally different amounts of moisture even at the same temperature. So chemistry impacts the measurement in the liquid phase, whereas in the gaseous phase, all chemicals, with only rare exceptions, behave according to the ideal gas laws.
Equilibration - This is another behavior that can be compared between gaseous-phase and liquid-phase streams. Equilibration is reached much more rapidly in gaseous-phase streams than in liquid-phase streams because of the density of the stream. The water molecules have the same energy and attraction to equilibrate and normalize across whatever volume they encounter, but they have much more resistance to overcome in the liquid.
Aluminum Oxide Measuring Technique
The aluminum oxide sensor is one of the few sensing techniques that can be used in gases and liquids. The sensor is constructed as a sandwich of aluminum, aluminum oxide (an insulating layer), and gold that forms a thin film capacitor. The aluminum oxide insulating layer is porous and moisture molecules are moved into and out of the structure by changes in the water vapor pressure surrounding the sensor. The capacitance value changes with the population of water molecules in the dielectric layer and is directly related to dew point (water vapor pressure). The change in capacitance controls an oscillator that provides a frequency output. The frequency is related to the dew point at the sensor and a frequency versus dew point table is developed to describe each sensor's response to moisture. This calibration data is recorded in actual tests of exposure to water vapor over a wide range in a calibration fixture at the factory. This calibration is traceable to NIST standards and should be verified every 6-12 months as with any other type of analytical test equipment.
The sensor is mounted at the end of a probe that can be inserted into the stream or used in conjunction with a sample conditioning system. Sample conditioning systems must be designed with care since any change in the temperature of the stream will impact the behavior of the moisture.
Moisture Measurement in Liquids
So how does General Eastern’s make a moisture measurement in liquids? The aluminum oxide sensor reacts to changes in the partial pressure of water dissolved in the liquid by sensing the resultant change in dielectric. After this capacitance is related to the vapor pressure, the real question is how can we convert this to a useful unit of measure parts per million by weight?
The following procedure illustrates this conversion. These calculations begin with the fact that we measure the percent of saturation by looking at the partial vapor pressure in the stream that our probe sees and divide it by what we know to be the saturation vapor pressure at the temperature. If this were a gas, we could multiply that fraction by 100 to get the percent RH. The relative humidity/vapor pressure relationship follows:
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Because percent RH is analogous to percent of saturation, the only change needed for measuring in liquids is to express this in percent of saturation:
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Chemical computations often require information in parts per million by weight. Ppmw can be found by multiplying percent of saturation by Cs or concentration saturation. Because Cs is expressed in ppmw, the result is ppmw of water in the liquid stream. So the formula becomes:
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The source for this Cs value can be found in many published reference texts. The rest of the calculation is handled exactly like the relative humidity of a gaseous stream.
General Eastern moisture analyzers do this calculation from dew point (water vapor pressure) to ppmw automatically. But they do need the data for the behavior of that particular chemical with respect to its ability to carry moisture. That information is input into the analyzers as the solubility constant or SC at various temperatures. The analyzer then will do the calculations automatically from the measured partial water vapor pressure in the stream. The following table is an example of the SC data for benzene.
Solubility of water in Benzene
Temp. 10°C 20°C 30°C 40°C 50°C
Sat. PH2O (mm Hg) 9.029 17.54 31.82 55.29 92.47
Cs (ppmw) 454 639 870 1178 1570
SC 50.28 36.43 27.34 21.29 16.97
The Cs data shown here for benzene is from published tables. This table was further developed by using the formulas discussed earlier. Published data can be found for many other fluids, but because chemistry is such a rapidly growing field, with new chemicals are being formulated every day, tables and the data may not be available. So, in many cases, empirical testing must be done on new chemicals to learn what Cs values are at various temperatures.
Measuring in a liquid with an unknown Cs
In many cases, published data is just not available. Testing to determine the Cs values at varying temperatures must be done first in order to measure water in such a liquid.
The formula for the SC used in General Eastern’s analyzers is as follows:
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And:
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Substituting:
In practice, determining an unknown Cs is very simple to do with a General Eastern’s moisture analyzer, but it is an investment in time and should be done with precision. Throughout the test, test samples drawn from a process should not be exposed to the ambient air as the humidity in the air can influence the results of the test.
We begin by measuring the amount of moisture in a dried fluid. The fluid can be dried by bubbling dry nitrogen through the liquid for several hours. After the nitrogen removes most of the water dissolved in the fluid, a moisture measurement can be taken with the analyzer. A moisture probe immersed in the fluid is connected to the analyzer set to read in units of percent relative humidity (in liquids this is percent saturation), and after equilibration, a reading is made. Then a very small amount, say 10-50 ppmw of carefully measured water is added to the fluid. This should be mixed until the water is totally dissolved in the fluid. During the mixing step, which may take several hours to get the water dissolved in the liquid, the temperature should be kept constant. Then a second set of readings for moisture and temperature measurements are taken.
The change in percentage reflects what was added to the fluid at the temperature at which the measurements were taken. The quantity of water added, expressed as a fraction of the entire sample in ppmw, divided by the change in percent saturation, gives the saturation values. Below are two examples: