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Liquefied hydrocarbon gases. Liquefied petroleum gas - classification, properties and technological advantages

PHYSICAL AND CHEMICAL PROPERTIES OF LIQUEFIED HYDROCARBON GASES

The term "liquefied hydrocarbon gases" refers to propane, butane, iso-butane, mixtures of propane and butane. Among the commonly used fuels, liquefied hydrocarbon gases are so far the only fuels of their kind that can be transported and stored in liquid form at relatively low pressure and normal temperature. However, at normal pressure and relatively low temperatures these mixtures are capable of evaporating, in which case they are used as gases. The transition of liquefied hydrocarbon gases into a gaseous or liquid state depends on three factors - pressure, temperature and volume.

Gaseous hydrocarbons, which are part of liquefied gases, have a density that significantly exceeds the density of air, and are characterized by slow diffusion into the atmosphere, a low ignition temperature, low explosive limits in air, and the possibility of condensate formation when the temperature drops to the dew point or when the pressure rises. In accordance with GOST 20448--80, liquefied hydrocarbon gases of three grades are produced for domestic consumption: SPBTZ, SPBTL - a mixture of propane and technical butane, respectively, winter and summer, BT - technical butane. Pure propane like liquefied gas can be used as a fuel without regasification and at temperatures up to 253 K.

Butanes without regasification can be used as fuel only at temperatures exceeding 273 K. At lower temperatures, their vapor pressure is less than atmospheric pressure. At temperatures of 318, 313, and 258 K, the pressure of n-butane is 0.49, 0.42, and 0.06 MPa, respectively.

If liquefied gases are used at high temperatures, then the use of butanes is desirable, since at the same temperature the pressure of their saturated vapors is approximately three times lower than that of propane. This makes it possible to store the liquid phase of butanes at ordinary temperatures (313 K) in tanks designed for a pressure of 0.7 MPa, and at temperatures up to 353 K in tanks designed for a pressure of 1.6 MPa.

The density of the liquid phase of liquefied gas at a temperature of 273 K and a pressure of 0.1 MPa, depending on the composition, is 0.58--0.6 of the density of water, i.e., the liquid phase of liquefied gas is about two times lighter than water. Consequently, water settling will occur at the bottom of the tanks and apparatuses.

With intensive extraction of the vapor phase from the reservoir, the temperature of the liquid phase will decrease due to the consumption of liquid heat for evaporation. Maximum temperature, at which the liquid does not evaporate, for propane is 231 K, for butane 273 K. For mixtures of propane and butanes, this temperature is variable. It depends on the composition of the mixture.

FEATURES OF LIQUEFIED HYDROCARBON GASES AND THEIR IMPACT ON THE HUMAN BODY

Liquefied hydrocarbon gases are saturated (boiling liquids) in the presence of a free surface of the liquid phase. In this case, a two-phase system always arises (liquid - vapor). The vapor pressure depends on the temperature of the liquid phase and can reach a significant value with a change in temperature external environment. This property of liquefied hydrocarbon gases in the event of a rupture of the apparatus or pipelines determines the maintenance of pressure in them for a long time (up to full release from the liquid phase), which creates a significant great danger to surrounding objects than when an oil pipeline or natural gas pipeline breaks, in which the pressure at the break quickly decreases to zero.

The density of the vapor phase of liquefied hydrocarbon gases is much greater than the density of air. The vapor density of liquefied hydrocarbon gases at a temperature of 273 K and a pressure of 0.1 MPa ranges from 19.6 to 26.46 kg/m 3 . The relative density (in air) of propane is 15.62, iso-butane is 20.64, n-butane is 20.91.

The vapor phase of liquefied hydrocarbon gases does not dissipate in the atmosphere, rising up (like natural gas). It spreads along the surface of the earth or the floor of a room (like CO 2 and other heavy gases). In this regard, it is necessary to arrange ventilation of the premises at floor level, cross-ventilation of the design bureau site (GNS) at ground level, avoid deepening and pits both in the premises and on the site itself.

Liquefied hydrocarbon gases at atmospheric pressure do not have a toxic (poisonous) effect on the human body, since they are slightly soluble in the blood. However, when they get into the air, they mix with it and reduce the oxygen content in the air. A person in such an atmosphere experiences oxygen starvation, and with a significant content of liquefied hydrocarbon gas in the air, he can die from suffocation. Inhalation of air containing 1% propane or butane for 10 minutes does not cause any symptoms of poisoning. A two-minute inhalation of air with a 10% content of liquefied gases causes dizziness. Propylene and butylene have narcotic properties. With a content of 15% propylene in the air, loss of consciousness occurs after 30 minutes, at a content of 24% - after 3 minutes, at a content of 35--40% - after 20 seconds. In this regard, all components of liquefied hydrocarbon gases are included in the list of substances harmful to the human body. Sanitary standards set the maximum allowable concentration of liquefied hydrocarbon gases in the air of the working area of industrial premises, equal to 300 mg/m 3 (in terms of carbon). These standards must also be observed in the working area of outdoor installations. Such a concentration is approximately 15-18 times less than the lower explosive limit.

The dangerous effect on humans of liquefied hydrocarbon gases increases significantly if they contain hydrogen sulfide and other sulfur compounds, which are strong poisons. When the content of hydrogen sulfide in the air is from 150 to 230 mg / m 3, after a few hours, a person develops symptoms of mild poisoning, at a content of 310 mg / m 3, after 5-8 minutes, severe irritation of the mucous membrane of the eyes, nose and throat occurs. An increase in concentration from 770 to 1080 mg / m 3 after 1 hour causes serious poisoning, and at a concentration of 1540-4620 mg / m 3 death occurs.

Vapors of liquefied hydrocarbon gases mixed with air form an explosive mixture. At a temperature of 273 K and a pressure of 0.1 MPa, the explosive limit of propane occurs when its volume content in air is 2.3--9.5%, n-butane--16, 1.5--8.4%, iso -butane - 1.8 - 8.4%. As a result, and also because of the very slow dispersion of liquefied hydrocarbon gas vapors in the atmosphere, the mixture of their vapors with air for a long time and at a great distance from the place of evaporation remains explosive and flammable.

Uncontrolled combustion of liquefied hydrocarbon gases indoors or outdoors leads to fires. The fire hazard of these gases is characterized by a heat output that exceeds 2273 K and provides a flame temperature measured by instruments in the range of 2103-2198 K, significant heat released during the combustion of the gas-air mixture, low flammability and explosive limits, low autoignition temperature, a large need for air during combustion and large quantity resulting products of combustion.

An explosion of a gas-air mixture occurs when it ignites and burns in a limited space ( industrial premises, basement, channel, tank, boiler furnace, furnace, etc.). When the gas-air mixture explodes in a room, a large number of heated gases, as a result of an increase in the volume of which the pressure increases (up to 0.858 MPa). Under the influence of such pressure, building structures are destroyed. The actual volume of the vapor phase of propane during the evaporation of its liquid phase at a temperature of 273 K and a pressure of 0.1 MPa is 269 m 3, mzo-butane - 229 m 3, n-butane - 235 m 3.

This term refers to the entire spectrum liquefied hydrocarbon gases of various origins (ethane, propane, butanes and their derivatives - ethylene, propylene, etc.) and their mixtures. But most often under LPG understand a mixture of liquefied propane and butanes used as domestic fuel and. AT recent times the names and abbreviations of SPBF began to be used more often ( liquefied propane-butane fraction), SPBT ( liquefied propane-butane technical), LPG ( liquefied carbon gas), CIS ( liquefied petroleum gas).

The physical properties of LPG are determined by the physical properties of its main components. It can be stored in liquefied form at relatively low pressures up to 1.5 MPa in a wide temperature range, which makes it possible to transport LPG in tanks or cylinders. The composition of LPG, depending on the specification, may also include isobutane and ethane. When the volume of LPG is approximately 1/310 of the volume of gas under standard conditions.

The physical properties of propane and n-butane, which determine the method of their transportation in liquefied form in tanks, are presented in the table.

LPG used as household fuel (heating, cooking), and also used as an environmentally friendly motor fuel, in particular for public transport in major cities. Liquefied gas is a raw material for the production of olefins (ethylene, propylene), aromatic hydrocarbons (benzene, toluene, xylene, cyclohexane), alkylate (an additive that increases the octane number of gasoline), synthetic motor fuels. AT winter time Butane is added to gasoline to increase the RPV (Reid Vapor Pressure). In the US, LPG, after being diluted with nitrogen and/or air (to bring the specific calorific value to that of network gas), is used as an additional source of gas to smooth out peak loads on gas distribution networks.

As a raw material for the production of LPG, natural gas and , oil and petroleum associated gases. The technology for the production of liquefied gas depends on the industry: oil and gas processing and petrochemistry. In the oil refining industries, liquefied carbon gas is actually an additional product in the production of gasoline. In gas processing, liquefied gas is the main product for final sale or further processing.

Due to the depletion of the deposits of the Cenomanian "dry gas" deposits of the Neocomian-Jurassic horizons, characterized by high content hydrocarbon gases of the C 2+ series ( "wet and condensate gas"). In petrochemistry, fat content is understood as the average number of carbon atoms per gas molecule (for methane, the fat content is 1, for ethane - 2, etc.). From the point of view of preparing gas for transportation by pipeline transport, fat content refers to the excessive presence of hydrocarbons of the C 3+ series in the gas, leading to their condensation in the gas pipeline during transportation. The fat content of gas increases its value as a feedstock for petrochemicals.

LPG produced in Russia is mainly used in three areas: 1) LPG as a raw material in petrochemistry; 2) in the public utility sector; 3) export.

Liquefied petroleum gas, more commonly used as an automotive fuel, is a mixture of propane (C3H8), butane (C4H10) and a small amount (about 1%) of unsaturated hydrocarbons.

Liquefied gas can be produced both from oil and from the condensate fraction of natural gas. The mixture of hydrocarbons formed in the course of processing enters the absorption-gas-fractionation plant, where it is separated into separate fractions in special columns.

Propane and butane are purified from sulfur compounds, alkali, water and other components, so burning gas brings only minor harm to the atmosphere. Compared to propane, butane has a poorer volatility and is therefore mixed with propane. Depending on the brand of GOS, propane and butane are mixed in the required ratios.

Physiochemical properties

Density the liquid phase of the gas depends on the temperature, with an increase in which the density decreases. At normal atmospheric pressure and a temperature of 15 degrees C, the density of the liquid phase of propane is 0.51 kg / l, butane - 0.58 kg / l. The vapor phase of propane is 1.5 times heavier than air, butane is 2 times heavier. The boiling point of gasoline is higher than environment, and liquefied gas evaporates at lower temperatures. This means that the gasoline in the tank can be liquid state at atmospheric pressure, and liquefied gas in a container - at a pressure corresponding to the ambient temperature.

Octane number gas fuel higher than that of gasoline, therefore the knock resistance of liquefied gas is greater than that of even the highest quality gasoline. The average octane number of liquefied gas - 105 - is unattainable for any brand of gasoline. This allows you to achieve greater fuel efficiency in a gas boiler.

Diffusion. The gas mixes easily with air and burns more evenly. The gas mixture burns completely, so no soot is formed in the furnaces and on the heating elements.

pressure in the container. In a closed vessel, LPG forms a two-phase system consisting of liquid and vapor phases. The pressure in the tank depends on the saturated vapor pressure, which in turn depends on the temperature of the liquid phase and the percentage of propane and butane in it. Saturated vapor pressure characterizes the volatility of LPG. The volatility of propane is higher than that of butane, therefore, its pressure at low temperatures is much higher. Calculations and experiments have established that at low ambient temperatures it is more efficient to use LPG with a high propane content, since this ensures reliable gas evaporation, and hence the sufficiency of gas for gas consumption. In addition, sufficient overpressure in the tank will ensure a reliable gas supply to the boiler in very coldy. At high positive ambient temperatures, it is more efficient to use LPG with a lower propane content, since in this case a significant overpressure will be created in the tank, which can cause the relief valve to operate. In addition to propane and butane, LPG contains no significant amount methane, ethane and other hydrocarbons that can change the properties of LPG. During the operation of the tank, non-evaporable condensate may form, which adversely affects the operation of gas equipment.

Change in the volume of the liquid phase during heating. The rules of the United Nations Economic Commission for Europe provide for the installation automatic device, limiting the filling of the container to 85% of its volume. This requirement is explained by the large volumetric expansion coefficient of the liquid phase, which is 0.003 for propane and 0.002 for butane per 1°C increase in gas temperature. For comparison: the expansion coefficient of propane is 15 times, and butane is 10 times greater than that of water.

Change in the volume of gas during evaporation. When liquefied gas evaporates, about 250 liters are formed. gaseous. Thus, even a minor LPG leak can be dangerous, since the volume of gas during evaporation increases by 250 times. The density of the gas phase is 1.5–2.0 times greater than the density of air. This explains the fact that in case of leaks, the gas is difficult to disperse into the air, especially in a closed room. Its vapors can accumulate in natural and artificial recesses, forming an explosive mixture. SNiP 42-01-2002 provides for the mandatory installation of a gas analyzer that gives a signal to the shut-off valve to close in the event of gas accumulation at a concentration of 10% of the explosive concentration.

Odoration. The gas itself practically does not smell, therefore, for safety and timely diagnosis of gas leaks by human olfactory organs, small amounts of strong-smelling substances are added to it. At mass fraction mercaptan sulfur less than 0.001% LPG must be odorized. For odorization, ethyl mercaptan (С2Н5SH) is used, which is an unpleasantly smelling liquid with a density of 0.839 kg/l and a boiling point of 35°C. The threshold of odor sensitivity is 0.00019 mg/l, the maximum allowable concentration in the air of the working area is 1 mg/m 3 . In the case when the toxicity is normal or slightly below the norm, the smell of the odorant is practically not felt and its accumulation in the room is not observed.

Do you know that....

The cost of autonomous gasification averages from 5 thousand rubles per kilowatt of thermal power, the cost of gasification with main gas from 9 thousand rubles per kilowatt, the cost of allocating electric power from 20 thousand rubles per kilowatt.

Introduction

Liquefied hydrocarbon gases (LHG) - a mixture of light hydrocarbons liquefied under pressure with a boiling point of? 50 to 0 ° C. Designed for use as a fuel. Main components: propane, propylene, isobutane, isobutylene, n-butane and butylene.

It is produced mainly from associated petroleum gas. It is transported and stored in cylinders and gas holders. It is used for cooking, boiling water, heating, used in lighters, as fuel for vehicles.

Liquefied hydrocarbon gases(propane-butane, hereinafter referred to as LPG) - mixtures of hydrocarbons that under normal conditions are in a gaseous state, and with a slight increase in pressure or a slight decrease in temperature, they pass from gaseous state into liquid.

The main components of LPG are propane and butane. Propane-butane (liquefied petroleum gas, LPG, in English - liquifiedpetroleumgas, LPG) is a mixture of two gases. The composition of liquefied gas also includes in small quantities: propylene, butylene, ethane, ethylene, methane and a liquid non-evaporating residue (pentane, hexane).

The raw materials for the production of LPG are mainly petroleum associated gases, gas condensate deposits and gases obtained in the process of oil refining. liquefied hydrocarbon propane refinery

From LPG plants in railway tanks it goes to gas filling stations (GFS) of gas facilities, where it is stored in special tanks until sold (released) to consumers. LPG is delivered to consumers in cylinders or tank trucks.

In vessels (tanks, tanks, cylinders) for storage and transportation, LPG is simultaneously in 2 phases: liquid and vapor. LPG is stored and transported in liquid form under pressure, which is created by its own gas vapors. This property makes LPG a convenient source of fuel supply for domestic and industrial consumers, because liquefied gas during storage and transportation in the form of a liquid occupies hundreds of times less volume than gas in its natural (gaseous or vaporous) state, and is distributed through gas pipelines and used (burned) in gaseous form.

Liquefied hydrocarbon gases (LHG) consist of simple hydrocarbon compounds, which are organic substances containing in their composition 2 chemical element- carbon (C) and hydrogen (H). Hydrocarbons differ from each other in the number of carbon and hydrogen atoms in the molecule, as well as the nature of the bonds between them.

Commercial liquefied gas should consist of hydrocarbons, which under normal conditions are gases, and with a relatively small increase in pressure and ambient temperature or a slight decrease in temperature at atmospheric pressure, they pass from a gaseous state to a liquid state.

The simplest hydrocarbon containing only one carbon atom is methane (CH 4). It is the main component of natural as well as some artificial combustible gases. The next carbon in this series - ethane (C 2 H 6) - has 2 carbon atoms. A hydrocarbon with three carbon atoms is propane (C 3 H 8), and with four - butane (C 4 H 10).

All hydrocarbons of this type have the general formula C n H 2n + 2 and are included in the homologous series of saturated hydrocarbons - compounds in which carbon is saturated to the limit with hydrogen atoms. Under normal conditions, only methane, ethane, propane and butane are gases from saturated hydrocarbons.

To obtain liquefied gases, natural gases extracted from the bowels of the Earth are currently widely used, which are a mixture of various hydrocarbons, mainly of the methane series (saturated hydrocarbons). Natural gases from pure gas fields are mostly methane and are lean or dry; heavy hydrocarbons (from propane and above) contain less than 50 g/cm 3 . Associated gases released from wells of oil fields together with oil, in addition to methane, contain a significant amount of heavier hydrocarbons (usually more than 150 g/m 3) and are oily. Gases that are produced from condensate deposits consist of a mixture of dry gas and condensate vapor. Condensate vapors are a mixture of vapors of heavy hydrocarbons (C3, C4, gasoline, naphtha, kerosene). At gas processing plants, propane-butane fraction is separated from associated gases.

WFLH - a wide fraction of light hydrocarbons, includes mainly a mixture of light hydrocarbons of ethane (C 2) and hexane (C 6) fractions. In general, a typical NGL composition is as follows: ethane from 2 to 5%; liquefied gas fractions C 4 -C 5 40-85%; hexane fraction C 6 from 15 to 30%, the pentane fraction accounts for the remainder.

Given the widespread use of LPG in the gas industry, it is necessary to dwell in more detail on the properties of propane and butane.

propamn is organic matter class of alkanes. Contained in natural gas, formed during the cracking of petroleum products. Chemical formula C 3 H 8 (Fig. 1). Colorless, odorless gas, very slightly soluble in water. Boiling point? 42.1C. Forms explosive mixtures with air at vapor concentrations from 2.1 to 9.5%. The self-ignition temperature of propane in air at a pressure of 0.1 MPa (760 mm Hg) is 466 °C.

Propane is used as a fuel, the main component of the so-called liquefied hydrocarbon gases, in the production of monomers for the synthesis of polypropylene. It is the raw material for the production of solvents. In the food industry, propane is registered as food additive E944 as propellant.

Butamn (C 4 H 10) -- organic compound class of alkanes. In chemistry, the name is mainly used to refer to n-butane. Chemical formula C4H10 (Fig. 1). The mixture of n-butane and its isomer isobutane CH(CH 3) 3 has the same name. Colourless, flammable gas, odorless, easily liquefied (below 0 °C and normal pressure or at high blood pressure and ordinary temperature - a highly volatile liquid). Contained in gas condensate and petroleum gas (up to 12%). It is a product of catalytic and hydrocatalytic cracking of oil fractions.

The production of both liquefied gas and NGLs is carried out at the expense of the following three main sources:

? oil production enterprises - the production of LPG and NGL occurs during the production of crude oil during the processing of associated (bound) gas and the stabilization of crude oil;
? gas production enterprises - obtaining LPG and NGL occurs during the primary processing of well gas or free gas and condensate stabilization;
? oil refineries - the production of liquefied gas and similar NGLs occurs during the processing of crude oil at refineries. In this category, NGL consists of a mixture of butane-hexane fractions (C4-C6) with a small amount of ethane and propane.

The main advantage of LPG is the possibility of their existence at ambient temperature and moderate pressures, both in liquid and gaseous states. In the liquid state, they are easily processed, stored and transported, in the gaseous state they have best performance combustion.

The state of hydrocarbon systems is determined by the totality of the influences of various factors, therefore, for complete characteristics you need to know all the parameters. The main parameters that can be directly measured and affect the LPG flow regimes include pressure, temperature, density, viscosity, concentration of components, and phase ratio.

The system is in equilibrium if all parameters remain unchanged. In this state, there are no visible qualitative and quantitative changes in the system. A change in at least one parameter violates the equilibrium state of the system, causing that

or some other process.

During storage and transportation, liquefied gases constantly change their state of aggregation, part of the gas evaporates and turns into a gaseous state, and part condenses, turning into a liquid state. In cases where the amount of evaporated liquid is equal to the amount of condensed vapor, the liquid-gas system reaches equilibrium and the vapor above the liquid becomes saturated, and their pressure is called saturation pressure or vapor pressure.

pressure and temperature. The gas pressure is summary result collisions of molecules against the walls of a vessel occupied by this gas.

The elasticity (pressure) of saturated gas vapor * p p is the most important parameter by which operating pressure in tanks and bottles. The temperature of the gas determines the degree of its heating, i.e. a measure of the intensity of the movement of its molecules. The pressure and temperature of liquefied gases strictly correspond to each other.

The vapor pressure of LPG - saturated (boiling) liquids - varies in proportion to the temperature of the liquid phase (see Fig. I-1) and is a value strictly defined for a given temperature. All equations relating the physical parameters of a gaseous or liquid substance include absolute pressure and temperature, and equations for technical calculations (strength of the walls of cylinders, tanks) include overpressure.

The vapor pressure of LPG increases with increasing temperature and decreases with decreasing temperature.

This property of liquefied gases is one of the determining factors in the design of storage and distribution systems. When a boiling liquid is taken from tanks and transported through a pipeline, part of the liquid evaporates due to pressure losses, a two-phase flow is formed, the vapor pressure of which depends on the flow temperature, which is lower than the temperature in the tank. In the event that the movement of a two-phase liquid through the pipeline stops, the pressure at all points equalizes and becomes equal to the vapor pressure.

Liquefied hydrocarbon gases are transported in railway and automobile tanks, stored in tanks of various volumes in a state of saturation: boiling liquid is placed in the lower part of the vessels, and dry saturated vapors are in the upper part (Fig. 2). When the temperature in the tanks decreases, part of the vapors will condense, i.e. the mass of the liquid increases and the mass of the vapor decreases, a new equilibrium state occurs. As the temperature rises, the reverse process occurs until the phases are in equilibrium at the new temperature. Thus, evaporation and condensation processes occur in tanks and pipelines, which in two-phase media proceed at constant pressure and temperature, while the evaporation and condensation temperatures are equal.

Figure 2. Phase states of liquefied gases during storage.

AT real conditions Liquefied gases contain some amount of water vapor. Moreover, their amount in gases can increase to saturation, after which moisture from gases precipitates in the form of water and mixes with liquid hydrocarbons to the limiting degree of solubility, and then free water is released, which settles in tanks. The amount of water in LPG depends on their hydrocarbon composition, thermodynamic state and temperature. It has been proven that if the temperature of LPG is reduced by 15-30 0 C, then the solubility of water will decrease by 1.5-2 times and free water will accumulate at the bottom of the tank or fall out in the form of condensate in pipelines. The water accumulated in the tanks must be periodically removed, otherwise it can get to the consumer or lead to equipment failure.

According to the LPG test methods, the presence of only free water is determined, the presence of dissolved water is allowed.

Abroad, there are more stringent requirements for the presence of water in LPG and its amount, through filtration, it is brought to 0.001% by weight. This is justified, since dissolved water in liquefied gases is a pollutant, because even at positive temperatures it forms solid compounds in the form of hydrates.

Density. Mass per unit volume, i.e. the ratio of the mass of a substance at rest to the volume it occupies is called density (notation). The unit of density in the SI system is kilogram per cubic meter (kg / m 3). AT general case

When moving liquefied gases with a pressure below the vapor pressure, i.e. when moving two-phase flows, to determine the density at a point, you should use the ratio limit:

In numerous calculations, especially in the field of thermodynamics of gases and gas-liquid mixtures, it is often necessary to use the concept of relative density d - the ratio of the density of a given substance to the density of a given substance to the density of a substance, taken as specific or standard c,

For solid and liquid substances, the density of distilled water at a pressure of 760 mm Hg is taken as standard. and a temperature of 3.98 ° C (999, 973 kg / m 3 1 t / m 3), for gases - the density of dry atmospheric air at a pressure of 760 mm Hg. and a temperature of 0 ºС (1.293 kg / m 3).

Figure I-2 shows the density curves of the saturated liquid and vapor phases of the main components of liquefied gases as a function of temperature. The black dot on each curve indicates the critical density. This point of inflection of the density curve corresponds to the critical temperature at which the density of the vapor phase is equal to the density of the liquid phase. The branch of the curve located above the critical point gives the density of the saturated liquid phase, and below - the saturated vapor. The critical points of saturated hydrocarbons are connected by a solid line, and those of unsaturated hydrocarbons by a dashed line. Density can also be determined from state diagrams. AT general view the dependence of density on temperature is expressed next

T \u003d T0 + (T-T 0) + (T-T 0) 2 + (T-T 0) 2 ±.

The influence of the third and other members of this series on the density value due to small values is insignificant, therefore, with an accuracy quite sufficient for technical calculations, it can be neglected. Then

T \u003d T0 + (T-T 0)

Where = 1.354 for propane, 1.068 for n-butane, 1.145 for isobutane.

The relative change in the volume of a liquid with a change in temperature by one degree is characterized by the temperature coefficient of volumetric expansion, W, which for liquefied gases (propane and butane) is several times greater than for other liquids.

Propane - 3.06 * 10 -3;

Butane - 2.12 * 10 -3;

Kerosene - 0.95 * 10 -3;

Water - 0.19 * 10 -3;

When the pressure increases, the liquid phase of propane and butane is compressed. Its degree of compression is estimated by the coefficient of volumetric compressibility vszh, the dimension of which is inverse to the dimension of pressure.

Specific volume. The volume of a unit mass of a substance is called the specific volume (designation). The unit of specific volume in the SI system is a cubic meter per kilogram (m 3 / kg)

Specific volume and density are reciprocals, i.e.

Unlike most liquids, which slightly change their volume with temperature changes, the liquid phase of liquefied gases increases its volume quite sharply with increasing temperature (15 times more than water). When filling tanks and cylinders, you have to take into account the possible increase in the volume of liquid (Fig. I-3).

Compressibility. Estimated by the coefficient of volumetric compression, m 3 / n,

The reciprocal of p is called the modulus of elasticity and is written as follows:

The compressibility of liquefied gases compared to other liquids is very significant. So, if the compressibility of water (48.310 -9 m 2 / n) is taken as 1, then the compressibility of oil is 1.565, gasoline is 1.92, and propane is 15.05 (respectively 75.5610 -9, 92.7910 -9 and 727, 4410 -9 m 2 / n).

If the liquid phase occupies the entire volume of the reservoir (cylinder), then when the temperature rises, it has nowhere to expand and it begins to shrink. The pressure in the tank in this case increases by an amount, N / m 2,

where t is the temperature difference of the liquid phase, .

The increase in pressure in the tank (cylinder) with an increase in ambient temperature should not exceed the allowable calculated value, otherwise an accident is possible. Therefore, when filling, it is necessary to provide a steam cushion of a certain size, i.e. fill the tank not completely. Hence, it is necessary to know the degree of filling, determined by the relation

If it is necessary to find out what temperature difference is permissible with the existing filling, it can be calculated using the formula:

Critical parameters. Gases can be converted into a liquid state by compression, if the temperature does not exceed a certain value characteristic of each homogeneous gas. The temperature above which a given gas cannot be liquefied by any increase in pressure is called the critical temperature of the gas (T cr). The pressure required to liquefy a gas at a critical temperature is called critical pressure (p cr). The volume of gas corresponding to the critical temperature is called the critical volume (Vcr), and the state of the gas, determined by the critical temperature, pressure and volume, is called the critical state of the gas. The density of vapor over a liquid in a critical state becomes equal density liquids.

The principle of corresponding states. Usually, to generalize experimental data on the study of various processes and substances, criteria systems based on the analysis of the equations of motion, thermal conductivity, etc. are used. To use such similarity equations, tables of physical properties of working media are needed. The inaccuracy in the determination of physical properties or their absence does not make it possible to use the similarity equations. This is especially true for little-studied working fluids, in particular, for liquefied hydrocarbon gases, about physical properties which there are rather contradictory data in the literature, often at random pressures and temperatures. At the same time, there are accurate data on the critical parameters and molecular weight of the substance. This allows, using the given parameters and the law of the corresponding states, which is confirmed by numerous studies and theoretically substantiated by the modern kinetic theory of matter, to determine unknown parameters.

For thermodynamically similar substances, and liquefied hydrocarbon gases are thermodynamically similar, the reduced equations of state, i.e. the equations of state written in dimensionless (reduced) parameters (р pr = р/р cr =) have the same form. AT different time up to fifty equations of state for real substances have been proposed by various authors. The most famous and commonly used of them is the van der Waals equation:

where a and b are constants inherent in a given chemical compound;

Having expressed the parameters of the gas in dimensionless reduced quantities, it can be established that for gases there is a general equation of state that does not contain quantities characterizing a given gas:

F(r pr, T pr, V pr) = 0.

The laws of the gas state are valid only for an ideal gas, therefore, in technical calculations related to real gases, they are used with real gases within the pressure range of 2-10 kgf / cm 2 and at temperatures exceeding 0. The degree of deviation from the laws of ideal gases is characterized by the coefficient compressibility Z = (Fig. 1-4 - 1-6). It can be used to determine the specific volume if the pressure and temperature are known, or the pressure if the specific volume and temperature are known. Knowing the specific volume, you can determine the density.

Specific gravity. The weight of a unit volume of a substance, i.e. the ratio of the weight (gravity) of a substance to its volume is called the specific gravity (designation. In the general case, where G is the weight (gravity of the substance, V volume, m 3. Unit of specific gravity in SI = newton per cubic meter (n / m 3). Specific gravity depends on the acceleration of gravity at the point of its definition and, therefore, is a parameter of matter.

Heat of combustion. The amount of heat that is released during the complete combustion of a unit mass or volume of gas is called the heat of combustion (notation Q). The unit of heat of combustion in SI is joule per kilogram (j/kg) or joule per cubic meter (j/m3).

Ignition temperature. The minimum temperature to which the gas-air mixture must be heated in order for the combustion process (combustion reaction) to begin is called the ignition temperature. It is not a constant value and depends on many reasons: the content of combustible gas in the gas-air mixture, the degree of homogeneity of the mixture, the size and shape of the vessel in which it is heated, the speed and method of heating the mixture, the pressure under which the mixture is, etc.

Gas flammability limits. Gas-air mixtures can ignite (explode) only if the gas content in the air (or oxygen) is within certain limits, beyond which these mixtures do not burn spontaneously (without a constant influx of heat from the outside). The existence of these limits is explained by the fact that as the content of air or pure gas in the gas-air mixture increases, the flame propagation speed decreases, heat losses increase and combustion stops. As the temperature of the gas-air mixture increases, the flammability limits expand.

Heat capacity. The amount of heat required to change the temperature of a body or system by one degree is called the heat capacity of the body or system (notation C). The unit in SI is joule per degree Kelvin (J/K). 1 j / K - 0.2388 cal / K \u003d 0.2388 * 10 -3 kcal / K.

In practical calculations, the average and true heat capacity are distinguished, depending on the temperature range in which it is determined. The average heat capacity C m is a value determined in a finite temperature range, i.e.

With m \u003d q / (t 2 -t 1).

The true heat capacity is the value determined at a given point (for given p and T or and T), i.e.

There are heat capacities determined at constant pressure (C p) or at constant volume (C v).

Thermal conductivity. The ability of a substance to transmit thermal energy called thermal conductivity. It is determined by the amount of heat Q passing through a wall with an area F of thickness over a period of time at a temperature difference t 2 -t 1, i.e.

where is the thermal conductivity coefficient characterizing the heat-conducting properties of a substance, W / (m * K) or kcal / (m * h * C).

Viscosity- this is the ability of gases or liquids to resist shear forces, due to the forces of adhesion between the molecules of a substance. The force of resistance to sliding or shear F, arising from the movement of two adjacent layers of liquid or gas, is proportional to the change (gradient) of the velocity along the axis normal to the direction of the flow of liquid from gas, i.e.

where - coefficient of proportionality, ns / m 2 (in SI); it is called the coefficient of dynamic viscosity (internal friction) or dynamic viscosity; dw is the velocity gradient in two adjacent layers located at a distance dy.

In many technical calculations, kinematic viscosity is used, which is the ratio of the dynamic viscosity of a liquid or gas to their density, i.e. =/. Unit kinematic viscosity in SI - square meter per second (m 2 / sec).

The viscosity of the liquid phase decreases with increasing temperature, while the viscosity of the gas and vapor increases.

Octane number gas fuel is higher than that of gasoline, so the knock resistance of liquefied gas is greater than that of even the highest quality gasoline. The average octane number of liquefied gas - 105 - is unattainable for any brand of gasoline. This allows you to achieve greater efficiency in the use of fuel in a gas boiler.

Diffusion. The gas mixes easily with air and burns more evenly. The gas mixture burns completely, so no soot is formed in the furnaces and on the heating elements.

pressure in the container. In a closed vessel, LPG forms a two-phase system consisting of liquid and vapor phases. The pressure in the tank depends on the saturated vapor pressure, which in turn depends on the temperature of the liquid phase and the percentage of propane and butane in it. Saturated vapor pressure characterizes the volatility of LPG. The volatility of propane is higher than that of butane, therefore, its pressure at low temperatures is much higher. Calculations and experiments have established that at low ambient temperatures it is more efficient to use LPG with a high propane content, since this ensures reliable gas evaporation, and hence the sufficiency of gas for gas consumption. In addition, sufficient overpressure in the tank will ensure reliable gas supply to the boiler in severe frosts. At high positive ambient temperatures, it is more efficient to use LPG with a lower propane content, since in this case a significant overpressure will be created in the tank, which can cause the relief valve to operate. In addition to propane and butane, LPG contains a small amount of methane, ethane and other hydrocarbons that can change the properties of LPG. During the operation of the tank, non-evaporable condensate may form, which adversely affects the operation of gas equipment.

Change in the volume of the liquid phase during heating. Regulations of the United Nations Economic Commission for Europe provide for the installation of an automatic device that limits the filling of the container to 85% of its volume. This requirement is explained by the large volumetric expansion coefficient of the liquid phase, which is 0.003 for propane and 0.002 for butane per 1°C increase in gas temperature. For comparison: the expansion coefficient of propane is 15 times, and butane is 10 times greater than that of water.

Change in the volume of gas during evaporation. When liquefied gas evaporates, about 250 liters are formed. gaseous. Thus, even a minor LPG leak can be dangerous, since the volume of gas during evaporation increases by 250 times. The density of the gas phase is 1.5--2.0 times greater than the density of air. This explains the fact that in case of leaks, the gas is difficult to disperse into the air, especially in a closed room. Its vapors can accumulate in natural and artificial recesses, forming an explosive mixture. SNiP 42-01-2002 provides for the mandatory installation of a gas analyzer that gives a signal to the shut-off valve to close in the event of gas accumulation at a concentration of 10% of the explosive concentration.

Odoration. The gas itself practically does not smell, therefore, for safety and timely diagnosis of gas leaks by human olfactory organs, small amounts of strong-smelling substances are added to it. With a mass fraction of mercaptan sulfur less than 0.001% LPG must be odorized. For odorization, ethyl mercaptan (С2Н5SH) is used, which is an unpleasantly smelling liquid with a density of 0.839 kg/l and a boiling point of 35°C. The threshold of odor sensitivity is 0.00019 mg/l, the maximum allowable concentration in the air of the working area is 1 mg/m 3 . In the case when the toxicity is normal or slightly below the norm, the smell of the odorant is practically not felt and its accumulation in the room is not observed.

Conclusion

Thus, it is possible to summarize and highlight the main properties of propane-butane mixtures that affect the conditions for their storage, transportation and measurement.

1. Liquefied hydrocarbon gases are low-boiling liquids capable of being in a liquid state under saturated vapor pressure.

Boiling temperature:

Propane -42 0 С;

Butane - 0.5 0 C.

2. Under normal conditions, the volume of gaseous propane is 270 times greater than the volume of liquefied propane.
3. Liquefied hydrocarbon gases are characterized by a high coefficient of thermal expansion.
4. LPG is characterized by low density and viscosity compared to light oil products.
5. Instability of the aggregate state of LPG during the flow through pipelines depending on temperature, hydraulic resistance, uneven conditional passages.
6. Transportation, storage and measurement of LPG is possible only through closed (sealed) systems, designed, as a rule, for a working pressure of 1.6 MPa.
7. Pumping, measuring operations require the use of special equipment, materials and technologies.

All over the world, hydrocarbon systems and equipment, as well as the arrangement of technological systems, are subject to uniform requirements and rules.

Liquefied gas is a Newtonian fluid, so the pumping and measurement processes are described by the general laws of hydrodynamics. But the function of hydrocarbon systems is reduced not only to the simple movement of the liquid and its measurement, but also to ensure that the influence of the "negative" physical and chemical properties of LPG is reduced.

Fundamentally, systems pumping LPG differ little from systems for water and oil products, and, nevertheless, it is necessary optional equipment, which guarantees the qualitative and quantitative characteristics of the measurement.

Based on this, the technological hydrocarbon system, at a minimum, must include a reservoir, a pump, a gas separator, a meter, a differential valve, a shut-off or control valve, and safety devices against excess pressure or flow rate.

Liquefied hydrocarbon gases(propane-butane, hereinafter referred to as LPG) - mixtures of hydrocarbons, which under normal conditions ( Atmosphere pressure and T air \u003d 0 ° C) are in a gaseous state, and with a slight increase in pressure (at a constant temperature) or a slight decrease in temperature (at atmospheric pressure), they pass from a gaseous state to a liquid state.

Main Components LPG are and .

Propane-butane (liquefied petroleum gas, LPG, in English - liquified petroleum gas, LPG) is a mixture of two gases. The composition of liquefied gas also includes in small quantities: propylene, butylene, ethane, ethylene, methane and a liquid non-evaporating residue (pentane, hexane).

The raw materials for the production of LPG are mainly petroleum associated gases, gas condensate deposits and gases obtained in the process of oil refining.

From LPG plants in railway tanks it goes to gas filling stations (GFS) of gas facilities, where it is stored in special tanks until sold (released) to consumers.

LPG is delivered to consumers in cylinders or tank trucks ().

Scope of LPG

Due to its environmental friendliness (cleanliness of combustion) and relatively low production and processing costs, propane-butane gas has been widely used for industrial and household needs of the population. The scope of liquefied petroleum gas is wide. So, for example, LPG is used as a source of heat, fuel for vehicles, raw material for the production of aerosols, as a fuel for truck loaders, etc.

Industry
In industry, liquefied hydrocarbon gases (propane-butane, isobutane) are used as raw materials and fuel. In the construction industry, SPBT (a mixture of propane and butane) is used in the processing of metals, with gas welding works.
There is a wide range of LPG applications at large warehouse enterprises. So, for example, SPBT is used for heating large warehouses and retail areas (in infrared heaters (emitters). Due to its environmental friendliness, lack of smell, gas is used as a fuel for forklift trucks in grocery warehouses and in the food industry.
LPG is widely used in the petrochemical industry. In the cosmetic industry, isobutane is used in the production of sprays, auto chemicals.
Isobutane is also used in the production of foamed insulating materials.
Motor transport
Propane-butane - liquefied petroleum gas - is used as a motor fuel as an alternative traditional look fuel - gasoline. And successfully competes on them at the price.
Today, with the advent of new advanced systems of the 4th generation of LPG, the transfer of vehicles to gas is becoming increasingly popular. A number of regional programs for the conversion of vehicles to gas are currently being adopted. But due to the lack of proper funding, unfortunately, the process is slowed down.
Basically, the conversion of cars to gas occurs privately.
But many car enthusiasts who use gas instead of gasoline report significant improvements in engine performance and real cost savings.
Experts note the undeniable advantages of using liquefied hydrocarbon gas instead of gasoline. So, for example, the engine resource is increased by 10 - 15%, the consumption of engine oil is reduced by 10%. When running on gas, detonation does not occur in any mode of engine operation. A car running on gas has additional protection from theft and fuel drain.
A car enthusiast using gas does not experience the inconvenience associated with refueling, because. Outwardly, the process of refueling a car with gas is very similar to refueling with gasoline. And the number of gas stations is growing every day.
Conversion of cars to gas is real savings your funds.
Communal sector
The traditional use of LPG is domestic use: for propane heating at home and cooking. Gas consumption volumes vary depending on the consumer: from small household plots to cottage villages and large construction projects.
In private homes, at enterprises where there is no way to supply natural gas, it is advisable to use liquefied hydrocarbon gas as a fuel in boiler houses.

Liquefied hydrocarbon gases, supplied to settlements, must comply with the requirements of "Hydrocarbon liquefied fuel gases for domestic consumption" (PT, SPBT or BT brands), which is quite suitable for domestic purposes (filling cylinders, for example).

And for automotive gas equipment, meters at gas filling stations, fine filling equipment need the same gas, but manufactured according to “Liquefied hydrocarbon gases for road transport” (PA or PBA brands).

For domestic consumption and industrial purposes, the standard provides for the production and sale of LPG of three grades: