Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات FERTILIZER INDUSTRY LECTURE (3) H EATING V ALUE OF NATURAL GAS The heating value (or calorific value) of a combustible material is the negative of the standard heat of combustion, usually expressed per unit mass of the material. The higher heating value (or total heating value or gross heating value) is with H 2 O (l) as a combustion product, and the lower heating value (or net heating value) is the value based on H 2 O (v) as a product.

 Since is always negative, the heating value is positive.  The higher heating value (HHV) of commercial, processed natural gas is about 40 MJ per normal cubic meter which, in the United States, is equivalent to about 1,015 Btu per standard cubic foot. However, those values can vary by several percent from one natural gas to another depending upon their source reservoir and upon their degree of processing.  The heating value of a fuel gas when the water formed during combustion does not condense is called the lower heating value (LHV) and can be as much as 10% less than the HHV.

To calculate a lower heating value of a fuel from a higher heating value or vice versa, you must calculate the moles of water produced when a unit mass of the fuel is burned. If this quantity is designated n, then: HHV= LHV+ n ∆ H v ^ (H2O at 25º C) The heat of vaporization of water at 25°C is: ∆ H v ^ (H2O at 25º C)= kJ/mol=18934 btu/lbmol If a fuel contains a mixture of combustible substances, its heating value is: ∆ H v =∑ x i(∆ H v )i

Where (HV) i is the heating value of the i(th) combustible substance. If the heating values are expressed in units of (energy)/ (mass), then the x i ’s are the mass fractions of the fuel components, while if the dimensions of the heating values are (energy)/ (mole) then x i ’s are mole fractions. Example (1): Calculation of heating value A natural gas contains 85% methane and 15% ethane by volume. Calculate the higher heating value of this fuel in kJ/g from the standard heats of combustion of methane and ethane given below:

CH4 (g) + 2O 2 (g) = CO2 (g) +2H2O (g) ∆ H c º^ = -802 kJ/mol C2H6 (g) + 7/2O2 (g)= 2CO2 (g)+3 H2O (g) ∆ H c º^ = kJ/mol

Uses of Natural Gas 1-Power generation: Natural gas is a major source of electricity generation through the use of gas turbines and steam turbines.electricity generationgas turbinessteamturbines 2- Domestic use: Natural gas is supplied to homes, where it is used for such purposes as cooking in natural gas-powered ranges and/or ovens, natural gas- heated clothes dryers, heating/cooling and central heating. Home or other building heating may include boilers, furnaces, and water heaters. Compressed natural gas (CNG) is used in rural homes without connections to piped-in public utility services, or with portable grills. However, due to CNG being less economical than LPG which is liquified propane, butane or a mixture of both, LPG is the dominant source of rural gas.cookingclothes dryersheatingcoolingcentral heatingfurnaceswater heatersruralpipedpublic utilitygrills LPG

3. Transportation fuel: Compressed natural gas (CNG) is a cleaner alternative to other automobile fuels such as gasoline (petrol) and diesel fuel. The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. 4. Fertilizer industry Natural gas is a major feedstock for the production of ammonia used in fertilizer production.

5. Hydrogen production: Natural gas is also a major feedstock for the production of hydrogen, with one common method being the steam-methane reforming (SMR) process. Hydrogen has various applications: it is a primary feedstock for the chemical industry, a hydrogenating agent, an important commodity for oil refineries, and a fuel source in hydrogen vehicles.steam-methane reforminghydrogen vehicles 6. Aviation: The advantages of liquid methane as a jet engine fuel are that it has more specific energy than the standard kerosene mixes and that its low temperature can help cool the air which the engine compresses for greater volumetric efficiency, in effect replacing an intercooler. Alternatively, it can be used to lower the temperature of the exhaust.keroseneintercooler

Natural Gas Processing A-Upstream (Field) processes in natural gas processing: It consists of: Purification (Slug removal) Mercury removal Sweetening(de-souring) Dehydration (absorption, adsorption or injection) Compressing

B-Downstream process (NGL plants): It consists of: This operation varied from liquefied the gas to separate the heavy components in a form could be stored and transferred easily or liquefied the light components in the gas to transfer the liquid via tankers then degasification again in another place The main products of natural gas processing: Petrochemical feed stock (C2 & C3 mixture): Propane as commercial product. LPG Finally, condensate which is the heaviest product plus sales gas which is the exhausted gas after processing (sales or residue gas).

A-Upstream (Field) processes in natural gas processing: 1- Purification: The main objectives of the process are: 1) Remove entrained solids (mud, and sand), which are normally present with the gas as natural surround in the well. 2) Separation of the gas from liquids such as crude oil, in case of associated gas. 3) Separation of the gas from liquids such as, hydrocarbon condensate 4) Separation of the gas from liquids such as free water accompanied present originally in the well or from water injection to left the gas from the naturally in well.

Removal of entrained solids (mud, and sand This operation performed using sand removal units and slug catchers. Desander is drilling equipment with a set of hydro- cyclones that separate sand and abrasive solids from the drilling fluid. The fluid is passed through the desander vessel which incorporates a cyclone to remove the solid particles by means of the centrifugal force whilst the water and condensate are carried on.

Why dust removal is necessary?  To prevent erosion of pipes and metallic equipments and prevents plugging of small orifices in various controlling and process equipment.

Separation of the gas from liquids Separation of oil and gas is a critical field processing operation. Substances to be separated are crude oil, water and gas.  Crude Oil: this is a complex mixture of hydrocarbons produced from the reservoir in liquid form. Its density usually ranges from around 40 lb/ft3 to 55 lb/ft3.  Natural Gas: which is associated with an oil accumulation may be termed free gas or dissolved gas.  Water: water produced with oil or gas may be in the form of liquid or vapour. The liquid water may also be in the form of free water or it may be emulsified in the oil.

 According to the form of water (either free or emulsified) the separation processes can be classified into:  Three phase separation [if water is present in the form of free water],  Two phase separation [if water is emulsified with the oil] In three phase separation, however, in addition to the removal of gas from liquids, the oil and water are separated from each other.

Principles of separation: The objective of ideal separation is to separate the hydrocarbon stream into liquid-free gas and gas-free- liquid. For the separator to perform this function there are two necessary factors, namely; 1. The fluids to be separated must be immiscible 2. Difference in density must be exist (one fluid must be lighter than the other).

Factors affecting separation: The factors that affect the operation and separation between the oil and gas phases in a separator are: A. Fluid stream composition. B. Separator operating pressure. C. Separator operating temperature

Acid Gas Removal Sweetening(de-souring)  Natural gas usually contains some impurities such as H2S, CO2, CS2, COS and heavy hydrocarbons such as RSH.  These compounds are known as acid gases because in the presence of water it gives rise to an acidic aqueous solution.  Natural gas with H2S or other sulfur compounds (such as COS, CS2 and mercaptans) is called “sour gas,’’ whereas gas with only CO2 is called “sweet gas’’.

The presence of these compounds in natural gas is objectionable because: 1. S causes corrosion problem (cause metal embrittlment) 2. CO2 reduces calorific value of natural gas 3. CO2 cause dry ice formation during liquefaction To prepare the gas with the required specifications for either transportation (field) or processing (plant). The reasons for acid gases removal are: 1. Safety and environmental protection 2. Product specification 3. Corrosion prevention 4. Improved heating value

Factors that must be considered an acid gas removal process: 1.Type of impurities to be removed 2. Inlet and outlet acid gas concentrations 3. Gas flow rate and conditions (T and P) 4. CO2/H2S ratio 5. Feasibility of S recovery 6. Relative economic