Wax precipitation temperature

The practical range of operating temperatures is limited on the high side (about 150° F.) by the high vapor pressure of propane and on the low side by the tendency for emulsion formation as wax precipitates or possibly as phenol freezes as in high phenol content Selecto. Operation between these limits provides no difficulty because the solvent power of Selecto can be altered by changing the ratio of phenol to cresylic acid. 

Pedersen, K.S. Prediction of cloud point temperatures and amount of wax precipitation. SPE Prod, and Fac. 1995, 46. 

The wax appearance point is the temperature at which wax begins to precipitate (hence it is also called the wax precipitation point) from an oil under specified cooling conditions. Although more applicable to distillate fuel oil, the wax appearance point can also have implications for mineral oil use. In... 

Ronningsen, H. P., Bjorndal, B., Hansen, A. B., Pedersen, W. B. (1991). Wax precipitation from North Sea crude oils. 1. Crystallization and dissolution temperature, and Newtonian and non-Newtonian flow properties. Energy and Fuels. 5 895-908. 

Wax and asphaltene precipitation are serious problems in production from some hydrocarbon reservoirs. Wax, which is a solid precipitate, can occur in the well, in the production facilities, and in the pipelines. Ther( is a field case in which wax has been reported in a natural state in the reservoir. Wax precipitation can occur for gas-condensate, light-oil and hoavy-oil fluids at temperatures as high as 150 F. Asphaltene precipitation may occur in the reservoir, in the production facilities, and in the pipelines. Asphaltenes may precipitate from some light oils, but there may be no precipitation from, some heavy oils. Asphaltene precipitation has not been reported from gas condensate reservoirs. 

Wax and asphaltene precipitation are old problems, but only recently have attempts been made to develop a thermodynamic description for these processes. The asphaltene thermod3mamic models have just begun to evolve. In this chapter, we will first present the thermodynamics of wax precipitation and discuss the effects of pressure, temperature, and composition. Then we will present a thermodynamic micellization model for asphaltene precipitation, and study the pressure and composition effects. All the calculations for the equilibrium between the precipitated phase and the crude oil will be based on direct minimization of the Gibbs free energy of the total system. 

Components in a gas-condensate fluid contain hydrocarbons from methane, C, ethane, C2, and other hydrocarbons as heavy as or Qo or even heavier. Reservoir crudes may contain hydrocarbons as heavy as C oo- At room temperature (75T) and atmospheric pressure, Cl, C2, C3, and C4 are in the gas state, nC to nC- are in the liquid state, and normal alkanes heavier than nC- are in the solid state. The broad volatility and melting-point range of these hydrocarbon components found in petroleum fluids cause formations of gas, liquid, and solid phases in response to changes in pressure, temperature, or composition. Let us consider a mixture of two hydrocarbons—nC and nC. The melting-point temperature of nC. is 57" C at atmospheric pressure. The solubility of n Cqs in nC at atmospheric pressure is 0.5 mole percent at 14"C. At 40"C and atmospheric pressure, the solubility of nC2R in increases to 12 mole percent. [it is therefore natural that when the temperature falls, heavy hydrocarbons in a crude or even a gas condensate may precipitate as wax crystals. In the petroleum industry, wax precipitation is undesirable because it may plug the pipeline and processing equipment.)... 

Here, we provide both the equilibrium and the material balance equations for wax precipitation calculations for solid-liquid equilibria. At fixed temperature and pressure, for every component i, the multisolid-phase model must satisfy... 

Effect of pressure on wax precipitation. Pressure increase causes the cloudpoint temperature (which is the temperature at which wax first begins to precipitate) to increase for a liquid mixture of fixed composition. 

Figure 5.7 shows the effect of pressure on the cloudpoint temperature for three different stock tank oils. Note that these results are based on calculations from the multi solid model. For all three oils, the CPT increases as pressure increases. However, oil 2 is more sensitive to pressure than the other two oils. As we will see in Problems 5.7 and 5.8, the effect of pressure on wax precipitation in gases is different than in liquids. 

Effect of composition on wax precipitation. Wax precipitation is often measured using the stock-tank oil at atmospheric pressure. When some gas is dissolved into the crude oil, the cloudpoint temperature may decrease, which is desirable. Low cloudpoint temperature implies that one may be less concerned about wax precipitation. 

It was stated earlier that wax and asphaltene precipitation are different processes the effects of composition, pressure, and temperature on wax precipitation can be in the opposite direction to those on asphaltene precipitation. As an example, an increase in pressure often increases the CPT in a crude oil that is, pressure increase enhances wax precipitation from crude oils (see Fig. 5.7). On the other hand, as... 

Wax precipitation is strongly effected by temperature temperature may weakly effect asphaltene precipitation, and may enhance or inhibit it. The composition effect is also very different on wax and on asphaltene precipitation. An increase in concentration of light hydrocarbons such as C3 and nC and nonhydrocarbons such as CO2 decreases the CPT (see Fig. 5.24, to be discussed later). On the other hand, an increase... 

Example 5.3 Wax precipitation from natural gases Consider a natural-gas fluid of composition shown in Table 5.9 at P = 563 bar and T = 338 K, where the fluid is in the gaseous state (Ungerer et al., 1995), Use the multisol id-wax model to calculate the temperature at which the solid precipitates at 563 bar. Ungerer et al, measured the phase transition from gas to (gas+solid) at a temperature of 62,0 C at constant pressure of 563 bar. Also calculate the amount of the precipitated solid vs. temperature at P — 563 bar. 

Effect of pressure on CPT for gases UnliKc liquids where a pressure increase often enhances the wax precipitation at constant temperature, for gases pressure has often the opposite effect pre.s.> ure increase may inhibit the precipitation at constant temperature. The solubility of nC- s in CO2 at 308 K. and the solubility of nC2o in Cj are provided in Tables 5.11 and 5.12 below. First derive the expression... 

Ronningsen, H.P., B. Bjorndal, A.B. Hansen, and W.B. Pedersen Wax Precipitation from North Sea Crude Oils. 1. Crystallization and Dissolution Temperatures, and Newtonian and Non-Newtonian Flow R operties, Energy Fuels, vol. 5,895,1991. 

Pedersen, W.B. Hansen, A.B. Larsen, E. Nielsen, A.B. and Ronningsen, H.P. (1991b). Wax Precipitation from North Sea Crude Oils. 2. Solid-Phase Content as a Fimction of Temperature. Energy Fuels, Vol.5, No.6, pp.908-913, ISSN 0887-0624 Radulescu, A. Schwahn, D. Richter, D. Fetters, L.J. (2003). Co-Crystallization of Poly(Ethylene-Butene) Copolymers and Paraffin Molecules in Decane Studied with Small-Angle Neutron Scattering. Journal of Applied Crystallography, Vol.36, No.4, pp.995-999, ISSN 0021-8898... 

When the high Mw PEB-7.5 (30 K) copolymer is mixed with C36 wax in decane, almost the same morphological and structural features are observed as in the case of low Mw copolymer and C24 wax [14]. The only difference is that, according to the elevated precipitation temperature of both components [12,13], the joint wax-copolymer correlated platelets occur at much... 

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