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Crude Oil Foams Testing and Ranking of Antifoams with the Depressurization Test Christian Blázquez, Christine Dalmazzone, Eliane Emond, and Sophie Schneider Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02567 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017
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Crude Oil Foams: Testing and Ranking of Antifoams with the Depressurization Test Christian Blázquez, Christine Dalmazzone1*, Eliane Emond2, Sophie Schneider2 1
IFP Energies nouvelles, 1-4 avenue de Bois-Préau, 92852 Rueil-Malmaison Cedex – France
2
Bluestar Silicones, 55 rue des Frères Perret, 69192 Saint-Fons – France
e-mail:
[email protected] –
[email protected] *–
[email protected] –
[email protected] * Corresponding author Abstract: The addition of chemicals is the most widely applied solution to prevent the formation of foam or to destroy it immediately after its generation, due to its simplicity and efficiency. Among the different chemicals that can be used as antifoams or defoamers, PDMS (polydimethylsiloxanes) are the most common followed by fluorosilicones for the most severe cases. Nonetheless, there is no clear management on the selection of these additives so it is still based on a trial-and-error basis. For this reason, we have studied the properties and effectiveness of different chemical additives by defining two parameters based upon the logistic model developed for the study of the defoaming kinetics of crude oils foams formed by depressurization: the effect on the foamability or Antifoamability Effect (AE) and the effect on foam stability or Destabilization Effect (DE). Finally, we have tried to go further in the understanding of the mechanisms involved in foam breaking looking for similitudes on the defoaming behavior in the different crude oils tested. Keywords Crude oil foam; antifoam; defoaming rate; PDMS
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Introduction Dealing with unwanted foams during crude oil exploitation is a major issue. Big efforts have been carried out to prevent or to destroy foams formed in the gas-liquid separator as the presence of foam reduces the production capacity. Furthermore, the presence of foam may damage the separation unit and the equipment downstream, leading to a shutting down of the refining process causing a big economic loss.1-3 Several techniques have been developed to prevent foam formation or to improve its breaking up, such as spraying water onto the surface, mechanical breaking, increasing of the operational temperature, removal of the foamy agents or using specially designed units.2, 4 However, the most common method for foam control is still the addition of chemicals because of its ease of use as well as its effectiveness.5-11 In this case, special chemical agents are added to prevent foam formation (antifoamers) or to destroy the foam already created (defoamers). Industrial experience from the oil industry has shown that among the different molecules that can be used as antifoaming or defoaming agents, silicone oils are the most effective ones. Actually, polydimethylsiloxane oils (PDMS) are the most common antifoam agents used in the oil industry followed by fluorosilicone oils for the most severe cases.7, 12-14 Actually, some recent works are focused on the evaluation of these chemicals as antifoams in crude oil samples.15-17 Nevertheless, there are other kinds of molecules that can be used as antifoams, principally phosphate esters, metallic soaps of fatty acids, sulphonate compounds, amides, polyglycols, glycol ethers and alcohols. These molecules usually exhibit less antifoaming effect than silicone oils but they biodegrade faster than PDMS. In addition, their impact on the refining crude oil catalytic processes is negligible, contrary to silicone and silica which are usually catalytic poisons. For instance, several nonsilicone agents have been tested in a heavy crude oil from North Alberta (diluted with 10% heptane),
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showing that a sulphosuccinate of sodium and a dispersion of long-chain fatty alcohols in water have a significant antifoaming effect.11 In spite of the efforts carried out to understand the non-aqueous foams breaking mechanism, the selection of industrial antifoaming agents is still based on a trial-and-error basis with no clear management. There are other considerations to take into account apart from the chemical structure of the surfactant: the solubility of the antifoaming agent, the presence of solids or gels onto the gas-liquid interface, the oil and gas composition and properties, the operating conditions (thermal stability of the agent), or the impact that the agent may have in the post-treatment of crude oil. It is noteworthy that most of the literature on defoaming and antifoaming chemicals is focused on the breaking of aqueous foams. So the theoretical developments concerning the mechanisms of defoaming were mainly done on these systems.18 Two different mechanisms have been proposed for aqueous foam breaking by non-soluble oils: fluid entrainment or pinch-off.5, 6 In the first one, an antifoam droplet spreads onto the gas-liquid surface entraining the subjacent fluid with it and, by this way, creating a local film thinning and rupture. On the other hand, in the second mechanism, an antifoam droplet bridges the interface creating a capillary instability which destroys the liquid film. In both cases, the first condition needed is the entering of the antifoam droplet into the film lamellae. The definitions of the entering coefficient (Ea/i), the spreading coefficient (Sa/i) and the bridging coefficient (Ba/i) have been made based on thermodynamic criteria for capillary stability of the foam lamellae as a function of the different surface energies in the system:
Ea / i = σ w / g + σ w / a − σ a / g
(1)
Sa /i = σ w/ g − σ w/ a − σ a / g
(2)
Ba / i = σ w2 / g + σ w2 / a − σ a2 / g
(3)
Where σ is the surface tension and the subscripts w, g and a refer to water, gas and antifoam respectively. On the other hand, Denkov differentiates between two types of antifoams: “fast” and 3 ACS Paragon Plus Environment
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“slow”.19 The main property which differentiates between both types is the barrier to drop entry. This entry barrier has been defined as a function of the critical capillary pressure or the disjoining pressure6, 1921
where the threshold value of the entry barrier is somewhere between 15 and 20 Pa, which separates the
fast antifoam region from the slow one. These mechanisms have been developed for three immiscible phases, i.e. water, gas and an oily phase containing the antifoam. However, having positive values of these coefficients does not guarantee foam destruction but they provide a basis for using these types of antifoams. Regarding crude oil foams, other considerations must be taken into account. First of all, the antifoam is usually injected dissolved or dispersed in an organic phase instead of an aqueous one so it is hard to predict the aggregation state of the antifoams and thus the validity of these mechanisms. Furthermore, the solubility of the antifoaming agent, the presence of solids or gels onto the gas/liquid interface or the effect of the additive on downstream processes are also important to the selection of the additive. However, some authors have hypothesized that antifoams should work by spreading onto the gas-liquid interface.11 As highlighted previously, the selection of additives and the prediction of foam behavior after chemicals addition is quite complex and no mechanism has been yet clearly established for its understanding. The scope of this work was to develop a methodology for the study and ranking of chemical additives. This methodology is mainly based on the depressurization foaming test that has been presented and thoroughly described in a previous paper.22 Furthermore, we propose a new way to study and to compare the effect of the antifoaming chemical additives in terms of impact on both foamability and foam stability.
Materials and Methods Fluids selection. Four crude oils with different API degrees have been chosen to perform antifoaming tests. Their properties are summarized in Table 1. In all cases, crude oils were dehydrated (100%) to 0%. This parameter A’1, which represents the ratio between the maximum foam volume that it is possible to create in the tests conditions and the total volume of liquid in the system (A’1= Vf0/Vlinf ) can be considered as a Foamability Index. On the other hand, parameter A’2 results from the multiplication of A2 by A’1.
Vf V
0 f
Vf Vl
inf
=
1 − A2 t 1 + t0
=
+ A2
p
A '1 − A ' 2 t 1 + t0
p
+ A'2
(4)
(5)
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Antifoamability Effect and Destabilizing Effect definitions. We have defined two criteria based on Equations 4 and 5 parameters to simplify the comparison between the different chemical additives in terms of effect on the foamability (i.e., the effect on the initial foam volume) and foam stability (i.e., the effect on the foam volume after a certain period of time). The antifoamability effect (AE) can be defined as the capacity of a surfactant to prevent foam formation or to reduce the amount of foam created. In our case, we have related this property to the Foamability Index, defined as the maximum amount of foam that the system can create at the test conditions. Mathematically, it can be expressed as shown in Equation 6:
AE =
A'1,with antifoam − A'1,without A'1,without
antifoam
x100
(6)
antifoam
This new index is expressed as a percentage and shows the reduction rate of the foam created compared to the oil system without additive. So antifoamers can be easily classified according to their effectiveness: •
If AE>0, then the chemical shows a foaming effect.
•
If AE=0, then the chemical has no effect.
•
If AE
