Wax Deposition of Diesel Oil and Consequent Contamination of Gasoline in Sequential Transportation of Product Oil Pipeline

Abstract

Wax deposition of diesel oil and contamination of gasoline by diesel wax deposit is a severe problem in sequential transportation of product oil in pipes. However, it has long been neglected by the pipeline transportation industry. In response, this work aims to present a unique perspective on wax deposition of diesel oil and consequent contamination of gasoline. A cold finger apparatus was designed and constructed. Model oil composed of diesel and refined wax was prepared for wax deposition. Shear dispersion was excluded for wax deposition of diesel oil. Moreover, dissolution experiments of diesel wax deposit in gasoline were conducted. It was found that the dissolution rate increases with oil temperature and decreases with cold finger temperature and shear stress. Analysis on gasoline quality after diesel wax deposit dissolution indicated that contamination of gasoline caused by diesel wax deposit severely deteriorates its distillation range, but the octane number remains acceptable. This work features an interesting investigation on the wax-deposition mechanism of diesel oil, dissolution characteristics of diesel wax deposit, as well as quality deterioration of subsequent gasoline. It would be helpful in scheduling a product oil-transportation program.

1. Introduction

Energy is among the most significant driving forces for socioeconomic development. For the petroleum industry, the geographical disparity between refineries and consumption markets necessitates long-distance transportation to address the imbalance between petroleum resource allocation and sales demand [1]. Transportation of product oil is often carried out through sequential delivery in pipes, where different types of products are transported continuously in the same pipeline in a specific order [2], forming various operational modes such as diesel pushing gasoline or gasoline pushing diesel. Diesel has a higher wax content compared with gasoline. In pipeline transportation, when ambient temperature falls below the wax appearance temperature of diesel, wax components tend to crystallize, precipitate, and adhere to the pipe wall. Subsequent gasoline continuously flushes and dissolves the deposited wax, resulting in a significant increase in mixed oil, degradation of gasoline quality, and substantial economic loss. This problem is particularly severe in cold winter, where the large temperature difference between the oil and pipe wall exacerbates the wax deposition of diesel. For example, a product oil pipeline in northeast China experienced two severe incidents of diesel wax deposition within a short period of time. When diesel with a cold filter plugging point of 8 °C was transported at ambient temperature ranging from 4 °C to 8 °C, significant wax deposition occurred in the pipeline. The mixed oil increased from a normal value of 50~60 tons to 2433 tons and 1994 tons, respectively, and the time required for cutting the mixed oil extended from the usual 10–15 min to over 7 h. Therefore, contamination of subsequent gasoline by diesel wax deposit is an important problem in sequential delivery of oil products.

However, most works on wax deposition are mainly focused on crude oil. Molecular diffusion is the most widely accepted wax-deposition mechanism [3,4,5]. When pipe wall temperature drops below wax appearance temperature of crude oil, the radial solubility difference induced by the temperature gradient between the oil and the pipe wall drives wax molecules to diffuse and deposit onto the wall [6,7]. Fasano et al. [8] proposed that shear dispersion is also responsible for wax deposition. However, Huang et al. [9] conducted flow loop experiments and found that wax deposition is not affected by shear dispersion. Moreover, mechanisms such as Brownian motion [10] and gravity settling [11] were also proposed to explain wax deposition. Currently, most wax-deposition models of crude oil are based on molecular diffusion [12,13,14,15,16]. The applicability of these models to diesel wax deposition remains uncertain.

Compared with crude oil, wax deposition of diesel in pipeline transportation draws much less attention. Yan et al. [17] conducted a preliminary investigation on wax deposition of diesel oil. A certain amount of diesel was loaded into a beaker for wax deposition. The beaker was placed in a thermal water bath for temperature manipulation. It was found that when the oil was agitated, the mass of deposited wax was less than that in static condition, which proves that shear of oil flow restrains wax deposition of diesel oil. This corresponds well with the crude oil situation. However, lack of temperature gradient between oil and pipe wall makes the beaker experiments far from real pipeline transportation. Moreover, Yan et al. [17] also conducted wax-dissolution experiments with diesel wax deposit and found that it could be quickly dissolved by gasoline at low temperatures. This explains the severe contamination accidents of gasoline by diesel wax deposit in northeast China. Studies were also conducted to understand wax deposition regarding diesel engine performance. Coutinho et al. [18,19] proposed a thermodynamical model to describe the phase behaviour of different diesels. Pauly et al. [20] analyzed the impact of high pressure in a diesel engine on wax formation of diesel fuel. These works present some interesting perspectives on diesel wax deposition, but the key concerns for the pipeline transportation industry including wax deposit dissolution in subsequent gasoline and consequent gasoline contamination are scarcely involved.

Given the severity of diesel wax deposition in sequential transportation of product oil pipeline, this paper aims to investigate wax deposition of diesel oil and consequent contamination of gasoline. A cold finger apparatus was built and the COMSOL Multiphysics 5.6 was used to optimize its temperature distribution. The roles of molecular diffusion and shear dispersion in diesel wax deposition were explored. The impacts of gasoline temperature, cold finger temperature, and shear stress near the pipe wall on wax deposit mass were clarified. Deterioration of some key indexes of gasoline quality including distillation range and octane number when contaminated by diesel wax deposit was also investigated.

2. Experimental

2.1. Materials

Model oil composed of diesel and refined wax was prepared for wax deposition. Certain amount of diesel was initially heated to 60 °C in a beaker by a thermal water bath. Then refined wax was carefully weighed, sliced into thin pieces, and added into the preheated diesel. An electric stirrer was used to agitate the system for another 30 min at this temperature to guarantee a complete dissolution of wax. Weight ratios of wax added to the solution included 2%, 3%, and 4%.

2.2. Cold Finger Apparatus

A cold finger apparatus was built for wax deposition of diesel oil in this work. Ascribing to oil viscosity, fluid near the cold finger surface presents flow patterns in good agreement with actual oil flow in a pipeline, which overcomes the shortage of azimuthal rotation of the stirrer in cold finger experiments. The oil tank was double-layered with a circular water jacket for temperature manipulation. Inside the jacket, a spiral guide groove was employed to ensure an even temperature distribution, which will be discussed in detail in the next section. The height and inside diameter of the oil tank were 24 cm and 16 cm, respectively, which gave an oil volume of around 4.8 L. The cold finger was a hollow cylindrical structure with an inlet conduit extending to the bottom and the outlet was located at the top. The surface temperature of the cold finger was controlled by another thermal water bath to create a temperature gradient for wax deposition. Inside and outside diameters of the cold finger were 1.4 cm and 5.4 cm, respectively, with height being 18 cm. An electric stirrer was used to agitate the model oil to simulate oil flow in pipeline transportation. The stirring paddle was an anchor type with blade diameter and height both being 12 cm. The diameter of the stirring rod was 0.8 cm, which allowed it to pass through the cold finger. A schematic diagram of the cold finger apparatus is showcased in Figure 1.

During experimentation, oil temperature was controlled by circulating water through the water jacket on the oil tank. When the water jacket was hollow with nothing, temperature was unevenly distributed. To improve the uniformity of the temperature distribution on the inner wall of oil tank, deflector plates and spiral guide groove were designed to regulate water flow and its temperature distribution was simulated.

COMSOL Multiphysics 5.6 was used to obtain the temperature distribution of the water jacket. Water flow was simulated using the turbulence module [22]. According to the actual situation, inlet flow velocity was set to 1.27 m/s and temperature to 40 °C. The outlet condition was specified as a pressure outlet. The influence of gravity on water flow was considered. The ambient temperature was set to 20 °C. The k-epsilon model was employed for turbulence simulation. The computational domain was discretized into unstructured meshes with a total number of 124,878. Mesh independence verification was conducted to ensure an optimal balance between computational accuracy and speed.

Three cases of temperature distributions inside the oil tank were simulated: hollow, with deflector plates and with spiral guide grooves. The results are given in Figure 2. It can be seen that temperature distribution of the spiral guide grooves case was more uniform when compared with the other two cases. The maximum temperature difference was only 0.3 °C, which helps to enhance the accuracy of oil temperature manipulation.

2.3. Test Procedure

Both wax deposition and gasoline-dissolving experiments were conducted. After loading the prepared model oil into the oil tank, it was kept agitating for 12 h and the wax deposit was obtained on the cold finger surface. Then wax deposit was collected and weighed for further analysis.

For gasoline-dissolving experiments, wax deposition was the first step to obtain wax deposit. Then the cold finger covered by the deposited wax was placed into a beaker with gasoline, whose temperature was controlled by a thermal water bath. The amount of dissolved wax was determined by weighing the beaker after the desired dissolving duration. Agitation was required to simulate oil flow in the pipeline.

3. Results and Discussion

3.1. Repetitive Experiments

Repetitive experiments were conducted under different conditions, which are summarized in

Table 1. Experimental conditions for repetitive experiments $\Delta T$.

Oil Temperature To (°C)	Cold Finger Temperature Tc (°C)	Temperature Difference (°C)	Agitating Speed V (rpm)	Wax Content M (wt%)
5	0	5	140	2
15	10	5	70	4
10	−5	15	210	3

. After wax deposition, wax deposit mass was carefully weighed and the deviation was found to be within ±0.75 g (see Figure 3), which illustrates the repeatability and reliability of the cold finger apparatus and test procedures.

3.2. Wax-Deposition Mechanism of Diesel Oil

For crude oil, molecular diffusion has been widely recognized as the primary wax-deposition mechanism and the effect of shear dispersion remains controversial [23,24]. To check the applicability of shear dispersion to wax deposition of diesel oil, experiments with and without temperature gradient and agitation were conducted. Images of cold fingers after experimentation are shown in Figure 4.

Figure 4a presents an experiment conducted with a temperature gradient in static condition, which means that molecular diffusion works while shear dispersion plays no role in this situation. Observation of wax deposit on a cold finger surface illustrates that molecular diffusion is responsible for wax deposition of diesel oil. To investigate the role of shear dispersion, we conducted an experiment on the occurrence of agitation while keeping oil temperature and cold finger surface temperature the same, which implies there is no temperature gradient in the experiment. In Figure 4b, we hardly see any wax deposit on the cold finger surface, which proves that shear dispersion is not a key contributor for wax deposition of diesel oil. Moreover, when temperature gradient and agitation both occur, a compact and smooth wax deposit was observed; see Figure 4c. This is because shear stress near the pipe wall scours the loose and soft wax deposit; thus, only the hard part remains on the surface. To conclude, shear dispersion is not a determinant factor that determines whether wax deposition of diesel oil occurs or not, but once a wax deposit forms, shear dispersion dramatically impacts its physical properties due to the scouring effect.

3.3. Dissolution of Diesel Wax Deposit in Gasoline

3.3.1. Impact of Gasoline Temperature

The effect of oil temperature on the wax dissolution rate in gasoline was investigated. Gasoline temperature was set to 11 °C, 14 °C and 17 °C, respectively. Stirring speed was set to 70 rpm and cold finger temperature to 5 °C. The result is given in Figure 5. Obviously, higher gasoline temperature facilitates wax deposit dissolution. On the one hand, higher gasoline temperature means larger solubility of diesel wax deposit in gasoline. On the other, according to the molecular diffusion mechanism, an increased temperature gradient enhances migration of wax molecules into gasoline, resulting in an increased wax-dissolution rate. Moreover, we found a decreasing trend of wax deposit-dissolution rate over time. This may be caused by the fact that gasoline becomes saturated with solubilized wax over time, which decreases the driving force (concentration difference) for dissolution and molecular diffusion.

3.3.2. Impact of Cold Finger Temperature

Wax-dissolution rates in gasoline at different cold finger temperatures are showcased in Figure 6. Oil temperature and stirring speed were maintained at 10 °C and 70 rpm, respectively. Larger wax-dissolution rate was observed at lower cold finger temperature. Similar to the impact of gasoline temperature, wax molecules are more likely to migrate into gasoline at larger temperature gradients created by low cold finger temperature. Furthermore, in wax deposition, more light wax components would precipitate and deposit onto the pipe wall [25]. These components more easily dissolve in gasoline. Therefore, lower cold finger temperature brought about larger wax-dissolution rate in gasoline.

3.3.3. Impact of Shear Stress

Since shear stress is positively correlated to stirring speed, we set different stirring speeds to investigate the impact of shear stress on wax dissolution in this work. Stirring speeds of 70 rpm, 140 rpm and 210 rpm were employed. Cold finger temperature and gasoline temperature were 0 °C and 10 °C, respectively. Judging intuitively, we would assume that large shear stress triggered by high stirring speed would enhance wax dissolution. However, somewhat strangely, wax dissolution was weakened by increasing stirring speeds in experiments; see Figure 7. This is because, in wax deposition, stronger shear stress scours the soft and light wax components off the cold finger surface; thus, hard and tough wax with large carbon number was left. The solubility of the heavy components is small in gasoline and they are hard to dissolve. Therefore, the dissolution rate of the wax deposit of diesel oil decreases at higher stirring speeds.

3.3.4. Deterioration of Gasoline Quality by Diesel Wax Deposit

In sequential transportation of product oil, gasoline easily becomes contaminated by wax deposit of diesel oil, especially in winter in high-latitude areas. In this work, we take the distillation range and octane number as key indexes of gasoline quality to investigate the contamination of gasoline by diesel wax deposit. The deposited wax was collected for wax-deposition experiments of diesel oil, then added into gasoline with weight contents of 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt% and 10 wt%, respectively.

3.3.5. Distillation Range

Variations in the distillation range of the gasoline against diesel wax deposit content are given in Figure 8. It can be seen that 10% distillation temperature fluctuates around 55–65 °C and remains below 70 °C. This demonstrates that igniting performance of automobile engine is not dramatically affected by dissolution of diesel wax deposit in gasoline. A 50% distillation temperature reflects the volatility of gasoline and greatly affects the accelerating ability of an automobile. From Figure 8, we see that 50% distillation temperature of contaminated gasoline non-monotonically varies against the amount of dissolved wax deposit of diesel oil. It reaches the minimum (100 °C) at 6 wt% of wax deposit content. When wax deposit content increases to 10 wt%, the 50% distillation temperature is 113.5 °C, which is higher than the upper limit of 110 °C according to the Chinese standard (gasoline for motor vehicles, GB 17930-2016 [26]); thus, gasoline is no longer acceptable. As for the 90% distillation temperature, it is positively correlated with content of dissolved diesel wax deposit. At 10 wt% of wax deposit, it reaches 209.5 °C, which is way higher than the acceptable level (lower than 190 °C). This means excessive heavy components from diesel wax deposit cause gasoline quality to severely deteriorate.

The dry point is the highest temperature of the whole distillation process, it reflects the content of heavy components in gasoline. From Figure 8, we see that the dry point of the gasoline is very sensitive to dissolution of the diesel wax deposit. A handful of wax deposits at 2 wt% disqualifies the dry point of the gasoline. Therefore, contamination of diesel wax deposits on gasoline cannot be ignored. A quadratic relationship can be fitted to describe the relationship between the dry point of the gasoline and the content of the dissolved diesel wax deposit.

$$T=-0.52m^2+14.95m+203$$

(1)

where T is the dry point of the gasoline, °C; m is the weight content of the diesel wax deposit.

When the weight content of the dissolved diesel wax deposit reaches 0.14 wt%, the dry point of the gasoline becomes 205 °C, which no longer meets the qualified specifications of gasoline according to the Chinese standard (Gasoline for Motor Vehicles GB 17930-2016).

3.3.6. Octane Number

Octane number is one of the most important properties of gasoline as it determines the antiknock performance. Indeed, gasoline is divided into different brands based on octane number. As shown in Figure 9, when 0–10 wt% of diesel wax deposit was added into 95# gasoline, its octane number remained above 95, which demonstrates that the antiknock performance of gasoline is not greatly affected when limited diesel wax deposit is dissolved.

4. Conclusions

Wax deposition of diesel is a severe flow-assurance challenge faced in pipeline transportation of product oil. This paper systematically investigates wax deposition of diesel oil and contamination of gasoline by diesel wax deposit in sequential transportation of product oil pipeline. A cold finger apparatus was designed and constructed. Spiral guide grooves were employed to improve uniformity of temperature distribution of the oil tank. Repetitive experiments were conducted to illustrate the repeatability and reliability of the experimental setup and test procedure. Experiments with and without temperature gradient and agitation showed that shear dispersion is not responsible for the formation of wax deposit of diesel oil. Moreover, higher oil temperature and lower cold finger temperature facilitate diesel wax deposit dissolution in gasoline and shear stress plays an opposite role. Deterioration of gasoline quality by diesel wax deposit was also investigated. The distillation range of gasoline was dramatically affected by diesel wax deposit. An empirical model was developed to describe the relationship between the dry point of a gasoline and the content of the dissolved diesel wax deposit. The octane number of the gasoline was not greatly affected when a limited amount of diesel wax deposit was dissolved.