Shen Zhibing; Zhang Juntao; Zhang Jie; Liang Shengrong

(School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065)

The Caustic Alkali-free Water Extraction Agents for Treating Inner Mongolia Oil Sands

Shen Zhibing; Zhang Juntao; Zhang Jie; Liang Shengrong

(School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065)

The caustic alkali-free water extraction agents were studied for treating the oil sands excavated from Inner Mongolia, China. Several kinds of chemical reagents were evaluated, among which sodium carbonate (SC), sodium dodecyl benzene sulfonate (SD) and sodium chloride were confirmed as composite solutes. Their proportion was optimized by an orthogonal test. The optimum proportion of the composite agent covered 0.03% of SD, 0.50% of sodium chloride, 3.00% of SC, with the rest composed of water. The optimal operating condition was also confirmed. The oil sands were extracted at the following optimized conditions: a treating time of 15 min, a temperature of 80 ℃ and an extraction agent/feed ratio of 1:1, with the bitumen yield reaching more than 96%. The extraction agent after separation from the bitumen product can be recycled for reuse to carve out a good environmentally friendly route.

oil sands, extraction agents, bitumen, orthogonal experimental design

1 Introduction

Oil sands, tar sands or, more technically, bituminous sands, are a mixture of sand, water, clay and bitumen, which represents a type of unconventional petroleum deposits. Bitumen is a kind of oil that is too heavy or too thick to flow or to be pumped without being diluted or heated. The estimated worldwide deposits of oil equivalent in oil sands are more than 2 trillion barrels (320 billion cubic meters), most of which are found in Canada, America, Venezuela and Russia. In recent years, the latest reports on Chinese resource evaluation showed that the Chinese deposits of bituminous oil sands reach up to 5.97 billion tons, and the exploitable reserves are equal to 2.258 billion tons[1]. Therefore, the exploitation and processing of oil sands reserves are quite important to the energy security of China.

Oil sands can be mined and processed to extract the oilrich bitumen, which is then refined to yield several petrochemical products. The extraction process applying hot sodium hydroxide solution has widely been studied[2-4]and has also been used in the large-scale commercial bitumen recovery process for Athabasca oil sands by the Syncrude Canada Ltd. and Suncor Inc.[5]However, sodium hydroxide can easily lead to the formation of caustic residue, which is not environmentally friendly and is difficult to be disposed of. In recent years, a series of environmentally friendly water extraction agents that contain low caustic alkali or are free from alkali have been attracting many researchers’ attention. Han, et al.[6]prepared some different series of YSFL agents with lower amount of sodium hydroxide. Their study results showed that the YSFL agents had better property compared to sodium hydroxide and the bitumen extraction rate could reach more than 95%. Liu, et al.[7]studied some kinds of surfactants (EL 110, SG20, Span 60 and Tween80), which were used to extract bitumen from oil sands, and their results showed that the ethanol solution with addition of 0.5% of Span 60 gave a bitumen yield of 93.3%.

In order to develop a kind of extraction agents for exploiting the oil sands excavated from Inner Mongolia of China, we dug into the chemical-flooding mechanism for enhanced oil recovery in the field of petroleum engineering and eventually chose sodium carbonate, sodium tripolyphosphate, sodium silicate and sodium dodecyl benzene sulfonate as main extraction agents. And then, by comparing the yields of bitumen obtained from differentextraction agents, the better agents were chosen to prepare a kind of composite agents and the optimized ratio of chemical reagents could be also confirmed by an orthogonal experiment. Finally, the optimum operating conditions of the composite reagents for extracting bitumen from oil sands and their property after recycling was investigated.

2 Experimental

2.1 Materials and chemicals

Oil sands samples were obtained from the oil sand reserve of Zhalaiteqi of Inner Mongolia, China. The composition of the samples collected from different blocks and depths of the reserve is presented in Table 1, denoting that oil sands of Zhalaiteqi have high oil content.

Table 1 Compositions of oil sands

All reagents used in the experiments are shown in Table 2.

Table 2 Description of reagents

2.2 Methods

2.2.1 Preparation of the extraction agents

The extraction agents were prepared using a certain concentration of sodium carbonate (SC), sodium silicate (SS), sodium tripolyphosphate (ST), sodium dodecyl benzene sulfonate (SD) solution, respectively. And then, by comparing the yield of bitumen extracted by the sodium hydroxide (SH) solution, the two kinds of better reagents were preferred to prepare a kind of high-efficiency and environmentally friendly extraction agent.

2.2.2 Separation method

Oil sands and the extraction agent were mixed according to a proper proportion, and then the mixture was put into the reactor, heated, and stirred. The separation process conditions should be controlled strictly to make the sands and bitumen in the oil sands separate thoroughly.

The yield of bitumen is calculated according to the following formula:

whereYstands for the yield of bitumen, andX1andX2represent the bitumen content in the oil sands and in the grain of sands after extraction, respectively. The bitumen content of oil sands is measured according to the way reported in the literature[8].

3 Results and Discussion

3.1 Extraction mechanism applicable to the caustic alkali-free reagents

The alkaline compounds in the extraction agents (for example sodium carbonate) could react on naphthenic acid, aliphatic acid or other acidic compounds to form surfactants (such as sodium aliphatate, etc.), according to the following equation:

This kind of anionic surfactants can decrease the interfacial tension and viscosity of bitumen, which can enhance the separation of bitumen from sand.

Figure 1 Mechanism for recovery of bitumen from oil sands

Dai, et al.[3]have disclosed the mechanism for recovery of bitumen from oil sands, as shown in Figure 1. This process experiences three stages, namely: (a) Bitumen droplets are covered with sand particles. (b) When NaOH is added, the oil/water interfacial tension is reduced andsand particles slide down and are detached from the bitumen droplets. (c) Bitumen droplets then float in the solution. Therefore, we chose preliminarily some kinds of basic salts instead of NaOH and then preferred two better agents to blend a kind of environmentally friendly extraction agent.

In addition, Xu, et al.[9]showed that sodium chloride could increase the electric double layers of compressible bitumen droplets, reduce the repulsion between bitumen droplets, contribute to the coalescence between bitumen droplets and increase the density difference between different phases, which could help bitumen droplets float and improve the extraction efficiency.

3.2 Selection of the extraction agents

Figure 2 depicts the properties of pure water and different agents used in extraction of oil sands. It can be seen from the test results that pure water is not able to extract bitumen from oil sands. After addition of the salts, the yields of bitumen are improved significantly, especially for the case of SC and SD. The performance on extracting bitumen by these two reagents is next to that of sodium hydroxide. Therefore, we chose SC and SD with addition of a small amount of sodium chloride for blending the composite agent.

To confirm the optimum proportion of the composite extraction agents, the orthogonal experiments on three factors at three levels were designed. Three kinds of chemicals (SC, SD and sodium chloride) comprised three factors and their three different contents covered three levels. All experiments were performed at a reagent/feed ratio of 1:1, a treating temperature of 80 ℃ and a reaction time of 30 min. The process and results of the orthogonal experiments are displayed in Table 3.

Figure 2 Properties of different extraction agents

The study results showed that the composite agents had better performance on the recovery of bitumen than mono-component solutions. Based on the orthogonal experiment and range analysis, the order of the factors could be obtained as follows: SD ＞ sodium chloride ＞ SC. The optimum proportion of the composite agent covered 0.03% of SD, 0.5% of sodium chloride and 3.0% of SC. The yield of bitumen reached up to 96% when the optimum extraction agent was adopted.

Table 3 Results of the orthogonal design

3.2 Operating conditions test

The separation efficiency of the extraction agent could be influenced by temperature, reaction time and the extraction agent/feed ratio. The optimum operating condition was confirmed by a series of experiments as shown below.

3.2.1 Temperature effect

The effect of temperature on the yield of bitumen was tested at an extraction agent/feed ratio of 1:1 and a treating time of 15 min. The results are shown in Figure 3.

At first, the bitumen yield was increased rapidly with an increasing temperature. And then, when the temperature increased beyond 80 ℃, the bitumen yield gradually leveled off to reach a stable value of around 96%. It can beseen that temperature had an important impact on the yield which is in agreement with the result of Dai, et al.[5]They have indicated that an increase in temperature not only reduces the viscosity of bitumen, but also changes the physicochemical property of the silica-water-bitumen system. They have discovered that the negative zeta potential of silica increases significantly with an increasing temperature. Therefore, the strength of the double-layer repulsive force between silica and bitumen is expected to increase, which can facilitate the recovery of bitumen from oil sands. Based on the conclusion, the repulsion between sand particles and bitumen might have soared to a peak value at 80 ℃. Therefore, when temperature increases beyond 80 ℃, the bitumen yield cannot be improved. Therefore, the optimum extraction temperature was chosen as 80 ℃.

Figure 3 Effect of the temperature on bitumen yield

3.2.2 Time effect

The effect of time on the yield of bitumen was tested at 80 ℃ and an extraction agent /feed ratio of 1:1. The results are depicted in Figure 4.

Figure 4 shows that the bitumen yield only reached 76% when the system was heated and stirred for 5 min. The extraction effect was weak mostly because the bitumen was combined closely with the sand particles[6]. As time continued to increase, the bitumen yield was obviously improved. When the time was extended to 15 min, the bitumen yield reached up to 95% and tended to remain at a stable value. Therefore, the optimized extraction time was chosen as 15 min.

3.2.3 Effect of the extraction agent/feed ratio

The mass ratio between the extraction agent and oil sands was also investigated at a temperature of 80 ℃ and an extraction time of 15 min. The results are displayed in Table 4. The data clearly showed that the yield of bitumen was improved with an increasing extraction agent/feed ratio. At the same time, the sand (or clay) content in the bitumen gradually decreased with increase in the extraction agent/feed ratio. Table 4 also shows that the bitumen yield became a constant when the extraction agent/feed ratio reached 1.5:1. Therefore, on account of the energy and water consumption, the better extraction agent/feed ratio was confirmed to be 1:1.

Figure 4 Effect of the time on bitumen yield

Table 4 Effect of the extraction agent/feed ratio on extraction efficiency

3.3 Recycle of the composite extraction agent

To investigate the reusable property of the optimum extraction agent at the optimized operating condition, the agent was re-used for five times and the results are shown in Table 5.

Table 5 Relation of the recycle times and the bitumen yield

Table 5 shows that the yield of bitumen was stable when the agent was used for two times. Beginning from thethird time of reuse, the bitumen yield began to decline, which might be caused by the consumption of alkaline chemicals in the extraction agent. Petroleum acids, silica and other minerals contained in the oil sands could react on alkaline chemicals, leading to a decreased concentration of the extraction agent. Hence, the fresh agent should be added into the regenerated agent periodically and quantitatively to maintain the extraction efficiency.

3.4 Properties of bitumen derived from oil sands

The properties of bitumen derived from oil sands (or oil sands bitumen) in Inner Mongolia were analyzed with the results presented in Table 6. The oil sands bitumen features high viscosity, high Conradson carbon residue and ash content, low hydrogen/carbon ratio and high asphaltenes and resins content. Therefore, this bitumen is not an ideal feed for FCC and hydrotreating processes, but a good raw material for producing road asphalt.

Table 6 Properties of the oil sands bitumen

4 Conclusions

The water extraction agents were studied for treating the oil sands excavated from Inner Mongolia in China. Some kinds of chemical reagents were evaluated. Then sodium carbonate, sodium dodecyl benzene sulfonate and sodium chloride were confirmed as composite solutes. Their proportion was optimized by an orthogonal test. The optimum proportion of the composite agents covered 0.03% of sodium dodecyl benzene sulfonate, 0.5% of sodium chloride, 3.0% of sodium carbonate, respectively, with the rest composed of water.

The optimal operating condition was also investigated. The results showed that when the oil sands were extracted at a heating time of 30 minutes, a treating temperature of 85 ℃, and an extraction agent/feed ratio of 1:1, the yield of bitumen could reach more than 96%, and the oil sands bitumen could be a good raw material for producing road asphalt. The extraction agent after separation from bitumen product can be recycled for reuse with good environmentally friendly advantages.

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Received date: 2014-05-23; Accepted date: 2014-07-31.

Shen Zhibing, Telephone: +86-15388631667; E-mail: shen_zhibing@163.com, or SZB@xsyu. edu.cn.