Study on the influence of desalination factors for high-temperature coal tar

CHANG Qiulian

(1.Beijing Research Institute of Coal Chemistry,China Coal Research Institute Corporation Ltd.,Beijing 100013,China;2.State Key Laboratory of Coal Mining and Clean Utilization,Beijing 100013,China)

Abstract:All inorganic salts usually present in water droplets in coal tar,which play a key role during equipment corrosion.It is very important to remove these salts to prevent corrosion.It has already mature for the desalination technology of crude oil.Therefore,the crude oil desalination technology is generally used to carry coal tar desalination.But because of the great differences in structure and properties between coal tar and crude oil,the application of crude oil desalination technology in coal tar has many questions.A high temperature coal tar was used in this research to explore the impact of operational desalting parameters.In order to obtain optimum static desalting results,seven kinds of demulsifiers were used,in of which,five were organic amine demulsifier,and the others were block polyether demulsifier.Different operating parameters were explored to find the optimal desalting conditions,such as the temperature,the pH of water and the demulsifier injection rate.The results indicate that,to get a higher desalting efficiency for this high temperature coal tar,the block polyether demulsifier has a better demulsification efficiency than those of organic amine.The temperature range of 125 ℃ to 135 ℃ is required.The maximum dehydration rate occurs both in acidic and alkaline environments.The suitable pH value is 2 and 11 respectively.Using the optimal conditions obtained above,a field tests on a tar enterprise was conducted at different demulsifier injection rates.An optimum separation rate of 79.2% was obtained when the demulsifier injection volume was 90 mg NaCl/kg,and the salt content decreased from 25 mg NaCl/kg to 5.2 mg NaCl/kg.Thus,the technical requirements of coal enterprises could be met.

Key words:high temperature coal tar;demulsification;effect of electric desalination;emulsification stability;inorganic salt;dehydration rate

0 Introduction

The inorganic salts(NaCl,CaCl2,MgCl2,and so on) containing in coal tar brought out serious corrosion to reactor,high pressure heat exchanger,vacuum distillation column and the pipeline.Problems such as partial rupture of pipelines and leakage caused by which not only disrupt the normal transportation,but also caused serious environmental pollution,and even catastrophic accidents[1].In order to solve the problems above,enterprises generally use desalination technology to remove salts in coal tar[2].At present,the research on demulsification mechanisms of crude oil has been relatively mature[3-5].Considering the similarity between coal tar and crude oil,the urgency to solve corrosion problems,coal tar enterprises mostly use the crude oil desalting technology to carry out the desalination problems of coal tar[6].However,previous studies have found that,although there are natural interfacial active substances both in cruid oil and coal tar,such as colloid and asphaltene,but the structure of which are distinctly different[7],as a result,the mechanisms of promoting interfacial stability are vastly different.

Over the past few years,many studies have been conducted with an aim to understand the demulsification mechanisms of oil-water emulsions for coal tar.However,the existing research are mostly confined to a relatively single compound,and the mechanism of special compounds with a larger local resistance,such as coal tar,is still not clear.In order to find the final factors to affect desalting quality,this study has been undertaken.

According to the differences in density and properties,coal tar is classified into high-temperature and medium-low temperature[8-11].In this study the high-temperature coal tar was used.The desalting efficiency usually affected by process parameters,such as the type of demulsifier,the temperature,the water pH and so on.In this research,the desalting experiments were conducted in a laboratory to find the performance of this desalting process system.In order to find the differences between laboratory and platform at the same experimental conditions,comparing between them have also been explored.Several demulsifiers(a,b,c,d,e,f,g) have been adopted to find the optimum demulsifier for the coal tar.Further more,the pH value of water was explored to find the impact of which on desalting efficiency.Additionally,the desalting temperature was also explored to find the influence on emulsion stability.

1 Experimental materials

In this paper,the attempts have been made to optimize the type of demulsifier,the amount of fresh water,the temperature and the pH of water,as to get a greater desalting efficiency.The high tempera- ture coal tar was then obtained from one of the Neimeng platform.The characteristics of this coal tar were showed in Table 1 and the range of operating parameters were showed in Table 2.

Table 1 Properties of this high temperature coal tar

Table 2 Range of operating parameters

2 Experimental methods

Inorganic salts were removed from coal tar by first emulsifying oil with chemicals and wash water.Usually,stability of oil-water emulsion is a key factor in determining desalt efficiency.By changing experimental conditions,such as temperature,water content,type of demulsifier and so on,the emulsion stability was studied.The data showing the desalting efficiency and the interfacial tension between oil and water were collected in this research.

2.1 Analysis methods

Analysis methods for interfacial tension is from standard ISO 9101—1978,for the density is from ISO 3838-2004,for the salt content is from SY/T 0536-2008,for the water content is from ISO 9029—1990,and for the asphaltene content is from GB/T 2292—1997.

2.2 Experimental procedure

In the laboratory experiments,the coal tar was first preheated to 80 ℃～100 ℃ with oven in order to reach the required viscosity level.About 8 vol%～15 vol% fresh water was metered ahead of demulsifiers as an extraction agent to dissolve salts.Acid or caustic might be added to get a different PH value.Then,intense mixing took plates using a high-shear emulsifying machine to get oil-water emulsion.The needed demulsifier was then injected into the emulsion above,and a mixing was required to promote a thorough contact of emulsion.20 mL of prepared emulsion was poured into the 50 mL mixing cylinder with stopper,and place it in an appropriate oil bath to keep the temperature constant,after which,such parameters as salt removal efficiency were checked regularly.

3 Results and discussion

3.1 Demulsifier performance

In this study,the optimum demulsifier for maximum salt removal from oil was determined in laboratory experiment.Comparison of salt removal efficiency in different demulsifiers was presented in Fig.1,and water removal efficiency was showed in Fig.2.Among them,a,b,c,d and e were organic amine demulsifier,and f and g were block polyether demulsifier.

As shown in Fig.1 and Fig.2,f and g have the highest separation efficiency and duly selected for the field tests.For these block polyether demulsifiers,the lipophilic groups in oil phase and the hydrophilic groups in water phase have stronger force respectively,which can be used as an alternative of emulsifiers,such as asphaltenes,on oil-water interface to reduce interfacial tension.So that,the block polyether demulsifier has a better desalting efficiency.

Fig.2 Comparing of water removal efficiency of different demulsifiers as a function of time (demulsifier content was 50 mg NaCl/kg)

3.2 Temperature of desalting

The relationship between temperature and desalting efficiency was evaluated.Densities of each output oil and water at different temperature from the desalter were determined.

As shown in Table 3,the density of water and oil decreases with the increasing of temperatures.However,the reduction in water has a sharper gradient than that of oil.The highest density difference between water and oil is observed in the range of 120 ℃to 130℃.According to Storkes’ Law,the higher the density differences between the two phase,the faster settling rate of the two phase.This shows that the optimum temperature range for desalting experiment lies in 120 ℃ to 130 ℃.

Table 3 Densities of oil and water at different temperatures

The coal tar viscosity at different temperatures were illustrated in Fig.3.It is found that with the increasing of temperature,emulsion viscosity decreases and desalting efficiency increases.

Fig.3 Desalting efficiency and viscosity at different temperatures

The relationship between viscosity and dispersion velocity of two phase can be described by Storks’ Law[8]:

WhereUis the settling velocity of water droplets,m/s;dis the diameter of water droplet,m;ρ1,ρ2is the viscosity of water and oil,respectively,kg/m3;νis the kinematic viscosity of emulsion,kg/m·s;gis the gravitational acceleration,m/s2.

The separation velocity of two phase increases with the decreasing of kinematic viscosity of emulsion.However,when the temperature rises to about 130 ℃,the desalting efficiency decline and the slope of viscosity becomes smaller.An excessive temperature makes a sllipage and evaporation of water droplets,limiting there binding and causing shrinkage[12].And,the viscosity is shown to be sensitive to the mass but less sensitive to the mass distribution[13].

When water droplets are less than a certain calculated diameter,it will not follow this law and settle in the tank,which result in a decreasing of desalting efficiency.Otherwise,considering the economic factors,an excessive temperature should be avoided.

3.3 The pH of water

The HCl and NaOH were used to get the emulsion of different PH value.The temperature of this part was fixed at 60 ℃.The effect of water pH on stability of emulsion,interfacial tension and dewatering efficiency were showed in Table 4 and Table 5.

Table 4 Dehydration efficiency by increasing associated water pH

Table 5 Desalination data in coal tar enterprise

According to Table 4,percentage of water separated from emulsion increases both with the increasing and decreasing of pH from the point of original material.The maximum in dehydration rate occurs both in acidic and alkaline environments.And the dehydration rate plummetes to a lower point at strong acid/alkali environment.

The regularity of emulsion stability for coal tar at alkaline environment is consistent with that discovered by Leila and Duanming.Leila[14]found a different rule in crude oil emulsion.Leila found that the lowest percentage of water separated from the crude oil emulsion occurred in acidic environments and the maximum occurred in an alkaline environment.And Duanming[15]found that the maximum dehydration rate occurredin neutral and alkaline trending condition.The explanation was that soaps could be formed from naphthenic acids in crude oil at a high pH values,which lead to a more emulsion stability and lower dehydration rate.Moreover,particles in emulsion could accumulate on the surface of W/O droplets and lead to a hard layer.High pH would result in removal of the precipitation of OH ions on this layer.Hence,increasing the surface electric potential would result in removal of the precipitation particles from surface[16].This in turn brought an enhancing of instability of emulsion.When the pH was greater than 13,OH-would harden on the surface of emulsion and increasing of stability.

Nevertheless,for coal tar,in acidic environm- ents,the dehydration rate and interfacial tension increased with the decreasing of pH.While,when the pH was lower than 2,dehydration rate declined substantially.The oil-soluble acid in coal tar was mainly phenol that could be transformed into sodium phenolate at a lower pH values,which destroyed equilibrium of emulsion.As a result,water combined with phenol was released,which brought about an increasing of dehydration rate.

However,the wetting properties of solid particles or electrical properties can be changed by adjusting pH value.as a result,surface potential of particles can be regulated,the adsorption of particles on oil-water interface is promoted or inhibited.So,by regulating the concentration of aqueous electrolyte,the surface potential can be greatly reduced,thereby interface adsorption of particles is promoted and emulsion stability is improved.

3.4 Field tests on the tar enterprise

As is shown in Fig.1 and Fig.2,demulsifiers f and g had the highest separation efficiency and were selected for field tests.

In this part,water pH was fixed at a specified amount,demulsifier injection rate varied in different experiments.Experimental results were showed in Table 5.An optimum separation of 79.2% was obtained at the demulsifier injection rate of 90 mg NaCl/kg,when salt content could be decreased from 25 mg NaCl/kg to 5.2 mg NaCl/kg which met technical requirements of coal enterprises.

4 Conclusions

The effect of operating parameters on desalting efficiency of coal tar has been explored in this article.Some factors,such as type of demulsifier,temperature,water pH,were conducted.

Results show that,for this high temperature coal tar,block polyether demulsifier has a better demulsification efficiency.The maximum dehydration rate occures both in acidic and alkaline environments.And dehydration rate plummeted to a lower point at strong alkali environment.It is also demonstrate that the increasing of temperature makes emulsion viscosity decreased and desalting efficiency increased.

Due to the different nature of coal tar from different sources,an appropriate operating parameter for this coal tar is usually not the best for another one that come from different sources.So that,the results obtained from this research is satisfied to the field data finding.

高温煤焦油电脱盐影响因素研究

常 秋 连

(1.煤炭科学技术研究院有限公司 煤化工分院，北京 100013; 2. 煤炭资源高效开采与洁净利用国家重点实验室，北京 100013)

摘 要:煤焦油中的无机盐通常溶解于煤焦油种的水中，无机盐是造成设备腐蚀的重要因素，煤焦油脱盐对预防设备腐蚀起关键作用。目前，我国原油脱盐技术已相当成熟，很多煤焦油加工企业直接套用原油脱盐技术进行煤焦油脱盐。但由于煤焦油与原油在结构和性质上存在巨大差异，原油脱盐技术在煤焦油脱盐中的应用存在各种问题。以高温煤焦油为原料，研究操作参数对脱盐效果的影响。选用7种不同类型的破乳剂，其中5种为有机胺类破乳剂，其余为嵌段聚醚类破乳剂，探讨温度、pH值、破乳剂注入量等操作参数对脱盐效果的影响。结果表明：嵌段聚醚破乳剂对高温煤焦油的脱盐效果优于有机胺破乳剂，脱盐试验的最佳温度范围为120 ℃～130 ℃；脱水速率的最大值分别出现在酸性和碱性环境中，pH值的适宜值分别为2和11；利用上述最佳条件对某企业煤焦油进行不同破乳剂注入速率的现场试验，在破乳剂注入量为90 mg NaCl/kg时，含盐量由25 mg NaCl/kg降至5.2 mg NaCl/kg，分离效率达到79.2%，达到了企业脱盐需求。

关键词:高温煤焦油; 反乳化作用; 电脱盐影响; 乳化稳定性; 无机盐; 脱水速率

中图分类号:TQ524文献标志码:A文章编号:1007-7677(2021)03-048-06