Theoretical and experimental investigation of the continuous process of desalination of heavy crude oil using microwaves and ultrasonic waves.

Doctoral thesis in the field of chemical engineering

Abstract

Theoretical and experimental investigation of the continuous process of heavy crude oil desalination using microwaves and ultrasonic waves

Crude oil desalination is the first step in oil refining, by which the undesirable compounds with the oil are separated before they reach the main unit. In order to reduce the salt present in oil, different methods should be used, such as adding suspending chemicals, heat and electric current. However, with the increase in the extraction life of the well, the salt and water content of the oil increases and the quality of the processed oil decreases. Since there are limitations in the use of conventional methods, it seems necessary to use new technologies in the desalination process. In this thesis, ultrasonic and microwave waves have been investigated as two new technologies in the desalination process, and the desalination process has been completely modeled with these waves. In this part, momentum balance, energy balance and population balance are used as modeling principles. The equation of the movement of water drops has been obtained and based on that, the relative movement and collision of two water drops under the influence of microwave and ultrasonic waves has been predicted. Using the two-drop motion model, the population size model of water droplets is presented. From the solution of the population size model, the change in the number of drops due to collision with each other and their settling is predicted. MATLAB software was used to program the models. In addition, the pilot equipment of the desalination process with these waves has been designed and built to conduct experiments. Using the results of the experiments, the parameters of the written models were determined. Finally, suggestions for the industrialization of the desalination process using microwave and ultrasonic waves have been presented.

- Introduction

1-1- General information about salt water in crude oil

Crude oil is usually accompanied by some water when it comes out of the well. In the water that comes out from the depths of the earth along with the oil, there are some salts in solution. NaCl solution or sodium chloride NaCl and magnesium and calcium salts are also among these. In the oil-rich regions of the world, salt water has been observed along with oil. The outflow of salt water from a well increases due to the increase in the exploitation life of that well, because this causes the volume of gas and oil in the source to gradually decrease and the level of salt water that is located in the lower part rises and occupies the place of the reduced oil and gas, and as a result, more salt water comes out of the well. Wells with a large amount of salt water mixed with oil are called wet wells or oil mixed with salt water.

The amount of salt in the well can be controlled by reducing the output oil, in some cases, in order to control the amount of salt water, they have to close the wells that have a lot of salt water so that the oil of the other wells that will be extracted with it later will not be contaminated with salt water. Of course, if the exploitation of an oil-rich area is at the maximum level, the flow of one or more wells that produce salt water may not have a significant effect on raising the salt of the total oil. They consider the actual amount of salt in the tanks of the port of loading. The closer you get to the end of the well's production, the oil and gas of the well will gradually decrease, and finally the day will come when the only product that comes out of it is salt water.

If the amount of salts in the crude oil exceeds 10 pounds per thousand barrels, it must be salted, because the salt deposit will cause clogging and corrosion of the facilities. For this purpose, in order to prevent the waste caused by the presence of these materials in oil facilities, it is necessary to establish a desalination plant on the way of the oil exiting the operating factories and before entering the refineries.

Advantages of desalination:

A- Reduction of corrosion in oil facilities

B- Reduction of clogging of heat exchange system ducts

P- Protection of refinery catalysts

D- Improvement of crude oil quality

1-2- Common processes of crude oil desalination

The common methods of separate desalination are:

1- Settling connection coalescing

2- Chemical coalescing

3- Thermal coalescing

4- Mechanical coalescing

5- Electrical coalescing

First method: In this method, 3 tanks with equal capacity are usually used for each desalination unit. One of the tanks is always being filled, one is being settled and the other is being emptied. This process is continuously repeated for each tank. As a result of stopping the flow in the reservoir, the salt water molecules that are present in the oil in the form of large water droplets settle down.

The second method: In this method, by injecting chemicals, the speed of the salt water droplet settling increases. Since the outer wall of salt water drops is surrounded by a layer of oil, the injection of separating chemicals, which is a type of demulsifier, breaks this wall. As a result, small drops of water are attracted to each other and form larger drops and settle. The injection amount of these substances depends on the characteristics of the oil, the amount of salt water and the temperature of the oil and varies between 5 and 100 parts per million (ppm). If the suspension breaker is injected more than this amount, the opposite of this operation is done. This means that the drops of salt water are broken into smaller drops and as a result the process of settling becomes more difficult. Therefore, it should be noted that the precisely calculated chemical substance is injected.

Third method: One of the effective factors in the separation of salt water from oil, or in other words, the settling of salt water droplets with the help of heat. Heat reduces oil viscosity and weakens the outer wall of salt water droplets. As a result, water droplets are attracted to each other and settle. The amount of heat should not exceed the specified limit. The best temperature is about 130 degrees Fahrenheit. High heat separates light hydrocarbons from oil. As a result, the relative density of oil increases and its value decreases. For this reason, there is no need for heating in oil-rich areas where the temperature of the well is high.

Fourth method: Mechanical connection is done by various methods, the most important of which are:

A: Filtering - in this method, the oil is passed through special filters. These filters should be cleaned or replaced after working for a while.

B: Centrifugal action - In this method, the property of centrifugal force is used. Oil is placed in special containers and those containers are rotated around its central axis. As a result, salt water drops are connected to each other and settle. Since the amount of production in the oil industry is high, the use of mechanical methods is not practical in the industry and is mostly used in laboratory work.

Fifth method: In cases where salt water is difficult to separate from oil, electricity is used. In electric devices that separate salt water from oil, there are two positive and negative poles of high voltage electric current (about 17000 to 21000 volts). Spherical drops of water are stretched under the influence of electrostatic force and become oval. As a result, adjacent droplets are connected to each other and settle. Of course, if the water drops are too big, the electric current will break them up and it will be more difficult to settle. For this reason, for proper separation, the distance between the droplets should be small, and their diameter should not be large.