The Application of Cyclonic Dissolved Gas Flotation Separation Technology in Electrochemical Desalination Water Cutting Treatment
This article is published in the September 2022 issue of "Petrochemical Technology."
Authors:
Zhao Rui, Cangzhou Branch of China Petrochemical Corporation;
Jian Xiaowen, Shenzhen Sinokle Technology Co., Ltd.
[Abstract] The Cangzhou Branch of China Petrochemical Corporation's continuous vacuum desalination unit faces challenges such as low qualification rate and excessive oil content in the desalination wastewater due to the wide and frequently changing sources of crude oil. This situation has a serious impact on the environmental operation of the unit and subsequent treatment, affecting the stable operation of downstream sewage treatment facilities. The use of cyclonic dissolved gas flotation technology can effectively reduce the oil content in wastewater, ensure the standard discharge of desalination wastewater, and has good economic and environmental benefits.
[Keywords] Desalination dehydration; Oil content; Cyclonic dissolved gas flotation
I. Background
The Cangzhou Refinery's continuous vacuum desalination unit processes crude oil from domestic sources such as Jinzhou and Shengli, as well as from foreign sources such as Oman and Pazfloro. Due to the wide and frequently changing sources of crude oil, the existing desalination unit is prone to unstable cutting oil content, with the oil content often exceeding the standard. The normal effluent oil content is below 1000mg/L, but it often exceeds the standard, sometimes reaching as high as 5510mg/L, and in more serious cases, there is so much oil that it cannot be detected. The impact is as follows:
(1) A large amount of crude oil enters the sewage treatment plant, which places a huge burden on the sewage treatment plant, seriously affecting the normal operation of the sewage plant's facilities, resulting in the entire sewage treatment plant's facilities operating overloaded, with various unit treatment water quality indicators exceeding the standard, and at the same time, increasing the operating cost of the sewage treatment plant.
(2) After a large amount of crude oil enters the sewage treatment plant and is mixed with other sewage, this sewage oil, due to its mixing with other sewage and the addition of a large amount of chemical agents, leads to serious emulsification of the recovered oil, with a large amount of suspended solids, sludge, metal ions, flocculants, etc. It is a heavy, poor-quality sewage oil. Once it is recycled, it will further increase the difficulty of recycling equipment. Over time, the accumulation will seriously affect normal crude oil refining, leading to high oil content in the effluent of the unit and further deterioration of water quality.
(3) The cutting water from the desalination unit has a high oil content and serious emulsification. Currently, this high-oil sewage is directly sent to the sewage treatment plant without pretreatment. While causing a serious impact on the treatment plant, it also produces a large amount of high-water, difficult-to-treat sewage sludge, greatly increasing the company's sewage sludge treatment costs.
II. Sewage Overview
The sewage from the continuous vacuum desalination unit is sent to subsequent treatment after heat exchange and cooling. The water temperature after primary heat exchange is 90°C, and after secondary heat exchange, the water temperature is 60°C. The sewage has poor water quality, with a black color, and the oil content can reach over 20%. The sewage is highly emulsified and contains a large amount of fine suspended solids, with a high salt content and mineralization degree, which causes severe corrosion to equipment, greatly increasing the operating pressure and cost of downstream sewage treatment plants.
III. Solution
To address the above wastewater treatment challenges, a survey of existing treatment technologies on the market was conducted. After collecting and comparing multiple treatment technologies, a new desalination pretreatment technology combining a high-efficiency flocculation pressure oil remover and a CDFU (high-efficiency cyclonic dissolved gas flotation unit) was selected. This technology is currently being used in projects such as the desalination wastewater treatment at PetroChina's Dushanzi Petrochemical, Tuha Oilfield, and CNOOC's Bohai Sea offshore platform.
CDFU-Cyclonic Dissolved Gas Flotation Unit is a new generation of high-efficiency cyclonic dissolved gas flotation unit that combines cyclonic centrifugal separation technology with dissolved gas flotation separation technology. The unit greatly improves the flotation effect through the combined action of cyclone and micro-nano bubbles generated by dissolved gas, shortening the flotation time and achieving efficient and rapid oil removal. The unit is suitable for various types of difficult-to-treat oil-containing (containing suspended solids) sewage. The process diagram is shown in Figure 1 below.
3.1 Working Principle
The desalination wastewater containing oil, after heat exchange and cooling, enters the high-efficiency flocculation separator. In the separator, the floating oil, dispersed oil, and some emulsified oil in the wastewater gather and flocculate into large oil masses. These oil masses are quickly separated from the wastewater and enter the oil collection tank for discharge. After the initial oil-water separation, the wastewater still contains a large amount of emulsified oil, which then enters the high-efficiency cyclonic dissolved gas flotation unit for further treatment. The emulsified oil is captured by ultrafine bubbles and, under the action of cyclone, continuously gathers and flocculates to form oil masses, which are rapidly separated from the wastewater. After flocculation and aggregation, the oily sludge is discharged, and the treated wastewater enters the sewage treatment plant's network for further treatment.
IV. Analysis of Treatment Effect
4.1 Operational Data Analysis
After the commissioning of the new desalination pretreatment facility, the purification effect on saline wastewater has been good, and the online table has been running normally and steadily to date. In addition to a significant improvement in the appearance of water quality, there has also been a significant improvement in the water quality analysis data. The main water quality indicators affecting the discharge of saline wastewater are COD and oil content.
4.2 Economic Benefit Calculation
4.2.1 Cost Calculation
① Electricity Cost: The electricity cost for treating each ton of wastewater is 0.130 yuan/ton.
② Nitrogen Gas Consumption: The cost of gas used for treating each ton of wastewater is 0.104 yuan/ton.
③ Chemical Reagent (PAC) Consumption: Purely physical separation process, no chemical reagent consumption.
④ Comprehensive Cost of Water Treatment per Ton: The comprehensive cost of treating each ton of wastewater = electricity cost + nitrogen gas cost = 0.234 yuan/ton.
4.2.2 Profit Calculation
The new desalination pretreatment facility adopts a purely physical separation process without adding any chemical reagents. It efficiently captures emulsified oil droplets with ultrafine bubbles and breaks the emulsion under the action of bubbles, resulting in a low impurity content and low water content in the recovered oil, which can be directly used in coking facilities.
To calculate the crude oil recovery based on the average oil removal before and after commissioning:
Average value before commissioning: 236.54 mg/L, average value after commissioning: 53.60 mg/L, average decrease of 182.94 mg/L, a decrease of 77.34%;
Total annual recovery of oily sludge: 79.03 tons, total annual recovery cost of oily sludge is about 158,100 yuan, calculated at 2,000 yuan per ton; total annual savings on hazardous waste treatment of oily sludge, calculated at 2,700 yuan per ton (incineration process treatment fee) is about 1,066,900 yuan
Therefore, the total annual environmental benefit is 1,225,000 yuan.
V. Conclusion
After the new desalination pretreatment facility was put into stable operation, the quality of discharged saline wastewater was stable. The average value of petroleum decreased from 236.54 mg/L before commissioning to 53.60 mg/L, a decrease of 182.94 mg/L, or 77.34%; the average value of COD decreased from 1382.32 mg/L before commissioning to 938.31 mg/L, a decrease of 444.01 mg/L, or 32.12%.
On the other hand, the water content of the oily sludge after static dewatering is between 2.5% and 10%. The recovered oily sludge can be reused in coking, maximizing the utilization of petroleum resources and reducing crude oil losses. It also reduces the treatment load on downstream water plants, achieving a win-win situation in terms of economic and environmental benefits.
