June 2024

Special Focus: Process Optimization

Research and application of proprietary cold oil absorption technology in the recovery of refinery dry gases

With the increasing shortage of global energy and resources, the recovery and utilization of refinery dry gases have become important topics of concern.

IP: 3.15.148.70

With the increasing shortage of global energy and resources, the recovery and utilization of refinery dry gases have become important topics of concern. Separating light hydrocarbons from refinery dry gases to be used as the raw material for ethylene production is an efficient pathway to improve resource utilization and help achieve the goal of carbon peaking and carbon neutrality for integrated refining and chemical enterprises. The authors’ company’s proprietary cold oil absorption technology^a (SCOAT) is an advanced and efficient technology for the recovery of refinery dry gases. This article introduces the principles and process flow of SCOAT, along with the optimal operating conditions, by investigating the influence of operating parameters of the absorption and desorption towers on separation efficiency. Industrial applications show that SCOAT has several benefits, such as a high separation efficiency, a straightforward process and long operating cycles, among others. The process also obtains a higher C₂ recovery with lower energy consumption and less equipment investment—therefore, offering significant economic and social benefits to processing facilities.

Recovering refinery dry gases

Refinery dry gases are the byproduct of the refining process, accounting for 3%–5% of crude oil. They are rich in light hydrocarbons such as ethane, ethylene, propane, propylene and hydrogen (H₂).^1,2 Presently, the yield of dry gases and light hydrocarbons is significantly increasing with the expansion of new refinery capacity in certain geographic areas. In the past, most refinery dry gases were used as fuel or directly discharged to the flare due to the lack of effective treatment methods; this not only caused resource waste, but also increased carbon dioxide (CO₂) emissions.

Conversely, ethylene is one of the most important petrochemical raw materials. The production of ethylene consumes a large amount of light oil, such as naphtha, which results in the cost of raw materials exceeding 60%.³ As a result, pathways to optimize ethylene raw materials and reduce ethylene production costs have become a key focus in the petrochemical industry. In view of this, separating light hydrocarbons—such as C₂ and C₃—from refinery dry gases as ethylene raw materials can not only improve the comprehensive utilization of refinery dry gases significantly and reduce CO₂ emissions effectively, but also expand the source of ethylene raw materials and reduce ethylene production costs. Therefore, the recovery and utilization of refinery dry gases have received increasing attention as effective ways for refiners to reduce costs, improve quality and increase efficiency, as well as help achieve the goals of carbon peak and carbon neutrality.

The primary technologies for recovering refinery dry gases include the cryogenic separation method, the pressure swing adsorption (PSA) method,^4–6 the membrane separation method^7–10, the hydrate separation method^11–13, the oil absorption method and the combined operation of two or more separation methods.^14–19 The cryogenic separation method utilizes the difference in the relative volatility to separate every component through distillation at low temperatures of approximately –100°C. This process is mature, and both the product purity and recovery are high. However, this method has strict requirements for the pretreatment of raw materials, high equipment investment cost and high energy consumption; therefore, it is generally only suitable for treating large amounts of refinery dry gases in areas with concentrated refineries.

The principle of PSA is that different components have diverse adsorption rates and adsorption capacity on the same absorbent. The notable feature of PSA is low energy consumption; however, it has several disadvantages, such as significant equipment requirements. It is also a complex process with a low recovery and poor product quality.

The membrane separation method utilizes the difference in the dissolution and permeation rates of gas molecules in membrane materials to achieve the separation of different components. While this method has the advantages of a simple flow, a clean production process and low energy consumption, it is difficult to obtain satisfactory recovery and product purity by relying on this single method due to the complex composition of refinery dry gases.

The hydrate method is a new type of separation technology developed for separating mixed gases with low boiling points, primarily based on the temperature and pressure differences in the formation of water hydrates for different components. However, the relative research on the hydrate separation mechanism is still ongoing.

The authors’ company’s oil absorption method^a accomplishes the separation target by means of the solubility difference of the components to be separated. The process is simple, and the operating conditions are mild, so that a higher recovery of C₂ can be obtained under the premise of lower energy consumption and less equipment investment. The authors’ company has carried out research on the oil absorption method for decades, developing the first cold oil absorption technology with temperatures at –40°C to –30°C in the early 1970s. This technology was primarily used for the separation of small-scale ethylene-cracked gases and has been applied in nearly 20 sets of small ethylene units throughout China, with remarkable results.

With the urgent need to recover refinery dry gases, the authors’ company has made a breakthrough in the oil absorption process through a large number of phase equilibrium experiments. On the basis of the optimization of absorbent components and operating conditions, the company has developed a comprehensive technology for the recovery of refinery dry gases (i.e., SCOAT) that is able to economically and efficiently separate low-carbon hydrocarbons (such as C₂ and C₃) from refinery dry gases at temperatures of 10°C–15°C.

Two technical principles and process flow

SCOAT combines the advantages of the previous cold oil absorption technology and the refinery absorption-stabilization technology, mainly based on the principle of “like dissolves like.” It accomplishes the separation of different components in terms of the unequal solubility for different components.

The key steps of SCOAT are the absorption process and the desorption process. As shown in FIG. 1, under the desired conditions of temperature and pressure, refinery dry gases contact the lean absorbent countercurrent—accompanied by the mass and heat transfer—inside the absorption tower. In this absorption process, C₂ and heavier components in the dry gases are absorbed by the lean absorbent and sent to the desorption tower. Methane (CH₄), H₂, nitrogen (N₂), oxygen (O₂), nitrogen oxides (NO_X), carbon monoxide (CO) and other impurities in the dry gases are difficult to be absorbed because they are dissimilar to the absorbent and, therefore, are removed from the top of the tower in the form of absorption offgas. The separation of C₁ and C₂ components is accomplished through the absorption step. The rich absorbent enters the desorption tower from the bottom of the absorption tower to desorb C₂ and C₃ components dissolved in it. As a result, the C₂-rich gas—primarily consisting of C₂ and C₃—is obtained from the top of the desorption tower, and the lean absorbent returns to the absorption tower from the bottom of the desorption tower. The C₂-rich gas is suitable as the raw material for ethylene plants. In this desorption step, the separation of the C₃ and C₄ components is achieved. In addition, refinery dry gases usually contain a small number of heavy components, such as C₄ and heavier hydrocarbons, that can be extracted from the bottom of the desorption tower.

FIG. 1. A schematic diagram of the SCOAT principle.

 

Since SCOAT accomplishes the recovery of C₂ and C₃ with the help of the solubility difference, the requirement for the absorbent is that the solubility of C₂ and C₃ is far greater than that of CH₄, H₂ and other impurities in the absorbent inside the absorption tower. Meanwhile, C₂ and C₃ are easily desorbed inside the desorption tower. Therefore, C₃–C₆ components are commonly used as the absorbent based on the principle of “like dissolves like.”

The typical process flow of SCOAT is shown in FIG. 2. Refinery dry gases are first pressurized and cooled to the required pressure and temperature. Then, by means of the aforementioned absorption and desorption processes, the C₂-rich gas and the offgas are obtained at the top of the desorption tower and at the top of the absorption tower, respectively. Although the vast majority of CO₂, hydrogen sulfide (H₂S), O₂ and other impurities in refinery dry gases are discharged into the offgas, a small amount will inevitably be absorbed by the lean absorbent—ultimately going into the C₂-rich gas. For example, the O₂ content in the C₂-rich gas is around 10 parts per million (ppm). Since downstream ethylene plants have stricter requirements on impurity content, the decarbonization and deoxygenation processes are employed in the SCOAT unit. The purpose of the decarbonization process is to remove CO₂ and H₂S from the C₂-rich gas, which is accomplished in the decarbonization tower with the help of a methyldiethanolamine (MDEA) solution or sodium hydroxide (NaOH) solution. The deoxygenation process is a hydrogenation reaction in which O₂ and other oxygenates—such as NO, nitrogen dioxide (NO₂) and dinitrogen trioxide (N₂O₃)—convert to H₂O and N₂ in the presence of a proprietary catalyst. After decarbonization and deoxygenation, the impurities in the C₂-rich gas are generally:

1. CO₂ < 100 ppm
2. H₂S < 1 ppm
3. O₂ < 1 ppm
4. NO_x < 10 parts per billion (ppb).

These impurities fully adhere to the requirements of ethylene plants.

FIG. 2. A typical process flow of SCOAT.

 

As a result of the phase equilibrium, the offgas obtained at the top of the absorption tower usually contains a small amount of absorbent, so the reabsorption tower is utilized to reduce the loss of absorbent. The offgas enters the reabsorption tower from the bottom and has countercurrent contact with the lean reabsorbent. The absorbent entrained in the offgas is dissolved into the reabsorbent. The gas obtained at the tower’s top contains a large amount of CH₄ and H₂, which can be further separated for H₂ usage or discharged into the fuel gas pipeline directly. There are no strict limitations to the reabsorbent. Some common refinery products—such as stabilized gasoline, heavy naphtha and aromatics raffinate oil—can be used as the reabsorbent. In addition, if the content levels of C₄ and heavier hydrocarbons in refinery dry gases are low, then a small amount of fresh absorbent must be added into the lean absorbent to maintain the absorbent balance in the system.

It should be noted that if refinery dry gases—such as hydrogenation dry gases, reforming dry gases and coking dry gases—contain only a small amount of unsaturated hydrocarbons or none at all, the C₂-rich gas is primarily composed of ethane and propane; therefore, it can be sent to the ethylene cracking furnace directly without requiring decarbonization, deoxygenation or other purification processes. In this case, the process flow is simplified, and the investment and energy consumption are much lower.

Analysis of major factors affecting the separation process

The core of SCOAT is the absorption and desorption processes, so the separation efficiency is mainly governed by the phase equilibrium and material balance, and is closely related to the operating conditions of the absorption and desorption towers. In view of this, the optimal operating parameters to improve the efficiency and economy are determined by studying the influences of the major factors of the separation process.

One petrochemical producer in China constructed a SCOAT unit for the recovery of refinery dry gas, and the process flow was the same as the one shown in FIG. 2. With the help of proprietary process simulation software^b, this unit is used as an example to examine the effects of major operating parameters of the absorption and desorption towers. Refinery dry gas specifications are shown in TABLE 1.

 

For the convenience of subsequent discussions, two parameters—namely R_c2 and R_c3—are proposed. R_c2 and R_c3 are defined to be the C₂ recovery for the absorption tower and the C₃ recovery for the desorption tower, respectively. The calculations of R_c2 and R_c3 are shown in Eqs. 1–2:

R_c2 = (mass flow of ethylene and ethane in the rich absorbent / mass flow of ethylene and ethane in the refinery dry gas) × 100%           (1)

R_c3 = (mass flow of propylene and propane in the C₂-rich gas / mass flow of propylene and propane in the rich absorbent) × 100%         (2)

R_c2 is an important parameter characterizing the performance of the absorption tower, and it reflects the separation efficiency of C₁ and C₂ components under a certain CH₄ content in the rich absorbent. Similarly, R_c3 is an important parameter characterizing the performance of the desorption tower. It indicates the separation efficiency of C₃ and C₄ components under a certain C₄ content in the C₂-rich gas.

Operating parameters of the absorption tower

The absorption process is a key step for SCOAT, and the separation of C₁ and C₂ is achieved inside the absorption tower. Therefore, the recovery of C₂ and impurities (e.g., CH₄) in the C₂-rich gas is closely related to the operating parameters of the absorption tower.

The number of theoretical trays

The number of theoretical trays is an important factor affecting the separation efficiency of the tower. For the absorption tower with other conditions unchanged, the gas-liquid contact time becomes longer when the number of theoretical trays is increased, which results in a better absorption efficiency. Of course, the increase in the number of theoretical trays will increase the cost of the tower.

Under the premise that the absorbent is cooled to 15°C before entering the absorption tower, the CH₄ content in the rich absorbent is 1 mol% and the tower’s pressure is 3.8 megapascal gauge (MPaG). The influence of the number of theoretical trays has been studied, and the results are presented in TABLE 2 and FIG. 3.

 

R_c2 improves significantly when increasing the number of theoretical trays, and the loss of C₂ in the offgas is reduced. For example, R_c2 is < 83% with 27 theoretical trays. It rises rapidly to 90.85% when the number of theoretical trays increases to 30 and will exceed 95% if the number of theoretical trays increases to 36. However, C₂ recovery slows (as shown in FIG. 3) when the number of theoretical trays is increased beyond 36. In addition, the reboiler duty decreases with the increase of theoretical trays, but this impact is small.

FIG. 3. The effect of the number of theoretical trays in the absorption tower on adsorption efficiency.

 

Considering the comprehensive effect of C₂ recovery, the investment and other factors, the desirable number of theoretical trays in the absorption tower is 30–36. In this case, C₂ recovery reaches > 90%.

Operating pressure

The operating pressure of the absorption tower has a substantial influence on the C₂ recovery and separation efficiency of C₁ and C₂. According to the law of gas-liquid phase equilibrium, the equilibrium constant reduces when increasing the pressure at a given temperature, which means that more C₂ components dissolve into the absorbent, improving the C₂ recovery at higher pressures. Accordingly, the dissolved amount of CH₄ in the absorbent will also increase to make the separation of C₁ and C₂ more difficult. Moreover, the energy consumption of the dry gas compressor and tower pressure increases. If the tower pressure increases to the value that is close to the critical pressure of the components to be separated, the minor difference between the gas and liquid densities will make the separation difficult.

Under the premise that the absorbent is cooled to 15°C before entering the absorption tower, the CH₄ content in the rich absorbent is 1 mol% and 33 theoretical trays are employed. The influence of the tower pressure was studied, and the results are presented in TABLE 3 and FIG. 4.

 

As shown, R_c2 is < 90% when the tower pressure is lower than 3.3 MPaG. As the tower pressure increases, R_c2 gradually increases, along with the bottom temperature and the reboiler duty. However, when the tower pressure exceeds 4.2 MPaG, the growth of R_c2 slows, but the reboiler duty increases quickly. Taken together, the tower operating pressure is reasonable within 3.6 MPaG–4.2 MPaG, which can ensure a C₂ recovery of > 90%, along with a smaller reboiler duty and lower energy consumption of the compressor. Additionally, the bottom temperature is low within this pressure range, so the polymerization of unsaturated hydrocarbons is also avoided.

Operating parameters of the desorption tower

The desorption tower is an important part of SCOAT, and its major objective is to separate C₂ and C₃ components from the rich absorbent to the C₂-rich gas. At the same time, the content of C₄ and heavier components should be controlled within a certain range (< 5 mol%), since these heavier components will exacerbate the operation of the gas cracking furnace and cryogenic system in the downstream ethylene plant.

To reduce the investment and energy consumption of the refrigeration system, the overhead reflux temperature is set at 15°C, so only low-temperature water of 5°C–7°C is necessary for the tower condenser. Under these parameters, the influences of the number of theoretical trays and the operating pressure of the desorption tower on the separation efficiency were examined. In this part of the study, the operating parameters of the absorption tower were:

• The absorbent was cooled to 15°C before entering the absorption tower.
• The CH₄ content in the rich absorbent was 1 mol%.
• The tower pressure was 3.8 MPaG.
• Thirty-three theoretical trays were employed.

The number of theoretical trays

The desorption tower is a typical distillation column. The more theoretical trays employed, the smaller the reflux ratio required to meet the same separation objectives with reduced energy consumption. Obviously, the increase in theoretical trays will increase the investment cost.

Under the parameters that the tower pressure is 2.6 MPaG, the C₃ recovery for the desorption tower R_c3 was 90%, and the content of C₄ in the C₂-rich gas was 4.5 mol%. The influence of the number of theoretical trays was investigated, and the results are presented in TABLE 4 and FIG. 5.

 

From these results, in the case of certain separation targets (i.e., the same R_c3 and C₄ content in the C₂-rich gas), both the overhead reflux ratio and the reboiler duty were reduced substantially when the number of theoretical trays was increased. However, when the number of theoretical trays exceeded 35, the impact of these theoretical trays on the reflux ratio and reboiler duty lessened, but the tower cost increased. Considering the balance between energy consumption and the tower investment, it is more reasonable to design the desorption tower with 25–35 theoretical trays.

Operating pressure

The operating pressure of the desorption tower has a major influence on both the composition of the C₂-rich gas and the energy consumption of the tower. On one hand, the boiling points of the components to be separated increase with higher tower pressures, which will reduce the content of C₄ and other heavy components in the C₂-rich gas at a given reflux temperature. Conversely, the relative volatility of different components becomes larger in pace with the decrease of tower pressure, which will contribute to separating C₃/C₄ at a lower bottom temperature. Under these parameters, the number of theoretical trays is 25, the reflux temperature is 15°C and R_c3 is 90%. The influence of the desorption tower’s pressure on separation efficiency is shown in TABLE 5 and FIG. 6.

 

These results show that, with an increase in the tower pressure, the C₄ content in the C₂-rich gas will be less, so the quality of the C₂-rich gas will improve. For example, when the tower pressure increases to 2.3 MPaG, the C₄ content in the C₂-rich gas will be < 5 mol%. However, with the increase in tower pressure, the bottom temperature and the reboiler duty both rise sharply. When the tower pressure increases to 2.9 MPaG (as shown in TABLE 5), the heat load of the reboiler is > 5,100 kW and the bottom temperature exceeds 135°C, which could increase the polymerization risk of the unsaturated hydrocarbons.

Industrial applications of SCOAT

The 100,000-tpy fluid catalytic cracking (FCC) dry gas recovery unit in Sinopec Qilu Petrochemical Corp.’s plant is the first industrial application of SCOAT, which began operations in 2011. The C₂-rich gas separated from the FCC unit (FCCU) dry gas is sent to the caustic washing tower of the ethylene plant as raw material—the C₂ recovery exceeds 93%, and the product quality is better than the design specification (TABLE 6). Since operations began, this unit has performed as intended and has continued to provide high-quality raw materials for the ethylene plant with remarkable economic and social benefits.

 

In recent years, SCOAT has been adopted to recover refinery dry gases in most newly built or expanded large-scale integrated refining and chemical projects in China. As of August 2023, 14 SCOAT units are in operation in China and another eight SCOAT units under construction (TABLE 7). The largest single unit produces approximately 1.6 MMtpy. All units in operation have exceeded the expected results and achieved remarkable economic benefits. Moreover, several other domestic and overseas refining enterprises are working on preliminary plans to utilize the SCOAT process to recover refinery dry gases.

 

As for economic benefits, Zhejiang Petrochemical Co.’s 400,000-bpd refinery produces 900,000 tpy of dry gases. Using the SCOAT process, the operator can generate economic benefits of > $20.5 MM/yr, with the potential to reduce carbon emissions by 1.5 MMtpy.

Takeaways

The production of high-value-added ethylene by using low-value refinery dry gases can expand the feedstock source of ethylene plants, as well as reduce the cost of ethylene production. It also helps achieve the goals of carbon peak and carbon neutrality. The industrial applications of several units for > 10 yr have proven the reliability and advancement of the SCOAT process. The primary process characteristics and technical advantages of SCOAT are:

• High C₂ recovery: The C₂ recovery of SCOAT units in operation is > 93%, while several exceed 95%. In operation, the C₂ recovery can be adjusted according to demand by changing the liquid-to-gas ratio or the operating pressure of the absorption tower.
• Uncomplicated process and stable operation: Since SCOAT units are close to the refinery absorption-stabilization system, they are simple and can be operated for an extended period, since only a small number of conventional equipment is needed.
• High product quality: Depending on the technical principles, SCOAT has the unique advantage of removing impurities such as H₂, O₂, N₂, CO and NO_X. The CH₄ content in the C₂-rich gas is low and controllable (generally < 5%), so it can reduce the energy consumed by the compressor and cryogenic system of downstream ethylene plants.
• Low investment and small floor plan: The major equipment of SCOAT units are the dry gas compressors and towers. The amount of equipment is relatively small (ordinary carbon steel is acceptable to use), so both the investment and the footprint requirements are minimal.
• Less energy consumption: The refrigerant and heat medium used in SCOAT units are both low-grade, so the refinery surplus waste heat can be fully utilized to drive the refrigeration and heating systems to reduce total energy consumption.
• Strong adaptability to feedstock and large operating flexibility: SCOAT units have no special limitations for the feedstock composition, and it is suitable for various types of refinery dry gases such as FCCU gas, coking dry gas, reforming dry gas and hydrogenation dry gas, among others. At the same time, SCOAT units have a wide operating flexibility, and they can run normally within the range of 30%–120% of the designed load. HP

NOTES

^a Sinopec’s Cold Oil Absorption Technology

^b SLB’s Symmetry process simulation software

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