M. Koizumi, Kurita Water Industries, Tokyo, Japan; and D. WRAZIDLO, Kurita Water Industries, Minneapolis, Minnesota (U.S.)
Throughout various refining and petrochemical processes, steam is critical for production. However, the steam generation process can corrode assets, increasing fuel use and carbon dioxide (CO2) emissions. Regulations have tightened across industries, and many corporations are now incorporating sustainability goals into their corporate social relationships, requiring plants to use steam more efficiently and reduce fuel consumption.
Traditional systems rely on filmwise condensation for heat transfer; however, for large energy consumers, dropwise condensation will help improve heat transfer efficiency and reduce steam consumption and carbon footprint. This can be achieved through water treatment technologies—specifically the dropwise condensation technology—that help facilities increase productivity and reliability while reducing energy consumption. Compared to traditional techniques, dropwise condensation allows for up to ten times greater heat transfer coefficient due to its distinct droplet formation as the surface remains unwetted.
Basic mechanism of action. Steam heating has many advantages. A major feature of steam is that it heats up quickly compared to other methods. When steam changes phase and condenses, the latent heat is transferred to achieve rapid heat transfer. However, there is one drawback to using steam: when heat is transferred across the exchanger, the condensed water forms a water film on the heat transfer surface (i.e., steam/water interface). This film inhibits heat transfer.
The most common condensation is filmwise, which is when fresh steam condenses on the outside of the film and heat is transferred by convection—instead of conduction—through the film to the metal surface. This heat flows through the film, and like all energy exchanges, it is associated with heat loss.
This film of water acts as a heat transfer barrier, and its resistance to heat transfer accounts for the difference between the effectiveness of filmwise and dropwise condensation. Due to the nature of the materials used in the construction of condensing heat exchangers, filmwise condensation is normal.
The next generation of heat condensation treatment is the authors’ company’s dropwise condensation technologya, which prevents the water film and enables more efficient heat transfer with steam. As the steam condenses, many spherical water beads cover the surface. As the condensation proceeds, the beads become larger, coalesce and fall over the surface. The dry surface offers very little resistance to heat transfer and much higher heat fluxes are possible.
Specifically, by continuously injecting the dropwise condensation technologya into the steam line and forming a water-repellent film on the heat transfer surface (i.e., steam condensation surface), the company’s technology continuously prevents this water film, thus the heat transfer coefficient improves. This makes it possible to improve the overall heat transfer coefficient (U value) in heat exchangers. The overall heat transfer coefficient (U) is influenced by the thickness and thermal conductivity of the materials through which heat is transferred. The larger the coefficient, the more efficiently heat is transferred from its source to the product being heated. In a heat exchanger, the relationship between the overall heat transfer coefficient and the heat transfer rate (Q) can be expressed by Eq. 1:
Q = UAΔTLM (1)
where,Q = Heat transfer rate, W = J/secU = Overall heat transfer coefficient, W/(m2/°C)A = Heat transfer surface area, m2ΔTLM = Logarithmic mean temperature difference, °C
From this equation, the heat transfer is directly proportional to the U value. Assuming the heat transfer surface and temperature difference remain unchanged, the greater the U value, the greater the heat transfer rate. In other words, for a certain heat exchanger, a higher U value could lead to shorter batch times and increased production/revenue.
The U value can be improved by increasing the film heat transfer coefficient on the heating side. The differences in surface conditions for filmwise and dropwise condensation technologies are shown in FIG. 1.
Application overview. Every industry in which the performance of the process is related to condensing efficiency, the authors’ company’s dropwise condensation technologya has the potential to minimize fuel consumption and maximize production. The application of this technology (FIG. 2) leads to operational improvements and significant energy savings:
Chemicals are continuously injected into the steam line.
The injection point is just in front of the target heat exchanger.
The typical dosage is 10 milligrams per liter (mg/L)–20 mg/L to steam.
There is no need to change the specifications of the existing boiler water treatment.
Condensate with the company’s technologya can be reused in the boiler feedwater.
If there is a condensate polisher (ion exchanged resin), this chemical is removed by the polisher. Note: Concerning the concentration in boiler feedwater and polisher inlet water, a certain limit value is established.
The chemical concentration can be measured in the condensate water (i.e., DW Index) onsite.
Condensate, including this chemical, will not adversely affect wastewater treatment (biological treatment).
Laboratory evaluation. The authors’ company performed the following evaluations at its research and development (R&D) center. The company created a heat exchanger (shown in FIG. 3) and confirmed that the application of this technology improved the U value. Details of the double-pipe heat exchanger evaluation tube shown in FIG. 3 include:
Material: C6871T (BsTF2 : Aluminum brass)
Diameter: Outer Φ 3/4 in. (19 mm)
Thickness: 1 mm
Length: 1,500 mm (Heat transfer area: 0.09 m2)
Application chemical: The authors’ company’s dropwise condensation technologya.
The chemical used was the authors’ company’s dropwise condensation technologya. The results (FIG. 4) show that when this chemical was added at a concentration of 20 mg/L or more to the steam, the U value increased by approximately 25% after filmwise condensation was previously used. It also shows that the effect is insufficient below 20 mg/L, and the residual effect lasts approximately a week after chemical injection ceases.
Next, to confirm that this improvement in the U value was the effect of the company’s dropwise condensation technologya, an internal team calculated the film heat transfer coefficient on the steam side using industry-known Xjpe (Version 9), a heat transfer calculation software for double-pipe heat exchangers provided by HTRI, a third-party engineering vendor. The results are shown in TABLE 1.
APPLICATIONS AND CASE STUDIES IN REFINERIES AND CHEMICAL PLANTS
Application overview. This technology works throughout multiple industries but has shown increased capabilities in two main applications (TABLE 2) in refining and petrochemical plants (hydrocarbon process) for the turbine condenser. In the power industry, for example, the efficiency in the heat extraction from the condenser will determine the vacuum level, which will have a critical effect on the energy output of the turbine.
Surface condenser of the steam turbine driving a centrifugal compressor. Perhaps due to increased global warming in recent years, cooling capacity has decreased in the summer, causing the vacuum in the surface condenser to deteriorate. When the degree of vacuum deteriorates, the heat drop across the steam turbine decreases, resulting in a decrease in output (FIG. 5).
Output can be maintained by increasing the consumption of steam introduced into the turbine, but there is a limit to how much this can be done. Once the valve opening reaches its limit, output can no longer be maintained. The application of the authors’ company’s technologya can increase the cooling capacity of the condenser, improve the vacuum level and maintain the turbine output. Furthermore, even in cases where turbine output does not decrease, the application of the company’s technologya improves the degree of vacuum and increases the thermal drop, reducing unnecessary steam consumption.
Steam heater. Steam heaters, which use steam to heat through a heat exchanger, are primarily used as preheaters and reboilers in distillation towers (FIG. 6). In particular, they are used to heat the reboiler with steam to maintain the temperature inside the tower (tower bottom).
In situations like this, the company’s technologya will help accomplish the following:
Increase the amount of throughput (i.e., feed rate)
Increase the recovery rate at the top of the distillation tower
Improve the purity at the bottom of the distillation tower.
The application of this technology increases the U value of the heat exchanger, consequently increasing the amount of heat provided per unit time (heat duty).
CASE STUDIES
The following are three case studies of the application to plants:
Application to the surface condenser of steam turbine-driven compressors (debottlenecking)
Application to the surface condenser of steam turbine-driven compressors (steam reduction)
Application to distillation tower reboilers (debottlenecking).
Surface condenser: Hydrogen recycle gas compressor in a residue oil desulfurization (RDS) unit (debottlenecking). Problem: The RDS had problems due to the decrease in the recycled gas volume in the summer. The main cause was that the rising cooling water (seawater) temperature in summer caused the condenser vacuum to deteriorate, reducing the steam turbine output (FIGS. 7–10). Solution: The author’s company’s technologya was applied, and the following effects were obtained:
Increased recycle gas volume by > 15%
Stable operation for the steam turbine.
The condenser settings and chemical information in FIG. 7 are detailed here.
Condenser settings:
Equipment type: Shell and tube (shell side: Steam)
Tube material: Aluminum brass (C6871)
Operating hours: 24 hr/d
Steam flowrate: 20 tph–25 tph
Steam pressure: 1 MPaG
Cooling water: Seawater.
Chemical:
Applied: The authors’ company’s dropwise condensation technologya
Dosage: 20 mg/L vs. steam flowrate
Injection point: Upstream of the steam turbine.
Surface condenser: Main air blower in a residual fluid catalytic cracking unit (RFCCU). Problem: Although there was no operational bottleneck in the RFCCU, the steam consumption of the main air blower (MAB) was very high (> 100 tph) (FIGS. 11–12). Solution: The author’s company’s technologya was applied and the following effects were obtained:
The condenser vacuum was improved, and the heat drop increased
As a result, steam consumption was reduced by an average of 3.6% (–3.9 tph)
The condenser settings and chemical information in FIG. 11 are detailed here.
Steam flowrate: 108 tph (average)
Steam pressure: 4.3 MPaG
Injection point: Between turbine and condenser
Cooling water: Circulation cooling water.
Injection point: Outlet of the steam turbine using a spray nozzle.
Steam heater: Reboiler of a distillation tower. Problem: Even when the reboiler heating steam control valve was fully opened, the bottom temperature of the ammonia stripper did not reach the target temperature [212°C (414°F)], and the ammonia recovery rate was poor (FIGS. 13–14). Solution: The authors’ company’s dropwise condensation technologya was applied, and the following effects were obtained:
The bottom temperature of the ammonia recovery tower increased (+1.3°C), improving the ammonia recovery rate
In addition, the valve opening can be adjusted.
The reboiler settings for FIG. 13 are:
Equipment type: Vertical shell and tube (shell side: Steam)
Tube material: Titanium (TTH340)
Steam flowrate: 5.8 tph
Steam pressure: 0.8 MPaG
Target set temperature: 121°C (250°F).
Takeaways. Dropwise condensation offers improved steam quality in industrial facilities, especially in refining and petrochemical plants. The authors’ company’s dropwise condensation technologya is the next generation of condensation technique: a chemical injection into the steam line that resolves the insulating layer of condensate film in steam heating and enables more efficient heat transfer. This improves the steam-side film heat transfer coefficient and results in an increased overall heat transfer.
A facility’s steam is not reduced but consumed in a better way, allowing plants to increase productivity and prevent unplanned shutdowns. Along with those benefits, plants can reduce their fuel consumption and decrease greenhouse gas emissions to help meet operational and corporate sustainability goals. HP
NOTES
Kurita Dropwise Condensation Technology
Masakazu Koizumi is a Senior Chief Engineer for Kurita Water Industries. Since joining Kurita in 1995, Koizumi has led the chemical engineering group for the oil refining and petrochemical markets after spending 10 yr as a boiler water treatment chemical engineer. He graduated from Waseda University Graduate School of Science and Engineering with an MS degree from the Department of Resource Engineering and is a member of the Process Section of the Refining Division in the Japan Petroleum Institute.
Darin Wrazidlo is Kurita America's Manager of CSV technologies—technologies that create shared value for customers and society by reducing waste and energy while improving operations and reducing carbon emissions—and has nearly 20 yr of industrial water treatment experience. He works with many sectors, including biofuels, power generation, food and beverage, and healthcare and institutional applications. Wrazidlo earned a BS degree from the University of Minnesota.
