Integrated Vapor Collection Unit and Low NOx RTO Design For a Chemical Tank Loading / Unloading Station
Project No 1370
The Challenge
A major industrial chemical distributor was expanding their tank loading/unloading terminal operation in Texas to meet the growing demands, and required a new emissions capture system and emission control equipment to keep the facility operating in accordance with strict EPA compliance and new requirements.
Tank loading/unloading terminals are a vital liaison between chemical producers and transporters. Typical tank loading/unloading terminals consist of chemicals or petroleum storage tanks and loading/unloading racks to meet consistent delivery of bulk chemicals to satisfy local demands. There are two major sources of emissions associated with storage tank and loading terminal operation, often referred to as standing losses and working losses. Standing losses generate emissions from storage tanks at the idle times when there is no loading and unloading activity. The amount of vapor generation in the tanks depends on available vapor space in the tank, as well as temperature variation and vapor pressure inside the tank. The other category of tank emissions, working losses, are released while material is actively being loaded or unloaded out of the tank. The fluid movement and turbulence effect during this transfer process generates excessive airborne vapors which are collected at the top of the tank.
Due to increasingly stringent EPA emissions requirements, the tank terminal owners must utilize vapor collections and emission control technologies; the most common application is the installation of discrete individual vapor collection systems attached to a single common emission control equipment such as a flare, vapor combustor or Direct Fired Thermal Oxidizer. However, negative drawbacks such as high fuel and operational cost due to fluctuating large volumes of incoming vapors from multiple emission sources and varying concentrations of hydrocarbons. During the initial loading/unloading period with the higher concentration of hydrocarbons released, the emission control equipment burner does not draw on the higher assist fuel gas (i.e. natural gas). However during idle mode, when there is no loading/unloading activity taking place there are significantly lower hydrocarbon concentrations released from tank standing losses. Due to compliance regulations, emission control equipment is required to be operation all the time, regardless of cost. This is a major disadvantage for this application approach as it significantly impacts the overall facilities’ environmental and financial performance, as excessive fuel is required to operate the emissions control equipment.
The Solution
Integration of Vapor Collection Unit (VCU) with Thermal Oxidizer System
When designing the unique system, the primary focus was on optimizing energy efficiency and fuel savings, as well as accommodating a new automation system, considering operational reliability and maintenance issues. A single vapor collection tank, also referred to as a bladder tank, was utilized for effective vapor collection from storage tanks and loading/unloading racks across the entire terminal operation. The vapor collection tank technology consisted of a floating bladder mechanism to capture the emission losses from the storage tanks, loading and unloading racks, container filling stations, and ductwork into a common collection tank. The vapor collection tank was incorporated with different position level sensors and pressure measurement instrumentation for effective control over vapor collection in the bladder tank, and the subsequent release into the emission control system, the Thermal Oxidizer. Depending on the loading and unloading activity and rates, the collection tank offered the new capability to hold vapors ranging from 4 to 7 hours before releasing into the Thermal Oxidizer system. A key advantage offered in utilizing a vapor collection tank technology is that it minimizes the required capacity of the Thermal Oxidizer system, allowing for a more flexible and efficient design.
While the vapor collection tank is engaged in the normal operation mode, the Thermal Oxidizer system is synced and automated to heat up to an operating temperature of 1400 °F, and remain in idle mode until the collection tank reaches the preset discharge collection levels and diverts vapor towards the oxidizer system. Designed with smart self-operated and self-controlled systems, the Thermal Oxidizer utilizes the latest state-of-the-art programmable logic controller (PLC) mechanism to automate the comprehensive system. In addition, a series of control devices were placed between the vapor collection tank and Thermal Oxidizer, ensuring a streamlined transition and enhanced control over both systems. Several safety features were key considerations in the initial design phase to mitigate any safety concerns and avoid potential shutdown of the Thermal Oxidizer System in the event of excessive hydrocarbon concentrations being diverted to the Oxidizer. Since the volatile emissions generated from product transfer and carry a wide variety of materials being stored and then loaded into an array of transportation tanks and containers, it becomes critical to control, monitor and dilute the hydrocarbons concentration below safe explosive levels before entering into the Thermal Oxidizer system. To address this important safety concern, custom specialized lower flammability limit (LFL) monitors were incorporated in the design to measure flammability levels in the incoming vapors and provide feedback signals to control dilution air volume to achieve fine system controls. The Thermal Oxidizer system design also incorporated flame arrestors to address any potential concerns of flashback from the Thermal Oxidizer’s source of ignition, a typically burner system, to a larger vapor collection tank. A flashback explosion can be very dangerous when storing volatile vapors. To ensure added precaution a series of blocking valves and temperature monitoring controls were utilized in the design to react in the event of any system malfunctioning, and flashback dangers.
Higher Destruction Efficiency Design with Low NOx Configuration
The Thermal Oxidizer system was also specially designed for achieving lower nitrogen oxide emissions (NOx), with a low NOx burner configuration with electronic controller managed air and gas valves were employed to achieve NOx levels below 30 ppmv throughout the entire firing capacity range of burner system. The completely automated system design enabled the chemical distributor to operate the emission control system in most efficient and effective manner, while optimizing operating fuel requirements. The inlet flow control mechanism also controlled high concentration overloading into the Thermal Oxidizer unit by managing inlet vapor flow rates and mitigated excess vapor from accumulating in the collection tank.
The Results
This new design application provided initial capital cost savings up to 50%, in comparison to the conventional approach of designing a full capacity emission control device to handle irregular and often infrequent process loads.
The integrated approach of combining a vapor collection unit with the Thermal Oxidizer system resulted in significant cost savings to the chemical distributor while achieving a higher destruction rate efficiency (DRE up to 99.99%) during the regulatory compliance testing phase. The solution also offered lower operating cost for emission control system with lower downtime by maintaining the Thermal Oxidizer in hot (idle) mode, with the exception of the vapor collection tank discharge periods; and more operational reliability with lower down time by capturing the vapors in the collection tank, while also performing system maintenance on emission control equipment. Ultimately the custom-engineered solution, with the integration of the custom Vapor Collection Unit and comprehensive system controls, proved to be operationally more efficient, saving on excessive fuel combustion, and a greener solution for the environment and the customers.