Increasing Efficiency of Hot Potassium Carbonate CO2 Removal Systems
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Increasing Efficiency of Hot Potassium Carbonate CO2 Removal Systems Stanislav Milidovich, P.E. and Edward Zbacnik UOP LLC ABSTRACT The UOP Benfield™ Process is the original hot potassium carbonate technology widely used in the ammonia industry for CO2 removal downstream of the steam reformer. With experience in over 700 licensed units, UOP has developed significant advances in the technology and continues to support the industry with opportunities to improve existing unit performance. This technical paper will provide information on three types of unit upgrades: addition of advanced chemical activators, implementation of energy-saving semi-lean solution flash technology, and increasing capacity with high-efficiency tower internals and packing. Introduction The Hot Potassium Carbonate Process (HPC) originated from research work done by the US Bureau of Mines (USBM), between 1940 and 1960. The original justification was to determine how to convert coal to gaseous and/or liquid fuels. If the coal could be gasified, followed by the removal of CO2 and sulfur compounds, the resulting product would be mostly a mixture of hydrogen and CO, also called synthesis gas. The synthesis gas can be used as a chemical plant feedstock, converted to substitute for natural gas, or processed further to produce synthetic gasoline. While gasifying the coal is relatively easy, treating the resulting hot gas is more difficult. Scrubbing this gas while hot is more desirable than cooling it because heavier hydrocarbons and tar-like compounds can condense at cooler temperatures. The solvents available at that time for acid gas removal were water, monoethanolamine (MEA), and other amines. These solvents require operating temperatures below 125°F (50°C) in order to prevent solvent degradation. Water could be used to remove CO2, but it is very inefficient and does not provide acceptable treated gas purity. Caustic solutions, either NaOH or KOH could remove the acid gases very effectively, but could not be regenerated. Using a solution of KOH to first pick up CO2 would generate potassium carbonate in solution, which could absorb still more acid gases. Thus, the “Hot Potassium Carbonate” Process was born. Since the technology was developed by the U.S. Government, the basic process remained the property of the U.S. Government. Several U.S. citizens further developed the technology and started businesses to assist the industry to use this technology. A partnership was formed by Benson, Field and Epes, who were former employees of USBM, to help design some 150 units for use in treating town gas produced from coal at locations throughout Europe, mostly in the United Kingdom before the advent of North Sea gas. Eventually this partnership found patentable improvements to the technology and started designing and licensing their improved versions. This became known as the Benfield™ Process. © 2013 UOP LLC., A Honeywell Company All rights reserved. 1 The improvements and process developments included addition of small amounts of amines/other proprietary additives to the HPC solution to increase the rate of reaction with CO2, and using corrosion inhibitors to permit the use of carbon steel for the majority of the process unit. When evaluating economics, the HPC Process, was a relatively low capital investment option when compared with other CO2 removal process technologies available on the market. The low capital cost can be attributed to the simple unit configuration and to most materials of construction being low-cost carbon steel. This, coupled with relatively low cost of utilities, made the HPC Process popular. With the growing global economy, the demand for many products that are created by utilizing the HPC Process has increased motivating many HPC Process owners to increase capacity and improve process efficiency. During recent years, multiple process improvements developed by UOP have been introduced to the market in order to modernize existing HPC Processes by reducing operating costs or increasing unit capacity. These improvements include a new chemical activator to improve HPC solution performance, LoHeatTM Technology for the reduction of energy required to remove CO2 from the feed gas, and Raschig Super-Ring packing and internals for unit capacity increases. These improvements are discussed in more detail below. Benfield ACT-1TM Activator The first available unit upgrade is addition of much more advanced chemical activator to the HPC Process. Almost all HPC Process uses a chemical solution based on 30% potassium carbonate (K2CO3) dissolved in water, some kind of a chemical activator, and a corrosion inhibitor. The activator is a low-concentration additive designed to improve the rate of CO2 absorption. For many years, diethanolamine (DEA) has been the standard activator and it is still used today at many operating plants around the world. Unfortunately, like most other organic chemicals, DEA is subject to degradation. Some of the reasons DEA tends to degrade are listed below: DEA will break down from overheating (thermal degradation). DEA reacts with oxygen from air contact or from overuse of reoxidation agents such as potassium nitrite (KNO2), used to regenerate the corrosion inhibitor (vanadium). By absorbing CO2, a secondary amine activator such as DEA forms a carbamate chemical that normally is easily regenerated. However, because further reactions can occur, some by-products are formed that are not regenerable, and thus a degradation compound is formed. Typically, these compounds are high molecular weight, polymer-type chemicals. Evidence of extensive DEA degradation can be visually seen. The potassium carbonate solution samples appear black and opaque similar in appearance to liquid coal. Such DEA degradation will cause interference with analytical procedures such as carbonate titrations and vanadium valence determinations. Foaming upsets are also frequent due to degradation products and constant addition of antifoam may be required. Often there is also a rapid reduction of valence state of the vanadium corrosion inhibitor from the active V+5 to passive V+4. 2 One result of the breakdown of the DEA molecule is formation of potassium formate and a few other carboxylic acid salts. These salts can be analytically measured and are usually benign at low concentrations. However, when they are found at concentrations of 5% or more, they interfere with operations by altering the physical properties of the potassium carbonate solution. The solution becomes much harder to regenerate. Most of the other known and unknown DEA degradation compounds are notoriously difficult to analyze since most of these compounds, being large polymer-based molecules, are still reactive. Some amine degradation compounds are even considered to be corrosion accelerators in that they may solubilize iron, keeping it in solution and preventing it from formation of the passivation coating. UOP has found an alternative to DEA which has been commercialized as Benfield ACT-1TM activator. This activator, which is a proprietary chemical from UOP, is also an amine but with a more stable molecular form that is considerably more resistant to degradation. To measure the improved performance of the ACT-1 activator, side-by-side accelerated laboratory degradation tests were performed to compare a potassium carbonate solution with DEA and with ACT-1 activator. The first test was to heat samples of both solutions to 167°F (75°C) and expose them to oxygen by continuously injecting air. The DEA was 15% degraded within 45 days, but the ACT-1 activator was still 100% available. Please refer to Table 1 for laboratory data summary. Table 1: Effects of Oxygen on HPC Process Activators (Lab Test Conditions: CO2 saturated, constant air injection, at 75°C) Days of Test % of Active ACT-1 % of Active DEA Day 0 100 100 Day 10 100 97 Day 18 100 93 Day 37 100 87 Day 46 100 86 In another test, both solutions were heated to 121°C to 132°C (250°F to 270°F) and saturated with CO2 at autoclave pressures of 9 to 14 bar (135 to 200 psi). After 15 days, only 25% of the DEA remained; 100% of the ACT-1 activator remained and was reactive after another 50 days. Please refer to Table 2 for laboratory data summary. 3 Table 2: Effects of Temperature and CO2 on HPC Process Activators (Lab Test Conditions: 121-132°C and continuous exposure to CO2 at 9-14 bar) Days of Test % of Active ACT-1 % of Active DEA % of Active MMEA Day 0 100 100 100 Day 3 100 75 70 Day 8 100 50 50 Day 10 100 42 46 Day 15 100 26 40 Day 18 100 - 33 Day 20 100 - 30 Day 50 100 - - Note: MMEA is 2-methyl-methanolamine. The ACT-1 activator is currently in use in many units worldwide, including ammonia plants. It has been used in new units where no DEA was present and in existing units that had used DEA for more than 20 years and later converted to ACT-1 activator. Concentrations most effectively used in plant solutions are 0.3 to 1.0 weight% ACT-1 activator compared to about 3 weight% for DEA. The performance of ACT-1 activator is far superior to that of DEA when looking at the CO2 absorption rates. In all comparisons, the ACT-1 activator in the potassium carbonate solutions substantially reduced the CO2 slippage typically to about 50% of the levels achieved by DEA activation. This improved unit performance is available at no additional energy demand for solution regeneration and no additional solution circulation is required. In fact, plants frequently find slight reductions in regeneration duty and solution circulation rates when compared to the requirements for the same units operating with DEA. Please refer to Figure 1 for a simple graph comparing relative rate of CO2 absorption between DEA and ACT-1 activator for various CO2 partial pressures. Figure 1: DEA vs. ACT-1 Activator 2.5 ACT-1 2.0 Absorption 2 DEA + ACT-1 1.5 DEA 1.0 0.5 Relative Rate CO of Rate Relative 0.1 1 10 CO2 Partial Pressure 4 The ACT-1 activator benefits are fully achievable in new green-field units and in units fully converted from DEA to ACT-1 activator1.