Development and Application of the Controlled-Freeze-Zone Process

Abstract

Summary The controlled-freeze-zone (CFZ) process is a cryogenic distillation technique for separating CO2 and heavier compounds from methane (C1). This paper describes how the process controls the freezing and melting Of CO2 in a specially designed section of an otherwise conventional distillation tower. In addition to describing the process, this paper compares the CFZ process to other gas-treating processes, discusses pilot-plant operational results, and gives examples of potential CFZ applications. The CFZ process is a simple, cost-effective method to process gas streams containing CO2 using proven facilities, equipment, and control philosophies and has been demonstrated through the operation of a large-scale pilot plant. Introduction Natural gas processing has developed over the last 2 decades from lean-oil technology to cryogenics to separate C1 from heavier components present in the gas streams. Cryogenic processing has been proved to reduce capital and operating costs of recovering natural gas liquids (NGL's). Cryogenic fractionation, however, encounters potential problems when more than about 5 % CO2 is present in the gas stream. When a gas containing large quantities of CO2 encounters the process conditions of a cryogenic demethanizer, the CO2 may freeze, thereby plugging the trays or packing and preventing tower operation. To avoid such problems, CO2-rich gas processing involved an extra treating step. A physical or chemical solvent was used to absorb the CO2 or a freeze-suppression additive was used to prevent the CO2 from solidifying. In both cases, the extra treating step involved the recovery of the solvent or additive after the C1/CO2 separation. The CFZ process was developed to take advantage of cryogenic processing for C1/CO2 separation without additives. The process uses a cryogenic distillation tower with a special internal section designed to handle the solidification and melting of CO2. The separation occurs in a single column with no solvent or additive recovery required. Fig. 1 illustrates the major steps involved for the CFZ process compared with solvent or additive processes. After dehydration, the CFZ process separates acid-gas components by cryogenic distillation through the controlled freezing and melting of the CO2 in a single column. The solvent and additive processes require a second column to regenerate the solvent or to recover the additive. The solvent or additive is then recirculated. The CFZ process eliminates the regeneration/recovery column and fluid recirculation. Additionally, the acid components leave the CFZ column as a liquid enabling economical transportation by pumps. This paper describes how the process controls the freezing and melting of CO2 in a specially designed section of an otherwise conventional distillation tower. In addition, the paper compares the CFZ process to other gas-treating technologies, discusses operational results of a large-scale pilot plant, and gives examples of potential CFZ applications. CFZ Process The CFZ process uses distillation, solidification, and melting to separate C1 and CO2 physically in a single column. The process achieves direct separation without use of solids-processing equipment, Solid CO2 forms in a vapor space and falls into a specially designed melting tray. Fig. 2 illustrates the CFZ column. The top and bottom sections of the column are similar to a conventional cryogenic demethanizer. The top section removes CO2 and heavier components from the overhead product. The bottom section strips C1 from the liquid CO2 product. The solidification and melting of CO2 occurs in the specially designed CFZ middle section. That section does not contain packing or trays like a conventional column; instead, it contains spray nozzles and a melting tray. Liquid containing 3 to 8 mol% CO2 is spray contacted with vapor containing about 15 mol% CO2 that is rising from the melting tray. The liquid that is sprayed is collected in a chimney tray located at the bottom of the top section. Solid CO2 forms and fats onto the warm liquid layer below. Vapor from the lower section heats the liquid on the melting tray, maintaining the temperature above the freezing Point of CO2. All solids are confined to the CFZ section and do not enter or leave the, section, thereby eliminating any need for solids-processing equipment. The liquid from the melting tray is fed into the conventional bottom section. Fig. 3, based on the work reported in Ref. 2, illustrates the necessity for the CFZ section in cryogenic distillation. The pressure/temperature diagram shows the pure-C1 and pure-CO2 saturation curves. For various C1/CO2 mixtures, a locus can be drawn connecting the mixture critical points. Operations above the critical pressure of C1 (Feed B in Fig. 3) approach the critical locus, where the liquid and vapor phases become indistinguishable and thereby prevent further distillation. The limitation of the critical locus is reached before a C1 product of less than 2 to 4 mol% CO2 is obtained. Operating a distillation column at pressures below the critical pressure of C1 (Feed A of Fig. 3) would result in the formation of solid CO2 before the desired C1 purity is attained. A CFZ section allows the distillation path to proceed straight through the solid-CO2 region and produces a high-purity C1, product. The operation of a CFZ tower can be described with the temperature/composition diagram for the C1/CO2 system in Fig. 4. Regions for a given pressure phase are mapped to show the presence of vapor, liquid, vapor and liquid, and vapor and solid. The limits of the vapor/liquid regions at the vertical axes are defined by the saturation temperatures of both pure C1 and pure CO2). Operation of a CFZ column would occur in the vapor/solid and vapor/liquid regions. Vapor in the lower section of the column becomes leaner in CO2 and colder while rising through the liquid of each stage. The vapor eventually reaches a phase region where, upon further cooling, a liquid mixture of C1 and CO2 cannot exist. Rather, a solid, pure-CO2 phase will form in equilibrium with a C1/CO2 vapor. Further cooling will continue to form pure, solid CO2 from the vapor. Solidification from the vapor phase continues as long as the CO2 vapor concentration is greater than about 5 mol %. At that point, further cooling of the vapor results in the formation of a liquid mixture of C1 and CO2. Conventional vapor/liquid distillation can then be used to reduce further the amount of CO2 in the vapor. In the CFZ section, solid CO2 is formed by simultaneously cooling a vapor of about 15 mol% CO2 and evaporating a liquid being sprayed that has about 3 to 8 mol% CO2. Points on the temperature/composition diagram can be related to the schematic of the tower. Warm vapor from the bottom section (Point Av) is used to melt the solids on the CFZ melting tray. The liquid at the free surface of the CFZ melting tray is slightly warmer than the solid CO2 condition (Point BL). The cold, C1-rich liquid (Point DL) is sprayed into the CFZ section. Vapor exiting the spray section is cold, depleted in CO2, and enriched in C1 (Point Cv). The CFZ section may represent more than one equivalent vapor/liquid-equilibrium theoretical stage for heat and mass transfer the solids formed are essentially pure CO2. SPEPE P. 265^