(54d) Significant Reduction in Catalyst Cost of Ethylene Production in a Gas Cracker Plant

The gas cracker plant in which this catalyst change was made had an initial nameplate capacity of 350000 TPA ethylene, with feedstock flexibility for segregated cracking of ethane and/or propane as fresh feed. Initially, acetylene was removed in reactors located at the discharge of the CGC, to hydrogenate wet raw gas before drying. Palladium based catalyst suggested by the process licensors was used in the three adiabatic reactor beds in series, with cooling of gas between reactor beds. This design had the inherent problem of formation of propionic acid in the reactors, which damaged the catalyst and resulted in corrosion, with products of corrosion also contaminating the catalyst in the downstream reactor beds. Consequently, catalyst life was severely limited; and catalyst change was needed every 8 or 9 months. The problem was not immediately understood, so studies were conducted with help from the process licensors till the root cause of the problem was established. It was recommended by the process licensors to change the process sequence, so that dry raw gas would be the feed to the reactors, to minimize these problems. The change was implemented by rerouting piping within the plant during a plant turnaround in 1998. After this change, CGC discharge gas was first dryed, then fed to the reactors. A secondary dryer was provided by the process licensors to remove traces of moisture that they expected to be formed in the reactors due to hydrogenation of CO. The gas was then chilled for separation of products. With this change, the problems due to propionic acid and corrosion products contaminating the catalyst were eliminated. Catalyst life improved significantly; however it still was a concern to operations. Changing of catalyst was still needed outside of planned plant turnarounds, either because of runaways that subsequently interfered with smooth, sustainable operations; or because the inlet temperature limit of 100 Celsius was exceeded within a year or two of putting in service the new charge of catalyst. Additionally, there were plant realities that had to be managed. Changes in CO content in feed gas did occur when DMDS injection to the furnaces fluctuated at times; and this would lead to disturbances in reactor operation, which sometimes led to runaway reactions. Operation of this reactor was viewed by operations as critical, needing a lot of panel operator attention; with some additional plant shutdowns required just to attend to the reactor for sustaining acetylene removal as desired. It was at this time that E-Series? Catalyst became proven in commercially operating gas cracker plants in raw gas service. Eastman Chemicals were satisfied that the catalyst was stable, and could handle CO variations without putting reactor operation on the verge of a runaway. A year after successful implementation of a catalyst change in one of their cracker plants that had to hydrogenate raw gas, Eastman Chemicals decided to implement the change in a second plant as well. This was clear endorsement of the effectiveness of E-Series? Catalyst to hydrogenate acetylene in raw gas in a gas cracker plant. A year later, when Eastman chemicals decided to use E-Series? Catalyst in a third gas cracker plant as well, there was no further doubt of the effectiveness of E-Series? Catalyst in raw gas service. After discussions with operations personnel of Eastman Chemicals, it was decided to try E-Series? Catalyst in the reactors, replacing the other Pd based hydrogenation catalyst with established low life, that had been used for 14 years. Since September 2006, when the E-Series? Catalyst was put in service, it has functioned stably. When DMDS injection to furnaces drops, resulting in an increase in CO content, the panel operator is able to increase reactor inlet temperature, to maintain acetylene spec. at reactor outlet. And when the DMDS injection pump is restored to function properly again, and CO content drops, the operator has adequate time to lower the reactor inlet temperature, maintaining on-spec operation while avoiding reactor runaway, and for the heat content in the reactor vessels and catalyst to get reduced adequately to reach the new equilibrium conditions, without the reactor taking off on a runaway. (This was a big risk with the previous Pd based catalyst, and the panel operators were often on tenterhooks to avoid a runaway; or risk offspec operation for some time with consequent loss of production ). The performance of E-Series? Catalyst has been monitored periodically, to confirm that selectivity of hydrogenation of acetylene to gain ethylene is being maintained. Feedback from the panel operators about the stability of operation afforded by the switch to E-Series? Catalyst has been given considerable weightage. Where runaway reactions were previously an annual occurrence, there has been no runaway during normal operation of the plant with E-Series? Catalyst, even during plant upsets. Ingress of air/ oxygen, or oxygenates, into raw gas during operation of the reactor must be avoided. There have been two separate and unrelated occasions when this has been reported by operations. Catalyst activity increased considerably, with uncontrolled increase in temperatures as the consequence. High temperature trips of the reactors occurred on both occasions. Till now, such events have not led to significant deterioration of catalyst performance. CPChem have indicated that selectivity is maintained even with reactor inlet temperature of 120 Celsius. After seeing some increase in reactor inlet temperature during the first 6 months of service of the new catalyst, the rate of temperature increase has dropped significantly. At present, the reactor operates with inlet temperature around 80 Celsius. At the rate of inlet temperature rise seen during the past year, it will take several years to reach reactor inlet temperature of 110 Celsius. Though one projection indicates that catalyst life could exceed 10 years before a change becomes necessary, it has been decided that close monitoring and periodic evaluation will dictate when the change will become necessary. A method of monitoring catalyst performance by using methane as an internal standard for online analysis has been initiated. Once established, the effectiveness of this monitoring will become apparent in the coming years. CPChem have extended their full support for monitoring the catalyst. At present, the indications are that catalyst life should exceed another 4 years, provided there are no external contaminants or ingress of air that could reduce catalyst activity. It is intended to monitor the catalyst internally, and get inputs from CPChem as necessary, so that catalyst life and performance will fulfill the expectations with which this change of catalyst has been made. All indications at present point to the fact that the catalyst cost of ethylene production has dropped sharply, to less than 50% of what it had been even during the best two years of catalyst performance during the first 14 years of plant operation. This is an ongoing improvement, based on cost and life of catalyst, which could well end with a more significant reduction of catalyst cost of ethylene production. However, that will be revealed and established in future.