Title : Fixed bed reactor and catalyst: an intimate relationship
In this presentation, the reaction chosen as an example is the Fischer-Tropsch reaction in a tubular fixed bed reactor.
It often happens that a good catalyst is identified, but upon loading the pressure drop exceeds the design limits. There are a few ways to solve this, like decrease the reactor height, the linear velocity, the overall pressure, increase the catalyst size, etc. the easier one seems to be to increase catalyst size. This may, however, change the reaction selectivity in a negative manner, particularly increased methane, increased deactivation rate due to diffusional effects, etc. A change in reactor height is not recommended due to its effect on the capital cost as well as on the overall per stage conversion. A change in the compressor could be expensive besides being one of the long delivery items in the plant. Changes in linear velocity could result in heat transfer problems, lower per pass production, higher deactivation rate by catalyst oxidation, etc. Therefore, before beginning the reactor and plant design as well as catalyst development a range envelope for the above variables should be identified. The financial effect of the particle size is a good indicator to reach an optimum compromise: change in the products value (alpha), amount of light HC recycled to SMR or ATR, cost of catalyst deactivation rate, per pass and per stage conversion, carbon efficiency, number of stage, etc.
Activity is another of the main variables. It will determine the necessary temperature to reach the expected per pass conversion. This will have an effect on the selectivity, overall productivity/unit volume therefore also having an influence on the diameter of the tubes and reactor design. That is, the activity will have an influence on the maximum desired temperature to reach the maximum productivity without too much deviation of the expected product selectivity. A decrease in the diameter of the tube will allow for higher productivity as long as the maximum chosen temperature is not exceeded. The catalyst activity also has a strong influence on the linear velocity, generally favoring a high one. This in turn also has an effect on the pressure drop, discussed above.
Another topic that will be discussed in the presentation is the effect of the crystallite size, specifically for cobalt, although some variables are also applicable to precipitated iron-based catalysts. One of the manners to increase the mechanical strength of the catalyst is by using mechanical strength promoters and/or high-temperature calcination. Regardless of the technique used, it will have an effect on the pore size distribution, which in turn will influence the crystallite size. In general, and for any given system, smaller pore diameters are linked with higher mechanical strength. For iron, a similar effect is seen when the crystallite size is decreased. This has an influence on a number of catalyst performance parameters like reducibility, activity, stability, selectivity, etc. It follows that the crystallite size could easily affect the reactor design as well as the plant configuration.
This presentation will discuss how to reach a compromise between some of the competing variables. Another goal is to remind the audience that the catalyst and reactor are not independent but intimately related to each other and one cannot be designed without taking the other into full consideration.