The challenges of designing a Fischer-Tropsch reactor

FT flow-sheet

Syngas clean-up  Syngas contaminants are sulphur containing materials e.g H 2 S and organic sulphur  Cleaning methods available incude chemical and physical ones - Rectisol - activated carbon guard beds - membrane separation

Process Synthesis Methodology Use experimental results to modify flowsheet Start by designing flowsheet Do experiments/calculations at required accuracy Experimental and simulation results Use flowsheet to design experimental programme Iterate until design acceptable Process Design Decisions Involve lab and designers from concept to flowsheet Complete Process Synthesis (Comps)

What is an FT reaction  Produces gas, liquid and solid hydrocarbons from syngas over group VIII metal catalysts e.g. cobalt or iron.  CO + 2H 2 → -CH H 2 O ∆H o rxn = -165 kJ/mol syngas FT product monomer

FT reaction kinetics  Modelled usually using a simple power law equation:  -r co = Aexp(-E/RT) P co m P h2 n  m often lies between -0.1 to and n ~ 1  Activation energy values are reported in literature

Definition of fundamental processes  Process synthesis for a system begins with identifying all fundamental processes.  Reaction and heat transfer describe the possible outputs for an FT system, within constraints.  Process synthesis methodology

F-T Reactor Configuration  F-T reaction is highly exothermic  Heat removal has influenced the design and operating philosophy of the F-T reactor  Selectivity to methane increases with increasing temperature  Close temperature control necessary in order to maintain desired selectivity  Three major types of reactors are considered for F- T process: – Tubular fixed bed reactor – Fluidized bed reactor – Slurry reactor

Industrial types of FT reactors a. slurry bed column – catalyst suspended in recycled liquid product b. multitubular fixed bed – tubes loaded with catalyst having external cooling c. circulating fluidised bed d. fluidised bed # Heat removal has influenced the design and operating philosophy

Tubular Fixed Bed Reactor  First reactor type to be exploited commercially e.g. Arge Reactor  TFB reactors used in Sasol 1 and Shell plant in Malaysia  Catalyst in pellet shape are placed in the tubes and the cooling medium flows around the outside of the tubes, similar to a shell and tube heat exchanger  Tube diameter limited to cm because of heat transfer limitations within the tubes

Tubular Fixed Bed Reactor (2)  Relatively simple design, but expensive construction with its large number of tubes required for a commercial scale plant  Easy to scale-up from a single tube pilot plant  Catalyst replacement is a major undertaking  Iron catalysts inherently unstable have to be replaced periodically  More stable cobalt catalysts can be regenerated and reactivated in-situ; may not have to be replaced for several year  May experience temperature gradients in the tubes which can lead to catalyst sintering and deactivation  Can also experience high pressure drop in tubes

Fluidized Bed Reactors  Two types of FB reactors:  Circulating fluidized bed (CFB) reactor such as the SASOL Synthol reactor  Fixed fluidized bed reactor (SAS reactor)  Better heat removal  Better temperature control  Less pressured drop problems  Online catalyst removal and addition possible

Fluidized Bed Reactors  A vessel with a gas distributor at the bottom and heat exchange tubes suspended in the fluidized bed  Improved stability and slight less catalyst consumption  Catalyst inventory and product selectivity similar to the Synthol reactor  Less erosion than Synthol reactors

Fluidized Bed Reactors  Major disadvantage of fluidized beds for F-T applications: Product must be volatile at the reaction conditions  Non-volatile hydrocarbons can accumulate on catalyst particles, making them sticky and adhering to each other; the bed looses its fluidization properties  Thus, the F-T products of fluidized bed reactors must be of relatively low molecular weight  More difficult to scale-up than TBF reactors, because the fluidization behavior function of the reactor diameter  However, there is a wide experience with large fluidized bed reactors in refining, such as FCC units; thus fluidized bed reactors can be scaled-up with confidence

Slurry Bed Reactors – SBCR  The slurry- phase bubble column reactor (SBCR) is a newer generation FT reactor  In a SBCR syngas is bubbled up through a slurry made up of the hydrocarbon wax, the liquid at the reaction conditions, and the catalyst suspended in it  The catalyst is a finely divided powder with an average particle size of 50 – 80  m

Slurry Bed Reactors – SBCR  Advantages provided by the use of SBCR: – Good heat transfer – Excellent temperature control – Better selectivity control – Ideal for higher boiling products – Low pressure drop – Ease of adding and removing catalyst – Simple design and construction – Potentially high capacity ( bbl/d)  The gaseous products are removed from the top of the reactor, while the heavy non-volatile wax must be separated from the catalyst which remains in the reactor  Various proprietary internal or external filtration systems have been devised

SBCR - Challenges  If internal filter is used, must avoid plugging by: – By careful design of filtration system – Use of catalysts with adequate particle size distribution – Use attrition resistant catalysts to avoid formation of fines  If external filter is used, must provide adequate means to return recovered catalyst to the reactor without breakage  Sparger design important to provide satisfactory catalyst suspension along the height of the SBCR without introducing excessive catalyst attrition  For scale-up purposes, sophisticated mathematical models are required to describe a SBCR  SBCR hydrodynamics heavily influenced by the reactor size  Not commercially proven in large size with Co catalysts

Operating Conditions  Operating conditions influenced by reactor type as well as the nature of the catalyst  Main controlling parameters: – Temperature: o C for iron; o C for cobalt; high temperature required for fluidized bed reactors – Pressure: atmospheric – 500 psi – Space velocity: h -1 – H2/CO ratio depends on the source of syngas and type of catalyst – Optimum per pass conversion may be more of an economical decision than a technical one

Features of a Multitubular fixed bed reactor  Ease of scale up  Consists of long tubes filled with catalyst  However the fixed bed multi-tubular reactor dimensions depend on the available standard tubes in the market.  This means m ID and 12m long tubes

Why design for the MFBR  Catalyst patents result in expensive technology  Develop suitable catalyst  Test for thermal conductivity – depends on porosity and support material  Test for activity – depends on the loading of the active metal  Determine space velocity that achieve the design reaction rate

Parameters to consider  catalyst activity – depends on Co or Fe loading  heat transfer dynamics – thermal conductivity  production rate → sets the target reaction rate R co  Heat removal ∞ reaction rate  Carry out a heat balance based on the Maximum tolerable tube radial temperature difference

Pressure drop effects  Pressure affects pumping costs as well as catalyst integrity.  Setting a limit on the pressure drop value, the intersection of heat transfer and pressure drop contours is interpreted as the target for design.  The pressure drop is more sensitive to tube radius than the constant temperature difference curve.

Conclusion  FT reactor design should balance the catalyst activity to the tube size and the pressure drop.  Catalyst choice and reactor operating conditions must be done according to the process development methodology proposed here.  Because of the high heat of reaction, matching the reaction rate with the heat removal becomes the constraint.