Practical Process Engineering

57- Outlet CO for Naphta steam reforming

56- Find outlet CO2 composition graphically based on P, T, and S/C. (Feed: Natural Gas)

55- Find outlet CO composition graphically basd on P, T and S/C. (Feed: Natural Gas)

54-Find methane slip graphically based on P, T and S/C ratio (Feed: Natural Gas)

53- Effect of pressure and temperature on methane slip at fixed S/C ratio. Higher temperature at constant pressure and lower pressure at constant temperature yields more reforming.

52- Effect of temperature and S/C ratio on methane slip at fixed pressure= 32 barg. Higher temperature at constant S/C ratio and higher S/C ratio at constant temperature gives more reforming.

49- There are three significant variable affecting the reactions: a) Temperature b) Pressure c) Steam/ Carbon ratio (mole of steam/ mole of carbon) 50- What are the key performance indicators (KPI's) of a steam reformer? Suppose your client asks you to evaluate the steam reformer operation. Here is the list of items you should review carefully: a) Reactor pressure drop b) Methane slip (amount of methane at reformer outlet) c) Tube skin temperatures. Steam reforming is an endothermic process. Heat is supplied using burners. Heat is transfered to the tube wall and then to the gas and catalyst. Tube skin temperature are critical and should always be monitored. d) Radiant box temperature and pressure 51- Because natural gas is usually under elevated pressure and the reforming reaction entails an increase in volume, significant savings in compression energy can be achieved if the process is performed under elevated pressure. But thermodynamically this is unfavorable: on account of the volume increase, an increase in pressure will reduce the conversion of methane. To compensate this, higher temperatures will become necessary (limited by reformer tube material). On the other hand, higher steam-to-carbon (S/C) ratio have a beneficial effect on the equilibrium methane concentration and could to some extent mitigate the negative influence of the increased pressure, but the penalty is a higher energy consumption.

The second reaction is named Water-Gas shift (WGS) reaction. It is exothermic.

48- Natural gas (mostly Methane) and Naphta are the typical feeds for steam reforming. The earlier is the main feed. Let's look at the steam reforming thermodynamics. The reactions are listed here:

47- The radiant box enclosed the reactor tubes. The main heat transfer from the burners to the tubes is radiation. The radiant box pressure is always kept under atmosphere.

46-Steam reformer schematic in cluding radiant box at the right side, convection duct with coils at the middle and flue gas stack located at the left side.

45- Steam reforming is the most economic commercialized method to produce large amount of synthesis gas (syn gas). By syn gas we mean, a stream contains mainly hydrogen and carbon monoxide. Many chemicals are produced using this gas; Methanol, Ammonia, Fischer-Tropsch, to name but a few.

Now you haw a qualitative understanding of catalytic reactions and catalyst deactivation concept. Let's move forward to the next part and focus only on steam reforming catalysts and its problems.

42- Poisioning: Removal of active sites via the strong chemisorption of impurities on the surface, thus blocking access of the reactants. Catalyst poisons can be classified in various ways, such as by their degree of affinity for the surface, as temporary or permanent, or as selective ornonselective. In particular, we will make a distinction between temporary and permanent poisons, a point which rests upon the degree of reversibility of chemisorption on the surface, and inhibitors of reaction rate which are either reactants or products of the main reaction being carried out. 43- Since we are really considering this process in terms of chemisorption, various degrees of reversibility can be associated with poisoning under different circumstances or in different reaction systems. Conversely, the problem of regeneration of poisoned catalysts is also a function of the degree of reversibility involved in the poisoning process. 44- Coke often encountered in catalyst coking by large amount, but even micromoles per gram of certain poisons are sufficient to completely deactivate some catalysts.

Fresh catalyst at t=0 VS. coked catalyst at t=t

40- Sintering is usually negligible at temperatures below 40% of the melting temperature of the solid. 41- Coking or Fouling: carbonaceous material (coke) deposits on surface. It is called self-posioning as well. In many reactions involving hydrocarbon molecules (or even carbon oxides) there are side reactions on the catalyst surface that lead to the formation of carbonaceous residues which tend to cover over the active surface. In many instances the "mechanism" of coke formation can be visualized as a kind of condensation polymerization on the surface resulting in macromolecules of empirical composition approaching CH. Since coke deposition normally manifests itself as macroscopic deposits of carbonaceous material on the active surface, the quantity of such residues may be measurable on the same scale as the amount of catalyst—coked catalysts containing 15 or even 20 wt. % of the deposit (grams per gram of catalyst) are not unusual in certain services. First, the deactivation effect is accomplished primarily by the covering over of active sites by the residue; reactants are denied access to active sites by physical screening rather than by the competitive chemisorption at work in poisoning. Second, since the amounts of coke formed can be quite large, such deposits can eventually build to the point of blocking pores in the internal volume of the catalyst and further restricting access of reactive molecules to active surface. Two types of coking; Pyrolytic and Whisker, would be explained in more detail later in this discussion.

Sintering of Cu catalyst

Sintering: Agglomeration of active sites; loss of reactive surface area

38- Catalyst deactivation mechanism are catagorized into 3 groups: a) Aging or Sintering b) Coking or Fouling c) Poisiong 39- Aging or Sintering: Sintering, also referred to as aging, is the loss of catalytic activity due to a loss of active surface area resulting from the prolonged exposure to high gas-phase temperatures. Most commonly this is a thermally activated process and is physical rather than chemical in nature. Sintering can occur in both supported metal catalysts and unsuppor­ted materials such as zeolites or amorphous silica-alumina. In the former case, active surface is lost via the agglomeration of small metal crystallites into larger ones with smaller surface-to-volume ratios, while in the latter instance the process may involve actual collapse of the internal pore struc­ture.