High-temperature steam reforming reaction tubes

Absolute values of the heat transfer parameters that ean be used for seale-up are dififieult to determine in bench-scale units due to the very high gas velocities and heat fluxes in industrial units. Attempts to determine them in pilot plants operating at industrial conditions but without reaction are also highly uncertain due to small driving forces. The steam reforming reaction is, however, strongly endothermic and limited by chemical equilibrium. This implies that for a new catalyst, the reaction will be close to chemical equilibrium in the major part of the tube, so variation in catalyst activities will only have a small impact on the temperature profile (refer to Section 3.3.7). If, however, heat transfer... 

Transferred Duty. In order to supply the heat for the overall endothermic steam reforming reaction, the catalyst is loaded into a number of high alloy tubes placed inside a furnace equipped with burners. Typical inlet temperatures are 450-650 C, and the product gas leaves the reformer at 700-950°C depending on the applications. 

The continual development of the steam reforming process ensured that this became the most practicable way to produce synthesis gas and ammonia on the large scale. A mixture of hydrocarbons and super-heated steam is passed through the reactor tubes that are packed with the nickel catalyst, and suspended in a furnace that operates at temperatures around 1000°C (Figs. 9.1 and 9.2). The steam reforming reaction is extremely endothermic and the heat of reaction must be supplied continually at a very high operating temperature. The catalyst must... 

A promoted nickel type catalyst contained in the reactor tubes is used at temperature and pressure ranges of 700-800°C and 30-50 atmospheres, respectively. The reforming reaction is equilibrium limited. It is favored at high temperatures, low pressures, and a high steam to carbon ratio. These conditions minimize methane slip at the reformer outlet and yield an equilibrium mixture that is rich in hydrogen. ... 

The subsequent steam reforming section is operated at very high temperatures 850-900 °C. The SMR catalysts themselves are already active below 400 °C, but high temperatures are necessary to drive the strongly endothermic reaction forward [8]. In industry, nickel catalysts are used in high-alloy reaction tubes, which are heated by external burners. This design is expensive and leads to heat losses, although much of the heat is recuperated. Noble metal catalysts such as sup-... 

Their thermal efficiency is not very different and in a top-fired furnace can be as high as 95 %. The enthalpy difference between inlet and exit, often referred to as reformer duty, is made up of the heat required to raise the temperature to the level at the tube exit and the enthalpy of the reforming reaction. In a typical tubular steam reforming furnace, about 50% of the heat generated by combustion of fuel in the burners is transferred through the reformer tube walls and absorbed by the process gas (in a conventional ammonia plant primary reformer 60 % for reaction, 40% for temperature increase). 

Pyrolytic carbon is formed mainly by three different reactions, namely, the reversible decomposition of methane (Reaction 2.5), the irreversible cracking of higher hydrocarbons (Reaction 2.6), and/or coke formation (Reaction 2.7). The formation of these carbon deposits leads to the breakdown of the catalyst and hot spots in the reactor. Pyrolytic carbon is usually found as dense shales on the reformer wall or encapsulating the catalyst particles. The process leads to the deactivation of the catalyst and increase of pressure drop across the reformer tubes. The thermal cracking of hydrocarbon occurs at high temperatures and at low steam to hydrocarbon ratios. 

In the CAR process, the natural gas feed is mixed with steam and introduced into the CAR reactor via a tube sheet to the catalyst-filled tubes in which reforming to synthesis gas takes place. The natural gas is partially converted, and the slip methane is allowed in the lower chamber where partial oxidation takes place. In this lower section, temperatures are about 1300-1400°C. The resulting hot synthesis gas then passes upward and supplies heat to the primary reforming reaction inside the catalyst tubes. An important element in the CAR reactor is the tube sheet, which acts as a feed stream distributor to the reformer tubes. In addition, there are enveloping tubes around the catalyst tubes, which constrict the flow of the autothermal product gas, thereby increasing the convective heat transfer coefficient. The CAR reactor, due to the high temperatures, is also jacketed with water. 

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