From Naphtha to Synthesis Gas: The Steam Reforming Process

Synthesis gas, a crucial mixture of hydrogen (H2) and carbon monoxide (CO), serves as a building block for numerous petrochemicals. Steam reforming of naphtha, a hydrocarbon mixture derived from crude oil, is a prominent method for producing this valuable gas. Let's explore the process flow, step by step.

Synthesis gas production from naphtha by steam reforming

Naphtha Steam Reforming Process

Step 1: Naphtha Vaporization and Preheating

The journey begins with liquid naphtha. It's first vaporized and then preheated to about 220°C. This initial heating step is crucial for preparing the naphtha for the subsequent reactions.

Step 2: Hydrogenation and Desulfurization

Next, the vaporized naphtha is heated further to around 380°C in a fired heater. This temperature is optimal for the hydrogenation reaction. During this stage, sulfur compounds present in the naphtha react with hydrogen, transforming into hydrogen sulfide (H2S).

Removal of Sulfur Compounds

Sulfur is a catalyst poison for the downstream reforming process, so its removal is essential. The H2S is then absorbed and removed, typically using a zinc-based absorbent.

Step 3: Adiabatic Pre-Reformer

The desulfurized naphtha, now free from sulfur, enters an adiabatic pre-reformer. Inside the pre-reformer, the heavier hydrocarbons in the naphtha undergo partial reforming, converting into simpler molecules like methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and more hydrogen (H2).

Step 4: Steam Reformer

The partially reformed gas mixture proceeds to the steam reformer. This is where the bulk of the reforming takes place. The reformer consists of catalyst-filled tubes positioned vertically within a furnace.

Reforming Reactions

The high temperature and steam-rich environment in the reformer further convert methane and other hydrocarbons into more H2, CO, and CO2.

Step 5: Secondary Reformer (with Air Injection)

The gas stream from the steam reformer enters a secondary reformer. Here, air is injected into the mixture. The oxygen in the air reacts with the remaining hydrocarbons and some of the CO, further increasing the hydrogen content.

Step 6: Waste Heat Recovery

The hot synthesis gas exiting the secondary reformer contains valuable thermal energy. This heat is recovered using a waste heat boiler to generate process steam, which can be used elsewhere in the plant, improving the overall energy efficiency of the process.

Step 7: Synthesis Gas Composition

The resulting synthesis gas from the secondary reformer has a typical composition (in mole percent) of approximately: 

• H2: 51.52% 
• N2: 22.79% 
• CO: 13.57% 
• CO2: 11.25% 
• Ar: 0.27% 
• CH4: 0.60%

Here are the reforming reactions for the naphtha steam reforming process:

Reforming Reactions

Primary Reforming Reaction

• C_nH_m + nH₂O → nCO + (n + m/2)H₂

Secondary Reforming Reaction (with Air Injection)

• CO + 1/2O₂ → CO₂
• CH₄ + 2O₂ → CO₂ + 2H₂O
• C_nH_m + (n + m/4)O₂ → nCO₂ + (m/2)H₂O

Shift Conversion Reactions

• CO + H₂O → CO₂ + H₂ (High-Temperature Shift)
• CO + H₂O → CO₂ + H₂ (Low-Temperature Shift)

Methanation Reaction

• CO + 3H₂ → CH₄ + H₂O
• CO₂ + 4H₂ → CH₄ + 2H₂O

Key Advantages of Naphtha Steam Reforming

Established Technology

Steam reforming of naphtha is a well-established and widely used technology.

High Efficiency

The process can achieve high conversion rates and efficient heat recovery.

Versatile Feedstock

Naphtha is readily available in many refineries.