Question 2 of 5: Conversion processes; crude sulphur material balance

Question 2 — Conversion processes; crude sulphur material balance (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Purposes, feedstocks and reactions

(i) Catalytic reforming. Purpose: upgrade low-octane heavy naphtha into high-octane, aromatics-rich reformate for the gasoline pool, co-producing hydrogen for the refinery. Feedstock: hydrotreated heavy straight-run naphtha (C₇–C₁₀, ~90–180 °C) on Pt–Re/alumina catalyst. Desirable reactions: naphthene dehydrogenation ($\text{C}_6\text{H}_{12}\rightarrow\text{C}_6\text{H}_6+3\text{H}_2$), paraffin dehydrocyclisation and isomerisation — all octane-boosting. Undesirable: hydrocracking of paraffins to light gas (yield loss) and coke laydown that deactivates the catalyst.

(ii) Catalytic cracking (FCC). Purpose: crack heavy gas oil into gasoline-range material and C₃–C₄ olefins (LPG, alkylation feed). Feedstock: vacuum gas oil (VGO), sometimes with residue, on acidic zeolite catalyst. Desirable: β-scission of long paraffins to gasoline olefins/paraffins, plus isomerisation and mild aromatisation that raise octane. Undesirable: over-cracking to dry gas and heavy coke deposition on the catalyst (burned off in the regenerator).

(iii) Hydrocracking. Purpose: crack difficult heavy stocks into high-quality, low-sulphur naphtha, jet and diesel under high hydrogen pressure. Feedstock: VGO, coker/FCC cycle oils on a bifunctional (metal + acid) catalyst. Desirable: cracking accompanied by hydrogenation (olefin/aromatic saturation) and hydrodesulphurisation/-denitrogenation. Undesirable: excessive gas make (over-cracking), very high hydrogen consumption and thermal runaway if exotherm is not controlled.

(iv) Fluid coking. Purpose: continuously convert vacuum residue into distillates and petroleum coke in a fluidised bed of hot coke particles. Feedstock: vacuum residue and other heavy bottoms. Desirable: thermal cracking of residue to gas oils, naphtha and gas. Undesirable: substantial coke make (part burned to supply heat) and a lower liquid yield than hydrogen-addition routes.

(b) Sulphur material balance

Given. A crude of $^{\circ}\text{API}=28$ (so $\mathrm{SG}=141.5/(28+131.5)=0.887$) carries $1750\ \text{kg/hr}$ of sulphur at $1.0\ \text{wt\%}$ S. The distillate cuts and their sulphur contents are:

Find. The complete material balance (mass flow and sulphur flow of every stream) and, in particular, the sulphur content of the 427⁺ °C residue, which the table leaves blank.

Approach. Fix the basis from the sulphur specification, distribute mass by the assumed yields, compute the sulphur carried by each distillate cut, and close the overall sulphur balance to obtain the residue sulphur.

1. Total crude feed from the sulphur specification. With $1.0\ \text{wt\%}$ S and $1750\ \text{kg/hr}$ of sulphur, $$\dot m_{\text{crude}}=\frac{\dot m_{\text{S}}}{x_{\text{S}}}=\frac{1750}{0.010}=\boxed{175{,}000\ \text{kg/hr}}.$$
2. Mass of each cut from the assumed yields. Multiplying the crude rate by each yield gives, for example, heavy naphtha $0.14\times175{,}000=24{,}500\ \text{kg/hr}$ and residue $0.37\times175{,}000=64{,}750\ \text{kg/hr}$ (full column in the table below).
3. Sulphur carried by each distillate cut. Sulphur flow $=\dot m_i\,x_{\text{S},i}$. The light ends are taken as sweet ($\approx 0$). Thus $$\dot m_{\text{S,naphtha}}=24{,}500(0.0017)=41.65,\quad \dot m_{\text{S,kero}}=24{,}500(0.0080)=196.0,$$ $$\dot m_{\text{S,LGO}}=21{,}000(0.0050)=105.0,\quad \dot m_{\text{S,HGO}}=26{,}250(0.0090)=236.25\ \text{kg/hr}.$$ Their sum is $\sum\dot m_{\text{S,dist}}=578.9\ \text{kg/hr}$.
4. Residue sulphur by overall balance. Sulphur is conserved, so the residue must carry the balance: $$\dot m_{\text{S,resid}}=1750-578.9=1171.1\ \text{kg/hr},\qquad x_{\text{S,resid}}=\frac{1171.1}{64{,}750}=\boxed{1.81\ \text{wt\%}}.$$ The residue is the most sulphur-rich stream, as expected — heteroatoms concentrate in the heavy bottoms.

The completed balance (mass and sulphur both closing on 175,000 kg/hr and 1750 kg/hr):