Question 6 of 6: Refinery environmental risks, hydrocracker design, and a sieve-tray balance

Question 6: Refinery environmental risks, hydrocracker design, and a sieve-tray balance (20 marks — equal value)

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) Environmental risks and their prevention (3 + 3 marks)

(i) Potential risks.

• Air: SO₂ and H₂S, NOₓ, particulates, CO₂, and volatile organic compounds (fugitive VOCs, benzene) from combustion, flares, tanks and leaks.
• Water: oily process wastewater, sour water (sulphides, ammonia, phenols), spent caustic, contaminated stormwater and thermal (cooling-water) discharge.
• Land: oily sludges (tank bottoms, DAF float), spent catalysts and spent clay, and hydrocarbon-contaminated soil/groundwater.

(ii) Prevention. Sulphur recovery (Claus + tail-gas treating) and low-NOₓ burners for air; leak-detection-and-repair (LDAR), vapour recovery and flare minimisation for VOCs; sour-water strippers and multistage wastewater treatment (API separators, dissolved-air flotation, biotreatment) before discharge; spill containment and secondary containment (dikes, impervious liners); and characterised handling, recycling or licensed disposal of sludges and spent catalysts, with groundwater monitoring — all within a compliant environmental-management system.

(b) Four key hydrocracking-reactor design parameters (2 marks)

• Reactor temperature (weighted-average bed temperature, WABT).
• Hydrogen partial pressure (and total operating pressure).
• Space velocity (LHSV — feed rate per catalyst volume).
• Hydrogen-to-oil ratio (treat-gas / recycle-gas rate).

(c) Why those parameters matter (2 marks)

Hydrocracking is a highly exothermic, catalytic process, so these variables jointly set conversion, product slate and catalyst life. Temperature is the main conversion handle, but too high a WABT over-cracks feed to light gas, accelerates coking/deactivation and risks a temperature runaway. Hydrogen partial pressure drives hydrogenation and saturation, suppresses coke formation and thus prolongs catalyst life and improves product quality. Space velocity (via catalyst volume) fixes the contact time and hence conversion per pass. The hydrogen-to-oil ratio provides the hydrogen for reaction, acts as a heat sink/quench that limits the exotherm, and keeps the catalyst clean — together they define a safe, selective operating window.

(d) Sieve-tray (single equilibrium-stage) balance

Given. Entering vapour $V = 550$ kg-mol/hr at $y_{in} = 0.40$ benzene; entering liquid $L = 700$ kg-mol/hr at $x_{in} = 0.45$ benzene. On the plate the leaving fractions obey $y = 1.3x$; the leaving streams keep the same total flows ($V = 550$, $L = 700$ kg-mol/hr).

Find. The benzene mole fractions of the vapour and liquid leaving the plate.

Approach. Write a benzene balance around the tray (constant total flows), substitute the tray relation $y = 1.3x$ to leave one unknown, and solve.

1. Benzene entering the tray. $$\dot n_{B,in} = V\,y_{in} + L\,x_{in} = 550(0.40) + 700(0.45) = 220 + 315 = 535\ \text{kg-mol/hr}.$$
2. Benzene leaving, with $y = 1.3x$. With equimolal leaving flows,$$V\,y + L\,x = 535 \;\Rightarrow\; 550(1.3x) + 700x = 535 \;\Rightarrow\; 1415\,x = 535.$$
3. Solve for the leaving compositions. $$x = \frac{535}{1415} = \boxed{0.378}\ (\text{liquid}), \qquad y = 1.3x = \boxed{0.492}\ (\text{vapour}).$$Check: $550(0.492) + 700(0.378) = 270.3 + 264.7 = 535$ kg-mol/hr benzene — the tray closes.

Physically the tray does its job: the rising vapour is enriched in the more-volatile benzene (0.40 → 0.49) while the descending liquid is depleted (0.45 → 0.38).