Module 2 - Reactor Design

Module from the Hydroprocessing Operations Curriculum curriculum.

REACTOR DESIGN & INTERNALS · Learning Objectives · 1. Describe fixed-bed reactor configurations (single-bed, multi-bed, stacked) 2. Identify reactor internal components and their functions 3. Understand quench zone design and mixing efficiency requirements 4. Recognize common reactor problems (maldistribution, channeling, hot spots) 5. Specify inspection requirements for reactor internals during turnarounds · Reactor Internal Components

Component	Function	Design Criteria	Common Problems	Inspection Points	Materials
Inlet Distributor	Evenly distribute feed + H2 across reactor cross-section	Flux variation <5% across bed, liquid/gas co-current downflow	Plugging, erosion, uneven flow, drip point blockage	Visual inspection, flow test, drip point count	316 SS or Alloy 825 (sour service)
Vapor/Liquid Distributor Tray	Redistribute liquid between beds after quench mixing	Bubble cap or chimney tray, 100+ distribution points per m²	Tray damage from catalyst dumping, seal leg flooding	Level check, cap inspection, weir height	316 SS or 321 SS
Quench Zone / Mixing Chamber	Mix cold quench H2 with hot reactor effluent between beds	ΔT variation <10°F across next bed inlet, mixing efficiency >95%	Insufficient mixing (radial ΔT >20°F), quench nozzle erosion	Thermocouple survey post-startup, nozzle inspection	316 SS, Alloy 625 quench pipe
Catalyst Support Grid	Support catalyst bed weight, allow effluent to pass	Support full bed weight + surge loads, screen opening sized to retain catalyst	Grid deflection, screen tearing, catalyst migration	Deflection measurement, screen integrity, weep holes clear	Carbon steel grid, 316 SS screen
Thermocouple Bundle	Monitor bed temperatures at multiple radial and axial positions	Minimum 3 radial positions per bed (center, mid, wall), 2-4 axial per bed	Broken TCs, incorrect location, slow response	Verify all TCs reading, compare redundant readings	316 SS thermowell, Type K TC
Outlet Collector	Collect treated product from bottom of last bed, direct to outlet nozzle	Sized for full flow without excessive ΔP, screen to retain catalyst	Plugging, erosion at high velocity points	Visual inspection, screen integrity	316 SS or CS with SS screen
Scallop Screens (if radial flow)	Contain catalyst in radial-flow reactor while allowing flow through	Slot width sized to retain smallest catalyst particle (typically 0.5mm)	Plugging with fines, erosion, buckling	Slot measurement, straightness check, attachment welds	Profile wire (316 SS or Alloy)
Reactor Configurations
Configuration	Description	Typical Service	Advantages	Disadvantages	Typical Pressure (psig)
Single-Bed	One catalyst bed, no quench	Naphtha HDT, light feed treating	Simple, low cost, easy catalyst change	No temperature control within bed, limited severity	300-600
Multi-Bed with Quench	2-5 catalyst beds separated by quench zones	Diesel HDT, VGO HDT	Temperature control per bed, higher conversion capability	More complex internals, quench H2 consumption	500-1500
Stacked-Bed (Series Flow)	Multiple reactors in series, same H2 loop	Hydrocracking, deep HDS	Independent catalyst systems, flexible reactor staging	Higher cost, more equipment, complex operation	1500-3000
Two-Stage Hydrocracking	Stage 1: HDN/HDS, Stage 2: Cracking with clean feed	Maximum conversion hydrocracking	Cleaner Stage 2 feed → longer catalyst life, higher conversion	Two reactor loops, H2 separation between stages, expensive	2000-3000
Counter-Current (upflow/downflow)	Treat feed in last bed first (reverse flow concept)	Deep ULSD desulfurization	H2S removal between stages improves equilibrium for deep HDS	Complex, limited commercial installations	500-1000
Source: FOS Chief Files — Hydroprocessing Design Manual, Example Calcs-Hydro, Reactor_Inspection_Reporting_Tool_v1.xlsx

Source: Hydroprocessing_Operations_Curriculum_v1.xlsx · Sheet: Module 2 - Reactor Design

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