Crude assays translate a complex barrel into data that planners can trust. A good assay shows how a crude behaves from the first vapor to the heaviest residue, and which units will love it or struggle with it. This article explains the Top 10 Crude Assay Parameters for Petroleum Refining Planning and Optimization and how each parameter feeds linear programming models, hydrogen and energy balances, and constraint management. You will learn what drives conversion severity, catalyst life, and product quality outcomes. Each section pairs the lab measurement with decisions it informs, so both basic and advanced readers can connect values to actions and margins.
#1 True Boiling Point Distillation and Cut Yields
True boiling point distillation maps a crude into narrow cuts and cumulative yields across the entire temperature range. It reveals where the molecules sit, from light naphtha through vacuum gas oil into residue, with associated properties for each cut. For planners, TBP data drives crude unit cut points, splitter design, and expected yields into downstream hydrotreaters, FCC, hydrocrackers, cokers, and lube trains. In an LP, TBP plus cut qualities link to product blending constraints, octane, cetane, and sulfur. Operationally, it supports furnace duty estimates, flash zone conditions, and heat integration, keeping energy targets realistic. It improves blend planning.
#2 API Gravity and Density Profile
API gravity and density profile indicate how light or heavy a crude is and where mass sits across cuts. Lighter barrels generally yield more valuable light products, but API must be balanced with sulfur, metals, and CCR to avoid overestimating margins. Assay tables often report whole crude API plus cut densities, which planners use to allocate hydrogen, set FCC versus hydrocracker feeds, and meet vapor pressure limits. Density also feeds Watson K factor and viscosity correlations in LP property models. Monitoring API drift across cargoes protects product slate forecasts, storage utilization, and ship scheduling commitments.
#3 Sulfur Content and Speciation
Total sulfur and sulfur speciation determine hydrotreating severity, hydrogen consumption, and sulfur recovery unit loading. Assays should separate whole crude sulfur, cut sulfur, H2S, and mercaptans, since reactivity changes by boiling range and chemical type. Planners translate sulfur into reactor temperature targets, catalyst volumes, and amine system circulation. Speciation also affects corrosion risks and caustic treating needs for light ends and naphtha. In optimization models, sulfur becomes both a quality and a capacity constraint that can bind across multiple units, so accurate sulfur distributions prevent infeasible LP solutions and protect diesel and gasoline pool compliance.
#4 Basic and Total Nitrogen
Basic and total nitrogen poison acid sites in FCC and hydrocracking catalysts and raise hydrotreating severity. Crude assays should report total nitrogen, basic nitrogen, and their distribution by cut, since heavy basic species concentrate in VGO and residue. Planners convert nitrogen numbers into make up catalyst costs, guard bed sizing, and allowable feed blending levels. Nitrogen also burdens sour water, ammonia, and NOx limits in furnaces. When nitrogen is understated, LP models overpredict conversion and run lengths. Reliable nitrogen data enables realistic hydrogen balances, protects octane in FCC gasoline, and reduces risk of unplanned catalyst changeouts.
#5 Total Acid Number and Naphthenic Acids
Total Acid Number signals the presence of naphthenic acids that can drive high temperature corrosion, especially in the 220 to 400 degree Celsius range. Assays need TAN by cut, not just whole crude, so planners can route corrosive material to compatible metallurgy and adjust dilutions. TAN informs desalter temperature windows, corrosion inhibitor doses, and vacuum column flash zone operations. In commercial planning, TAN becomes a routing constraint that can limit crude throughput or force blend ratios. Accurate TAN data protects run length, avoids unexpected fouling of heat exchangers, and preserves margin by preventing off spec corrosion related outages.
#6 Trace Metals and Inorganic Contaminants
Trace metals such as nickel and vanadium deactivate FCC and hydroprocessing catalysts and promote coke and gas formation. Sodium and calcium can hitchhike as salts and cause fouling or catalyst poisoning if desalting is ineffective. Assays should quantify Ni, V, Fe, Na, Ca, and their cut distributions. Planners translate metals into guard bed requirements, catalyst make up rates, and allowable FCC feed metals to protect gasoline yield. Desalter performance targets and wash water needs also depend on salt and sediment data. Getting metals right prevents LP optimism, protects SRU capacity, and supports long term catalyst life budgeting.
#7 Asphaltenes and SARA Fractions
Asphaltenes and SARA analysis partition a crude into saturates, aromatics, resins, and asphaltenes, which signal stability risk and fouling tendency. High asphaltene content can trigger precipitation during blending, heating, or pressure drop, creating exchanger fouling and filter plugging. Assays that include SARA by cut help planners route unstable material toward coking or residue hydrocracking while protecting FCC slurry circuits. In optimization, SARA supports viscosity and CCR correlations, enabling better predictions of vacuum resid behavior. Reliable SARA data also informs solvent deasphalting options and aids crude compatibility screening before switchovers, reducing unplanned downtime and cleaning campaigns.
#8 Conradson Carbon Residue and Coke Tendency
Conradson carbon residue or micro carbon residue quantify the coke forming tendency of heavy fractions and predict coker, visbreaker, and FCC performance. Assays should provide CCR or MCR by cut, since vacuum resid CCR drives coke drum yield, gas make, and furnace fouling. Planners use CCR to set conversion severity, drum cycle times, and to forecast LPG and fuel gas balances. In LP models, CCR correlates with hydrogen consumption in residue hydrocracking and with catalyst deactivation. Accurate CCR avoids overstating liquid yields, protects regenerator temperatures in FCC, and keeps heater duty and decoking schedules within safe and economical limits.
#9 Viscosity and Rheology Across Cuts
Viscosity and rheology across cuts govern pumpability, heat transfer, atomization, and blending behavior. Assays should include kinematic viscosity at multiple temperatures and viscosity index for distillates. Planners use these data to set furnace outlet temperatures, choose pump and seal designs, and forecast diesel cold flow blending with kerosene. Viscosity also feeds crude pipeline hydraulics, tank heating duty, and vacuum column pressure drop limits. In LP property pools, viscosity constraints interact with density and aromatics to meet finished product specs. Reliable viscosity profiles prevent bottlenecks during winter operations and reduce reprocessing that erodes margins and energy efficiency.
#10 Light Ends, RVP, and Gas Plant Balance
Light ends content and Reid vapor pressure define stability and vapor management from tanks to blenders. Assays that measure C1 to C4 distribution, dissolved gases, and naphtha RVP help planners avoid venting, cavitation, and off spec gasoline volatility. RVP and light ends affect crude unit overhead systems, condenser loads, and flare risk during startups and switches. In LPs, vapor pressure constraints link to butane availability, alkylate balancing, and seasonal gasoline strategies. Accurate light ends data enable safe storage set points, minimize losses, and maximize butane blending value without violating vapor pressure and safety regulations. It improves gas plant tuning.