Key Operating Parameters of Residue Hydrocracking Unit
Understanding the key process variables is critical for the safe and efficient operation of the hydrocracking unit as the hydrocracking process is exothermic, generating heat, and if not responded it can lead to temperature excursion and runaway situations. The proper operation of the hydrocracking unit depends upon the careful selection and control of the process conditions. By cautious monitoring of the process variables, the unit can operate to its full potential.
The typical range of Hydrocracking unit parameters is shown in the table below;
Key operating variables that impact hydrocracking unit performance with respect to catalyst activity, stability, product selectivity, product quality, and yields are discussed here;
1. Conversion
It defines the performance and severity of the hydrocracking operation, which has an impact on the catalyst temperature and catalyst stability. A high conversion will produce heavier products with high distillation endpoints to compensate this high reactor temperatures are required. A high conversion reduces catalyst life owing to the severe reactor operation. Conversion can also be increased by increasing the pulling of the desired product (diesel) by slightly changing the fractionator conditions but it will increase the endpoint of the desired product.
The simplest method of conversion calculation for the once-through hydrocracking process is the gross conversion that depends upon the volume of unconverted oil from the bottom of the fractionation tower.
Gross Volume % Conversion = (Fresh feed Flow -Unconverted Oil Flow/Fresh Feed Flow) *100
Or Conversion=100-Fractionator bottom UCO volume %
e.g. Fresh Feed 200 m3/hr, while UCO is 100 m3, then conversion can be calculated as follows;
Gross Volume % Conversion = (200-100)/200*100= 50% conversion
Alternative Method= 1- (100/200) *100 = 50 %
Other conversion calculations are; Conversion cut point (CCP) in a hydrocracker = cutpoint (TBP) of unconverted oil, sometimes is called RCP for recycle units.
A true conversion (weight) = {1-weight of cutpoint plus material in the product)/(weight of cutpoint plus material in the feed)}*100
True conversion (volume) = {1-volume of liquid product/volume of unconverted oil in the Feed}*100%.
Below is the relationship of conversion with other parameters;
2. Catalyst Temperature
A high temperature in the hydrocracking reactor increases the rate of reaction and, therefore, increases conversion and shifts yields to lighter products. Whereas a decrease in temperature enhances yields of heavier products. More heat is generated at higher conversion, so conversion needs to be controlled within allowable temperature rise limits, and meet the heat removal capability of the unit to avoid temperature excursion situations.
Higher catalyst temperatures will reduce the catalyst life by increasing the coking laydown rate over the catalyst. Hydrocracking catalysts can be kept on stream for several years however their gradual aging affects both activity and selectivity. A decrease in catalyst activity will be reflected in the loss of conversion. In order to maintain the conversion constant, the operating temperature is gradually increased.
Generally, WABT (Weighted Average Bed Temperature) is used to monitor the reactor temperature profile and deactivation rate. Deactivation rate can be expressed in terms of temperature rise per month or per day. Further, the axial temperature rise and the radial spread across the catalyst bed are also critical parameters to be controlled within the design limit.
3. Hydrogen Partial Pressure
The high hydrogen partial pressure is necessary for saturating cracking products into more stable components, hydrotreating reactions, and keeping the coke growth under control. High hydrogen partial is necessary for reducing coke formation, increasing the hydrogen availability for reaction on the catalyst surface, and increasing conversion. It is always good to maintain maximum hydrogen partial pressure for good product quality, catalyst stability, and a longer life cycle. For detailed discussion related to hydrogen partial pressure please the previous blog ” Improving Hydrogen Partial Pressure in Hydrotreating Unit“.
Recycle gas purity is dependent on feedstock, process severity (light end production), makeup gas H2 purity, purge gas rate, and recycle gas scrubbing. The consequence of operating below the designed hydrogen partial pressure is rapid catalyst deactivation by coke deposits. High system pressure is restricted due to design limitations, so it is essential to maintain high hydrogen purity, to improve the hydrogen partial pressure.
4. Recycle Gas Rate
Recycle gas rate is expressed as volumetric gas flow per volume of feed. Recycle gas rate can be expressed as standard cubic feet per barrel of fresh feed (SCFBFF) or normal cubic meter of gas per cubic meter of feed (nm3/m3). Recycle gas provides the hydrogen gas for reaction, maintains hydrogen purity, and removes the heat of reaction. Higher recycle gas rate improves the catalyst activity due to hydrogen availability for reaction resulting in increased catalyst life. Higher gas rates reduce the coking rate over the catalyst.
5. LHSV (Liquid Hourly Space Velocity)
LHSV is defined as the volumetric liquid feed rate per unit volume of the catalyst. Space-time or the residence time is inverse of LHSV. Increasing the fresh feed rate or LHSV increases the severity of the catalyst. Increasing the severity will require higher WABT for the same conversion and thus it will increase the deactivation rate. Moreover, lower LHSV is also critical in terms of flow distribution over catalyst beds. LHSV is usually a pre-established designed parameter that determines the amount of catalyst and therefore reactor capacity for a required production rate.
LHSV =Total liquid feed rate to the reactor (m3/h)/Total catalyst volume (m3) = h−1
WHSV=Total liquid feed rate to the reactor (kg/h)/ total catalyst weight (kg) = h-1
6. Fresh Feedstock Quality
The quality of the fresh feed affects catalyst life, reactions severity, and products quality. The organic nitrogen compounds, which are converted to ammonia, have inhibiting effect on the hydrocracking catalyst activity thus resulting in higher temperature requirements. The content of unsaturated compounds (such as olefins and aromatics) sulfur and nitrogen present in the feed has an effect in increasing hydrogen consumption, high exotherm over the catalyst, and greater tendency to deposit coke.
In general, the heavier the feedstock (higher feed endpoint), the more severe the operating conditions are required to obtain satisfactory product yields and quality. Heavier feeds are usually processed at higher temperatures, higher hydrogen partial pressures, lower product yields, and lower conversion. The more severe process conditions required by heavier feedstocks result in higher light ends (C1–C4) and light naphtha yields, as well as lower middle-distillate yields.
Heavier feeds contain higher contents of metals, asphaltenes, and Conradson carbon that affect catalyst stability and cycle length. Therefore, limits are often set for these contaminants to make a feedstock acceptable for operation. Normal range for these contaminants is; (Ni + V) <2 ppm; asphaltenes <500 ppm; Conradson carbon residue <1 wt %. Please view the previous blog ” Reducing Coke Formation in Hydrotreating Catalysts“.
7. Combined Feed Ratio (CFR)
CFR is the volumetric ratio of fresh feed plus recycle feed to fresh feed. CFR of 1 is a once-through operation that does not have recycle and as CFR is increased in recycle operation at fixed gross conversion the conversion per pass across reactor drops. In recycle operation, conversion per pass determines selectivity, which is a function of CFR. A hydrocracking unit, when designed for recycle operation, is designed to operate at a constant recycle rate (CFR), although higher CFR gives escalated conversion. Higher CFR may cause a temperature rise across the first bed of the reactor due to fast hydrogenation reactions if the feed is recycled from the thermal cracking unit.
CFR= (Fresh Feed Flow+Recycle Feed Flow)/Fresh Feed Flow
For example, 150 M3/hr fresh feed and 40 m3/hr recycle then CFR can be calculated as follows;
CFR = (150+40) /150 = 1.26
8. Hydrogen Consumption
It is an indication of the rate of hydrogenation in a hydrocracking reactor. Greater the hydrogen consumption greater will be the rate of hydrogenation reactions, hence significant heat release. Further, higher hydrogen consumption also shows the presence of a higher number of unsaturated compounds in the feed. The designed chemical consumption of a specific hydrocracking unit is based upon the feed type and the type of required products and the design conversion level. In general, for a given boiling range feedstock, a reduction in API gravity (increase in specific gravity) indicates an increase in the aromaticity and, therefore, higher heats of reaction and higher hydrogen consumption.
Total hydrogen consumption at hydrocracking unit is the summation of a. chemical hydrogen consumption for hydrotreating & hydrocracking reactions, b. hydrogen losses with hydrocarbon from reactor section to the stripping section, c. mechanical losses from the recycle gas compressor, d. purging of process gas from the high-pressure separator to control the hydrogen purity.
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