Q & A > Question Details
Can anybody help me to calculate the total gas flow in the riser section of FCCU
27/02/2012 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
The previous respondent has answered it nicely. The linear velocity can also be determined experimentally by radioactive tracing of the hydrocarbon and measuring the responses appropriately at different heights. Likewise the velocity of catalyst can also be determined by radioactively labeled catalyst and measuring the radioactive response
If you wish to calculate the mass superficial velocity near riser O/L it is simple. The mass of all the feed hydrocarbon entering streams is known in kg/sec, the mass of all the entering steam i.e. fluidization steam, atomizing steam are also known in kg /sec. Add all of them. Correct it by the coke content on the catalyst formed due to coking. All the hydrocarbons and fluidization medium are in vapor at that high temp and low pressure and their mass velocity is known. Divide the mass flow in kg/ per second by cross sectional area of riser. Thus you know superficial mass velocity.
In case you want to calculate volumetric or linear gas velocity , that is also simple. You know the mass flow rates of all the streams i.e. steam, dry gases, wet gases, naphtha, LCO, HCO, CLO, recycle from the fractionator. Their molecular weight can also be estimated by their properties. Thus the flow rate in kg-moles / sec can be calculated for all the streams from the process data at the riser outlet. Since you know the temperature, pressure and flow rate in kg-moles/sec, you may easily calculate the volumetric flow rate of the gas at that conditions.
15/02/2012 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Let’s analyze this project. Entering the bottom of the riser are hydrocarbon and water vapor, the latter being from fluidization steam at various injection points from the regenerated catalyst standpipe to the riser bottom and mixing and atomization steam to the feed nozzles. Those steam flow rates are often not metered, but if they are, you have the water vapor in lbs/hr or kg/hr.
Weight Units
There can be 3 or 4 hydrocarbon streams such as fresh feed, LCO recycle, slurry recycle, and naphtha recycle. An outside naphtha stream may also be present. Some of the hydrocarbon streams may not enter the riser at the bottom, rather at higher elevations. At a given elevation, add the lbs/hr of each hydrocarbon stream present. These are determined by meter readings and gravities. A meter can be calibrated, including sampling for gravity measurement and using the correct analysis and temperature for the meter factor.
Volume Units
If you want the flow in volume units, plots of the hydrocarbon physical properties along the length of the riser are needed. Pilot plants have the capability of taking samples at various elevations for the purpose of determining where and to what extent the conversion is occurring. Thus, specific volumes can be calculated and plotted for the riser length. Commercial units generally do not have riser elevation sample connections.
By adding all products and their analyses to determine a yield vector, one can determine the hydrocarbon analysis at the riser top. Knowing that most of the conversion occurs in the first 10 feet of riser (or whatever your licensor tells you), and having calculated the composition at the bottom of the riser, one can estimate a specific volume curve along the length of the riser. The temperature profile of the riser length must also be measured or assumed. Thus, SCF/H and ACF/H can be determined.