Refinery Delayed Coking units produce light components from the top of the fractionation section as one of the products. The extent of recovering LPG from this stream via the Gas Recovery Unit mainly depends on the costs involved in recovery.
Gas Recovery Units usually comprise of feed compression to a higher pressure followed by absorption with a solvent (i.e. naphtha) and a stripper column where the light components absorbed in the process stream are stripped. The rich solvent from the stripper bottom is then routed to a debutanizer column where LPG is separated. The recovered LPG and Fuel Gas undergo further amine treatment to meet H2S specifications.
The objective of this paper is to present the methodology applied to evaluate/optimize the different process schemes and main operating parameters of a Refinery Delayed Coking Gas Recovery Unit for optimum energy/cost effective LPG recovery.
The Delayed Coking Gas Recovery Unit has been simulated utilizing HYSYS® steady state simulation software linked to Aspen Process Economic Analyzer to evaluate the effect of different combinations of process schemes and operating parameters on the products recovery as well as equipment sizing and thus the capital cost, while estimating the required utilities and thus the operating cost.
Two main different process operating parameters have been studied:
Absorber operating pressure
Naphtha (Solvent) recycle ratio
Four different process schemes have been studied:
Including both Recontactor and Presaturator
Including Recontactor only
Including Presaturator only
Without both Recontactor and Presaturator
The Absorber operating pressure and the naphtha recycle ratio have major effect on the LPG recovery. Increasing either of the above parameters enhances the absorption and thus increases the LPG recovery on the other hand increases the utility consumption and thus the operating costs as well as increasing the equipment sizing and thus the capital cost.
The optimum operating parameters resulting in the higher net present value (NPV) and internal rate of return (i.e. higher economics parameters) was found to be an Absorber pressure of 22 bar (i.e. the pressure that can be achieved by 2 compression stages) and Naphtha recycle ratio of 0.42.
The optimum scheme was found to be that including both the Recontactor and Presaturator, although this scheme has extra equipment, however the addition of these equipment result in higher product recovery at lower Naphtha recycle ratio and thus result in savings in equipment sizing and thus capital cost as well as savings in the utility requirements (i.e. power and steam) and thus the operating costs.
Coker Gas Recovery Units are common in most refineries, although the design of this unit is conventional; however, this paper introduces new level of absorber operating pressure which has been proven to be economically attractive in recovering more LPG with reduced operating costs.