FILED OF THE INVENTION
[001] The present invention relates to hydrocracking process in oil refineries in
general, and to capacity enhancement of a hydrocracker unit by decreasing loading in low pressure (LP) section of hydrocrackers in particular.
BACKGROUND OF THE INVENTION AND PRIOR ART
[002] Hydrocracking is a common process used in oil refining industry and refers
to a catalytic cracking process which is carried out at an elevated partial pressure of hydrogen gas. Hydrocracking results in removal of various impurities in a feed (for example, sulphur, nitrogen etc.). Hydrocracking also results in rearrangement and breaking of aromatics and olefinic bonds to produce alkane and napthenes. Hydrocracking units are of major importance for refinery industry, especially those catering to diesel products.
[003] Although the process of hydrocracking is well known, study of the prior art
does not reveal any process or apparatus even distantly similar to the present invention. An analysis of related prior art documents is presented below.
[004] Patent Application US2002127159 discloses a device that comprises an
absorber and a cold separator from which is extracted a gaseous phase that feeds the absorber. Also disclosed is a process for a hydrogen-rich gas or a process for recovery of a hydrocarbon-enriched liquid that uses the disclosed device. However,
1. The device shown has much more equipment (columns, re-boilers, vessels, etc) and its process scheme is entirely different from invented device.
2. The process utilizes separate sub-processes like refrigeration, stripping and absorption unlike the integrated process of the present invention.
3. The process objective and methodology of achieving the objective is entirely different.
4. The process handles different feed as compared to the present invention.
5. Also, the variants described in the document are entirely different.
6. The document does not disclose load reduction in LP section of the Hydro-cracker unit.
[005] Patent No. US 5,462,583 discloses a process for separating a feed gas
stream having components with a spectrum of volatilities including volatile (light)

components, intermediate volatility components, and least volatile (heavy) components and is not related to hydrocracker process, various differences are noted. The process disclosed in this document is capital intensive because it has much more equipments and its process scheme is entirely different from the present invention. The process in this document is directed to generating products whereas our process rejects the lighters and generates an intermediate stream which is fed to downstream fraction section. As such there is no product. Most interestingly, US 5,462,583 discloses a separate solvent regeneration step, which is not required in our process. The process handles different feed as compared to that of ours. This document too does not talk about load reduction in LP section of the hydro-cracker unit.
[006] Patent No.US 3,574,089A does not disclose a hydrocracker process. This
process treats CRU effluents which are mainly Naphtha range hydrocarbon, whereas the feed in the present invention has a bigger range (C1 to VGO) hydrocarbon mix. The process handles different feed as compared to that of ours. The process described has much more equipment and its process scheme is entirely different from our invention. The process is to separate liquid and gases, not for load reduction in downstream section. The process also uses fractionating tower where products are obtained, whereas there is no fractionation in our process. This document too does not disclose load reduction in LP section of the Hydro-cracker unit.
[007] It is clear from the above that the prior art does not disclose any process or
device similar to the present invention.
[008] The present invention is aimed at using the existing equipment and increase
yield of the value added products like LPG. It was observed that when the capacity is increased along with a higher conversion, following difficulties were faced.
i) The main fractionator feed furnace hydraulic load could be increased and thereby may limit.
ii) Main fractionators and all associated equipment like overhead condenser, wet gas compressor, product side strippers and side stripper re-boilers were unable to meet the augmented demand heat duty.
iii) Wet gas compressor was found to be limiting in capacity due to increased demand of polytropic head.
iv) Most importantly, the primary absorber which has high foaming tendency because of low system factor was found to be affected most. This led to LPG loss in fuel gas, and

WE CLAIM
1. A process for enhancing capacity of a low-pressure (LP) section of a hydrocracker unit
with improved LPG recovery comprising of the following steps:
a) feeding the reactor effluent from CLPS (12) and from HLPS (11) to enhanced flasher (VC-1);
b) flashing of combined liquid (10) in surge-cum-flash vessel (VC-1B) into vapour (3) and liquid (4);
c) washing combined vapour (5) in re-absorber (VC-1 A) by cooled absorbent liquid (7) for selective absorption of C3+ components from combined vapour
(5);
d) releasing the lighter components (H2, Ci and C2) from rich absorbent liquid (6) due to mixing with hot liquid pool (14); and
e) withdrawal of lean reactor effluent (9) from (VC-1B) and routing it to normal LP section of hydrocracker at the upstream of MF furnace preheat exchanger train (13);
characterised in that the process increases capacity of Low Pressure section of hydrocrackers by 20 to 30% including wet gas compressor leading to corresponding increase in LPG, MS, ATF and Diesel products and in that the process generates the absorbent liquid for its own requirement.
2. The process as claimed in Claim 1, wherein the LPG recovery is increased from 82% to 94% even for enhanced unit capacity.
3. The process as claimed in Claim 1, wherein LP section column loadings are reduced by 20% to 25% even for enhanced unit capacity.
4. Enhanced Flasher (VC-1) consisting of the following major components:

a) a surge-cum-flash vessel (VC-1B);
b) a top mounted re-absorber (VC-1 A) having a mass transfer bed of packing material, (VC-1 A) and (VC-1B) being fully integrated and in fluid communication and
c) absorbent re-circulation pump (PM-1) along with a cooler (HM-1);
characterised in that (VC-1A) mounting on (VC-1B) is designed in such a way so that rich absorbent liquid (6) gets sufficient residence time to liberate lighter components (H2, C1 and C2).