Hydrogen hydroprocessing units

A mixing tee for the hot and cold make-up hydrogen to a hydroprocessing unit leaked near the weld to the downstream piping. SEM examination indicated decarburization and Assuring along the internal surface of the tee. 

Sulfur is removed from a number of refinery process off-gas streams (sour gas) to meet the sulfur oxide emissions limits of the Clean Air Act and to recover salable elemental sulfur. Process off-gas streams, or sour gas, from the coker, catalytic cracking unit, hydrotreating units, and hydroprocessing units can contain high concentrations of hydrogen sulfide mixed with light refinery fuel gases. 

A pretreated (1) residue Maya has been hydrogenated in a hydroprocessing unit provided with a continuous stirred tank reactor (CSTR). 

Due to the presence of a large excess of hydrogen, coke formation in hydroprocessing units is slow - so slow that it can be neglected in material balances for commercial units. However, coke formation is one of four major causes of catalyst deactivation. The other three are poisoning, fouling, and sintering (see Section 4.1). 

Most hydrotreaters and hydrocrackers are trickle-bed units. A classic article by Satterfield " describes the fundamental behaviour of such units, in which mixtures of liquid and gaseous reactants pass down over fixed beds of catalyst. In hydroprocessing units, the liquid reactants are petroleum fractions, and the gaseous reactant is hydrogen. 

Due to the presence of hydrogen, leaks in hydroprocessing units often cause fires. Such fires can be devastating, if not deadly. The replacement of a reactor and the reconstruction of other equipment damaged by the accident can take 12 months. The cost of lost production can exceed US 50 million. Safety concerns are responsible for several operating constraints, such as ... 

Figure 7.12 illustrates the recycle gas circuit throughout a hydroprocessing unit. First, the reaction product is sent to a HPS, where the liquid products are recovered from the gas mixture. Then the gas stream is sent to a scrubbing unit in order to remove hydrogen sulfide and ammonia by contacting with an amine. 

Industrially, hydrodesulfurization of oil fractions, like aU hydroprocessing, is carried out catalytically in a fixed bed trickle flow unit. The catalyst is stacked in a packed bed and gas (hydrogen) and liquid (oil) are fed downstream concurrently. The reactor operates in the trickle-flow regime, in which the catalyst pellets are fully wetted with the liquid and both gas and liquid flow along the external surface. 

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