[0007] Second, the yield of C2+ enriched
product gas is low, generally 70-85% or less, especially after the PSA enriched C2+ device has been in operation for more than 1-2 years, the yield of product gas drops to about 60%
Since the product obtained from the adsorption phase is subject to the saturated adsorption capacity of the product gas component on the adsorbent and the concentration of the product gas component in the
dead space of the adsorption
tower, the concentration of the product gas (C2+) component must be high. In the
desorption process, the product gas is used for replacement, so that the C2+ concentration in the adsorption
tower is at a higher level during the
desorption process, which is beneficial to the C2+ in the
desorption gas (i.e. product gas) formed in the reverse
discharge or evacuation step. The concentration is up to the standard, but the larger the amount of replacement gas (product gas) required for replacement, the more replacement
waste gas will be produced, the more effective components C2+ will be discharged with the
waste gas, and the yield of C2+ will be higher. Low, although a part of the replacement
waste gas is returned to the feed gas to recover C2+
If the C2+ concentration in the
raw material gas is lower than the
methane concentration, then, in order to ensure that the C2+ concentration in the C2+ enriched product gas reaches the standard, the replacement gas will be used more, and the replacement waste gas will carry more C2+, and the yield will be higher. will be lower;
[0008] Third, the contradiction between the purity and yield of C2+ enriched product gas is very prominent and complicated, and even serious "double drop" phenomenon will occur, and it is impossible to extract product gas from non-adsorbed phase PSA in actual operation. (such as H2), reduce the product gas purity to maintain or increase the yield, or, reduce the yield to maintain or increase the product gas purity
In addition, the product gas molecules of the latter are relatively small. When deep adsorption occurs, appropriately increasing the displacement gas circulation and vacuum degree is to a certain extent a method to solve the difficulty of regeneration caused by deep adsorption; while the former (the prior art ) product gas has a relatively
large molecular weight and strong polarity, and is prone to deep adsorption. At this time, increasing the displacement gas circulation or vacuum degree will not have a great effect, and it is difficult to regenerate the adsorbent.
When the C2+ concentration in the adsorption
tower is too high, it will further prevent the
mass transfer process of the adsorbed C2+ components from "dissolving" into the replacement
gas phase, and on the contrary, it will further deepen the deep adsorption, and then, the replacement gas volume of the former (prior art), Replacement time, replacement pressure and temperature have a great influence on the replacement efficiency
Therefore, the replacement effect in the prior art is more to
push out the methane co-adsorbed in the
dead space of the adsorption tower
bed layer in the replacement step, but returns to another group of adsorption with the replacement waste gas part as feed gas.
Methane co-adsorption and accumulation occurred in the tower
bed layer, resulting in the methane in the C2+ enriched product gas exceeding the standard, reaching more than 8-10%; About 4%, it can only minimize the replacement of waste gas and return to the raw material gas to avoid the accumulation of methane, which will also lead to a further decline in the yield of C2+, and the decline is very large, from the current design value of 86% to 70%.
If raw gas fluctuations occur in actual working conditions, the yield of C2+ will be further reduced; therefore, it is necessary to extract C2+ from the refinery dry gas adsorption phase PSA with a wide boiling range, especially in the working conditions where the methane content is relatively high. Starting from the first-stage adsorption, an additional replacement step is adopted, and its effect on improving the purity (concentration) of the C2+ product gas is very limited, and the inverse ratio between purity and yield is very prominent, and even the "purity and yield decrease at the same time" will appear. "Double drop" phenomenon;
[0009] Fourth, the service life of the adsorbent is short, which affects the stability of the C2+ enrichment device
For working conditions with a wide boiling range of feed gas components, such as refinery dry gas enrichment of C2+, since the polarity of the effective component (C2+) is between polar
impurity components such as CO2 and H2O and non-polar impurities such as CH4 and CO, Among the polar
impurity components, cross-
contamination of the composite
bed is more common. The cross-
contamination of the composite bed in the process of concentrating C2+ by primary (grade) PSA is inevitable, which makes the regeneration of the adsorbent itself incomplete and the service life will be greatly reduced. the reduction;
[0010] Fifth, the impurity components of the feed gas are complex, such as
sulfide,
heavy metals,
arsenic, mercury, CO2, water, and C6+, which are toxic to the adsorbent, which greatly shorten the service life of the adsorbent in the prior art
However, impurity components such as sulfides,
heavy metals,
arsenic, mercury, water, and C6+ in the raw gas have a greater poisoning effect on the adsorbent loaded with
copper active components, deactivating the
active components and greatly reducing the adsorption capacity of the adsorbent. The ability to adsorb C2+ is high, and the service life of the adsorbent is also greatly shortened.
The purification process in the prior art is placed in the section after the output of the product gas, which cannot prevent the poisoning of the adsorbent in advance;
In a traditional multi-tower PSA
system, one or several towers are adsorbed, while the rest of the towers are in different desorption steps, resulting in frequent switching of cycle operations, making the timing arrangement relatively complicated
The existing technology adopts two sets of parallel connection, the chance of reverse
concentration gradient phenomenon in each adsorption tower in the
system is greatly reduced, and the regeneration cycle operation is relatively easy to control, so that there is enough time for the replacement step
However, since the cyclic operation of adsorption and desorption needs to be matched in time, the replacement step is added in the desorption process, which will correspondingly increase the adsorption time.
Therefore, for the C2+ stronger adsorbate component, deep adsorption is prone to occur, and the deep adsorption for a certain period of time further increases the capillary phenomenon in the adsorbent channel, making the adsorption and desorption mechanism more complicated and more deviated from the original design. The adsorption model used in the process, such as the multi-component
Langmuir-Friedrich (L-F) model, etc., the concentration distribution and
mass transfer process in the adsorption bed and in the pores of the adsorbent particles will change. The mismatch between adsorption and desorption becomes more and more serious, and finally affects the service life and separation efficiency of the adsorbent, directly affecting the purity and yield of the product
At this time, the raw material gas fluctuates, and the mismatch between adsorption and desorption becomes more serious, and the yield or purity will inevitably appear "double drop".
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