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Mar-2018

Improved economics with increased condensation

Attention to the influence of heat transfer mechanisms will promote better economic performance in heat exchanger operation.

TOM HIGLEY
Calgavin
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Article Summary
With the process industry and economic markets publicising the ever-changing oil and gas markets and areas where investment is being seen, growth sectors within the hydrocarbon business have changed considerably within the last five years. With the price of oil dropping from over $100/bbl to a low of $22/bbl in two years, then fluctuating and rising over the past year, return on investment has been key.

As it stands, expansion in upstream exploration has slowed down as raw crude value is no longer at the heady heights it once was, and this has translated down to refineries, many of which have no active expans ion plans. The value has been pushed downstream towards petrochemicals and chemicals, where we are seeing ethane as the feedstock of choice in the West and naphtha as the choice in the East. But overall worldwide, we are seeing olefin expansion, with investments being accelerated by the shale gas boom in the US.

With any case of expansion, there are two types of activity: pressure on producing more with existing plants, while in parallel new plant complexes are being built to utilise the ever-growing olefin feedstocks.

It has been reported that polypropylene production is seeing an increase in countries such as Brazil, while a new complex with $1 billion investment will be seen in Iran for a methanol to polypropylene plant. This illustrates that polyethylene and polypropylene production is all part of the petrochemical/chemical expansion story.

Polypropylene is a vital thermoplastic polymer. It is an important material in manufacturing for many everyday items, especially in the automotive industry, therefore efficient production of polypropylene will increasingly be needed to meet the demand.

Polypropylene production
The feedstock needed for polypropylene production is propylene processed via steam cracking of LPG or naphtha from refineries and first generation petrochemical plants. The general process flow diagram shown in Figure 1 illustrates the basic units within the process.

In layman’s terms, propylene feed, along with catalyst and some hydrogen and silane, enters the reactor and the reaction takes place. Any unreactive monomer vapour within the polypropylene effluent is recycled back to the reactor after being processed through the discharge vessel. The remaining effluent product then continues to a purge vessel. It is here, together with nitrogen, that polypropylene resin is purged and any off-gases exit together with the nitrogen. This off-gas stream is captured through membrane technology and any off-gas with traces of polypropylene enters a vent gas recovery condenser, to condense as much product as possible to re-enter the purge vessel. The gases that are not condensed will either be recycled back into the reactor or to flare. The process then continues with polypropylene powdered resin exiting the purge vessel and generally conveying to storage silos prior to the final extrusion process.

It is clear that in this specific processing practice, the heat exchangers required play a vital role in product quality and throughput. For example, if the vent gas recovery unit was not operating as per the design, any product still in the vapour phase would be either recycled or lost and flared to atmosphere, losing valuable product. There is another important heat exchanger unit to consider that cools the reactor vapour before being recycled back into the reactor. If the output temperature of the recycled stream is above the design specification, this will affect both the reaction kinetics of the reactor and product quality.

With heat transfer performance shown to affect two vital aspects of the product, quality and volume, added competency and understanding of heat exchangers is needed for efficient processing of products such as polypropylene.

Heat transfer considerations
Many commercial design software packages will use fundamental heat transfer correlations which give market standard accuracy, but these correlations are restricted by some of the assumptions made in the initial models.

One such assumption to highlight is the assumed uniform velocity flow per tube within heat exchangers, when in reality this is not necessarily the case. For viscous process fluids, extremes in differences of velocities per tube can occur and result in maldistributed flow within headers and bundles. This is not ideal for efficient heat transfer and modelling performance.

Another assumption is that when condensing or vaporising two-phase applications the two phases are thoroughly mixed along the tube length, when for various applications this is not necessarily true.

In recent years, software packages have been able to identify extreme occurrences of both, but due to mass transfer influences as well as heat transfer, they are inherently difficult to estimate.

For engineers, using modelling software alone is not always sufficient to improve processing. This takes a combination of on-site readings, discussions with maintenance engineers, and ‘auditing’ individual, specific, important exchangers to fully understand the problem and the root cause.

For consulting heat transfer engineers using a combination of commercial modelling heat transfer software, computational fluid dynamics (CFD) tools and asking the right questions, achieving practical solutions is possible.

In general, when taking an ‘auditing’ approach to individual heat exchangers, the typical possible root causes of poor performance can be failing to understand fouling mechanisms and how changes to process parameters can influence under-performance due to changes in environment from the design stage such as higher ambient air temperatures. Additional causes also include maldistributed flow due either to poor mechanical design distribution in the pipework/header nozzles or to extreme viscosities of process fluid and pumping limitations. Mechanical fatigue can also be brought about by factors such as tube vibration or thermal stresses. Closely linked to this are corrosion/erosion issues brought about by either galvanic corrosion/material resistance or high local velocities, and finally control issues with old instrumentation or lack of instrumentation to ensure correct operation of key heat exchangers within processes.

Condensation
In this article, the mode of heat transfer whereby vapour is cooled to the liquid phase by condensation is explained in detail. Estimating the performance of this heat transfer mechanism for certain process fluids will be addressed.

The rate of condensation is dictated by a number of factors, one being the temperature at which condensing takes place; just as important are the characteristics of the process fluid that is condensing. When evaluating condensation, one of the first questions asked by Calgavin engineers is whether the process fluid is a pure component or a multi-component mixture.
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