|
23/07/2007
|
A:
|
Ian Moore, Jacobs Consultancy, ian.moore@jacobs.com
It is certainly true that the industry has seen a large surge in interest from the refining industry to help define capital investment projects to improve CDU energy performance. This is from a combination of several factors: first, the increased cost of energy, combined in Europe with the desire to drive down CO2 emissions, and second, and perhaps more importantly, the availability of capital to invest outside of just safety-critical projects and stay-in-business projects such as clean fuels. In the last two years, we have executed over ten CDU revamp projects, and we firmly believe that the practical implementation of pinch technology, combined with good engineering judgment, is the optimum path to improving CDU energy performance.
As with all things, a little knowledge can be dangerous. Good engineering practice and know-how beat simplistic pinch analysis combined with poor engineering every time. Therefore, the first challenge is to overcome skepticism in some quarters as to the practical nature of this technology, which is strange considering that the actual track record is very strong. In fact, CDU analysis is almost simultaneously the best and worse place to apply pinch technology within a refinery. Although the benefits are high, the skills requirements for a successful study are also high. Due to the parallel nature of the composite curves, the CDU tends to have a large pinch region where the preheat train is constrained, rather than a single pinch point. This means that some of the tools in the pinch toolbox, such as cross-pinch heat transfer, do not automatically lead the engineer to a better design. As there is normally a single cold stream (crude) and multiple hot streams (products and pumparounds), applying MCP rules around the pinch to meet both energy and area targets leads to excessive splitting of crude, to a degree that is not practical to implement. However, there are other tools such as temperature driving-force plots that come in into their own for CDU analysis. Often, adding new exchangers on a new crude branch is an effective way of reducing pressure drop in the existing exchangers, which are probably operating well above their original design flows. Leaving the main preheat train intact can also make implementation easier and faster, which is an important factor.
A pinch technology study is normally split into two parts, targeting and network redesign. The methodology applied to most refinery pinch studies is to first generate an energy target based on fixed process conditions, as this is a conservative estimate and does not assume process modifications such as new pumparounds that can be capital intensive. The next step is then to systematically explore process changes (such as pumparound and cut point changes) that further improve energy performance.
If these look attractive, they can be verified by process simulation and included in the network redesign.
The benefits of the targeting stage should not be underestimated. How else can a refiner quickly identify the scope for energy saving ahead of any design work? How else can the likely revamp economics be evaluated without designing a single exchanger? Recently, a refiner asked us to perform an energy-reduction study of a crude unit after observing a large reduction in furnace inlet temperature compared to design. The pinch analysis quickly showed that the lower furnace inlet temperature could almost be entirely explained by the lower furnace outlet temperature and was not a result of poor preheat train performance or lack of heat exchanger area. In fact, this avoided the need for a detailed engineering study.
Any retrofit design will look to maximise the efficiency of using existing equipment, and would consider exchanger relocation and repiping to a new service if design pressures and temperatures permit. As the cost of adding new shells is always very high, and preheat trains are normally congested from a space perspective, there is a strong desire to improve the “UA” values of existing exchangers. For example, this can be done by improving “U” (on-line cleaning, tube inserts, helical baffles) or improving “A” (proprietary Twisted Tubes can allow up to 40% new area in an existing shell). Sensitivity analysis can be applied to easily identify these opportunities. Alternative heat exchanger technologies such as proprietary Compabloc exchangers can be considered where space is limited.
A significant challenge for all CDU revamp studies is that, fundamentally, you are capturing low-grade heat currently rejected to air cooling, forcing this through the length of the crude preheat train (or composite curves), to save high-grade furnace heating. Normally, adding just one heat exchanger at the cold end “bumps up” the rundown temperatures to trim coolers of the products matched in the upstream exchangers with little fuel saving. In effect, the gearing between heating at the cold end and saving at the hot end is unfavourable. It is necessary to add area both at the cold end to recover heat and through the rest of the preheat train to compensate for reduced temperature driving forces (to keep the heat in). It is possible to produce significant energy savings, but even with higher energy prices fast payout times should not be expected for major retrofits based purely on energy saving. Therefore, we always look for synergies with other benefits, such as capacity increase, yield improvement or debottlenecking of rundown coolers. A traditional revamp approach quantifies savings based on a single snapshot (test run) of the unit. In reality, the economics normally change between turnarounds as the preheat train fouls, and ultimately feed rate is cut as the crude furnace becomes bottlenecked. New proprietary technology such as HX-NET permits an economic evaluation over the full cycle between turnarounds, providing a much more accurate assessment of revamp benefits.
It should never be forgotten that a project idea identified using pinch technology has to be considered and challenged in an identical way to any other idea generated from best practices or experience. Many ideas generated direct from pinch analysis have to be adapted or modified in some way to meet with plant constraints, and pinch analysis should also be always considered as a tool to guide the engineer. However, conversely, it is rare to find any energy improvement project generated from best practices (such as pumparound duty and temperature changes) that cannot be explained in terms of pinch technology and temperature driving forces. Process know-how is an essential part of improving CDU energy performance, but is even more powerful when coupled with great tools, and pinch analysis is the best tool known today for revamping preheat trains.
|
|
23/07/2007
|
A:
|
Tony Barletta, Process Consulting Services Inc, tbarletta@revamps.com
Even with high fuel prices, it is difficult to obtain high rates of return on energy projects based solely on fuel savings. Staying within heater firing or emissions limits typically drives improving crude unit energy efficiency. For example, if energy improvements can eliminate the need for a new crude heater, the capital investment required to achieve the energy improvements are usually favourable. Achieving high gains in energy efficiency usually requires more than optimising network configuration. It usually requires identifying and rectifying one or more deficiencies in the process flow scheme.
This is the main challenge associated with the application of pinch technology and other simulation or modelling tools. The engineer using the tool must be versed not only in the ins and outs of the tool, but also the ins and outs of the crude unit. Every crude unit is different, each with their own peculiarities, constraints and opportunities. Crude units are highly integrated by definition; separating the distillation from the hydraulics to the heat exchanger train is a recipe for disaster. Process flow scheme changes, such as increasing the HVGO draw temperature by raising the LVGO cut point or adding an MVGO draw, are frequently overlooked. These changes often have enormous effects on the exchanger driving force (LMTD), greatly reduce surface area requirements for a fixed crude heater inlet temperature, and can be the difference between having to add a new heater or not. For new units, the challenge of pinch is not as great, because constraints of the existing unit do not exist. Innovative process flow schemes will only further enhance the application of pinch for grass-roots units.
|