Atmospheric Distillation Unit (CDU) – Step number one in any modern refinery energy assessment and rehabilitation program is to obtain a reliable, reconciled and validated plant wide mass and energy balances that reach to the unit energy balance level. Such exercise will enable the plant operators/engineers to reliably calculate and report the facility actual energy loss. Comparing such actual losses with the benchmark/targeted ones based upon agreed upon criteria will result in the unit(s) and total refiner gap in energy consumption.
Atmospheric Distillation Unit (CDU) | Energy Efficiency Guidelines in Refinery
The areas of inefficiency in any process unit can be pinpointed and evaluated.
The completion of this step will then be followed by energy conservation initiatives generation phase.
Atmospheric distillation unit simplified energy loss analysis can be conducted as follows:
The CDU graph below is the starting point.
We set performance criteria for all the CDU products and pumparound streams, from the target temperature/specifications point of view using tight tolerance(s), at given/certain unit feed conditions and product desired specifications assumed constant,(otherwise, develop a case for each feedstock type and/or each product mix case required (using CDU model/industrially verified CDU simulation model).
We set performance criteria to the CDU fired heater(s) stack temperature, excess air percentage, atomizing steam, if any.
We set performance criterion for stripping steam consumption Lbs/hr, converted to BOE/D units per certain type of crude oil processed (e.g., AH crude with feed rate of 100,000 B/D) and at given product mix with certain specifications( using industrially verified simulation models), (source of this steam, such as in-process generation; boilers, co-generation or mix is important to be considered) and We set performance criterion for power consumption per day (including crude oil charge pumps, air coolers and water cooling pumps) also converted to BOE/D units.(source of power purchased, in-house produced or mix is important to be considered).
We list the performance indices values current/actual, measured and/or calculated on timely basis/short time spans, at steady state conditions, or on daily average basis.
We calculate the delta/gap between actual energy consumption values and the set performance criteria values (using pinch technology and simulation software), due to its deviation from what we are calling it targets…calling it benchmarks (or as I prefer calling it guidelines) and express the gap/delta in each performance index in BOE/D and get the summation of all indices energy consumption gaps, again in BOE/D.
It is instructive to emphasize here on the importance of having properly “verified”
against the process simulation models for the refinery, especially for the distillation
columns and the heat exchangers network (pre-heat train). Such models will be used
for benchmarking, gaps calculations, testing/ prediction of fouling/proactive cleaning
needs and so on.
Upon the completion of the simplified process shown below, we start pinpointing the
major areas of inefficiencies in the CDU and start investigating how to improve it.
Crude Atmospheric Distillation Unit (Energy Loss Analysis)
Table 1 Illustrating table of a simplified energy loss for CDU:
*Assuming 100,000 B/D CDU and one BOE is equal to 5.73 MM Btu.
In brief, we measure and/or calculate actual consumption of fuel, power and steam, in BOE, and each criteria actual value listed above in our CDU. Then, for each of one of such
criterion, we calculate the excess energy being used because of the actual conditions which vary from criteria performance values, using equipment efficiency calculation models, process simulation models and heat integration software.
If criteria conditions are not specified using simulation and/or pinch technology software
use your best judgment values from experience and/or historical data.
It is important to mention here that we shall not forget the adjustment of energy consumption due to export, import and in-process generation.
In conclusion, besides the equipment based inefficiencies discussed earlier in this manual, the three “elephants” in energy efficiency optimization in the atmospheric distillation” unit” CDU are normally in, the furnace stack gas heat recovery, the overhead condenser and side streams; pumparounds; and bottoms heat integration with the incoming crude oil stream.
Some easy opportunities here can be captured via the recovery of the bottoms heat losses via the installation of steam generation or boiler feed water preheating systems.
To lower stack temperature an air preheating system or steam generators could be added to the convection section of the fired heater, provided that low sulfur fuel is fired and environmental regulations permit. Other losses in overhead condenser and side streams and pumparounds need more involved solutions using heat integration techniques (pinch technology and others) as will be shown in the next section.
However, it is important to note here that, fouling is also an important factor for efficiency losses in the CDU, and within the CDU, the crude preheater is especially susceptible to fouling. Initial analysis on fouling effects of a 100,000 bbl/day crude distillation unit found an additional heating load of 13.0 MJ/barrel processes. Reducing this additional heating load could results in significant energy savings.
Heat Integration in Atmospheric Distillation Unit (CDU)
The CDU process all the incoming crude and, hence, is a major energy user in all refinery layouts (except for those refineries that receive intermediates by pipeline from other refineries). In fact, it is estimated that the CDU is the largest energy consuming process of all refinery processes. Energy use and products of the CDU depend on the type of crude processed.
Process integration is especially important in the CDU, as it is a large energy consumer processing all incoming crude oil. Older process integration studies show reductions in fuel use between 10 and 19% for the CDU. An interesting opportunity is the integration of the CDU and VDU, which can lead to fuel savings from 10-20% compared to non-integrated units, at relatively short paybacks. The actual payback period will depend heavily on the layout of the refinery, needed changes in the heat exchanger network and the fuel prices.
The intra-unit integration of CDU in a modern refinery even though can be performing well with limited opportunities for further economic process integration will be better off upon integration with other refinery units. An analysis of such CDU upon integration with the refinery residue cracking unit can offer significant opportunities to reduce the combined heating demand by 35-40%.
In a typical crude atmospheric distillation unit feed get separated to different cuts according to the difference in boiling points. Crude feed get heated up from the ambient temperature to the desalting temperature, normally between 126 and 140ºC. After desalting operation that reduces the crude temperature between 3 to 6ºC, crude is fed to the pre-flash drum or sometimes even tower to remove some of the light hydrocarbons before the crude goes to the crude furnace.
As shown in the generic crude atmospheric distillation; above, the products from the crude tower and circulating pumparounds that are used as tower inter-coolers are used to aid the products in heating the incoming crude. The incoming crude then is fed to the heater to raise its temperature to the desired flash zone temperature in the atmospheric distillation column.
It is always of high importance in the design of this unit to integrate the cold stream represented by the crude feed stream and products and pump-around hot streams to reduce the heat load on the crude heater as much as possible. In this chapter, a real industrial application is introduced and discussed.
Table 2 below, shows data extracted from plant process flow diagrams for the crude as a cold stream and other hot streams. The cold stream is segmented into several streams using its simulation heating curve to avoid in-accuracy for assuming constant specific heat along long temperature range. The same is also practiced for some hot streams as shown in the table, using its simulation cooling curves.
If we accept the argument sometimes raised by some companies regarding the need for simply network for easy operation! And less headache due to fouling potential, we can start with the simplest one train, no split design and evolve in our modification to save energy with minimum addition of complexity to the HEN. Such approach is adapted in this brief of this energy conservation manual.
The first step in applying pinch technology is to search for a reasonable minimum approach temperature the minimum heating utility and minimum cooling utilities required for this crude unit. These targets are shown in figure below.
The heating utility which is our main focus in this application is about 48 MM Kcal/h.
Using pinch design method (PDM) to synthesis the crude pre-heat train, to reach desired level of waste heat recovery and to minimize the heating load of the crude heater results in the network below. The network exhibits three splits in the cold crude stream. First split happens before the crude desalter, the second after the desalter and before the flash drum and the third after the flash drum up to the crude heater.
As you notice both heating and cooling utilities are considered being of the same important. In most cases in oil refining heating utility is more expensive than cooling water. Therefore, many designer put more emphasis on saving in heating utility since it also reduce emissions than saving on the cold side. Another very important objective in designing new pre-heat train in crude distillation unit is the number of units of the heat exchangers, less sophisticated configuration and the easiness of heat exchanger network operation and maintenance.
This need sometimes push the plant designers to the extreme in considering the easiness of operation to the extent that they design networks that do not give enough attention at all to decent level of waste heat recovery and GHG emissions reduction compared with their emphasis on capital investment, maintenance and operation of the network.
The former approach is demonstrated here in the schematic pre-heat train design shown
below in Figure 12 for the same industrial application in hand. The schematic network
shown is only used for topology comparison.
The design exhibits less number of units, no split at all, but much less crude temperature
before the crude heater. This design means that the approach advocating the easiness of
operation and minimum number of units results in more energy consumption in the crude
heater and consequently more GHG emissions.
It is important to note here that maintenance and easiness of operation of heat exchanger networks in crude oil refining facilities are very legitimate issues. However, it needs to be handled with fear and at the end a balanced picture and right trade-off based upon economic should prevail over the argument of we did not do designs like this before, or our operators are not trained to operate such sophisticated network or even we do prefer this one since it has no splits and less potential for fouling and cleaning.
The third network shown below set the right compromise to some facilities. It is designed differently than the first network since it does not use the pinch design method.
However, some sense of systematic technique has been also used here. This network
design evolves from the simplest design desired by most of the process owners and plant
operators. It has less number of units, no splits, less possibility of fouling and frequent
cleaning needs, easy to operate and more importantly less area and capital cost.
The pre-heat train in Figure 13 kept the simple design before the de-salter almost as it is and focused only on the heating utility minimization. It tried to push the temperature before the crude heater from 265°F in the simple design to about 276ºF in the new design via better heat recovery in the area after the pre-flash drum only. A systematic technique can be used for that purpose via enumerating all matches possibilities between the crude stream, after the pre-flash drum, and all hot streams available at temperatures higher than its supply temperature.
Simulating these possibilities and rank them based upon the impact on the heating utilities requirement but with only one split in the crude stream after the pre-flash drum to avoid increasing fouling, will conclude this step and move the approach to the second step. The second step then is to reconcile the selection of the new matches after the pre-flash drum with that before it. The possibilities that can arise can also be ranked based upon the level of simplicity in it.
The point that we are trying to make here in this crude unit pre-heat train design is that we can design the heat exchanger network using pinch design method to get the best possible waste heat recovery and try in an ad hoc matter to simplify the network. By the way, this task is most of the time not easy doing as saying. On the other hand we can start from the simplest possible straight line network and evolve from it systematically to reach to better networks from waste heat recovery point of view and of-course GHG emissions reduction while keeping the easy to operate, clean maintain and build network objectives satisfied. The details of this new approach and its step-by-step implementation will be published latter elsewhere.
Pinch technology as a new systematic method for advanced waste heat recovery and heat integration in industrial facilities emanated in the early seventies during the first oil crisis is still nowadays the most widely used technique for energy integration in oil industry. It has been successfully used to systematically address the problems of energy efficiency optimization and the reduction of energy-based un-desired emissions.
Systematic waste heat recovery in oil and gas industry is very beneficial to plant operating cost reduction. Aggressive waste heat recovery is an essential approach for in-process GHG emissions avoidance. GHG and other energy-based undesired by-products atmospheric emissions, produced during oil and gas separation processes and crude oil distillation can be reduced significantly through proper application of heat integration concepts.
Every megawatt of heating utilities obtained from process boilers and/or furnaces that can be saved through efficient waste heat recovery system design and operation means less greenhouse gas and other harmful NOx and Sox emissions.
Improved heat recovery systems designs while can be attained systematically using Pinch Technology, it is important to consider it in a step-by-step approach especially for the heat exchangers network grassroots design modifications and existing plants retrofit to enable the decision makers selects the right scenario that best fit his/her capital investment budget.
Now remember the 10 compliance points below and assess your facility accordingly:
– Heat integration shall be applied with global ΔT_min less than or equal to 30ºF and the right use of utility level
– Process-Utilities system shall exhibit best possible synergy (using right amount of steam; minimum steam reserve; no vent minimum steam-air cooling)
– Efficiency of rotating equipment shall be more than 80-85%
– Efficiency of fired heaters shall be more than 90-92%
– Distillation columns shall be integrated with the rest of the process or among themselves and columns with long difference in its temperature profile shall be adapting inter-coolers or inter-heaters design features.
– Inlet feed to compressors shall be as cold as possible and to turbines as hot as possible.
– Turbines are used whenever possible and let down valves and let down drums shall be minimized.
– The process products’ temperatures shall not be higher than the feed temperatures, and process temperatures to air coolers shall be lower than 200ºF.
– “Source” Processes shall be integrated with adjacent ones and/or produce heating and/or cooling utilities.
– Heat transfer equipment shall always exhibit high U, as high as possible.