DMCplus控制器可以通过多种方式改善过程单元的操作和利润率。在多数情况下，增加利润的最好方式就是提高单位产量。对很多单元而言，通常将进料增加到系统接近激活到一些下游约束变量。
由于流程的多变量特性，可通过操作其它几个变量消除该约束，但任一其它变量的改变都会激活其它一些约束。
例如，在流化床催化裂化装置中（FCCU），将进料增加直到湿气压缩机达到它的速度上限。增加吸气压力设定值可以缓解速度限制，并允许更多的进料被引入，但吸气压力增大过多会激活再生催化剂滑阀压差下限。
这一限制可通过增加再生器压力设定值解除，但再生器压力设定值的增加会激活鼓风机速度上限，故也不能增加过多。
对过程进一步复杂化分析得到的事实是，进料、吸气压力以及再生器压力都会影响压缩机速度、再生催化剂滑阀压差和风机速度。
如上述例子所示，这是一个高度多变量过程。最佳操作点可能在压缩机转速、再生催化剂滑阀压差和风机转速都在极限内的最大产量处。
在化学反应单元中，提高利润的最佳办法是提高产率。大多数过程单元都是高度多变量的。加氢裂化装置有几个串联催化剂床层，每一床层入口都有急冷流股流入。通常情况下，每一床层都有一进口温度控制器控制急冷流股。为防止反应失控，每一催化剂床层受总床层温升（温度增量）约束。
为了增加产量需要更严格地对待关键指标，这可能意味着需要将其中几个催化床层运行在上部床层温升限或床层骤冷阀上限。
由于上游床层出口温度将对下游床层骤冷需求产生影响，有必要考虑过程多变量性质以获得最好收益。
若一单元中的某个产品相较于其它产品有较高的价值，通常可通过最大化回收该产品获得利润最大化。例如，天然气液态工厂（NGL）中，它有超过150＃的蒸汽（蒸汽是免费的），脱乙烷塔从丙烷（如液化石油气出售）中分离乙烷（燃料）。
由于蒸汽是免费的，并且丙烷价值比乙烷高得多，运行该塔利润最高的方式是最大限度地从乙烷中回收丙烷（顺带在产品规范允许的范围内于丙烷产品中留下尽可能多的乙烷）。
在另一些情况下，利润最大化通过公用工程消耗最小化得到。例如，一个分级精馏生产苯、甲苯和二甲苯的BTX工厂。相较于产品价值，在此工厂中蒸汽成本更为显著。
在这种情况下，将所有产品控制在产品规格所允许的最大杂质含量以节约能源是非常有必要的。这将节省可观的蒸汽消耗量。由DMCplus控制器带来的价值是通过努力监测过程干扰，推动产品卡边规范，主动采取动作使干扰在影响过程前得到修正。
在一些经常受到相当大且频繁干扰的单元中，稳定单元操作基础将DMCplus控制器严格限定。经济效益可以体现在下游控制器中。例如，加氢裂化装置中分离C4（塔顶）和C5（塔釜）的Pre-Frac塔。气态塔顶产物被送入气体厂，塔顶储液器液位由操作员控制。
单元频繁进料组成变化将引起反应器床层平均温度（WABT，重要指标）变化，这将会大大影响Pre-frac塔，自然也会影响天然气厂。
该塔委托运行的DMCplus控制器可以稳定运行。因为稳定了Pre-frac塔天然气厂进料，减少了天然气塔的能源消耗，天然气厂将获得经济效益。
一个典型的DMCplus应用程序将依据部分（或全部）上述列出的方式使利润最大化。DMCplus控制器可根据其内置的线性程序辨识最佳操作点（自动辨识是最大产量还是最小公用工程消耗）。
事实上，由于产品价值和原材料及公用工程成本的变化，最佳操作点可能从产量最大化变化到处理量最大化。例如，催化裂化装置（FCCU）在夏天可以运行最大化以生产更多汽油，然而在冬天则使燃油处理量最大化。DMCplus控制器的优点在于无需修改控制器以适应过程经济情况变化。
附原文：
A DMCplus controller can improve operation or profitability of a process unit in a variety of ways. In most cases, the best way to increase profit is to increase unit throughput. In many units, feed is increased until some downstream constraint becomes active.
Due to the multivariable nature of the process, several other handles exist that could relieve that constraint, but each of them will cause some other constraint(s) to become active.
For example, on a Fluid Catalytic Cracking Unit (FCCU), feed is increased until the Wet Gas Compressor is at its upper speed limit. Increasing the suction pressure setpoint will relieve the speed limit and allow more feed to be introduced, but the suction pressure can only be increased to the point that the Regenerated Catalyst Slide Valve differential pressure reaches its lower limit.
This limit can be relieved by increasing the Regenerator pressure setpoint, but it can only be increased to the point where the Air Blower reaches its upper speed limit.
Further complicating the picture is the fact that feed, suction pressure, and Regenerator pressure all affect the Compressor speed, the Regenerated Catalyst Slide Valve differential pressure,and the Air Blower speed.
As this example demonstrates, the process is highly multivariable. The best operating point might have theCompressor speed, the Regenerated Catalyst Slide Valve differential pressure,and the Air Blower speed all at their limits in order to truly maximize throughput.
In units that contain a chemical reaction, the best way to improve profit can be by improving yield. In most process units, once again the problem is a highly multivariable one. A Hydrocracker has several catalyst beds in series, each with a quench flow to the inlet of the bed. Normally, each bed has an inlet temperature controller manipulating the quench flow. Each catalyst bed is subject to a total bed temperature rise(delta temperature) constraint, to prevent a runaway reaction.
To increase yield will require increasing severity, which could mean running several of the catalyst beds at either the upper bed delta temperature limit, or the upper limit of the quench valve for that bed.
Since upstream bed outlet temperatures will affect downstream bed quench requirements, it is essential to consider the multivariable nature of the process to obtain the best yield.
In units where one of the products has a very high value relative to the other products, profit can often be maximized by maximizing product recovery. For example, in a Natural GasLiquids (NGL) plant which has an excess of 150# steam (steam is free), the Deethanizer is separating ethane (fuel value) from propane (sold as LPG).
Since steam is free, and since the propane value is considerably higher than ethane, the most profitable way to run the tower is to maximize propane recovery from the ethane (and incidentally, leave the maximum amount of ethane in the propane that products pecifications allow). The limit to recovery in this case is normally either atower flooding constraint or a condenser constraint.
In still other cases, the maximum profitis observed by minimizing utility consumption.For example, a fractionation train in a BTX Plantproduces benzene, toluene, and xylene. The cost of steam in this plant is significant when compared to the relative product values.
In this case, it is worthwhile to save energy and control all products at the maximum impurity allowed by products pecifications. This produces considerable savings in steam consumption. The value added by the DMCplus controller is in diligently monitoring the process for disturbances, which would drive the products off specification; and making proactive moves to correct for incoming disturbances before they affect the process.
In some units that are subject to frequent and sizable disturbances, it may be that a DMCplus controller is justified strictly on the basis of stabilizing unit operation.The economic benefits can show up in downstream controllers. For example, a Pre-Frac Tower in a Hydrocracker Unit separates C4's (overhead) from C5's(bottoms). The overhead product flow is a vapor that goes to a gas plant, and the overhead accumulator level control is performed by the operator.
Frequent feed composition changes to the unit cause changes in the reactor Weighted Average Bed Temperature (WABT, ameasure of severity), which dramatically affects this Pre-frac Tower, and consequently, the gas plant.
A DMCplus controller commissioned on this tower can stabilize the operation. The economic benefits are seen in thegas plant itself, since stabilizing the Pre-frac tower steadied out the gasplant feed, resulting in less energy consumption by the gas plant towers.
A typical DMCplus application will maximize profit in several (or all) of the ways listed above. The DMCplus controller, with its built-in linear program, can identify the optimum operating point whether that point is at maximum throughput or at minimumutilities consumption.
Indeed, as product values and costs of raw materials and utilities change, the optimum operating point can change from maximizing yield to maximizing throughput.For example, an FCCU can run at maximum severity in the summer to make gasoline and maximum throughput in the winter to make fuel oil. The advantage of a DMCplus controller is that it can account for changes in economic conditions and implement these changes on the process without modifying the controller.
2015.9.12