dx1/dt=-vm*x1/(km+x1)-p1*x1+u1;
dx2/dt=p1*x1-p2*x2;
dx3/dt=p2*x2-p3*x3;
dx4/dt=p3*x3-p4*x4;
dx5/dt=p4*x4-p5*x5;
dx6/dt=p5*x5-p6*x6;
dx7/dt=p6*x6-p7*x7;
dx8/dt=p7*x7-p8*x8;
dx9/dt=p8*x8-p9*x9;
dx10/dt=p9*x9-p10*x10;
dx11/dt=p10*x10-p11*x11;
dx12/dt=p11*x11-p12*x12;
dx13/dt=p12*x12-p13*x13;
dx14/dt=p13*x13-p14*x14;
dx15/dt=p14*x14-p15*x15;
dx16/dt=p15*x15-p16*x16;
dx17/dt=p16*x16-p17*x17;
dx18/dt=p17*x17-p18*x18;
dx19/dt=p18*x18-p19*x19;
dx20/dt=p19*x19-p20*x20;


